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1. Water Water plays a vital role in our life. It is most abundant, wonderful and useful solvent.
Although it is the most abundant commodity in nature it is the most misused one also.
80% of the earth’s crust is covered with water. The quantity available for actual use in
the form of rivers, lakes, wells and ponds is hardly 0.5% of the world’s water
resources. This is because more than 96% of water is locked in oceans which are too
saline to drink or to be used directly for agricultural, industrial or domestic purposes.
2% of the water is locked up in polar ice caps and glaciers. About 1% is deeply
underground and not accessible. Due to rapid industrialisation, urbanisation and
growth in population man has successfully polluted most of the water available on
earth. Industrial and domestic waste has caused significant—pollution of the aquatic
ecosystem (Trivedi and Goel 1986). Hence monitoring and control of pollution is
essential for better future. 1.1 - SOURCES OF WATER (i) Surface waters: (Rivers, lakes, seawater, etc.) Water present on the surface is
called surface water. River water, stream water (flowing water, moorland si-face
drainage) as well as water in the ponds, lakes and reservoirs (low and surface
drainage) is called surface water. (ii) Underground water: (Wells) Some part of rain water penetrates through the
soil. It goes down and down till it reaches impermeable rocks. If the top of this
rock is flat, it stays there. If the layers of rock have slope, water will flow the
slope down. We get this water in the form of well or spring water. Water from
lower measures of coal mines is also underground water. (iii) Rainwater: it is purest form of water obtained by natural distillation 1.2 - IMPURITIES IN NATURAL WATER When rainwater reaches the earth and flows on the earth, it becomes impure
because of absorption of impurities. The absorbed impurities are of the following
types. (i) Dissolved gases: Gases like oxygen, nitrogen, carbon dioxide, etc. from the
atmosphere dissolve in water and make the water acidic. Lake water contains
more carbon dioxide due to biological oxidation of organic matter present at the
bottom of the lake.
C6 H12O6 + 6O2 bacteriaAerobic
6CO2 + 6H2O
Hexose
C6 H12O6 Anaerobic
bacteria 2C2 H5OH + 2CO2 Hexose
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The colour and odour of the natural water is due to the presence of dissolved
organic matter. Underground water is colourless and odourless, but some deep
well water possess rotten egg’s smell which is due to dissolved hydrogen sulphide
(H2S). Well water of wells located in oil and gas areas contain dissolved methane.
(ii) Suspended matter: Surface water appears turbid due to the presence of finely
divided impurities, which remain suspended in water. These impurities—clay
particles, iron hydroxide, silica which are inorganic type while decaying vegetable
and animal matter which are organic type are called suspended impurities. They
are negligible in underground water because of filtering action of the soil.
(iii) Micro organism or bacterial impurities: Micro organisms like algae, fungi
and bacteria are present in surface water.
(iv) Dissolved mineral salts: When rainwater falls on the ground it reacts with
rocks and different minerals present on the earth. Salts like sodium chloride,
calcium chloride, sodium nitrate, dissolve in water; carbonates of calcium and
magnesium get converted to bicarbonates by the action of carbon dioxide from
water.
CaCO3
+ H2O
+ CO2
Ca(HCO3)2
Calcuim Carbonate
Calcium bicarbonate
MgCO3
+H2O +CO2
Mg(HCO3)2
Magnesium Carbonate
Magnesium bicarbonate
Thus, because of dissolution of many salts water becomes impure. Underground
water contains more soluble salts than the surface water.
1.3 - HARDNESS OF WATER
Hardness can be defined as the soap consuming capacity of water sample. Soaps
are sodium salts of fatty acids like oleic acids, palmetic acid and stearic acid. They
dissolve readily in water to form lather due to which it has cleansing property.
C17H35COONa C17H35COO-+ Na
+Sodium
stearate
But compounds of fatty acids with other metals do not dissolve in water If water
contains other metal ions like calcium and magnesium ions, they react with
sodium salts of long chain fatty acids to form insoluble soap which we observe as
curd.
2C17H35COONa + Ca++
(C17H35COO)2Ca
+ 2Na+
Calcium stearate These other metals ions are responsible for the hardness of water. Most important
metal ions which cause hardness to water are calcium and magnesium ions. The
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hardness of water can be calculated from the amounts of calcium and magnesium
ions present in water along with bicarbonates, sulphates, chlorides and nitrates.
The relation between the type of water and degree of hardness is as given below.
Type of water Hardness as ppm of CaCO3
Soft 0—75
Moderately hard 75 — 150
Hard 150— 300
Very hard above 300
Standards of water for drinking - As per Indian Standards (IS: 10500-1983)
Sr. No. Characteristics Desirable limit
1 pH value 6.5 to 8.5
2 Odour unobjectionable
3 Colour (Hazon unit), maximum 10
4 Test Agreeable
5 Turbidity (NTU) maximum 5
6 Total dissolved solids (TDS) ppm 500
7 Total hardness maximum as (CaCO3 ppm) 300
8 Calcium (ppm) 75 - 200
9 Magnesium (ppm) 30 – 150
10 Iron as Fe (ppm) 0.1 – 1.0
11 Chloride (as Cl-) pm 200 – 600
12 Nitrate (as NO3-) pm 45
13 Sulphate as (SO4) pm 200 – 400
14 Phosphate as (PO4) ppm 10 – 15
15 Organic matter (pm) 0.2 – 1.0
1.3.1 - TYPES OF HARDNES
Hardness due to the presence of calcium and magnesium bicarbonates is called
hardness.
(1) Temporary hardness: When water containing calcium and magnesium
bicarbonates is heated, soluble bicarbonates are converted into insoluble
carbonates and hydroxide. On filtering of such water, soft water is obtained. The
hardness which can be removed by boiling is referred as ‘temporary hardness’ or
bicarbonate hardness.
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(ii) Permanent hardness: ‘Permanent hardness’ is the term applied to the
hardness caused by dissolved chlorides, nitrates and sulphates of calcium and
magnesium and other heavy metal ions. This hardness cannot be removed by
boiling the water sample. Sum of temporary and permanent hardness is referred to
as total hardness. Permanent hardness can only be removed by lime-soda, ion
exchange or zeolite process.
(iii) Alkaline or carbonate hardness and non-alkaline or non-carbonate
hardness: Like all carbonates and bicarbonates, calcium and magnesium
carbonates and bicarbonates are alkaline. Then hardness due to the bicarbonates
and carbonates is called alkaline hardness or carbonate hardness. The alkalinity
can be measured by titration with standard mineral acid using methyl orange or
phenolphthalein as an indicator. As the sulphates and chlorides are neutral salts,
the hardness caused by the presence of calcium and magnesium sulphates,
chlorides and nitrates is termed non-alkaline hardness or non-carbonate hardness.
1.3.2 - UNITS OF HARDNESS
Hardness in water is expressed in terms calcium carbonate equivalents as:
1. Parts Per Million (ppm):
It expresses the concentration of hardness causing salt as the number of parts of
substance by weight in million parts by weight of water.
One part per million, i.e., 1 ppm hardness means one part of CaCO3 equivalent
hardness is present in one million parts of water. For calculation, the units of
weight used should be same for the substance and water (1 ppm = 1 mg/litre).
2. Degree Clark (°CI)
It is the number of grains of CaCO3 equivalent hardness per gallon of water. It is
also expressed as parts of CaCO3 equivalent hardness per 70,000 parts of water.
Thus, 1° Clark is equal to one grain of CaCO3 equivalent hardness in one gallon
of water which is same as 1 part of CaCO3 equivalent hardness per 70,000 parts of
water.
3. French Unit (°F)
It is the part of CaCO3 equivalent hardness per 105 parts of water.
The various units of hardness are inter-convertible and by using the following
information, hardness in one unit can be expressed in other units as -
1 ppm ≡ 1mg/litre ≡ 0.1℉ ≡ 0.07° Cl
1° Cl ≡ 14.3ppm ≡ 14.3mg /litre ≡ 1.43℉
1℉ ≡ 10 ppm ≡ 10 mg/litre ≡ 0.7° Cl
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1.4 - EFFECTS OF HARD WATER IN INDUSTRIES Industries like paper, sugar, chemical textile, pharmaceutical industries, etc.
require large amount of water—and for steam generation, heat exchangers and
condensers. Water free from all kinds of impurities with hardness below 25 ppm
is desirable for the industrial purpose. The pH of the water used in industries
should be 7 to 8.0 and free from all types of impurities. Water of higher hardness causes the following problems in industries. 1. In textile industry calcium/magnesium soap precipitates adhere to the fabric
material and interfere with dyeing process which affects the shades.
2. In boilers it leads to the formation of scales and sludges which reduces
efficiency of boilers.
3. In sugar industry presence of calcium magnesium salt interfere with the
crystallisation of sugar.
4. In paper industry smooth finish and proper colour cannot be obtained if hard
water is used.
5. Pharmaceutical industry: If hard water is used for preparing pharmaceutical
products like drugs, injections, lotions, syrups, etc., then the hardness causing
ions in water may react with them to produce undesirable products. This may
reduce efficiency of the material or create adverse action. 6. Concrete making: If the water containing chlorides, sulphates, etc. is used
may affect the hydration of compounds in cement and final strength of
concrete will be affected. 1.5 - ESTIMATION OF HARDNESS Hardness of water can be determined by two methods: 1.5.1 - Soap Titration Method Total hardness of water can be determined by titrating a fixed volume of water
sample (100 ml) against standard alcoholic soap solution. Formation of stable
lather which persists for two minutes is the end point of titration. In the beginning
sodium soap will precipitate all hardness causing ions as their respective stearates.
2C17H35COONa + CaCl2
(C17H35COO)2Ca
+ 2NaCl
Calcium stearate
2C17H35COONa
+ MgSO4
(C17H35COO)2Mg
+ Na2SO4
Magnesium stearate
(Thus, water which readily lathers with soap is called soft water whereas water
which forms scum or precipitate and does not form lather immediately is called
hard water.)
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1.5.2 EDTA Method (Complexometric Titration)
Principle: It is based on the fact that hardness causing ions like Ca++
, Mg++
form
unstable complexes with the indicator Eriochrome Black T. However, when such
a complex is treated with EDTA, since EDTA has more affinity to form stable
complexes with metal ions, it extracts the metal ions from the metal ion-dye
complex to form stable metal EDTA complex. The colour of dye -metal complex
and dye are different. However, the change in colour is sharper at pH 10.0 than at
other pH ranges. The metal-dye complex has wine red colour at pH 10.0 where the
dye itself has blue colour at pH 10.0. Hence, by observing the sharp change in
colour, the exact end point of reaction involving complete extraction metal ions by
EDTA can be determined. The results obtained by this method are more accurate
than those obtained by soap titration method.
Ethylene diamine tetra acetic acid (EDTA)
Metal - EDTA chelate Ca+2 or Mg+2 +EBT ⟶
CaEBT or MgEBT Wine red complex (unstable)
CaEBT or MgEBT + EDTA ⟶ CaEDTA or MgEDTA + EBT Blue Colurless
The various steps involved in estimation of hardness by EDTA method are given
as below.
Preparation of Solutions
1. Standard hard water
1.0 gm of pure CaCO3 dissolved in minimum quantity of cone. HCI and dilated to
a one litre with distilled water. Each ml contains 1 mg CaCO3.
2. EDTA solution
4 gm of pure EDTA (disodium salt) is dissolved in one litre of water.
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3. Eriochrome Black T Indicator 0. 5 gm of the dye is dissolved in 100 ml of pure alcohol 2 to 3 drops of indicator
is usually sufficient. Freshly prepared solutions are more suitable in order to
obtain more accurate results. 4. Buffer of pH = 10.0
67.5 of NH4Cl is mixed with 570 ml of liquor ammonia, and diluted upto a litre
with distilled water. Estimation of Hardness 1. 50 ml of standard hard water is pipetted out a clean 250 ml conical flask. Add
5 to 10 ml pH 10.0 buffer and mix well. Add 3 to 4 drops of Eriochrome
Black T. The colour of solution is wine red. 2. Fill the burette with EDTA solution and titrate against standard hard water in
flask. Let the volume of EDTA required be ‘A’ ml when the colour changes
to blue. 3. Pipette out 50 ml of sample of hard water adds to 10 ml buffer and 3 to 4
drops of indicator and titrate against EDTA from burette. Let the volume be
‘B’ ml. 4. Boil 50 ml of sample of hard water. Cool and filter, add 5 to 10 ml pH 10.0
buffer, 3 to 4 drops of indicator and titrate against EDTA till the colour
changes to blue. Let the volume of EDTA consumed be ‘C’ ml. Calculations Since standard hard water contains 1 mg/ml of CaCO3 hardness equivalent, 50 ml of standard hard water ≡ 50 mg of CaCO3 hardness 50 ml of standard hard water requires ≡ ‘A’ ml of EDTA
∴ ‘A’ ml of EDTA ≡ 50 mg of CaCO3 hardness
∴ Each ml of EDTA ≡ 50 mg of CaCO3 hardness. 50 ml of water sample requires ‘B’ ml of EDTA solution
≡ 50 mg of CaCO3 hardness (∴ 1 ml of EDTA = 50 mg of CaCO3 hardness equivalent)
∴ 1000 ml of water sample ≡ 50 100050 mg of CaCO3, hardness equivalent. Total hardness ≡ x 1000
mg of CaCO3
50 ml of water sample after boiling requires ‘C’ ml of EDTA
≡ × 50 mg of CaCO3 hardness equivalent
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(∴ 1 ml of EDTA = 50 mg of CaCO3 hardness equivalent)
∴ 1000 ml water sample after boiling
≡ × 50 × 100050 mg of CaCO3 hardness equivalent
≡ 1000 × mg of CaCO3 hardness equivalent. Permanent hardness = × 1000 mg of CaCO3 Temporary hardness = Total hardness - Permanent hardness
= × 1000 − × 1000
= 1000 × � − � mg of CaCO3
1.5.3 Problems on Hardness Calculations
Problem 1.1: 50 ml of standard and hard water containing 1 mg of pure CaCO3
per ml consumed 10 ml of EDTA solution. 50 ml of the given water sample
required 10 ml of same EDTA solution. Calculate the total hardness of water
sample in ppm.
Solution:
50 ml of standard hard water ≡ 10 ml of EDTA solution
∴ 1 ml of EDTA solution ≡ 5010 ml of std hard water
≡ 1050 mg of CaCO3
≡ 5 mg of CaCO3
50 ml of water sample ≡ 10 ml of EDTA solution ≡ 10×5 mg of CaCO3
≡ 50 mg of CaCO3
50 ml of water sample ≡ 50 mg CaCO3
∴ 1000 ml of water sample ≡ 50 × 100050
≡ 1000 mg CaCO3
Hardness of water sample ≡ 1000 ppm. Problem 2.2: In the determination of hardness by EDTA method, 50 ml of
standard hard water (containing 1 mg of CaCO3 hardness per ml of solution)
required 30 ml of EDTA solution, while 50 ml of the sample of hard water
consumed 20 ml of EDTA solution. After boiling 50 ml of the same sample which
required 10 ml of EDTA solution. Calculate the various hardnesses in ppm.
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Solution:
1 ml of std. hard water ≡ 1 mg of CaCO3
50 ml of std. hard water ≡ 50 mg of CaCO3
50 ml of std. hard water ≡ 30 ml of EDTA
≡ 50 mg of CaCO3
∴ 1 ml of EDTA ≡ 50 mg of CaCO3
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≡ 20 × 5030 mg of CaCO3 1000 ml of sample water ≡ 20 × 5030 × 20 mg of CaCO3 Total hardness ≡ 664 mg of CaCO3
≡ 664 ppm. 50 ml of boiled water sample ≡ 10 ml of EDTA solution
≡ 10 × 5030 mg of CaCO3
∴ 1000 ml of boiled water sample ≡ 10 × 5030 × 20 mg of CaCO3 ≡332mg of CaCO3 i.e.
Permanent hardness
≡ 332 ppm.
i.e.
Temporary hardness ≡ Total – Permanent
≡ 664 – 332 = 332 ppm.
1.6 - SOFTENING OF WATER Softening of water means removal of hardness. Since hardness is mainly due to
the presence of soluble salts of calcium and magnesium, softening methods aim at
removal of these components from water. The lime soda process involves
converting soluble impurities into insoluble precipitates by treatment with lime
and washing soda. The precipitates are then removed by sedimentation and
filtration. Other softening methods involving replacing the calcium and
magnesium by harmless ions through exchange as in zeolite and ion exchange
processes, are more effective and efficient in removal of hardness. These methods
are discussed in detail below. 1.6.1 - Lime Washing Soda Method Principle: Calculated quantities of lime and soda (10% excess) are added to hard
water to convert soluble impurities into insoluble one which can be easily
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removed by filtration. Reactions are as follows. If process is carried out at room
temperature it is called cold lime soda process. • Removal of temporary hardness
Ca(HCO3)2 + Ca(OH)2 2CaCO3 + 2H2O Mg (HCO3)2 + 2Ca(OH)2 Mg(OH)2 ↓ + 2CaCO3 + 2H2O
• Removal of permanent hardness causing magnesium compounds MgCl2 + Ca(OH)2 Mg(OH)2 ↓ + CaCl2 MgSO4 + Ca(OH)2 Mg(OH)2 ↓+ CaSO4
• Removal of ions like iron, aluminium, manganese
FeSO4 + Ca(OH)2 Fe(OH)2 + CaSO4 2Fe(OH)2 + 12 O2 + H2O O2 + H2O + 2Fe(OH)3 ↓ Al(SO4)3 + 3Ca(OH)2 2A1(OH)3 ↓ + 3CaSO4 2AlCl3 + 3Ca(OH)2 2A1(OH)3 ↓ + 3CaCl2
• Neutralisation of free acids
2HCI + Ca(OH)2 CaCl2 + 2H2O
H2SO4 + Ca(OH)2 CaSO4 + 2H2O • Removal of dissolved gases
CO2 + Ca(OH)2 CaCO3 + H2O
H2S + Ca(OH)2 CaS + 2H2O • Reaction with bicarbonate ions
2NaHCO3 + Ca(OH)2 CaCO3 +2H2O + Na2CO3 The above reactions also enable us to calculate the lime requirement on
quantitative basis assuming that purity of lime is 100% pure.
Reaction with washing soda
Permanent Calcium (Ca) hardness is removed by washing soda
CaCI2 + Na2CO3 CaCO3 + 2NaCl
CaSO4 + Na2CO3 CaCO3 + Na2SO4 It is to be noted that magnesium permanent hardness as well as those due to iron,
aluminium and neutralisation of acids also generate equivalent quantities of
calcium permanent hardness. Hence, while calculating the washing soda
requirement, these factors have to be taken into consideration.
As a result of lime soda treatment, hardness causing ions like Ca++
, Mg++
, Al+++
,
Fe+++
, etc. are converted into insoluble precipitates like CaCO3, Mg(OH)2,
AI(OH)3 and Fe(OH)3 which settle down and are removed. The anions, on the
other hand, combine with sodium ions to form sodium salts. Hence, in lime soda method, the hardness causing compounds are converted eventually into near equivalent amount of sodium salts. Since these chemical reactions take time and hence sufficient time should be allowed for the completion of reactions. Otherwise, precipitation can occur later causing problems. In order to ensure complete precipitation and settling, coagulants such as alum are used.
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1.6.2 - Cold Lime Soda Process Lime soda process can be carried either by batch or continuous process. a). Batch Process In this process, raw water and the required quantities of chemicals (lime, soda,
coagulants) are mixed thoroughly in big tanks provided with certain agitation
devices. Usually, two such tanks are constructed side by side so that tanks can be
used alternately. The softening process gets completed as the tank gets filled. The
stirring continues for another fifteen minutes so that the chemicals get uniformly
distributed throughout. As coagulants are included, the precipitates formed settle
down easily when stirring is stopped. The clear supernatant is then passed through
filter bed to remove any suspended particles which do not settle down easily. The
settled sludge at the bottom is removed through an outlet at the bottom of tank.
Batch process of softening is very useful to meet the requirement of soft water on
smaller scale. For industrial requirement, continuous softening treatment methods
are followed. b). Continuous Lime Soda Process In order to obtain soft water on large scale, continuous treatment methods are
used. This involves treating raw water with chemicals in continuous manner and
removing the precipitated material partly by settling and by filtration. The
equipment consists of two concentric vertical chambers. The inner chamber is
provided with stirrer whose action not only mixes the chemicals and raw water
intimately but also helps to gather the precipitated matter at the bottom in the
conical portion. The treated water containing some floating particles of precipitate
passes through a filter pad provided through which water passes. The treated
water flows out from the top of outer chamber and is filtered and used.
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Continuous Lime Soda
Process 1.7.2 - Hot Lime Soda Process
Hot lime soda softener
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The shortcomings of cold lime soda process like incomplete precipitation, slow
reaction and reduction of residual hardness only to 30-50 ppm are overcome by
carrying out the reaction at higher temperature 50-60°C. The softening is carried out in large steel tanks having two chambers. The upper
chamber is separated from lower chamber by funnel shaped inlet. The raw water
and chemicals flow into upper chamber where they are heated directly by high
pressure steam. The treated water passes down the funnel. The precipitated matter
settles down in the conical portion of chamber and is periodically removed. The softened water is removed from an opening at top lower chamber and passed
through filter bed to remove any suspended impurities still remaining in water. The main advantages of this method are: a. The time required for treatment is reduced considerably so that larger
volumes of water can be treated. Thus, it is more economical.
b. The chemical reactions take place faster, the precipitate settles faster. The
amount of coagulant if added is very low. b. Higher temperature of water, coupled with alkaline conditions reduces the
bacterial count to minimum.
c. Iron and manganese salts are precipitated out and their content in water is also
reduced.
d. The final hardness of water after treatment is between 20-25 ppm which is
almost 50% of the value obtained by the cold process.
e. The solubility of gases like oxygen, carbon dioxide is reduced at higher
temperature and hence corrosion of boilers due to dissolved oxygen and
carbon dioxide is reduced. Though there are many advantages, use of steam for heating will add on to cost of
production. Treating large volumes of water will also generate large volumes of
sludge material which has to be disposed of simultaneously. The residual hardness
of 20-30 ppm is high and such water cannot be used in high pressure boilers. 1.7.4 - Zeolite Process of Softening (Permutit’s Process) Zeolites are naturally occurring (hydrated) sodium aluminium silicates, having
different amounts of water of crystallisation. They are represented as Na2O.
A12O3. x SiO2. y H 2O, where x varies from 2 to 10 and y from 2 to 6. The naturally occurring mineral though more durable, non-porous and has lower exchange capacity. Synthetic zeolites, on the other hand, are porous and have more exchange capacity per unit weight. Whether natural or synthetic, zeolite
have the property of exchanging their Na+ ions for hardness causing ions like
Ca++
and Calcium and magnesium zeolite on treatment with a solution of NaCI
can replace Ca++
and Mg++
ion with Na++
ions, thereby regenerating the zeolite. The reactions taking place during the process of softening are presented below:
Ca(HCO3 )2 + Na2 Ze →CaZe + 2NaHCO3
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MgSO4 + Na2 Ze → MgZe + Na2 SO4
CaCl2 + Na2 Ze → CaZe + 2NaCl
Where Ze represents zeolite.
The zeolite mineral gets exhausted when all the Na++
ions are replaced by Ca++
and Mg++
ions. This indicates such an exhausted zeolite no longer has the
capacity to exchange any more Ca++
and Mg++
ions. Under such situation, the hardness of incoming water and outlet will be same. Zeolite can be regenerated by passing NaCI solution.
CaZe + 2NaCl CaCl2 + Na2Ze
MgZe + 2NaCl MgCl2 + Na2Ze
The regenerated zeolite can now be used for replacing Ca 2+
and Mg2+
from hard
water. Zeolite softening is carried out in large cylindrical tanks which holds the
zeolite material on a perforated platform. Sometimes it is contained between two
layers of sand. The tank is provided with two inlets, one for feeding raw water and
the other for passing saturated NaCI solution. There are two outlets, one for
softened water and the other to remove the CaCl2 and MgCl2, the wash water
formed by the regeneration process.
Fig. Zeolite process of water softening
In the process of softening, raw water is passed through the bed of zeolite where the hardness causing ions are exchanged for the sodium ion on zeolite. The water
coming out of zeolite bed now contains equivalent amount of Na + ions, instead of
Ca++
and Mg++
The presence of sodium ions does not impart any hardness to
water. However, the total dissolved solid content remains almost the same. By testing the hardness of emerging water from the zeolite bed, it would be possible
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to determine when the bed is exhausted. When the zeolite bed is exhausted, the
hardness of incoming and outgoing waler will be the same. Zeolite bed is then
regenerated by passing NaCl solution. CaCl2 and MgCl2 formed flow down the
bed and are drained. The bed is washed with soft water and made ready for
softening more raw water. The hardness of treated water is in the range of 5 to 15
ppm. Limitations of Zeolite Process In order to achieve best results, the following points should be noted. 1. Raw water should be free of turbidity and suspended impurities as they
interfere with the exchange process by forming a coat on the zeolite material. 2. Highly acidic or alkaline water is not suitable as it affects the mineral. 3. Calcium and magnesium zeolite can be easily regenerated by passing NaCl
solutions whereas iron and manganese zeolites cannot be so easily
regenerated. Hence, iron and manganese impurities in water should be
minimum. Disadvantages of Zeolite Process 1. As compared to the lime soda process the dissolved solid is more in zeolite
process since calcium and magnesium salts are replaced by sodium salts.
2. The presence of bicarbonate and carbonates generates NaHCO3 and Na2CO3 in
softened water. This alkalinity in water is not desirable since in boilers it leads to
caustic embrittlement due to formation caustic soda.
Na2CO3 + H
2O → 2NaOH + CO2 ↑
NaHCO3 → NaOH + CO2 ↑ NaOH formed in process react with iron at high temperature of boiler to cause
corrosion. Further CO2 evolved dissolve in condensed water and causes corrosion
of condenser tubes. Advantages of Zeolite Process 1. By careful monitoring it will be possible to achieve very low hardness of less
than 5 ppm.
2. The zeolite bed gets adjusted to any hardness of incoming water, i.e.,
variation in hardness of raw water does not affect the exchange process. The
rate at which regeneration has to be carried out will vary. 3. The equipment is compact and materials used are cheap and easily available.
Suitably trained people can handle the equipment without any problem.
4. The process can be operated under pressure also. 5. Since the reaction involves only replacement of Ca
++ and Mg
++ ions with Na
+
ions, there is no chance of sludge formation after precipitation at later stage.
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1.7.5 - Ion Exchange Process
Ion exchange resins are used for softening of water. Ion exchange resins are
organic polymers with long chains with cross links and having functional groups
through which various ions are exchanged. The resins are porous and insoluble in
water.
There are two types of ion exchange resins-cationic exchange resins which
exchange their H+ ions for cations present in water. These resins have functional
groups like − SO3 H + ,−COO H + , OH(phenolic) where the H+ ion get replaced
with other cations present in water. The anion exchange resins have functional
groups like —NH2, = NH, OH which can be exchanged with anions present in
The principle of ion exchange method is based on ability of the ion exchange
resins to exchange their functional group like H + with cations like Ca
++, M
++,
Na+ and (OH)
- with all anions present. The process of softening in the ion
exchange process involves passing raw water through cationic exchange resin and
followed by passing it through the anion exchange resin. The equipment consists
of two cylinders which contain the cation exchange resin and the anion exchange
resin. The outlet from cation exchange resin is connected to anion exchange
cylinder. Separate outlets are provided for draining purposes. Tanks provided at
the top of cylinders contain the regeneration chemicals.
As the raw water passes through the cation exchange resin, Ca ++
, Mg++
their ions
are exchanged with H+ ions of the resin.
R − H 2 + MgCl2 → R − Mg + 2HCl
Thus, sulphates, Chlorides, bicarbonates, get converted into sulphuric,
hydrochloric and carbonic acids.
The acidic water emerging from the cation exchange bed is passed through the
anion exchange bed where the anions are exchanged for the OH ions of resin.
R1 − (OH )2 + H2SO4 → R1 − SO4 + 2H2O
R1 − (OH )2 + 2HCl → R1 − Cl2 + 2H2O
The water emerging from the anion exchange bed is free from both canons and
anions and hence completely demineralised. It means it does not have any
hardness at all. However water may contain some dissolved gases. In order to
remove the dissolved gases, water is passed through degassifiers where the water
is heated, the escaping gases are removed by applying vacuum.
The cation exchange resin and the anion exchange resin are regenerated when they
get saturated with cations and anions. Cation exchange resins are generated by
passing dilute acids and anion exchange resins by passing alkali.
RCa + 2HCl → R(H)2 + CaCl2
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R1SO4 + 2NaOH → R1(OH)2 + Na2SO4 The regenerated resins can be used for treating further fresh raw water. Thus, the
same amount of resins can be used over and again after regeneration. Water obtained from ion exchange softening process has very low residual
hardness of less than 2 ppm. It can be safely used for high pressure boilers. Limitations due to presence of certain impurities As in the case of zeolites, ion exchange resins do not function effectively in the
presence of turbidity or suspended matter as they tend to cover the surface of resin
and prevent easy exchange of ions. Similarly, very high total solid content in raw
water will mean frequent regeneration of the resin. Hence, for efficient
performance, raw water is pretreated to reduce the total dissolved solid content.
Fig. 2.5 Ion exchange method of softening of
water Advantages of Ion Exchange Process 1. The process can be used for softening acidic or alkaline waters. 2. Where mineral free water is required as in the case of some pharmaceutical,
cosmetics and explosives and other manufacturing processes, ion exchange
process of softening is the only process available for getting such pure
3. The residual hardness after treatment is less than 2 ppm and this makes water
suitable for high pressure boilers. 4. Continuous supply of softened water can be made available by providing
storage facilities and two columns of each resin.
18
Disadvantages of the Ion Exchange Process
1. The resins used are costly, the regeneration chemicals like acids and alkalis
are costlier.
2. The initial investment in equipment is more. 3. Where water is highly turbid and contains a large amount of dissolved matter,
pretreatment of such water is essential to get the best results from the ion
exchange method.
1.8 - Calculation of Water Softening Reagents
1. Calculation of hardness
Hardness should be expressed in terms of weight of CaCO 3 i.e. in milligrams per
litre (mg/l) or parts per million (ppm) or degree clark (°Cl).
2. Lime requirements
Lime, i.e. Ca(OH)2 is required for
a) Temporary calcium hardness Ca (HCO3)2.
b) Temporary magnesium hardness, Mg(HCO3)2. Lime requirement temporary
Mg hardness is double that required for Ca hardness. c) Lime eliminates permanent magnesium hardness but introduces equivalent
permanent calcium hardness.
d) Lime also reacts with dissolved CO2. iron and aluminium salts, free acid and
introduces an equivalent Ca hardness. e) Lime also reacts with bicarbonates of Na and K to form carbonates.
Since 100 parts of CaCO3 is equivalent to 74 parts of Ca(OH)2 Lime required for
softening,
74 Temp..Ca.hardness + 2 × temp. Mg hardness + perm.(Mg + Fe + 3Al) hardness
=
1
100 HCl + H 2 SO4 + HCO3 + CO2 − NaAlO2 all in terms of CaCO3 equivalents
2
3. Washing soda requirement
Washing soda is required for eliminating salts of calcium other than temporary
hardness.Since 100 parts of CaCO3 is equivalent to 106 parts of sodium
carbonate, Washing soda requirement Perm.Ca hardness + Perm.(Mg + Fe + 3Al)hardness
106
1
= HCl + H 2 SO4 − HCO − − NaAlO 2
100 2 3
all in terms of CaCO3 equivalents
Above mentioned formula are used when CaCO3 equivalents are calculated
directly. When it is calculated by using multiplication factor with respect to
chemical reaction then HCI, Aluminium equivalents are to be added directly.
19
If NaAlO2is present in water, it undergoes hydrolysis to NaOH and Al(OH)3 as
follows:
NaAlO2+2H2O → NaOH + Al(OH) 3 ↓ NaAlO2 does not need lime or soda, but since one equivalent of it produces one
equivalent of (OH) ion it can be considered equal to one equivalent of lime,
Hence, in calculation involving NaAlO2, the corresponding CaCO3 equivalent
should be deduced from lime and soda requirement.
The conversion factor for CaCO3 equivalent is 100
82 × 2
When aluminium salt present in water is other then A12(SO4)3 then multiplication
factor will be with respect to the reaction with lime.
Table: Comparison of different softening processes
Lime Soda Zeolite Demineralisation
1. Capital cost is less; Capital cost is very high, Very high capital
operational cost is high. operational cost is low. cost; but operational
cost is low.
2. It can be used for turbid it cannot be used for turbid It cannot be used for
water. water turbid water.
3. Hardness is reduced to 15 Hardness is reduced below Hardness is reduced
-30 ppm. 10 ppm. to 0 to 2 ppm.
4. Total dissolved solids are Total dissolved solids are The total dissolved
reduced. not reduced. solids are removed
completely.
5. It removes mineral acids It cannot soften acidic It removes mineral
water water. from acids from
water. 6. It removes Fe++ and Mn++ Only small quantity of It removes all cations
ions. Mn++
and Fe++
ions can be present.
removed.
7. Water softened by this Water softened by this Water softened by this
method due to residual method due to dissolved method is free from
hardness and dissolved use sodium salts is not suitable all problems and is
salts is not suitable for use for boiler use; as it creates ideal for in boiler.
in boiler as it involves problems such as scale and
problems such as scales and sludge formation, priming
sludge formation, carry foaming corrosion, etc.
over, corrosion, etc.
8. Involves many steps like No such steps involved; No such steps
coagulation, settling of gets softened in one involved water gets
precipitate, filtration, operation. softened in none
removal and disposal of operation.
20
sludge etc.
9. Change in hardness of Process gets automatically Process gets
water requires change in adjusted to change in automatically adjusted
lime and soda dose. hardness. to change in hardness.
10. Due to sludge formation No sludge formation, thus No sludge formation
it is not a clean process. it is a clean process. thus it is a clean eel
11. Reagent used cannot be The exchange medium can The exchange
regenerated, be regenerated. medium can be
regenerated.
12. It removes dissolved It does not remove It removes all
CO2 hard water. dissolved CO2 from hard dissolved from gases.
water
1.9 – Reverse Osmosis
When two solutions of different concentrations are separated by a semi permeable
membrane, solvent flows from low region concentration to higher one until
concentration is equal in both sides. This process is known as osmosis. This
technique is used for the removal of dissolved salts from seawater called
desalination or desalting of water.
Demineralised water is produced by forcing water through semi permeable
membrane at high pressure.
Principle of reverse osmosis: In this process dissolved salts are separated from
water by using semi permeable membrane. When membrane is placed in between
water containing dissolved salts and pure water. Water flows through a membrane
into salty water due to osmotic pressures.
Reverse Osmosis
21
This natural tendency of water may be reversed by applying a higher pressure on
the salty water part. This tends to flow water from higher concentrations to lower
one. This reverse process of osmosis is called as reverse osmosis. The membranes
used are cellulose acetate, cellulose butyrate, etc. This method is also known as
super filtration. This is a single and continuous process, involves no phase
changes and needs low energy. This technique is also used for the separation of
toxic ions from plating wastes, concentration of radioactive waste and removal of
organics from vegetable and animal wastes. 1.10 – Ultra Filtration Some of the toxic chlorinated organisms are removed by filtering industrial waste
with activated charcoal as follows. Aldrin, Dieldrin, Endrin, DDT, etc. are removed nearly 99%. Synthetic organic ion exchange resins are very useful for reoval of industrial
waste chemicals. Styrene-divinyl-benzene copolymer can rmove chlorinated
pesticides by adsorption at the surface. Ionic dyes from text1e mill wastewater can
be eliminated by using cation and anionic ion exchange rfrsins.
Cation exchanger- COOH+ + M
+ → COOM+
+ H+
Anion exchanger—NH+ CI
- + A
- → NH+A
- + CI-
Fig. 2.11 Filtration of industrial wastes with activated charcoal The ion exchange membrane finds an important application in the removal of
toxic wastes by ultrafiltration In ultrafiltration, the solution is pushed under
pressure through a membrane which contains pores of size 2 to 10,000 nm (20 x
105A) whereby big molecules are retained and the effluent that passes off is free
22
of the big molecules. In reverse osmosis, the membrane pores are smaller— 0.04
to 600 nm—in size. Both these techniques have found extensive application in
purification of industrial wastewater in metal, textile, protein isolation, paper and
pulp and food industries.
Industrial wastewater purification by ultra
filtration 1.11 – Sterilisations and Disinfection of water
The most important and common disinfecting agent used to treat water is chlorine
and chlorine compound like bleaching powder. Most important ingredient of our
life is water it can be purified methods like filtration boiling bleaching powder
treatment solar water disinfection (Recommended by United Nations). Filtration
and Coagulation of water through sand purify it from suspended solids and partly
decrease its bacteriological contamination. Complete disinfection is attained by
chemical reagents which kill pathogenic bacteria or microorganisms.
Chlorine gas and chlorine compounds, such as chlorinated lime, chloramines,
chlorine dioxide, hypochlorite as well as ozone, and salts of heavy metals are
effective against microorganisms. Ultraviolet radiation, ultrasound and other
physical factors also kill pathogenic organisms.
Sterilization is carried by physical methods like boiling of water and exposure to
sunlight and ultraviolet light.
1.12 - Ozonisation
Drinking water is treated with ozonized oxygen. The plant consists of a tower
made of enamelled iron, and divided into several compartments by means of
perforated celluloid partitions. The tower is provided with two inlets at the bottom
and an outlet at the top. The ozonized oxygen and water to be treated are allowed
23
to pass through separate inlets provided at the bottom and sterilized water is
collected from the outlet provided at the top. The perforated partition breaks up
the gas and water stream into minute bubbles, as a result of which intimate contact
between gas and water is affected. Ozone is produced by passing a high voltage
current through dry air using either plate or cylindrical electrodes made of
stainless steel aluminum. In industry ozone is prepared by passing dry and clean air through an ozoniser
under constant pressure, when it is subjected to a silent electric discharge. The
ozonised air is then mixed with water in special chambers. Modern equipment is
provided with bubblers and jet ejectors. In nature, ozone is formed by discharges of atmospheric electricity during storms
and by oxidation of a number of organic substances. An allotropic modification of oxygen is ozone. Under normal condition it is a
bluish gas. In liquid state, ozone is dark blue and in the solid state it is almost
black. Its solubility in water is higher than that of oxygen. Small concentration of
ozone in the air is beneficial to man, especially in respiratory pathology. But
ozone becomes harmful when concentrations reach relatively high levels.
Prolonged exposure to ozone causes irritability, headache and fatigue. At higher
concentrations nausea, nasal bleeding, and inflammation of the eye mucosa
develop. Chronic ozone poisoning results in serious illness, the maximum
allowable concentration of ozone in industrial air is 0.1 mg/cu.m Due to high oxidation potential of ozone (2.076v) and the ease with which it
passes through the cell membranes of microbes. Ozone oxidises the organic
substances in the microbe cell in order to kill it. It has stronger bacterial action
than chlorine (1.36v). Experimental investigations show that if one ml of water contains 274-325 E. coli
type bacteria, 86% arc killed by an application of 1 mg/litre of ozone and 2
mg/litre of ozone fully disinfects the water. Spore forming bacteria are more
resistant to ozone than non -spore forming bacteria, but they are resistant to
chlorine as well. The dose of ozone required for water disinfection depends on the
degree of pollution, but usually varies from 0.5 to 4.0 mg/litre. Ozone consumption increases with water turbidity and higher doses are required
for turbid waters. The disinfecting action of ozone is almost independent of the
temperature of water. Ozonisation not only decontaminates water but also gives it
a pleasant taste, reduces its colour and deodorises produced by oxidation and
mineralisation of organic impurities. Humins are completely broken down by
ozone to give CO2 and H2O.
24
Ozonisation of water also has some advantages over chlorination.
(a) It improves the organolepetic properties of water and does not add to its
chemical pollution (b) Ozonisation does not require additional processes to
remove excess bacterial agents from purified water hence higher doses of ozone
can be used (c) Ozone can be prepared in situ. Only electricity is required, and a
single chemical reagent, silica gel, on which moisture is adsorbed from the air.
Sterilization with ozone has several advantages for example
(a) Ozone sterilizes, bleaches, decolourises and deodorizes water. (b) An excess of
ozone in water causes no danger because being unstable it decomposes into
oxygen. (c) It causes no Irritation of mucous membrane ax in case of chlorine
treatment (d) The taste of water is improved with ozone. Highly palatable water is
thus sterilised wish ozone. The most important disadvantage of the ozone
sterilization is the high cost involved in the treatment.
Ozonisation is not used widely because of the complexity of ozone manufacture
and the large amounts of high frequency and high voltage electricity required.
Ozonisation of water will be profitable only if a suitable material is found,
electricity is cheap, and the method of bringing water in contact with ozone is
improved.
Ozone is a corrosive agent the gas and its aqueous solutions destroy steel, cast
Iron, copper, rubber, and ebonite. All apparatus for the manufacture of ozone and
the pipes through which its solutions pass should be of stainless steel or
aluminium. Stainless steel can withstand the corrosion for 15-20 years and
aluminium for 5-7 years.
1.13 – Chlorination
Chlorination or shock chlorination is the process of flushing your well and water
system with a chlorine solution to kill bacterial and other micro organisms. It is
probably the best and cheapest method of sterilization of water and it is most
effective in checking pathogenic microorganisms. Chlorine may be used directly
in the liquid form or as bleaching powder. The excess of chlorine is removed by
suiphites anti- chlor.
The disinfecting action of chorine and its compounds depends on the oxidation-
reduction processes occurring in microbial cells subjected to the influence of these
chemicals. The Hypochlorous acid (HOCl) reacts with bacterial enzymes to
interfere with the metabolism inside the cell Free and bound active chlorine have
25
different oxidation potentials and the reaction rates and the required contact time
are also different. HOCl is most effective chlorine compound. The chemical action of chlorine is that it reacts with water to form hypochlorous
acid and nascent oxygen. Both these are powerful germicides.
Cl2
+ H2O
HOCl + HCl
Hypochlorous Acid
HOCl
HCl
+ [O] Nascent Oxygen
The chlorine effectiveness against microbes depends on the initial dose of
chlorine, the time it is in the water, and the pH of the water. Chlorine is consumed
to oxidise organic mineral impurities in water. Organic impurities in water are
destroyed with chlorine. Humins are mineralised to CO2, Fe2+
is oxidised to Fe3+
,
Mn2+
is oxidised to Mn4+
and stable suspensions are converted into unstable ones
because of decomposition of protective colloids. Sometimes plant and animal
organisms destroyed by chlorine in the water are converted into decay products
with a strong odour. Chlorination of water containing phenols and other aromatic
substances gives an especially unpleasant odour. Smack and odour develop in
water containing quantities of phenols as small as 1 : 10000,000. They strengthen
with time and do not disappear on heating. Large doses of chlorine are sometimes
required to destroy the aromatic compounds. Chlorination is very important for the purification of water. It discolours water
and provides good condition for clarification and filtration. When chlorine is
dissolved in water gives two acids, HCl and HOCl, the latter being a very weak
acid, its dissociation depends on the pH of the medium, The lower the pH of the
medium, the higher the concentration of HOCl, which disinfects water because of
its high redox potential.
Cl- + H2O ⇌ HOCl + H+ +2e- ; + 1.49 V
When chlorine compound added to water, they are hydrolysed to give HOCl.
For example 2CaOCl2 + 2H2O ⇌ CaCl2 + Ca(OH)2 + 2HOCl Chlorinated lime
Ca(OCl)2 + 2H2O
⇌ Ca(OH)2 + 2HOCl
Calcium Hypochlorite
NaOCl + H2CO3 ⇌ NaHCO3 Sodium Hypochlorite + HOCl
NaOCl
+ H2O
⇌ NaOH + HOCl
26
Hydrolysis of salts is slower than that of free chlorine, and the formation of HOCI
is therefore slower as well. But the further action of HOCI is the same as the
dissociation of Cl2 gas in water.
The quantity of molecular chlorine corresponding to the oxidizing power of a
given compound w.r.t potassium iodide in an acid medium is called active
chlorine. Each pair of electrons accepted by the oxidant is equivalent to 71 carbon
units of free chlorine. Therefore the compounds Cl2, NaOC1, CaOCl2, NH2Cl,
H2O2 correspond to 71 parts by weight of active chlorine, and the compound
NHCl2 to 142 parts.
The concept of active chlorine describes oxidizing power of a compound (w.r.t KI
in an acid medium) rather than the actual chlorine content of a given compound
example a gm. molecule of NaCl contains 35.5 g. of chlorine, but the active
chlorine content is zero. The actual chlorine content of a gm. molecule of NaOCl
is 35.5 g. and the active chlorine content is 71 g.
The active chlorine content in. a chlorine compound in percent can be calculated
by the relation.
Cl2 Percent = 0 x 10
n the number of hypochlorite ions In a molecule of a chlorine compound, M0, the
molecular mass of chlorine compound and M the molecular mass of chlorine. For
example the active chlorine content of chlorinated lime of the composition
3CaOC12.Ca(OH)2.5H2O is:
Cl2 % = 3 71 100
= 39.08 % 545
Here n = 3, M0 = 545 g. and M = 71 g.
1.14 – ELECTRODIALYSIS
In the electrodialysis method positive and negative ions are separated out of a
flowing current of saline or brackish water when it is allowed to pass through ion
exchange membranes under the influence of an electric field. Infect when a direct
current of electricity is passed through a saline water in a series of closely spaced,
alternately placed, cation exchanger and anion exchanger membranes, cations pass
through the cation exchanger membranes and anions through the anion exchanger
membranes. As movement of cations and anions result in the salinity decreases in
one space and increases in the next space, and so on throughout the stack. The
water containing more salt (increased salinity) is run to waste, while the water
containing less salt (decreased salinity) may either be re - circulated through the
stack or may be passed through a series of stacks in this manner, saline water may
be converted into drinking water.
27
Completely deminerlised water is not obtained by this method. The method
reduces the salinity of brackish water so as to make it suitable for drinking and
general use. The process is capable at reducing salt contents of brackish water
from 2000 to about 300 ppm, but it is very costly. For more efficient separation, Ion selective membranes, which are permeable to
only one kind of ions with specific charge, have been used in recent years. cation
selective membranes . Permeable to cation only and anion selective membranes
are permeable to anions only. The permeability of permeable ions, inside the
membrane pores. The ion selective membrane pores and designed with fixed
charge which exclusively allows one type of charged ions to pass through its pores
and does not allow oppositely charged ions to flow. 1.15 – BLEACHING POWDER
Bleaching powder CaCl2 (Calcium Hypochlorite is widely used as a bleaching
agent. After removing organic matter, suspended impurities etc, water is mixed
with required amount of bleaching power and mixture is allowed to stand for
several hours for the completion of sterilization.
CaOCl2 + H2O Ca(OH)2+ Cl2
Cl2 + H2O HOCl + HCl
HOCl HCl + [O]
Nascent Oxygen Both HOCl and Nascent oxygen are powerful germicides. Solubility of bleaching
powder is part 1 in 20 parts of water. It is very important to use calculated amount
of bleaching powder because excess of it gives bad odour and disagreeable taste,
28
while less quantity of it than required, will not sterilize the water completely. The
various factors on which the quantity of bleaching powder to be added for
complete sterilization depends are temperature, turbidity of water, time allowed
for sterilization and quantity of oxidisable micro organisms present in water.
The disadvantages of using bleaching powder are:
(a) Excess of bleaching powder in water causes unpleasant odour and disagreeable
taste. (b) As it introduces calcium in water as a result water becomes hard. (c) The
amount of chlorine liberated from a sample of bleaching powder with excess of
dilute acids or CO2 is called available chlorine, hence it is priced based on the
quantity of available chlorine.
CaOCl2+ H2SO4 CaSO4 + H2O+ Cl2 (Available chlorine)
More the available chlorine in bleaching powder better is its quality. A good
sample of bleaching powder contains 35-38% available chlorine.
Example 1: The water works department of a city, which has a population of
50,000 has to meet its water demand at the rate of 150 litres per capita per day.
Water is disinfected by making use of bleaching powder having 30% available
chlorine. Determine the quantity of the bleaching powder, is added annually. The
dose required at the works is 0.2 ppm of chlorine for disinfection.
Solution : Water required for the city per day =150 x 50000 = 7500000 litres =
7.5 x 106 litres. The dose of chlorine required per day = 0.2 ppm = 0.2 mg per litre
= 0.2 x l0-6
kg per litre. Hence amount of chlorine required = 7.5 x 10 6 x 0.2 x
10-6
kg = 1.50 kg.
The bleaching powder has 30% of available chlorine. So bleaching powder
required = 1.5 x 100/30 = 5.0 kg per day.
or 5.0 x 365 = 1825 kg per year.
Bleaching powder contains about 56% of chlorine (71 x 100/127) = 55.9%. whole
of it is not available for reaction because on standing it undergoes slow auto
oxidation and gets converted into calcium chloride and calcium chlorate. Hence
percentage of available chlorine in bleaching powder decreases on storage.
Whenever it is to be added, it analysed for its available chlorine content.
6Ca(OCI)2 5CaCI2 + Ca(ClO3)2
High test hypochlorite (HTH), Ca(OCI)2 has also been used for sterilization. It
has got an advantage over bleaching powder in that the percentage of chlorine in it
is higher than that in bleaching power. Ca(OCl)2 + 2H2O ⇌ Ca(OH)2 + 2HOCl
29 HOCI ⇌ HCl + [O]
Nascent oxygen Chlorination is the best method because of its various advantages. (a) liquid chlorine it more effective as well us cheapest. (b) Liquid chlorine can be
obtained in pure form and its storage without any problem. (c) on its storage no
deterioration or decomposition occur for many days because it is stable. (d) liquid
chlorine can be used at low, moderate or even at high temperatures. (e) No
impurities are introduced by adding liquid chlorine to water. Chlorination of water to such an extent that not only the living organisms. but
other impurities in water are completely destroyed is called break point
chlorination. Depending on the stage of treatment at which chlorine is added and also the
expected results of chlorination, various forms of chlorination are (a) Plain chlorination. (b) Prechorination, (c) Poatcodnation. (d) Double chlorination. (e) Super chlorination. (f) Breakpoint chlorination. (g) Decholrination (a) Plain chlorination only chlorine treatment is given to raw water, Water from
deep wells, lakes, reservoirs etc is comparatively dear with turbidity less than 30
ppm. In such cases no treatment such as sedimentation, coagulation etc is
necessary. The chlorine is added to raw Water in order to control the Forth of
algae and to remove pathogenic bacteria. it also removes organic matter and
colour from water. The quantity of chlorine to be added to raw water is about 0.50
ppm or more. Thus when no other treatment except chlorination is given before
supplying water to consumers, it is called plain Chlorination. (b) Pre chlorination: When chlorine ii added to raw water before any treatment,
it is called pre chlorination. It is usually done before raw water enters
sedimentation tanks. It reduces the taste and odour of water, improves coagulation
and less quantity of coagulant is required when this treatment is adopted. It also
controls the growth of algae in sedimentation tanks as well as in filters and
prevents the purification of sludge in the settling tanks (c) Post chlorination - After all the treatments of purification of water are
completed, it is called pent chlorination. Chlorine is applied to water. The dosage
of chlorine should be such that a residual chlorine of about 0.10 to 0.20 ppm
appears in water at the point of its entry into the distribution system. (d) Double chlorination- When more than one point, chlorine is added to raw
water the process is called double chlorination. Pre chlorination as well as post
30
chlorination is necessary when raw water is highly contaminated and contains a
large amount of bacteria or microorganism. The second unit of chlorination, in
addition, serves as a standby unit and as a result load of impurities is greatly
reduced.
(a) Super chlorination- Super chlorination is generally adopted for highly
polluted water. The application of chlorine beyond the stage of break point is also
known as super chlorination. Super chlorination is generally practicised in
waters where plain chlorination produces taste and odour, the water is coloured
and Mn and Fe are to be oxidised. This is also resorted to when the contact time is
limited at the pre chlorination stage. The super chlorination can also be adopted
when there is high content of organic impurities. The residual chlorine content
after break point may be 0.50 to 2.0 ppm. The excess chlorine may be added at the
end of filtration. Super chlorination effectively destroys organisms. The contact
period is generally 10-30 minutes. After super chlorination, it is necessary to be
removing excess chlorine by the process of dechlorination before water is sent for
consumption. The method of super chlorination followed by dechlorination
affords a maximum degree of security. The process can be installed in the form of
a 1oop. At one end of the loop, the water is chlorinated and at the other end of the
loop, it is dechlorinated.
(7) Dechlorination: Chlorine removal from water is called dechlorination. It is
done in such a manner that at the end for dechlorination process some residual
chlorine still remains in water to disinfect it when it is flowing through the
distribution system. The usual chemical compounds used are sodium thiosulphate,
sodium bisulphate, sodium sulphite activated carbon and potassium permanganate
but dechlorination is best carried out by the addition of sulphur dioxide or by
aeration. Water after breakpoint chlorination is subjected to dechlorination and
hence filtered through activated carbon which removes decomposition products as
well as excess of chlorine. A sulphur dioxide treatment is very common. Some
plants also make use of sodium bisulphite and sodium thiosulphate as antichlor.
H2O + SO2 H2SO3 H2O +Cl2 HClO + HCl
HClO +H2SO3 HCl + H2SO4
SO2+ Cl2+ 2H2O H2SO4 + 2HCl 1.16 – OTHER IMPORTANT METHODS
Chloramine process - this method consists in adding ammonia and chlorine to
water when mono and dichloroamines are formed, which destroy all the bacteria.
2NH3 + Cl2 NH4Cl + NH2Cl
3NH3+Cl22NH4Cl+NH4Cl2
31
The method does not impart chlorinous taste and odour to water. Further growth
of any bacteria is prohibited by the presence of high residual chlorine contents in
water. Ammonia used is generally half the quantity of chlorine. The chloramines
compounds are more bacterial than chlorine alone, because these are more lasting
than chlorine alone. Treatment with chloramines is slower than with free chlorine. The water and the
chloramines must be in contact for two hours. Chlorine consumption during
chlorination with ammoniation is the same as for treatment with chlorine alone.
But chloramines are good for disinfecting water containing large quantities of
organic matter, because the chlorine requirements are much lower in this case. If
water contains aromatic substances it acquires an unpleasant chlorophenolic
odour. The odour begins to develop at the point when chlorine stops binding Into
the chioramines. The latter do not react with aromatic hydrocarbons and do not
therefore, impair the organoleptic properties of the water. Chlorine dioxide - Chlorine dioxide, ClO2 has been found to be more effective in
the removal of bacteria than chlorine. Its advantage over chlorine is that ClO2
oxidises phenols to quinone and maleic acid, which do not give off the unpleasant
chlorophenolic odour. It also removes tastes and odours present in water. ClO2 is
very unstable and so it is used immediately after its production. It can be prepared
by passing Cl2 gas through sodium chlorite.
2NaClO2 + Cl2 2NaCl + 2ClO2 It can also be prepared by the action of HCl on sodium chlorite.
5NaClO2+4HCl 5NaCl+4ClO2+H2O
The dosage of ClO2 varies from 0.50 to 1.50 ppm. Its action is unaffected by pH
values between 6 to 10 and hence it is useful for water with high alkalinity. Iodine method - Water in swimming pools is iodinated, A saturated iodine
solution in water is used. The concentration of the solution increases with
temperature. For example, at 10C, the solubility of iodine in water is 100 mg per
litre, at 200C, 300 mg per litre, and at 50
0C, 750 mg per litre. At pH less than 7,
the iodine dose for the disinfection of water from natural sources varies from 03 to
1.0 mg/litre. The odour of iodine cannot be smelled because it can be sensed at
concentrations above 1.5 mg/litre. If the water contains chioramines, iodic acid
(because of lower oxidising power) remains inactive till the moment when a
strong oxidant is exhausted. This increases the time of the bactericidal action of
iodic acid. Water can also be disinfcted by organic iodine compounds, known as
Iodophores Potassium permanganate method— In villages the well water is sterilized by
adding calculated amount of potassium permanganate. This method is,
however, not popular because it is costly.
32
1.17 - CHEMICAL OXYGEN DEMAND (COD)
Chemical oxygen demand (COD) is the amount of oxygen used while oxidising
organic matter by means of strong oxidising agent. All organic matters are
converted into CO2 and H 2O. In chemical oxidation both biologically oxidisable
organic matter like starch, sugar, inert materials like cellulose, etc. are oxidised
and hence COD values are always higher than BOD. COD can be determined in 3
hours.
The wastewater sample is refluxed with a known excess of potassium dichromate
in a dilute sulphuric acid in the presence of silver sulphate as a catalyst or HgSO4.
The organic matter of the sample is oxidised to water, carbon dioxide and
ammonia. The unreacted excess of dichromate remaining is titrated with standard
solution of ferrous ammonium sulphate.
COD =
(V1 −
V
2
)× N
×
8
×100mg / L x
V1 = volume of ferrous ammonium sulphate required for blank
V2 = volume of ferrous ammonium sulphate required or test
N = normality of ferrous ammonium sulphate
x = volume of the sewage sample taken.
If an inorganic substances like chlorides, nitrates and organic substances like
benzene pyridine are present in wastewater they interfere as they are also oxidised
by dichromate and create an inorganic COD. Chloride interference can be
eliminated by adding mercuric sulphate prior to the addition of other reagents and
nitrite interference by adding sulphanic acid to the dichromate solution. COD is
much more useful than the BOD for estimating amount of oxygen in industrial
wastes. Ratios of BOD/COD can be employed to get an indication of the degree of
the bio-treatability of the waste. 0.8 or higher ratio indicates wastes are highly
amenable to biological treatment, while lower ratios indicate that the wastes is not
favourable to biological treatment. COD is important in calculating the efficiency
of treatment plants and proposing standards for discharge of domestic effluents.
Sewage: Water containing domestic or municipal waste is called sewage, which
contains nearly 99.95% water and 0.05% waste materials. Strength of sewage is
expressed in terms of Biological Oxygen Demand (BOD) and Chemical Oxygen
Demand (COD).
1.18 BIOLOGICAL OXYGEN DEMAND (BOD)
Is the quantity of dissolved oxygen required by bacteria for the oxidation of
organic matter under aerobic conditions or it is a measure of the oxygen utilised
by micro organisms during the oxidation of organic materials. The demand for
oxygen is directly proportional to the amount of organic wastes which has to be
broken down. Hence, BOD is a direct measure of oxygen requirement and an
33
indirect measure of biodegradable organic matter. Greater BOD greater is the
pollution. A known volume of sewage sample is diluted with known volume of dilution
water. This diluted sample is taken in two stoppered bottles of 300 ml. The
dissolved oxygen (DO) content of one of the bottles is immediately determined by
Winkler’s method (blank). Another bottle is incubated at 20°C for a period of 5
days. Then unused oxygen is determined. The different in the BOD of water
sample.
BOD = (DOb — DOS) x dilution factor
DOb = dissolved oxygen present in the blank.
D OS = dissolved oxygen of sewage after incubation BOD is expressed in mg/l. 5 days BOD of wastewater can be obtained in 2.5 days
if the temperature is 35°C rather than 20°C. BOD enables us to determine the
degree of pollution hence it has special significance is pollution control. BOD
values are useful generally in process design and loading calculations,
measurement of treatment efficiency and operation, self pollution control and in
determination of self purifying capacity of a steam. 1.19 - ACTIVATED SLUDGE PROCESS Activated sludge is a process for treating sewage and industrial wastewaters
using air and a biological floc composed of bacteria and protozoans. It is an
important part of the municipal wastewater treatment is the BOD-removal. The
removal of BOD is done by a biological process, such as the suspended growth
treatment process. This biological process is an aerobic process and takes place
in the aeration tank, in where the wastewater is aerated with oxygen. By creating
good conditions, bacteria will grow fast. The growth of bacteria creates flocks and
gases. These flocks will removed by a secondary clarifier.
34
Where:
Q = flow rate of influent [m3/d]
QW = waste sludge flowrate [m3/d]
Qr = flowrate in return line from clarifier [m3/d]
V = volume of aeration tank [m3]
S0 = influent soluble substrate concentration (bsCOD) [BOD g/m3] or
[bsCOD g/m3]
S = effluent soluble substrate concentration (bsCOD) [BOD g/m3] or
[bsCOD g/m3]
X0 = concentration of biomass in influent [g VSS/m3]
XR = concentration of biomass in return line from clarifier [g VSS/m3]
Xr = concentration of biomass in sludge drain [g VSS/m3]
Xe = concentration of biomass in effluent [g VSS/m3]
This system is usually placed between the primary clarifier and the disinfection of
a municipal wastewater treatment plant.
The parameters of which the symbols are shown in the schematic diagram, are
used to model a suspended growth process. In a Summary of all the Related
Calculations one can calculate all the necessary design characteristics of a
Complete - Mix Suspended Growth Process.
Process: The picture below shows a simplified flow diagram for biological
processes used for wastewater treatment. The influent wastewater (e.g. municipal
wastewater) goes through several stages in which different compound are
removed out of the wastewater.
Simplified flow diagram for a biological wastewater treatment with a activated-
sludge process.
▪ In the Bar Rack coarse solids are removed, such as sticks, rags, and other
debris in untreated wastewater by interception. By use of fine screening even
floatable matter and algae are removed.
35
▪ In the Grit Chamber grit is removed consisting of sand, gravel, cinders, or
other heavy solid materials that have subsiding velocities or specific gravities
substantially greater than those of the organic putrescible solids in
wastewater.
▪ The Primary Clarifier is a basin where water has a certain retention time
where the heavy organic solids can sediment (suspended solids). Efficiently
designed and operated primary sedimentation tanks should remove from 50 to
70 percent of the suspended solids and 25 to 40 percent of the BOD.
▪ The influent of the aeration tank is mixed with activated sludge and in the
Aeration Tank the mixed liquor is aerated. By aerating the mixed liquor the
aerobic processes will be stimulated, the growth rate of bacteria will be must
faster.
▪ Because the bacteria deplete the substrate, flocculation takes place . The
soluble substrate becomes a solid biomass. These flocks of biomass will
sediment in the Secondary Clarifier.
▪ At the end of the process the effluent water is treated to disinfect it and make
it free of disease-causing organisms.
36
Solved Problems for Practice
1. A sample of water found to contain following impurities in mg/litre.
Mg(HCO3)2 = 73 MgSO4 = 120 mg , CaCl2 = 222mg, Ca(NO3)2 = 164 mg.
calculate lime and soda requirement for treatment of 10000 liters of water.
Solution : Conversion of the impurities in CaCO3 equivalent .
Substance quantity conversion CaCO3 equivalent mg / liter
mg/liter factor
Mg(HCO3)2 73 100 100
× 73 = 50 146 146
MgSO4 120 100 100
× 120 = 100
120 120
CaCl2 222 100 100
× 222 = 200
111 111
Ca(NO3)2 164 100 100
× 164 = 100
164 164
Lime requirement for softening. (Temp. Ca hardness + 2 × Temp. Mg hardness + Perm. Mg hardness)
(0 + 2 × 50 + 100)
(200) = 148 mg/liter.
Lime requirement for 10,000 liters of water = 148 × 10000 × 10-6
= 1.480 Kgs.
Soda requirement for softening = 106 [ Perm. Ca + Perm. Mg]
100
= 106 [ 200 + 100 + 100]
100
= 106 [400] = 424 mg/liter
100
Soda requirement for 10,000 liter = 424 × 10,000 × 10-6 = 4.24 kg.
= 74 100
= 74 100
= 74 100
37
2. Calculate quantities of lime and soda required for softening of 20,000 liters of
water containing following salts in ppm (16.4 ppm NaAlO2 used as a coagulant) Ca2+
= 160 ppm, Mg2+
= 72 ppm, HCO3 - = 73.2 ppm, CO2 = 44 ppm, Al2(SO4)3 =
34. 2, HCl = 36.5 ppm
Solution : Conversion of the impurities in CaCO3 equivalent .
ion or salt amount conversion CaCO3 equivalent (ppm)
present (ppm) factor
Ca2+
160 100 100 × 160 = 400 40 40
Mg2+
72 100 100
× 72 = 300 24 24
HCO3 73.2 100 100 × 73.5 = 60
122 122
CO2 44 100 100
× 44 = 100 44 44
NaAlO2 16.4 100 100 × 16.4 = 10
164 164
Al2(SO4)3 34.2 100 100 × 34.2 = 10
342 342
HCl 36.5 100 100 × 36.5 = 100 36.5 36.5
Lime requirement for softening.
= 74 (300 + 3(10) + 1�2 (100) +100 + 60 - 10)
100
= 74 (530) = 392.2 mg/liter. 100
Lime requirement for 20,000 liters of water = 392.2 × 20000 × 10-6 = 7.84 Kgs.
Soda requirement for softening
= 106 [ 400 + 300 + 3(10) + 1�2 (100) – 60 – 10]
100
= 106 [710] = 752.6 mg/liter 100
Soda requirement for 20,000 liter = 752 × 20,000 × 10-6
= 15.05 kg.
38
3. A water sample contains following impurities per liter. Ca(HCO3)2 = 81 mg,
Mg(HCO3)2 = 73 mg, CaSO4 = 68 mg, MgSO4 = 60 mg, KCl = 100 mg. Calculate
(a) Temporary hardness and permanent hardness in water.(b) Quantity of lime
and soda required in kg for softening 50,000 liters of water if the purity of lime
and soda are 80 % and 90 % respectively .
Solution : Conversion of the impurities in CaCO3 equivalent .
Substance quantity conversion CaCO3 equivalent mg / liter
mg/liter factor
Ca(HCO3)2 81 100 100
× 81 = 50
162 162
Mg(HCO3)2 73 100 100
× 73 = 50
146 146
CaSO4 68 100 100
× 68 = 50
136 136
MgSO4 60 100 100
× 60 = 50
120 120
1. KCl does not react with lime or soda and its presence can be ignored
2. Temporary hardness in water = hardness due to Ca(HCO3)2 and Mg(HCO3)2
= 50 + 50 = 100 mg / liter
3. Permanent hardnes in water = hardness due to CaSO4 and MgSO4
= 50 + 50 = 100 mg / liter
4. Lime requirement for softening. = 74 (Temp. Ca hardness + 2 × Temp. Mg hardness + Perm. Mg hardness)
100
= 74 (50 + 2 × 50 + 50) 100
= 74 (200) = 148 mg/liter. 100
Since lime purity is only 80 %
So actual lime requirement for 50,000 liters of water
= 148 × 100 × 50000 × 10-6 80
= 9.25 kg.
39
5. Soda requirement for softening = 106 [ Perm. Ca + Perm. Mg]
100
= 106 [ 50 + 50 ] 100
= 106 [100] = 106 mg/liter 100
Since purity of soda is only 90 %
So actual Soda requirement for 50,000 liter
= 106 × 100 × 50,000 × 10-6 90
= 5.89 kg.
4. Calculate the amount of lime and soda required for softening 50,000 liters of
hard water containing the following salts in ppm.
Ca(HCO3) 2 = 162 ppm, MgCl2 = 9.5 ppm, Fe2O3 = 100 ppm, NaCl = 58.5 ppm,
SiO2 = 25 ppm, H 2SO4 = 98 ppm, MgSO4 = 60 ppm, CaCO3 = 100 ppm.
Also calculate cost of lime and soda if cost of lime is Rs. 530 / 100 kg and soda
is Rs. 450/10 kg.
Solution : Conversion of the impurities in CaCO3 equivalent .
Salt Amount conversion CaCO3 equivalent ppm
Present ppm factor
Ca(HCO3)2 162 100 100 × 162 = 100
162 162
MgCl2 9.5 100 100
× 9.5 = 10 95 95
Fe2O3 100 ppm does not contributes to hardness
NaCl 58.5 ppm -
-
SiO2 25ppm
H2SO4 98 100 100 × 98 = 100 98 98
MgSO4 60 100 100
× 60 = 50 120 120
CaCO3 100 100 100 × 100 = 100
100 100
40
Lime requirement for softening. = 74 (200 + 10 + 50 + 100)
100
= 74 (360) = 266.4 mg/liter. 100
Lime requirement for 50,000 liters of water = 266.4 × 50000 × 10-6 = 13.32 kg. = Rs. 70.59
Soda requirement for softening = 106 [ 10 + 50 + 100]
100
= 106 [160] = 169.6 mg/liter 100
Soda requirement for 50,000 liter = 169 × 50,000 × 10-6 = 8.48 kg. = Rs. 38.16
Total Cost = 70.59 + 38 .16 = Rs. 108.6
5. Calculate the quantity of lime and soda required for softening 1,00,000 liters of
water containing the following impurities. Ca(HCO3)2 = 30.2 ppm, Mg(HCO3)2 =
20. 8 ppm, CaCl2 = 28.1ppm, MgCl2 = 8.7 ppm, CaSO4 = 35.0 ppm, MgSO4 =
6.7 ppm. The purity of lime is 70 % and the purity of soda is 85 % (At. wt. for H
= 1, C = 12, O = 16, Na = 23, Mg = 24)
Solution : Conversion of the impurities in CaCO3 equivalent .
Impurity amount in conversion CaCO3 equivalent in ppm
ppm factor
Ca(HCO3)2 30.2 100 100 × 30.2 = 18.64 162 162
Mg(HCO3)2 20.8 100 100 × 20.8 = 14.24 146 146
CaCl2 28.1 100 100 × 28.1 = 25.31 111 111
MgCl2 8.7 100 100
× 8.7 = 9.15 95 95
CaSO4 35 100 100
× 35 = 25.73
136 136
MgSO4 6.7 100 100
× 6.7 = 5.58 120 120
41
Lime requirement for softening. = 74 (18.64 + (2 ×14.24) + 9.15 +5.58)
100 = 45.769 mg/liter or ppm
Since Lime is 70 % pure
So lime requirement for 100,000 liters of water = 45.769 × 100
70 × 100000 × 10-6
= 65.384 kg.
Soda requirement for softening = 106 [ 25.31 + 25.73 + 9.15 +5.58]
100 = 69.71 mg/liter
Since purity of soda is 85% So requirement of soda for 100,000 liter = 69.71× 10085 × 100,000 × 10-6
= 82.01 kg.
6. Calculate the quantity of lime and soda required for softening 10,000 liters of water containing the following impurities per liter. Ca(HCO3)2 = 7.8 mg,
Mg(HCO3)2 = 8.0 mg, CaSO4 = 12.2 mg, MgSO4 = 10.6 mg, NaCl = 5.5 mg,
SiO2 = 2.2 mg. (At. wt. for H = 1, C = 12, O = 16, Na = 23, Mg = 24)
Solution : Conversion of the impurities in CaCO3 equivalent .
impurity amount conversion CaCO3 equivalent mg / liter
mg/liter factor
Ca(HCO3)2 7.8 100 100
× 7.8 = 4.8
162 146
Mg(HCO3)2 8.0 100 100
× 8.0 = 5.4
146 146
CaSO4 12.2 100 100 × 12.2 = 8.9
136 136
MgSO4 10.6 100 100 × 10.6 = 8.8
120 120
Lime requirement for softening.
= 74 (4.8 + 2 × 5.4 + 8.8) 100
42
= 18.05 mg/liter. For 10,000 liters of water lime requirement = 18.05 × 10000 × 10-6
= 0.1805 kg. = 180.5 gm
Soda requirement for softening = 106 [8.9 +8.8]
100
= 18.23 mg/liter Soda requirement for 10,000 liter = 18.23 × 10,000 × 10-6
= 0.1823 kg. = 182.3gm
7. Calculate the amount of lime (90% pure) and soda (95% pure) required to
soften one million liters of water which contains the following impurities.
CaCO3 = 15 ppm, MgCO3 = 9 ppm, CaCl2 = 20 ppm, MgCl2 = 8 ppm, CO2 = 30
ppm, HCl = 9.2 ppm.
Solution : Conversion of the impurities in CaCO3 equivalent .
impurity amount in conversion CaCO3 equivalent in ppm
ppm factor
CaCO3 15.0 100 100 × 15.0 = 15.0
100 100
MgCO3 9.0 100 100 × 9.0 = 10.7 84 84
CaCl2 20.0 100 100 × 20.0 = 18.0
111 111
MgCl2 8.0 100 100 × 8.0 = 8.42 95 95
CO2 30.0 100 100
× 30.0 = 68.18 44 44
HCl 9.2 100 100
× 9.2 = 12.6 36.5 36.5
Lime requirement for softening.
= 74 (15 + 3(10.7) +8.42 +68.18 + 12.6) ×
100 (90 % purity of lime)
90 100
= 103.27 mg/liter Lime requirement for 10,00,000 liters of water = 103.27 × 10,00,000 × 10
-6
43
= 103.27 kg.
Soda requirement for softening
= 106 [ 18+ 8.42 + 12.6 –17.6] ×
100 (95% purity of lime)
95 100
= 41.07 mg/liter
Soda requirement for 10,00,000 liter = 41.07 × 10,00,000 × 10-6 = 41.07 kg.
8. Calculate of lime and soda required for softening 50,000 liters of water
containing following salts ( Purity of lime is 95 % and soda = 93 %) CaCO3 =
34.1 mg /liter, Mg(HCO3)2 = 29.2 mg / liter, Mg(NO 3) 2 = 29.6 mg, MgSO4 =
36.0 mg / liter , CaSO4 = 27.2 mg / liter, MgCl2 = 47.5 mg / liter, SiO2 = 105 mg /
liter , NaCl = 52 mg / liter, H2SO4 = 9.8 mg/lit.
Solution : Conversion of the impurities in CaCO3 equivalent .
Substance quantity conversion CaCO3 equivalent mg / liter
mg/liter factor
CaCO3 35.0 100 100 × 35.0 = 35
100 100
Mg(HCO3)2 29.2 100 100 × 29.2 = 20
146 146
Mg(NO3)2 29.6 100 100 × 29.6 = 20
148 148
MgSO4 36 100 100
× 36 = 30
120 120
CaSO4 27.2 100 100 × 27.2 = 20
136 136
H2SO4 9.8 100 100
× 9.8 = 10 98 98
Lime requirement for softening.
= 74 (35 + 2(20) + 100 +10)× 50,000 ×
100 ×10-6 (Purity of lime is 95%)
95
100
= 7.408 kg.
Soda requirement for softening
= 106 [ 20 + 100 + 10] × 50,000 ×
100 ×10-6 (Purity of soda is 93%)
93 100
44
= 15.05 kg.
9. Calculate amount of lime and soda required for softening of 20,000 liters
of water containing following salts in ppm CaSO4 = 13.6, Ca(HCO3)2 = 16.2,
MgCO3 = 16.8, HCl = 36.5, AlCl3 = 13.5, KCl = 5.1
Solution : Conversion of the impurities in CaCO3 equivalent .
impurity amount in conversion CaCO3 equivalent in ppm
ppm factor
CaSO4 13.6 100 100 × 13.6 = 10
136 136
MgCO3 16.8 100 100 × 16.8 = 20 84 84
AlCl3 13.5 100 100 × 13.5 = 10 133.5 133.5
Ca(HCO3)2 16.2 100 100 × 16.2 = 10
162 162
HCl 3.65 100 100 × 3.56 = 10 36.5 36.5
KCl 5.1 Do not contribute to hardness
Reaction
2AlCl3 + 3 Ca(OH)2 → 2Al(OH)3↓ + 3
CaCl2
Lime requirement for softening.
= 74 (Temp. Ca + 2 × Temp. Mg + Perm.(1.5Al) + 1�2 HCl)
100
= 74 (10 + 2(20) + 1.5(10) +1�2 (10))× 20,000 × 10
-6
100
= 1.036 kg.
Soda requirement for softening
= 106 [ Perm. Ca + Perm.(1.5Al) + 1�2 HCl]
100
= 74 (10 + 1.5(10) +1�2 (10))× 20,000 × 10
-6
100
= 0.636 kg.
10. Calculate the hardness in a given hard water sample having the following data.
1. 50 ml of standard hard water containing 1 mg of CaCO3 per
ml consumed. 2. 50 ml of standard hard water consumed 25 ml of EDTA using
eriochrome black T as indiacator. 3. 50 ml of water sample consumed 40 ml EDTA using the same indicator.
45
4. 50 ml water sample after boiling consumed 25 ml of EDTA using the
same indicator. Solution: 50 ml of std. hard water ≡ 25 ml of EDTA
≡ 50 mg of CaCO3
∴ 1 ml of EDTA ≡ 50
mg of CaCO3 25
Now 50 ml of sample water ≡ 40 ml of EDTA solution
≡ 40 × 5025 mg of CaCO3
1000 ml of sample water ≡ 100050 × 5025 × 40 mg of CaCO3 Total hardness ≡ 1600 mg of CaCO3
≡ 1600 ppm. 50 ml of boiled water sample ≡ 25 ml of EDTA solution
≡ 25 × 5025 mg of CaCO3
∴ 1000 ml of boiled water sample ≡ 25 × 5025 × 100050 mg of CaCO3 ≡1000 mg of CaCO3 i.e. i.e.
Permanent hardness ≡ 1000 ppm. Temporary hardness ≡ Total - Permanent
≡ 1600 - 1000 = 600 ppm.
11. 50 ml of hard water Sample required 8 ml of 0.05 N EDTA solution for
titration. 30 ml of the same water sample after boiling required 5 ml of 0.02
EDTA solution for titration. Calculate the hardness of water. Solution: 1000 ml of 1N EDTA ≡ 50 gm CaCO3 1 ml of 1N EDTA ≡ 50 mg CaCO3 Now 50 ml of hard water sample ≡ 8 ml of 0.05 N EDTA solution
≡ (8 × 0.05) ml of 1N EDTA solution ≡ (8 × 0.05× 50) mg CaCO3 ≡ 20 mg CaCO3
1000 ml of hard water sample ≡ 20
× 1000 mg of CaCO3
50
≡ 400 mg CaCO3
46 Total hardness ≡ 400 mg of CaCO3
≡ 400 ppm.
30 ml of boiled water sample ≡ 5 ml of 0.02N EDTA solution ≡ (0.02 × 5) ml of 1N EDTA ≡ (0.02 × 5 × 50) mg of CaCO3
∴ 1000 ml of boiled water sample ≡ 0.02 × 5 ×50 × 1000 mg of CaCO3 30
≡ 166.6 mg of CaCO3 i.e.
Permanent hardness
≡ 166.6 ppm.
i.e.
Temporary hardness ≡ Total — Permanent
≡ 400 - 166. = 234 ppm.
12. A standard hard water sample contains 0.20 mg of CaCO3 per ml. 100 ml of
this water consumed 25 ml 0.02 N EDTA. 25 ml sample water consumed 12 ml of
0.05N EDTA. The sample water is boiled and filtered, 50 ml of this water
sample consumed 4 ml of 0.01 N EDTA. Calculate the hardness of water.
Solution:
Given : a). 100 ml of std. hard water (0.2mg/liter of CaCO3) ≡ 25 ml of 0.02N EDTA mg of CaCO3
b). 25 ml of sample water ≡ 12 ml 0.02 N EDTA
c). 50 ml of boiled hard water
≡ 4 ml of 0.01N EDTA
∴ 25 ml of 0.02N EDTA ≡ (100 × 0.20) mg of CaCO3 1 ml of 1N EDTA ≡ 100×0.2025×0.02 mg of
CaCO3 ≡ 40 mg of CaCO3
Now 25 ml of sample water ≡ 12 ml of 0.02 N EDTA solution ≡ 12 × 0.02 ml of 1N EDTA
≡ 12 × 0.02 × 40 mg of CaCO3 ≡ 9.60mg of CaCO3
Total hardness ≡ 9.60 × 10025 mg of CaCO3
≡ 384 mg /liter ≡ 384 ppm.
50 ml of boiled water sample ≡ 4 ml of 0.01N EDTA solution ≡ (4 × 0.01) ml of 1N EDTA solution
≡ 40 × 4 × 0.01 mg of CaCO3 ≡ 1.6 mg CaCO3
47 ∴ 1000 ml of boiled water sample ≡ 1000 × 150.6 mg of CaCO3
≡32mg of CaCO3
i.e. Permanent hardness ≡ 32 ppm.
i.e. Temporary hardness ≡ Total - Permanent
≡ 384 - 32 = 352 ppm.
13. 1000 liters of hard water is softened by zeolite process. The zeolite was
regenerated by passing 20 liters of sodium Chloride Solution containing 1500 mg/
liter NaCl. Caculate the hardness of water. Solution: 20 liter of NaCl solution Contains = 20 × 1.5 = 30 gm of NaCl
2 NaCl = CaCO3 2 × 58.5 gm = 100 gm
Now 30 gm NaCl = 30 × 5850.5 gm of CaCO3 equivalent 1000 liters of water = 30 × 5850.5 gm of
CaCO3 emits 1 liter water = 1,00030 × 5850.5 = 0.02564 gm = 25.64 ppm Hardness of water = 25.64 ppm 14. By passing 50 liters of NaCl solution containing 250 mg/ liter of NaCl, a
exhausted zeolite softener bed was regenerated. Calculate the liters of hard water
sample ( Hardness equal to 200 ppm as CaCO3) which can be soften by
regenerated bed of zeolite softener. Solution: 50 liter of NaCl Solution Contains = 50 × 250 = 12,500gm of NaCl CaCO3 = 2 NaCl
100 gm = 2 × 58.5 gm
50 gm = 58.5 gm
Now 58.5 gm NaCl = 50 gm of CaCO3 equivalent ∴ CaCO3 equivalent would be = 12,500 ×50 gm
58.5
As hardness is 200 ppm ie. 200mg/liter of CaCO3 = 0.2 gm/liter ∴ 12500 × 58.5
50 gm of CaCO3 will be present in
48 = 12,5000.2 × 58.550 = 53, 418.89 liter of water Thus zeolite bed can soften 53,418.80 liter of water.
15. An exhausted zeolite softener was regenerated by passing 100 liters of NaCl
solution containing 150 gm per liter of NaCl. How many liter of a Sample of H2O
of hardness 300 ppm can be softened by this softener. ? (Given At. wt. for C = 12,
O = 16, Na = 23, Cl = 35.3, Ca = 40)
Solution:
1 liter of NaCl solution Contains = 150 gm of NaCl
∴ 100 liter of NaCl solution Contains = 100 × 150 = 15000 gm of NaCl Now 58.5 gm NaCl = 50 gm of CaCO3 equivalent ∴ 15,000 gm NaCl = 15,000 ×50 gm
58.5
As hardness is 300 ppm ie. 300mg/liter of CaCO3 = 0.3gm/liter
∴ 15,000 × 58.550 gm of CaCO3 will be present in = 15,0000.3 × 58.550 = 42,735.04 liter of water Thus zeolite bed can soften 42,735.04 liter of water.
16. A Hard water sample containing 4.5 gm/liter of CaCl2 is passed through a
permutit softener, what is the amount of NaCl present per liter of the soft water
(H2O)? (At. wt. Na = 23, Cl = 35.5, Ca = 40)
Solution:
Softening reaction is, CaCl2 + Na2Ze → CaZe + 2NaCl Mol. Wt. of CaCl2 = 40 + ( 2× 35.5) = 111
NaCl = 23 + 35.5 = 58.5
∴ 111 gm CaCl2 leaves 2× 58.5 gm NaCl in Soft water ∴ 4.5 gm CaCl2 will leave = 2 ×58.5 ×4.5 gm of NaCl
111
= 4.7 gm / liter of NaCl
17. How many liters of 10% Brine Solution will be required to regenerate an exhausted zeolite bed after softening 10 liters of hard water of 750 ppm hardness.
49
Solution:
Hardness of water = 750 ppm Total quantity = 10 × 750 = 7500 mg of CaCO3 equivalent.
NaCl used is 10% ie 100 gm / liter
Now 58.5 gm NaCl = 50 gm of CaCO3 equivalent ∴ 100 gm NaCl = 100 ×50 = 85.47 gm
58.5
= 85.47 mg/ml ∴ 7500 mg CaCO3 equivalent → 85
7500.47
= 87.75 ml of NaCl
18. Hardness of 77,500 liters of water was completely removed by zeolite
method. The exhausted zeolite softener then required 15 liter of NaCl(2%) for
regeneration. Calculate hardness of water sample.
Solution:
1 liter of NaCl Contains = 20 gm of NaCl
∴ 15 liter of NaCl Contains = 20 × 15 = 300gm of NaCl Now 58.5 gm NaCl = 50 gm of CaCO3 equivalent
∴ 300 gm NaCl = = 256.41 gm
Total quantity of water = 77,500 liters
77,500 liters of water = 256.41 gm of CaCO3 emits
1 liter water = = 0.0033gm = 3.30 ppm
Hardness of water = 3.30 ppm
Questions
1. Define soft and hard water, 2. what are temporary hardness and permanent hardness?
3. Distinguish between soft and hard water 4. Distinguish between temporary and permanent hardness
77, 500
256 .41
58. 5
300 ×50
50
5. What is the principle involve in the estimation of hardness of water by EDTA
titration method?
6. Why water is required to be softened ? mention the methods available for
softening.
7. What are zeolites? Discuss the zeolite process of softening of hard water .
8. Explain in detail the demineralization process. State advantages and
disadvantages.
9. Give the compairisn between ion exchange process and zeolite process.
10.Describe the process of lime soda method of softening of water. Mention its
advantages and disadvantages.
11. Explain with the help of chemical reactions the principle of softening of water
by lime soda method.
12. What is reverse osmosis? Explain in details.
13. What is ultrafiltration? Write its industrial applications.
14. What are the different methods to determine extent of water pollution?
Explain anyone in detail.
15. Write short note on:
a) BOD
b) COD
c) Chlorination process
d) Electro dialysis method
e) Effect of hard water in manufacturing sector.
f) Activated Sludge process
Numerical practice problems
1. Caculate the hardness of water sample whose 100 ml required 20 ml EDTA, 20
ml of calcium chloride solution (whose strength is equivalent to 4.5 gm of
Calcium corbonate per liter) required 30 ml of the same EDTA. (Ans. – 600 ppm)
2. 0.5 gm of CaCO3 are dissolved in dilute HCl and diluted to 500 ml, 25 ml of
this solution required 24.0 ml of EDTA using Eriochrom black T as indicator. 50
ml of hard water sample required 22.5 ml of the same EDTA , 100 ml of the water
sample after boiling required 12.0 ml of the said EDTA. Calculate the hardness in sample. (Total hardness = 468.75 ppm, permanent hardness = 125 ppm)
3. Calculate the quantity of lime and soda required for softening one million liters of
the following sample of water. The purities of lime and soda are 80 % and 85 %
51
respectively. The impurities are, Silica = 75 mg/liter, MgCl2 = 19 mg / liter, MgSO4
= 30 mg / liter, CaSO4 = 68 mg / liter, MgCO3 = 884 mg/ liter, CaCO3 = 120 mg/liter. (Lime =337 kg, Soda = 118.847 kg.) 4. Calculate the quantity of lime and soda required for softening one million liter
of hard water which on analysis was found to contain the following impurities.
Mg(HCO3)2 = 87.6 mg/liter, Mg(NO3)2 = 29.6 mg/liter, MgCl2 = 95 mg/liter,
CO2 = 33 mg/ liter, H2SO4 = 19.6 mg/ liter, KCl = 100 mg/ liter. ( Lime = 247.9
kg, Soda = 127.2 kg.)
5. A sample of water has hardness 304 ppm CaCO3 equivalent. Find the
hardness in terms of degree clark, degree French and mg/liter. 6. Calculate the quantities of lime (85% pure) and Soda (95% pure) for softening one
million liter of water if it has analysis as follows: CaCl2 = 49.95 ppm, MgSO4 = 12
ppm, NaHCO3 = 500 ppm, Mg(HCO3)2= 51.1 ppm, NaCl = 500 ppm, SiO2 =
10 ppm, CO2 = 3 ppm, Fe2+
= 3ppm, AlCl3 = 15 ppm. 7. Calculate lime (90% pure) and Soda (90% pure) required to soften 1,00,000 liters
of water containing, Mg(HCO3)2 = 146 mg/ liter, MgCl2 =95 mg / liter, Ca(HCO3)2
= 81 mg /liter, CaCl2 = 111 mg/ liter, Na2SO4 = 15 mg / liter, SiO2 = 10 mg/liter.
8. 50 ml of standard hard water (1.2 gm CaCO3/liter) requires 32 ml of EDTA
solution. 100 ml of water sample consumes 14 ml EDTA solution. 100 ml of
the boiled and filtered water sample consumes 8.5 ml of EDTA solution.
Calculate temporary hardness of this sample. 9. Calculate quantity of lime (90% pure) and soda (95% pure) required for
softening of one million liters of water containing CaCO3 = 140 ppm, CaSO4 =
136 ppm, MgCO3 = 8.4 ppm, MgSO4 = 60 ppm, MgCl2 = 38 ppm, SiO2 = 25 ppm. 10. A sample of water was found to contain following impurities in mg/liter
Mg(HCO3)2 = 7.3 gm, CaCl2 = 22.2 mg, HCl = 3.65 mg, H2SO4 = 9.8 mg, Ca(NO3)2
= 16.4 mg, MgSO4 = 12.0 mg, FeSO4 = 15.2 mg, Al2(SO4)3 = 340mg. Calculate
Amount of lime and soda required to softening or 10,000 liters of water.
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2. Polymers
Man used eight kinds of materials such as different metals, wood, ceramics,
glasses, skins, horns and natural fibers until nineteenth century. In the nineteenth
century, plastics and rubber were developed. The mass production of these
materials was possible only after the Second World War with the growth of oil
industry. Oil industry provided cheap raw materials for the production of synthetic
polymers and synthetic rubbers. Since then these materials are contributing in
raising the standard of living of mankind significantly. Everyday features of the
modem life such as motor cars, scooters, refrigerators, washing machines,
telephones, etc. depend for their existence on these materials. The construction of
printed circuit boards for electronic instruments and controls, computers,
televisions, etc. is possible only with the use of polymers.
Polymeric materials are extensively used as cheap substitute to older materials.
Sometimes polymeric materials are used because the properties shown by these
materials are unattainable by any other materials. The assets of polymers are—
they are most versatile materials available in the wide range of strength,
toughness, abrasion resistance and flexibility. They are resistant to corrosion.
Some of them have non-stick properties, electrical insulation capacity and
transparency. They can be produced in a variety of colours and show colour
fastness. They are available in wide range of chemical and solvent resistance.
Being light in weight their transportation and labour cost is low. The strength to
weight ratios is high. The ability of polymers to soften and flow at least once, one
of their most valuable assets, as it allows them to be formed into complex shapes
easily and inexpensively by processing them.
Fig. 1.1 Chronological development of important engineering polymers
53
Petroleum oil is the major source of raw materials required for manufacturing of
polymeric materials. The cost of polymeric materials is thus dependent on the cost
of oil. Fabrication is shaping of already processed parts. Thus, it involves additional
shaping operation, e.g., extruded sheets are vacuum formed into finished product,
such as case of some instrument or PVC plastic film is laminated to cloth, etc.
Finishing, assembly and integration, include operations as cutting, bonding,
painting, etc. 2.1- Definition of Polymers and Elastomers The word polymer derives from two Greek words “poly’ meaning many and “mer” meaning parts or units. The reactants from which such repeat units combine
are monomers (mono means single and titer means part or unit). In order to
facilitate polymerization, functionality of a monomer must be two or more than two. Such a monomer is known as polyfunctional monomer. Functionality of a
monomer refers to its ability to form new bonds. Thus, the functionality is the
number of reactive sites or functional groups in the molecule (e.g., —OH,—
COOH —NH2, —SH, etc.). Thus, ethylene glycol (HO—CH2-CH2----OH), adipic
acid (HOOC—(CH2)4—COOH) are bifunctional monomers. The unsaturated
compounds show polyfunctionality due to the presence of either double or triple
bond in them. Thus, ethylene, (H2C = CH2) is a bifunctional monomer as double
bond can open up and form two new sigma bonds. Plastics are the polymers which are shaped into hard and tough utility articles by
application of heat and pressure, e.g., polyethylene, nylon, polystyrene, PVC, etc. Elastomers or rubbers are the polymers which can be vulcanized into rubbery
product exhibiting good strength and can undergo large reversible elongation at
relatively low stress, e.g., natural rubber, synthetic rubbers such SBR (styrene
butadiene rubber), BR (butyl rubber), etc. Synthetic fibers are the polymers used for clothing. They can give rise to long
filament- like materials having good strength and low elongation, e.g., nylon,
terelene. Liquid resins are potentially reactive chemicals which on curing give cross-
linked polymers which can be used as adhesives, potting compounds, sealants etc.
Examples are epoxy adhesives, melamine formaldehyde resin, polysuiphide
sealants, etc. 2.2 - Degree of Polymerization (DP) The degree of polymerization (DP) refers to the average number of repeat units in
the chain. The number of repeat units (DP) in chain specify the length of polymer
chain. The molecular weight of a polymer can be calculated by multiplying DP by
molecular weight of repeat unit. The molecular weight of polyethylene with DP
54
equal to 1000 is 28,000 as the molecular weight of repeat unit involved-(-CH2-
CH2-) n is 28 n refers to DP. By controlling DP (chain length and thus the
molecular weight), it is possible to vary the physical properties of polymers. The
polymers having low molecular weight are quite soft and gummy and those
having higher molecular weight are tougher and heat resistant. This is because in
linear and branched chain polymers the individual chains are held together by
weak intermolecular forces of attraction. The strength of these forces increases
with the chain length or molecular weight. For the polymers to be used for plastic
films, etc., the molecular weight should be more than a certain critical value
referred to as Mc-critical molecular weight. DP increases with time and
temperature also depends upon concentration of monomer and the initiator.
Strength of a polymer increases with increase in DP. Polymers are classified as:
2.3 – Classification of Polymers
(i) Homopolymer and Copolymer: Whenever a polymer chain is made up of a
single repeat unit, (represented as A), it is known as a homopolymer. It can be
represented as
- A- A-A-A-A-A-A-A-A-A-
A homopolymer
Polyvinyl chloride is a homopolymer, the repeat unit [— CH2—CH-] is repeated
throughout the chain as shown in the structure.
Polyvinyl chlorider [A homopolymer]
The polymer which has more than one repeat unit, repeated throughout the chain
is known as a copolymer. If the two different repeat units are represented as A and
B, the copolymer can be represented as
– A – A – B – A – B – B – B – A – A – B –
A copolymer
SBR (styrene butadiene rubber) is a copolymer obtained from styrene and
butadiene.
55
SBR (styrene butadiene rubber), A copolymer When two different repeat units in a copolymer are distributed at random
throughout the chain, the polymer is called a random copolymer. It can be
represented as
– A – A – B – A – A – B – B – A – B – B – A random copolymer
When two repeat units are distributed alternately throughout the chain, the
polymer is known as alternating copolymer. It can be represented as
– A – B – A – B – A – B – A – B – A – B – A – B – A – B – An alternating copolymer
When the sequence or block of one repeat unit is followed by a block of other
repeat unit, which in turn is followed by a block of first repeat unit, and so on,
then the polymer is known as block copolymer. They are usually linear polymers
and can be represented as – [A – A – A – A] – [B – B – B]n − [A – A – A – A]m – [B – B – B] –
A block copolymer The branched polymer in which, main chain is made up of entirely one repeat unit
and the branch chain is made up of other repeat unit, is known as graft copolymer.
It can be represented as
Schematic copolymer arrangements, (a) A copolymer in which the different
units are randomly distributed along the chain (b) A copolymer in which the
units alternate regularly, (c) A block copolymer, (d) A graft copolymer. When the same type of atoms are present in the polymer backbone chain, it is
known as a homochain polymer, e.g.. polyethylene (polythene).
The backbone chain is made by
carbon atoms only
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Polyethylen
When the polymer chain is made up of more than one type of atoms, it is known
as heterochain polymer, e.g., polyamides (nylon), polyester, etc.
The backbone chain has
heteroatom (nitrogen)
Nylone - 6
(ii) Linear, branched or cross-linked polymers: The above classification is
based upon the structural shape of the polymer molecules.
2.3.1 Linear Polymers
Repeating units have been linked together in a continuous length to form polymer
molecules.
Linear polymer Branched polymer
2.3.2 Branched Polymers
Attached to main chain there can be short branches. e.g. (1) Low density linear
polyethylene (LDPE) (Fig. 1 .4a) or there can be long branches, e.g. (2) (Fig. 1
.4b) or there can be branched branches (Fig. 1 .4c).
Fig. 1.4 A schematic representation of different types of branched polymers
57
Linear and branched polymers can be amorphous or semi crystalline depending on
either secondary force between the polymer chains or close packing possible due
to regularity in their structure. 100% crystalline structure is not possible in
polymers because when solidification starts, the viscosity of material rises and
long chains of polymer find it difficult to move around and arrange them in a
symmetrical pattern needed for crystallisation. Examples of amorphous polymers: Polystyrene, Polyvinl chloride (PVC)
(rigid), Polymethyl methyl acrylate (PMMA), Some of the amorphous polymers
are rubbery at ambient temperature (e.g., natural rubber, SBR), while some are
rigid and transparent (e.g., PMMA, polystyrene, polycarbonates, PVC, etc.) Examples of semicrystalline polymers: Polyethylene, polypropylene, polyamide
(nylons such as nylon 6, nylon 66), etc. Semicrystalline polymers may be
transparent, translucent or opaque depending upon the size of crvstallites present
in amorphous matrix of the polymer (Crystallites are regions of crystallinity
embedded in amorphous matrix). Crystallites have dense packing of polymer
chains and thus there are strong intermolecular forces in this region. Thus,
presence of crystallinity enhances heat resistance, tensile strength, hardness while
amorphous region may constitute to toughness and flexibility of the polymer. 2.3.3 Cross-linked Polymers The cross-linked polymers have primary bonds
between polymer chains and thus resultant structure is strong and rigid, three-
dimensional structure. Most of the thermosetting polymers have such structure.
Greater the cross-linking, greater is the rigidity (less is the mobility of polymer
chains) of materials, less is its solubility and less it responds to remelting. Most thermosetting polymers have a cross-linked structure and some can
withstand high temperature. Linear polymers with their less complicated structure
can be rarely used at higher temperature. The cross-linking can be brought about
after polymerization by various chemical reactions. The number of cross-links and
their length can be controlled by using specific reaction conditions. Vulcanization
of rubber provides light cross-links due to which rubber gets good elastic
properties. High degree of cross-linking leads to impart high rigidity and
dimensional stability, e.g., urea formaldehyde (UF) or phenol formaldehyde (PF)
resins, ebonite.
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Cross-linked polymer (A schematic representation)
(iii) Organic and Inorganic Polymers: This classification is based on chemical
composition of polymer chain. The backbone chain of organic polymers is
essentially made up of carbon atoms. The hetero atoms such as 0, N, S usually
satisfy the side valencies of carbon atoms, e.g., polyethylene, polymethyl
methyacrylate, PVC.etc.
2.4 PLASTICS
A material consists of an essential ingredient, an organic material of high
molecular weight which has the property of plasticity.
Plastics are vet important materials. Polymers are the materials made by
polymerisation have repeated units in its structure. The plastic is the material in
finished form. It is processed by either forming or molding into a shape.
They are classified into thermoplastics and thermosetting on the basis of their
structure and thermal stability.
Termoplastics Thermosetting
(i) Linear structure (i) Cross-linked structure
(ii) Softens on heating and becomes (ii) Softens on first heating and
hard or rigid on cooling becomes hard on further heating
(iii) Hardening does not involve any (iii) Chemical change involved
chemical change
(iv) Low molecular weights as (iv) High molecular weights
compared to thermosetting
(v) They are soft (v) Harder, stronger and more brittle
than thermoplastics
(vi) E.g. polyethylene, (vi) Silicones, phenol formaldehyde,
polyvinyl chloride, polystyrene, etc. urea formaldehyde, etc.
2.5 - Compounding of Plastic
Properties of plastics are further improved by addition of certain additives and are
called compounding of plastics.
• Resin binds the various constituents.
• Plasticizers, are added to improve property of plasticity, e.g., vegetable oils,
camphor, esters of staric, oleic, phthalic acids, tricresyl, tributyl, triphenyl
phosphates.
59
• Fillers are added to improve workability, tensile strength and hardness. They
reduce cost, e.g., marble floor, paper pulp, carbon black, metallic oxides like
ZnO, PbO, metal powders like Al, Cu, Pb, etc. • Pigment and dyes—are resistant to the action of sunlight used to provide
• desired colour. TiO2, BaSO4, ZnO - white, ultramarine-blue, PbCrO4- yellow,
ZnCrO4-Green, quinacridone -violet. 2.6 - Glass Transition Temprature Amorphous polymers when cooled below certain temperature become hard, brittle
and glassy, but above this temperature they are soft, flexible and rubbery This
transition temperature of polymer is called ‘glass transition temperature’. (Tg). The hard brittle state is known as the glassy state and the rubbery is the soft one.
All chain motions are completely frozen in the glassy state, these are neither
segmental nor molecular motions.
When a polymer is heated beyond Tg the polymer passes from glassy state to
rubbery state. Only segmental motion while molecular mobility is forbidden is
rubbery state. Further heating much above Tg melt polymer starts flowing as each
polymer chain eventually obtains sufficient energy. The temperature below which the polymer is in rubbery state and above which it
is a liquid is called melting point of polymer (Tm) . As no sharp melting points are
shown by polymers. The transition temperature at which polymer passes from
rubbery state to liquid state is called its flow temperature (Tf). Determination of glass transition temperature: Polymer appropriately
contained in bulb at the bottom is kept immersed in a suitable liquid, usually
mercury so as to give a column of the liquid in the capillary up to a convenient
height for measurement.
The positioning of the glass plug, as shown enables heating the test specimen
avoiding overheating. The dilatometer placed in an outer bath may be heated at
the present rate and pattern. From the rise of the liquid in the capillary on heating
and consequent rise in the temperature the change in the volume of the specimens
may be conveniently obtained.
60
Dilatometer
Tm and Tg values of some of the polymers.
Polymer R-unit TmoC Tg
oC
Polyethylen -CH2-CH2- 137 - 115
Polystyrene -CH2-CH-C6H5 240 95
Polysiloxane -OSi(CH3)2 -85 -123
2.7 - Conducting Polymers
Initially, organic polymers are normally used as insulators because of their
excellent insulating properties. In 1977, Heegar, Macdianid and Shirkawa for the
first time showed that electrical conductivity of polyacetylene can be increased by
13 fold of magnitude by doping with electron acceptor and donors. Norman and
others have achieved conductivity as high as copper metal in polyacetylene.
Polymers have π backbone when dopped results in drastic electrical, electronic,
magnetic and optical properties. The important doping reactions are oxidative,
reductive and proton acid doping. An organic polymer with highly delocalised π -electron system, having electrical
conductance of the order of conductor is called a conducting polymer. These
compounds have various applications because of flexibility, ease of fabrication,
stability, ease of process ability with the low cost.
Conducting Polyaniline: (PANI) Alan Mediarmid in 1985 investigated
polyaniline as an electrically conductivity polymer. Polymerised form of aniline
monomer polyaniline can be found in one of three idealised oxidation states.
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Leucoemeraldine - white/clear, Emeraldine - greenor blue, Pernigraniline -
blue / violet.
As it shows semi-metallic properties it is considered an organic metal. It is
transparent and stable in air when heated. Specific conductivity is — 55 cm1. In
the conducting slate its redox active green material may change its colour and
conductivity when exposed to different media. Under reducing conditions it turns
yellow and blue under oxidising or basic ones. It has wide and controllable range
of conductivity with other interesting properties like multicolour, chemical
sensitivity etc. PANI has application potentials in electromagnetic interference
shielding, as gas sensors, in gas separation, as an electrode rechargeable batteries,
electrochromic and in static charge dissipation.
Polyacetylene
nHC ≡ CH → Polymerisation →− CH CH − CH =CH − CH = CH −
Conjugate structure makes it behave like a semiconductor as some of the π
electrons can be thermally excited out of the bonds giving rise to small electrical
conductivity. Other conducting polymers are polydiacetylene, polythiosphere, polypyrrole, poly
- phenylene sulphide (PT’S) are also synthesised by polymerisation.
2.8 - Photoconductivity Enhancement of electrical conductivity on exposure to light or irradiation is called
photoconductivity. These materials are commonly insulators in dark and they
behave like semiconductors when exposed to light, e.g. P(N-vinyl carbacole)
PNVC. Metals are used in the form of powder of flakes or reinforcing agents in a
polymer matrix by making various moulded articles. They impart good electrical
and thermal conductivity into the composites. Electrical conductivity of a
conductive composite depends on intrinsic properties of the filler material as well
as matrix filler interaction and processing conditions. Applications of conducting polymers ▪ Used for corrosion protection, printed circuit boards, conductive fabrics,
pipes and smart windows.
▪ Used for coating of films and semi finished articles.
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▪ It is very useful as a secondary electrode in rechargeable batteries and
electrochromic display devices due to its electrochemical response during
anodic and cathodic reaction (oxidation-reduction).
2.9 - Electrical Properties of Polymers
Till about the first few decades of the twentieth century common polymers were
used only as insulators in electronics and electrical areas. For these applications,
selection is done on the basis of electrical property parameters.
Resistivity: A material having high electrical resistance is a good insulator.
Volume resistivity of a material is the resistances between opposite faces of a unit
cube when the current flow is confined to the volume of the test specimen and it is
commonly expressed in ohm. cm. The reciprocal of resistivity is conductivity.
Moisture affects volume resistance of different insulators to markedly different
extent. Non-polar polymers such as polystyrene and polyethylene are unaffected
but polar organic polymers are affected to a greater extent. Affection depends on
their degree of moisture absorption. Porosity favours moisture absorption and
lowers volume resistance. However, polar inorganic polymers like quartz and
glass remain unaffected by moisture. Resistance suffers appreciably with rise in
temperature.
Dielectric constant: Dielectric constant depends on the geometry of the test
specimen and applied voltage. At high voltages failure of electrical insulation
occurs. The maximum potential gradient that an insulating material can withstand
without breakdown and passage of discharge is known as breakdown voltage. The
voltage required for breakdown is dependent on, rate of voltage application
thickness of test specimen, frequency of applied voltage, temperature, dimensions
and geometry of the electrodes and nature of the environment flexible materials
with high dielectric strength and mechanical strength are used as insulating tapes.
2.10 - Applications of Polymers in Medicine and surgery
Polymers are used as biomaterial in thereputic and diagonastic system they are
also used in many pharmaceutical preparations, for example, as coatings for
tablets or capsules or as components of transdermal patches. Biomaterials play a
central role in extra Corporeal devices, from contact lenses to kidney dialyses, and
are essential components of implants, from vascular grafts to cardiac pacemakers.
Biodegradable polymers take center stage in a great variety of research efforts.
Materials that can decompose and disappear from the body are desirable for short-
term applications in orthopedics, tissue engineering, and other areas, where, for
example, a physician may need a device to hold a bone in place long enough for
the body to heal. Listed some of polymers having medical applications
i. Cellophane: Often used in everyday life to package our products or to keep
our food fresh, cellophane is one of the most critical materials for the
treatment of many kidney malfunctions. ii. Polydimethyl siloxane (PDMS): The polymer polydimethyl siloxane
63
(PDMS) is used in pacemakers, the delivery of vaccines, and the construction
cerebrospinal fluid shunts. iii. PGA( polyglycolic acid), PLA, and PLGA : PGA, PLA, and PLGA allow
the polymers to be used for a wide variety of applications within the human
body. These polymers are then used for drug-delivery systems, to construct
synthetic scaffolding, etc. The latest treatment in treating brain tumors
involves attaching dime- sized wafers directly into the skull8. The wafers are
made out of PLA or PLGA and slowly distribute cancer-killing. iv. Polyethylene and Polymethylmethacrylate (PMMA): used in Joint
replacements, particularity at the hip, and bone fixation devices have become
very successful applications of materials in medicine. The use of pins, plates,
and screws for bone fixation to aid recovery of bone fractures. v. Polytetrafluoroethylene: Polytetrafluoroethylene is useful for some
orthopedic and dental devices. It also has Biomaterials are used in many
blood-contacting devices. These include artificial heart valves, synthetic
vascular grafts, ventricular assist devices, drug releases, and a wide range of
invasive treatment and diagnostic systems. vi. Polyurethane: polyurethane today is one of the most important materials in
use for ventricular assist devices. Differing from artificial hearts, VAD’s are
for short-term assistance to cardiac circulation attached to one or both of the
heart ventricles. Most commonly seen in the operating room during open-
heart surgery, postoperatively and of extreme cardiac trauma.
Polymer Application
PDMS Catheters, heart Valves
Polytetrafluoroethylene Heart valves, Vascular grafts, Nerve
repair
Polyurethane ventricular assist Devices
Polyethylene Catheters, hipprostheses
Polymethylmethacrylate (PMMA) Fracture fixation
PGA, PLA, And PLGA Drug delivery, devices
Cellophane Dialysis membranes
2.11 - Fabrication of Polymers
All plastic resins can be shaped into a variety of products by initially making them
plastic and then subjecting them to the action of temperature and pressure in a
mould. The different fabrication methods available are as follows. • Compression moulding • Injection moulding • Transfer moulding
64
• Extrusion moulding
2.11.1 - Compression Moulding
Thermoplastic and thermosetting resins can be moulded by this method. The die
used for moulding purposes consists of two parts, upper and lower parts of male
and female parts. In closed condition, the clearance between the two halves gives
the desired shape to the product. Generally, the lower part of mould is fixed, the
upper part moves up and down, the movement being properly aligned because of
guide pins present. The lower part of the die also has arrangement for heating and
cooling by circulating fluids through pipe work.
Compression moulding involves transfer of required quantity of polymer mix
consisting of other ingredients and polymer, into the cavity. A slight excess of
material is taken to ensure that the cavity gets completely filled with material
during the compression process. The charge in cavity is heated to make it easy to
mould. The upper part of mould is then lowered and the mould cavity closed by
applying the necessary pressure and heat. This ensures the plastic mass get
completely distributed uniformly in the mould, taking the shape of mould. Any
excess runs off in the form of ‘flash’.
Fabrication by compression moulding.
For thermoplastic material, the die is allowed to cool so that the article becomes
rigid enough to be expelled from the mould by the eject in mechanism. For
thermosetting resins, the temperature is maintained at the curing temperature for
the desired time to ensure the articles are properly cured. Moulding temperatures
and pressures for thermosetting polymers can be as high as 200°C and 70 kg/cm2
respectively.
The mould cycle starts with filling up of cavity with the material and end with the
ejection of product formed from the mould cavity. This may vary from article to
65
article depending upon its size and complexity. After the removal of article, the
mould is made ready to receive the next charge by cleaning the mould with a blast
of compressed air. 2.11.2 Transfer Moulding In compression moulding, there are limitations with regards to size of die,
effective heat transfer, and ability to mould intricate parts. Transfer moulding
overcomes many of these limitations. The charge is preheated in transfer chamber,
a pot which may sometimes form part of mould. The fluidised material from the
pot is transferred to mould cavity due to plunging action of plunger through
heated flow channels. This permits moulding of large and intricate parts, as the
melted polymer flows easily. It is also possible to include inserts into the article.
The mould itself is maintained at high temperature to facilitate curing of set resin
in the mould. Thick sections are uniformly cured so that dimensional accuracies
are maintained within limits. Cycle times in the case transfer moulding are shorter than those of compression
moulding as the initial charge is in fluidised state and the mould is maintained at
right temperature for proper curing. Thick portions and mechanically strong
section can be fabricated by transfer moulding technique.
2.11.3 - Injection Moulding Technique
66
This technique is used for high, speed moulding of thermoplastic resins. The
machine consists of two parts—injection unit and the clamping unit which carries
the mould.
Fig. 1.13 Fabrication through injection moulding
The injection unit is a hollow cylindrical device fitted inside with screw conveyer
or plunger. The end attached to mould narrows down to form the nozzle. Part of
the forward section carries electrical heaters which heat the charge as it moves
along the cylinder length. The movement of screw conveyer pushes the charge
forward where it gets heated and melts. The molten mass is then pushed through
the nozzle into the cold mould. It immediately solidifies to rigid form. The mould
is opened to eject the product and again closed and clamped tightly. Since the
molten mass is pushed at high pressure, arrangements for keeping the two halves
of mould should be secure. High pressure also ensures that molten material is
evenly distributed in the mould cavity.
2.11.4 - Extrusion Moulding
It is mainly used for continuous moulding of thermosoftering plastics. Pipes. rods,
hoses, tubes are some of the products manufactured by extrusion process. This
method is also used to coat cables with a layer of plastic insulating material. The
extruder is designed in such a way that as the raw mix passes along the length of
extruder, it melts and flows out at uniform rate towards the die section. The
extruded product is shaped according to die characteristics, into rods, pipes or
67
tubes and carried along a conveyer belt to be cut into specified lengths. Then
tubular films are also made by extrusion process. Besides these methods of
fabrication, blow moulding and calendering are some of the other methods used
extensively for fabrication purposes.
Fig. 1.14 Horizontal extrusion moulding of plastics.
2.12 - Rubbers Natural rubber, also called India Rubber or caoutchouc, is a mixture of
organic compound polyisoprene and small amounts of other organic compounds
as well as water. This polymer is the main component. This material is classified
as an elastomer (an elastic polymer). It is derived from latex, a milky colloid
produced by some plants. The plants are ‘tapped’, that is, an incision made into
the bark of the tree and the sticky, milk colored latex sap collected and refined
into a usable rubber. Polyisoprene can also be produced synthetically. Natural
rubber is used extensively in many applications and products, as is synthetic
rubber. It is normally very stretchy and flexible and extremely waterproof. The rubber latex can be mixed with the required compounding substance and
precipitated in the shape that is needed for use. For example, rubber gloves are
easily prepared in this manner 2.12.1 – Commercial forms of Rubber Rubber is made available in the following forms for commercial purposes. The
latex after dilution and coagulation yields the precipitated mass which is the
coagulum. The coagulum is separated by filtration and treated further, to obtain
the various forms of rubber. Crepe Rubber: The coagulated mass of rubber is made into sheets by passing the
coagulum repeatedly through rollers. Addition of sodium bisulphate bleaches the
colour of the rubber. The sheets obtained may be pressed and passed again through the
rollers to obtain the required thickness. The sheets are then dried in hot air at about
50°C. Smoked Rubber: The coagulum obtained after coagulation of rubber latex is made
into thick sheets by passing through rollers without using bleaching agents and dried at
about 40°— 50°C in the presence of smoke obtained by burning wood or shells. This
treatment prevents the growth of mould and bacteria and preserves rubber against
oxidation. Exposing the rubber sheets to smoke makes them stronger and brownish in
colour. Alternately the rubber sheets are prepared in long tanks provided with vertical
grooves fitted with metal plates. The latex, after initial purification is poured into
the tank a4d coagulated by adding formic or acetic acid and stirred. After inserting
the plates in the groove, the tank is kept at rest for nearly 16-18 hours. The slabs
of rubber obtained are removed and passed through a series of rollers having
decreasing clearance between them. Water is sprayed in between the rollers. The
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final roller can be adjusted to get ribbed pattern on the rubber sheets and facilitates
easy drying. The dried sheets are hung in smoke house at 40°-50°C.
Gutta-Percha: It is another kind of natural rubber obtained from the leaves of
Dichopsis gutta and palagium gutta trees found in Malaya, Sumatra and Borneo. The
leaves are ground and treated with water at about 70°C and poured into water when
the latex material floats on water. It can also be extracted by solvent extraction when
the resins and gums being insoluble get separated. Structurally, it is found to be trans-
polyisoprene.
2.12.2 – Properties of Natural Rubber
Structure
(i) It is a polymer of isoprene (2 methyl- 1, 3 -butadiene), the polymer consisting
over two thousand monomers linked together (C5H8) where stands for the number
of monomers. It may be represented as below.
It can exist in cis and trans forms. Natural rubber is cis-l, 4-poly isoprene and
gutta percha which is another form of natural rubber is a trans isomer.
The molecule of rubber in the unstressed condition is in the form of a coil which
can b stretched like a spring. It can be deformed to a large extent and yet can
recover its original shape and size after the removal of the applied stress.
General Properties
▪ Pure rubber becomes soft and sticky in summer and hard and brittle in winter.
▪ On heating, it decomposes to form isoprene (C5H8) an unsaturated
hydrocarbon.
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▪ Action of Ozone (O3) on rubber produces levulinic aldehyde, CH3—CO-
CFI2-CH2—CHO as the principal product showing that rubber is a head-to-
tail polymer of isoprene.
▪ Rubber shows marked resemblance to unsaturated hydrocarbons as indicated
by its reaction with different chemical reagents.o When rubber is made to react with hydrogen chloride, addition product,
i.e., Rubber hydrochloride (C5H9Cl)n.
o Chlorine reacts with rubber forming both addition and substitution
products (Chlorinated rubber). o Hydrogen reacts with rubber and produces addition product (C5H10).
o The effect of atmospheric oxygen is to cause hardness and brittleness in
rubber. o Corresponding derivatives are obtained by reactions with sulphuric acid,
sulphonic acid and oxides of nitrogen.
o Moderately strong acids and alkalies have no significant action on rubber.
▪ Rubber is insoluble in water and water - like solvents such as alcohol,
acetone, etc. but it disperses freely in benzene, toluene, gasoline, carbon
disulphide, turpentine, chloroform, carbon tetrachioride, etc. to form viscous
liquids which are used as adhesives. 2.12.3 Drawbacks of Natural Rubber Raw rubber shows the following drawbacks on account of which it needs to be
suitably compounded and heat treated. ▪ It is found to be unsuitable at low as well as at higher temperatures. At lower
temperatures, it is found to be brittle and at higher temperatures, it is soft and
sticky. It is found to be useful only in the temperature range of 10°C to 60°C.
▪ It has a low tensile strength, i.e. (200 kg/cm2).
▪ It has high water absorption property.
▪ It is easily oxidised by 02 of air and other oxidising agents like Nitric acid,
Sodium hypochlorite, Chlorine, Chromic acid, etc.
▪ It is not resistant to the action of solvents like vegetable and mineral oils,
benzene, gasoline, carbon tetrachloride, etc.
▪ It swells in organic solvents undergoing disintegration.
▪ It possesses marked tackiness: i.e. two pieces or sheets in fresh condition get
adhered to each other under pressure.
▪ It is less durable, non-resistant to scratches, and suffers permanent
deformation on being stretched strongly. 2.12.4 – Vulcanisation of Rubber The process was carried out first by Goodyear in 1839 using sulphur for effecting
cross-linking of the poly-isoprene molecules in natural rubber.
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Vulcanisation process is important for improvements in the properties of both
natural as well as synthetic rubbers. It is a process of cross-linking the rubber
molecules using a vulcanising agent. In addition to sulphur, certain compounds of
sulphur are also found to bring about the cross-linking in rubber molecules. The
cross-linking may take place either at the double-bond or even without affecting
the double-bond in the polymer followed by elimination of substances like I-lBS,
sulphur. The evolved sulphur may bring about further cross-linking reactions.
Some of the reactions are as follows:
Vulcanisation is also brought about by compounds of sulphur like thioacids,
mercaptans, etc. Addition of 0.5 to 5% sulphur gives soft and elastic rubber and
increasing the quantity of sulphur increases the hardness and stiffness of the
rubber.The time needed for the vulcanisation process depends on the quality of
product. The process can be accelerated by adding oxides of metals like zinc,
calcium, lead, magnesium, etc. which are the accelerators for the process.
The process of cross-linking can also be achieved by using peroxides, amine
derivatives and oximes in the case of certain varieties of rubber. Sulphur can be
partially replaced by selenium and tellurium in the case of diene rubbers.
The use of sulphur or any vulcanising agent together with the accelerator gives
only limited improvements in physical and mechanical properties. For improving
stability, flexibility, processability, resistance to abrasion, etc. various other
additives accelater, antioxidants, reinforcing agent are added and vulcanisation
together is carried out.
2.12.5 - Buna - S (styrene-butadiene rubber) describe families of synthetic rubbers derived from styrene and butadiene. These materials have good abrasion
resistance and good aging stability when protected by additives. About 50% of car
tires are made from various types of SBR. The styrene/butadiene ratio
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influences the properties of the polymer: with high styrene content, the rubbers are
harder and less rubbery. The material was initially marketed with the brand name Buna S. Its name derives Bu for butadiene and Na for sodium (natrium in several languages including Latin, German and Dutch), and S for styrene.
Types of SBR SBR is derived from two monomers, styrene and butadiene. The mixture of these
two monomers are polymerised by two basically different processes: from
solution (S-SBR) or as an emulsion (E-SBR). Structure of Buna - S
Emulsion polymerisation E-SBR produced by emulsion polymerisation is initiated by free radicals.
Reaction vessels are typically charged with the two monomers, a free radical
generator, and a chain transfer agent such as an alkyl mercaptan. Radical initiators
include potassium persulfate and hydroperoxides in combination with ferrous
salts. Emulsifying agents include various soaps. By "capping" the growing organic
radicals, mercaptans (e.g. dodecylthiol, control the molecular weight, and hence
the viscosity, of the product. E-SBR is more widely used. Typically,
polymerizations are allowed to proceed only to ca. 70%, a method called "short
stopping". In this way, various additives can be removed from the polymer. Solution polymerisation
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Solution-SBR is produced by an anionic polymerization process. Polymerisation
is initiated by alkyl lithium compounds. Water is strictly excluded. The process is
homogeneous (all components are dissolved), which provides greater control over
the process, allowing tailoring of the polymer. The organolithium compound adds
to one of the monomers , generating a carbanion that then adds to another
monomer, and so on. Relative to E-SBR, S-SBR is increasingly favored because it
offers improved wet grip and rolling resistance, which translate to greater safety
and better fuel economy, respectively.
Properties
Property S-SBR E-SBR
Tensile strength (MPa) 18 19
Elongation at tear (%) 565 635
Mooney viscosity (100 °C) 48.0 51.5
Glass transition temperature (°C) -65 -50
Polydispersity 2.1 4.5
Applications
The elastomer is used widely in pneumatic tires, shoe heels and soles, gaskets and
even chewing gum. It is a commodity material which competes with natural
rubber. Latex (emulsion) SBR is extensively used in coated papers, being one of
the most cost-effective resins to bind pigmented coatings. It is also used in
building applications, as a sealing and binding agent behind renders as an
alternative to PVA, but is more expensive. In the latter application, it offers better
durability, reduced shrinkage and increased flexibility, as well as being resistant to
emulsification in damp conditions. SBR can be used to 'tank' damp rooms or
surfaces, a process in which the rubber is painted onto the entire surface
(sometimes both the walls, floor and ceiling) forming a continuous, seamless
damp proof liner; a typical example would be a basement.
Additionally, it is used in some rubber cutting boards.
2.12.6 - Polyethylene (PE)
Polyethylene is obtained by polymerisation of ethylene. Depending on the
reaction conditions two types of polyethylene are available
(i) Low density polyethylene (LDPE) (density 0.915 to 0.940 g/cm3)
(ii) High density polyethylene (HDPE) (density 0.945 to 0.960 g/cm3)
Manufacture of LDPE
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Highly punifiea ethylene is compressed at 1500-2000 atm pressure in e presence
of traces of oxygen at 160-170°C.
nH C = CH 2
150 − 1700 C [CH 2
− CH ]
2 1500atm.
2 n
Ethylene LDPE Manufacture of HDPE Polymerisation reactions can be carried out at much high temperature and pressure
in the presence of catalyst containing metallic oxides, e.g., catalyst mixture
contain Cr203, Silica and aluminice is activated by heating at 250°C. The
activated catalyst is then dispersed in a solvent cyclohexane. The temperature of
polymerisation is around 130-150°C and 15 to 30 atm pressures.
LPDE HDPE
(1) Density 0.9 15 to 0.940 g/cm3
0.945 to 0.960 g/cm3
(ii) Temp. 160-170°C and pressure Temp. 130-150°C and pressure 15 to
1500-2000 atm. 30 atm.
(iii) Softening temp. 110-117°C 125-130°C
(iv) 2 to 50 branches for 1000 carbon 2.5 branches for 1000 carbon atoms.
atoms.
(v) Low tensile strength High tensile strength
(iv) Soluble in toluene at 60-70°C. soluble in toluene at 60-70°C.
2.12.7 - Polyurethane Polyurethane is a type of cross linked polymer prepared from two liquid i.e.a
polyol and isocynate
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Properties
It is also be foamed like polystyrene but unlike thermocoal, it is soft, spongy
known as “U foam”. It has low thermal conductivity. Its greatest advantage lies in
the fact that it can be made where they needed without any complex machinery
and two liquid ingrediants can be mixed and moulded.
Uses: it is used as insulating material in refrigerator. Due to its spongy nature used
in making pillow and mattresses. It is also used as coating on leather goods such
as shoes and hand bags, which improve appearances of leather goods. It is used in
making chair. It is also used in foundation garments and swim suits.
2.12.8 - Silicones
Silicones having alternate silicon – oxygen bonds and radical attached to silicon
atom
The monomers of silicon are prepared from alkyl silicon halides.
2 R – Cl + Si →Cu R2SiCl2
Or from Grignard reagent
SiCl4 + RMgCl ⟶ RSiCl3 + MgCl2 The monomer is obtained by fractional distillation of reaction products whereby
different organo silicon chloride are obtained
In next step chlorides are polymerized by hydrolysis by following steps ≡SiCl + H2O ⟶ ≡SiOH + HCl ≡SiOH + HOSi≡ ⟶ ≡Si –O – Si ≡+ H2O
Thus the Oh group of Si are involved in polymerization hence when there is one
or two – OH groups in Si, it leads to long chain polymers but when there are three
- OH groups, a cross linked polymer obtained reactions
2 R – Cl + Si →Cu R2SiCl2
From Grignard reagent
SiCl4 + RMgCl ⟶ RSiCl3 + MgCl2
Me2SiCl2 →H2O
Me2Si(OH)2
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HO.SiMe2 – [OSiMe2]n- OSiMe2OH Di – alkyl – di chlorosilicane and alkyl trichloro silicon undergo hydrolysis and
condensation polymerization to give a cross linked silicon polymer
Cross linked polymerisation Complete condensation of all the – OH gives rise to hard, insoluble product, thus
a mixture of monomers containing one or more – OH group along with sufficient
water for hydrolysis is heated for polymerization. Different types of silicones
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Depends on proportion of various alkyl silicon halides used, the final silicones
may be liquid, semisolid and solid. Their properties and uses also differ
accordingly
i. Silicone fluid - they are of relatively low molecular weight, sparkling clear
fluids with an oily feel, insoluble in water but soluble in aromatic and
chlorinated solvents. They possess good resistance to heat and oxidation, low
surface tension and show low change in viscosity with temperature. They
used as autofoam agents, high tempreture lubricants , used in cosmatics as
damping and hydrolic fluids and to give water repellent finish to textiles and
leather. ii. Silicone greases - these are formed from the oils by adding silica, carbon
black etc. they are used as lubricants particularly for very high and low
tempreture applications. iii. Silicone resins – ther are highly cross linked polymers, having good
insulating properties, heat resitance and good di electrical properties. Used as
high voltage insulator, high tempreture insulating foam, silicon glass
laminates for high tempreture application for different electrical and
electroninc equipments and parts manufacturing. iv. Silicon rubber - silicone rubber are formed by reaction of dimethyl silicone
fluid with peroxide and appropariate inorganic fillers like ZnO SiO2 TiO2
etc. they retain rubbery properties over much wide tempreture span, good heat
transfer properties, good resistance to dilute acid and alkalis. Using in tyre
manufacturing for fighter aircraft, as an insulator of electrical wires in ship, as
adhesive for artificial heartvalves, transfusion tubings, for special boots to be
used at very low tempreture, for making lubricants,paints , protative coatings
etc.
2.12.9 - PMMA (Polymethyl Methacrylate) or Lucite or Plexig lass
It is prepared by polymerisation of methyl-methacrylate an ester of methyl acrylic
acid, CH2=C. (CH3)—COOH in presence of actyl peroxide. It is an acrylic
polymer
Properties
Colourless thermoplastic, hard, fairly rigid material with high softening
temperature 130— 140°C, it becomes rubber like above 65°C. It has high optical
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transparency, high resistance cured conditions. Its refractive index is 1.59. Its
most important drawback is low resistance to hot acids and alkalis and low scratch
resistance. Uses ▪ Mainly used for protective coating, and for manufacture of safety glass as it
can be moulded easily to almost any shape.
▪ Emulsions of acrylic resins have been widely used as textile and leather
finish, base coats on rubberised surfaces, etc.
▪ Widely used in industry in making lenses, banber noses, transport models of
complicated mechanisms, artificial eyes, emulsions, paints, adhesives,
automobiles, wind screens, TV. screens, optical parts of instruments,
jewellery, etc. • Solution polymer in volatile solvents used for adhesive and for heat and fume
resistant enamels, luminecent paints, etc. 2.12.10 - KEVLAR Kevlar is the registered trademark for a para-aramid synthetic fiber, related to
other aramids such as Nomex and Technora. Developed at DuPont in 1965, this
high strength material was first commercially used in the early 1970s as a
replacement for steel in racing tires. Typically it is spun into ropes or fabric sheets
that can be used as such or as an ingredient in composite material components. Synthesis of Kevlar Kevlar is synthesized in solution from the monomers 1,4-phenylene-d iamine
(para-phenylenediamine) and terephthaloyl chloride in a condensation reaction
yielding hydrochloric acid as a byproduct. The result has liquid crystalline
behavior, and mechanical drawing orients the polymer chains in the fiber's
direction. Hexamethylphosphoramide (HMPA) was the solvent initially used for
the polymerization, but for safety reasons, DuPont replaced it by a solution of N-
methyl-pyrrolidone and calcium chloride.
Kevlar production is expensive because of the difficulties arising from using
concentrated sulfuric acid, needed to keep the water- insoluble polymer in
solution during its synthesis and spinning.
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Several grades of Kevlar are available:
1. Kevlar K-29 – in industrial applications, such as cables, asbestos replacement, brake linings, and body/vehicle armor.
2. Kevlar K49 – high modulus used in cable and rope products.
3. Kevlar K100 – colored version of Kevlar
4. Kevlar K119 – higher-elongation, flexible and more fatigue resistant.
5. Kevlar K129 – higher tenacity for ballistic applications.
6. Kevlar AP – has 15% higher tensile strength than K-29.
7. Kevlar XP – lighter weight resin and KM2 plus fiber combination.
8. Kevlar KM2 – enhanced ballistic resistance for armor applications
The ultraviolet component of sunlight degrades and decomposes Kevlar, a
problem known as UV degradation, and so it is rarely used outdoors without
protection against sunlight.
Molecular structure of Kevlar: bold represents a monomer unit, dashed
lines indicate hydrogen bonds.
Properties :
When Kevlar is spun, the resulting fiber has a tensile strength of about 3,620 MPa,
and a relative density of 1.44. The polymer owes its high strength to the many
inter - chain bonds. These inter-molecular hydrogen bonds form between the
carbonyl groups and NH centers. Additional strength is derived from aromatic
stacking interactions between adjacent strands. These interactions have a greater
influence on Kevlar than the van der Waals interactions and chain length that
typically influence the properties of other synthetic polymers and fibers such as
Dyneema. The presence of salts and certain other impurities, especially calcium,
could interfere with the strand interactions and caution is used to avoid inclusion
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in its production. Kevlar's structure consists of relatively rigid molecules which
tend to form mostly planar sheet-like structures rather like silk protein.
Thermal properties Kevlar maintains its strength and resilience down to cryogenic temperatures (−196
°C); in fact, it is slightly stronger at low temperatures. At higher temperatures the
tensile strength is immediately reduced by about 10–20%, and after some hours
the strength progressively reduces further. For example at 160 °C (320 °F) about
10% reduction in strength occurs after 500 hours. At 260 °C (500 °F) 50%
strength reduction occurs after 70 hours. Applications of Kevlar ▪ Cryogenics : Kevlar is used in the field of cryogenics for its low thermal
conductivity and high strength relative to other materials for suspension
purposes.
▪ Armor : Kevlar is a well-known component of personal armor such as
combat helmets, ballistic face masks, and ballistic vests.
▪ Personal protection : Kevlar is used to manufacture gloves, sleeves, jackets,
chaps and other articles of clothing designed to protect users from cuts,
abrasions and heat.
▪ Sports equipment: It is used as an inner lining for some bicycle tires to
prevent punctures. In table tennis, plies of Kevlar are added to custom ply
blades, or paddles, in order to increase bounce and reduce weight
▪ Shoes : With advancements in technology, Nike used Kevlar in shoes for the
first time.
▪ Audio equipment : Kevlar has also been found to have useful acoustic
properties for loudspeaker cones, specifically for bass and midrange drive
units.
▪ Strings: Kevlar can be used as an acoustic core on bows for string
instruments.
▪ Drumheads: Kevlar is sometimes used as a material on marching snare
drums. It allows for an extremely high amount of tension, resulting in a
cleaner sound.
▪ Woodwind reeds : Kevlar is used in the woodwind reeds of Fibracell.
▪ Fire dancing: Wicks for fire dancing props are made of composite materials
with Kevlar in them.
▪ Frying pans: Kevlar is sometimes used as a substitute for Teflon in some
non-stick frying pans.
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▪ Rope, cable, sheath:The fiber is used in woven rope and in cable, where the
fibers are kept parallel within a polyethylene sleeve.
▪ Electricity generation: Kevlar was used by scientists at Georgia Institute of
Technology as a base textile for an experiment in electricity-producing
clothing.
▪ Brakes : The chopped fiber has been used as a replacement for asbestos in
brake pads.
▪ Expansion joints and hoses: Kevlar can be found as a reinforcing layer in
rubber bellows expansion joints and rubber hoses, for use in high temperature
applications, and for its high strength.
▪ Particle physics: A thin Kevlar window has been used by the NA48
experiment at CERN to separate a vacuum vessel from a vessel at nearly
atmospheric pressure, both 192 cm in diameter.
▪ Smartphones :The Motorola Droid RAZR has a kevlar backplate, chosen
over other materials such as carbon fiber due to its resilience and lack of
interference with signal transmission.
▪ Composite materials: Aramid fibers are widely used for reinforcing
composite materials, often in combination with carbon fiber and glass fiber.
2.12.11 - Phenol formaldehyde resins (PF) are synthetic polymers obtained
by the reaction of phenol or substituted phenol with formaldehyde. Phenolic resins are
mainly used in the production of circuit boards. They are better known however for
the production of molded products including pool balls, laboratory countertops, and as
coatings and adhesives. In the form of Bakelite, they are the earliest commercial
synthetic resin.
Phenol-formaldehyde resins, as a group, are formed by a step-growth
polymerization reaction that can be either acid- or base-catalysed. Since
formaldehyde exists predominantly in solution as a dynamic equilibrium of
methylene glycol oligomers, the concentration of thereactive form of
formaldehyde depends on temperature and pH.
Phenol is reactive towards formaldehyde at the ortho and para sites (sites 2, 4 and 6) allowing up to 3 units of formaldehyde to attach to the ring. The initial reaction
in all cases involves the formation of a hydroxyl methyl phenol:
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In aqueous solution, formaldehyde exists in equilibrium with methylene glycol.
Depending on the pH of the catalyst, these monomers react to form one of two
general resin types: NOVOLAC RESINS and RESOL RESINS. Novolac Resins An acidic catalyst and a molar excess of phenol to formaldehyde are conditions
used to make novolac resins. The following simplified chemistry illustrates the
wide range of polymers possible. The initial reaction is between methylene glycol and phenol.
The reaction continues with additional phenol, and splitting off of water.
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The reaction creates a methylene bridge at either the ortho position or the para
position of the phenolic aromatic rings. The "rule of thumb" is that the para
position is approximately twice as reactive as the ortho position, but there are
twice as many ortho sites (two per phenol molecule) so the fractions of ortho-
ortho, para-para and ortho-para bridges are approximately equal.
Branching occurs because reaction can occur at any of three sites on each ring. As
the reaction continues, the random orientations and branching quickly result in an
extremely complex mixture of polymers of different sizes and structures. The
reaction stops when the formaldehyde reactant is exhausted, often leaving up to
10% of un-reacted phenol. Distillation of the molten resin during manufacturing
removes the excess phenol and water.
The final novolac resin is unable to react further without the addition of a cross-
linking agent.
Because an additional agent is required to complete the resin's cure, the industry
commonly refers to novolac resins as "two-stage" or "two-step" products. The
most common phenolic resin cross-linking agent is hexamethylenetetramine, also
known as hexa, hexamine, or HMTA. Ground and blended with the resin, hexa
serves as a convenient source of formaldehyde when heated to molding and curing
temperatures. A special attribute of hexa is that it reacts directly with resin and
phenol without producing appreciable amounts of free formaldehyde. Hexa cures
the resin by further linking and polymerizing the molecules to an infusible state.
Due to the bond angles and multiple reaction sites involved in the reaction
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chemistry, the resulting polymer is not a long straight chain but rather a complex
three-dimensional polymer network of extreme molecular weight. This tightly
cured bonding network of aromatic phenolics accounts for the cured materials'
hardness, and heat and solvent resistant properties. Certain catalysts can affect the orientations of the methylene linkages. Catalysts
that preferably promote ortho-ortho linkages tend to preserve the more reactive
para positions:
Novolac resins made with these catalysts tend to cure more rapidly than the
standard randomly linked resins. Novolac resins are amorphous (not crystalline)
thermoplastics. As they are most typically used, they are solid at room
temperature and will soften and flow between 150° and 220°F (65°C - 105°C).
The number average molecular weight (Mn) of a standard phenol novolac resin is
between 250 and 900. As the molecular weight of phenol is 94 grams per mole, a
Mn of 500 corresponds to a resin where the average polymer size in the entire
distribution of polymers is five linked phenol rings. Novolac resins are soluble in
many polar organic solvents (e.g., alcohols, acetone), but not in water. Resol Resins A basic (alkaline) catalyst and, usually but not necessarily, a molar excess of
formaldehyde is used to make resol resins. The following two stages describe a
simplified view of the reaction: First, phenol reacts with methylene glycol to form methylol phenol: Methylol phenol can react with itself to form a longer chain methylol phenolic:
or form dibenzyl ether:
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or react with phenol to form a methylene bridge.
The most important point in resol resin chemistry is that, when an excess of
formaldehyde is used, a sufficient number of methylol and dibenzyl ether groups
remain reactive to complete the polymerization and cure the resin without
incorporation of a cure agent such as hexa. For this reason, the industry commonly
refers to resol resins as "single-stage" or "one-step" type products. Resol resin
manufacture includes polymerizing to the desired extent, distilling off excess
water and quenching or tempering the polymerization reaction by rapid cooling.
Because resol resins continue the polymerization reaction at even ambient
temperatures, albeit at much slower rates than during manufacturing, they
demonstrate limited shelf lives dependent on the resin character, storage
conditions and application.
By manipulating the phenolic to aldehyde monomer ratio, pH, catalyst type,
reaction temperature, reaction time, and amount of distillation, a variety of resin
structures demonstrating a wide range of properties are possible. The typical
number average molecular weight (Mn) of a straight phenol resol resin is between
200 and 450. Plastics Engineering Company supplies resol resins as liquids or in
solvents with viscosities from 50 to 50,000 cps, or as solids in the form of lumps,
granules, or fine powders. Organic solvents and the amount of water or phenol
monomer left in the resin control the viscosity of the liquid resin products. Resol
resins are usually water-soluble to a certain degree.
Crosslinking and the phenol/formaldehyde ratio
When the molar ratio of formaldehyde : phenol reaches one, in theory every
phenol is linked together via methylene bridges, generating one single molecule,
and the system is entirely crosslinked. This is why novolacs (F: P <1) don't harden
without the addition of a crosslinking agent, and that’s why resoles with the
formula F: P >1 will.
Characteristics of phenol formaldehyde
Bonding Strength: The primary use of phenolic resin is as a bonding agent.
Phenolic resin effortlessly penetrates and adheres to the structure of many organic
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and inorganic fillers and reinforcements, which makes it an ideal candidate for
various end uses. A brief thermal exposure to complete the cross-linking or
"thermoset" process results in attainment of final properties. The unique ability of
phenolic resin to "wet out" and to cross-link throughout the fillers and
reinforcements provides the means to engineer the desired mechanical, thermal,
and chemically resistant properties. High Temperature Performance: A key characteristic of thermoset phenolic
resin is its ability to withstand high temperature under mechanical load with
minimal deformation or creep. In other words, cured phenolic resin provides the
rigidity necessary to maintain structural integrity and dimensional stability even
under severe conditions. Chemical Resistance: Phenolic resins accommodate the harsh exposure of severe
chemical environments. The inherent nature of phenolic resin provides an
impervious shield to protect a variety of substrates from the corrosive effects of
chemicals. Laboratory tests confirm minimal degradation from many chemicals
after prolonged exposure, often at elevated temperatures. Low Smoke and Toxicity: Burning phenolic resin typically generates hydrogen,
hydrocarbons, water vapor, and carbon dioxide. Phenolic resin produces a
relatively low amount of smoke at a relatively low level of toxicity. Manufacturers
use phenolic resins extensively to address the safety concerns of the transportation
industry. Automotive and mass transit industries choose phenolic resin for its high
heat resistance and excellent flame, smoke, and toxicity properties. Another
critical application is in air support systems for the mining industry and related
electrical conduit supports. Phenolic resins designed to meet specific flammability
ratings are available. Selective use of inorganic fillers and reinforcements often
enhances protection in the event of contact with an ignition source. High Carbon and Char Yield: Phenolic resins demonstrate higher char yields
than other plastic materials when exposed to temperatures above their point of
decomposition. In an inert atmosphere at high temperatures (600° - 2,000°F, 300 -
1,000°C), phenolic resin will convert to a structural carbon known as vitreous
carbon. In many ways, this material behaves similar to ceramic and may actually
contribute to structural integrity when exposed to fire situations. Manufacturers of
structural composite gratings and pipes for offshore oilrigs, where fires are a
constant threat, utilize phenolic resins for the characteristic. Phenolic resin is also
useful in designing vitreous carbon articles such as special analytical electrodes,
crucibles for melting rare earth metals, rocket nozzles, extremely high temperature
bearings and seals, and heat shields for missiles. Automotive applications that
benefit from the formation of a thin carbonized layer, such as brake blocks and
pads, brake linings, and clutch facings also use phenolic resins.
The aerospace, defense, and electrical industries are heavily reliant on phenolic
resins. Phenolic resin advantages include high heat resistance, excellent
dimensional stability, as well as having a United Laboratories rating. Phenolic
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molding compound applications within these industries include electrical
commutators, switches, business equipment, and wiring devices. Phenolic resin
retains its strength at high temperatures, resists creep under load, and possesses
chemical and corrosive resistance. Phenolic resins are widely incorporated in
household appliances because of their excellent electrical resistance, dimensional
and thermal stability, and resistance to water and solvents.
Applications of phenol formaldehyde
• Ablation: Phenolic resin chars when heated to temperatures greater than
480°F (250°C). This process continues at very high temperatures greater than
1,000°F (>500°C), until the resin completely converts to amorphous carbon.
This characteristic contributes to the unique ablative properties of phenolic
resins. Examples are rocket nozzles, rocket blast shields, and atmospheric
reentry shields. • Abrasives:The variety of abrasive products available in the market is
practically endless, as they have to meet the specific needs of the individual
grinding applications and substrates. Generally, there are three groups of
abrasive products: bonded, coated, and non-woven. • Bonded abrasives: Bonded abrasives like grinding wheels are
comprised of abrasive particles embedded in a bonding matrix.
• Coated Abrasives: Coated abrasives are flexible grinding materials
typically available as sheets, discs or belts. These applications require
abrasive grains fixed to the surface of a variety of backings, like paper
or fabric, by special liquid phenolic resin binders. • Non-Woven Abrasives: Household and industrial applications use non-
woven abrasives, also called abrasive pads. The characteristically green
pads used for cleaning the dishes are the most publicly visible non-
woven abrasive.
▪ Adhesives: Wood bonding applications such as particleboard or wafer-
board have traditionally used phenolic resin binders.
▪ Carbon: Phenolic resins have an excellent affinity for graphitic and
other forms of carbon. Manufacturers often use the resin simply as a
binder and adhesive for their carbon materials.
▪ Coatings: Cured phenolic resins demonstrate exceptional chemical
resistance. Railroad cars, storage tanks and heat transfer equipment are
coated using phenolic resins as part of baked phenolic coating systems.
▪ Composites: Phenolic resins are the polymer matrix of choice in
composite products especially when meeting high flame, smoke and
toxicity (FST) properties.
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▪ Felt Bonding: Fiber felt manufacturers use phenolic resins with reclaimed
or virgin fibers to produce thermal and acoustical insulation for the
automotive and household appliance industries.
▪ Foam:Special phenolic resins in combination with the proper cure
catalysts, surfactants and blowing agents produce foam products
▪ Foundry: Many technologies are available to foundries for the production of
dies for metal castings.
▪ Friction:Phenolic thermoset resin is the choice for composite friction
materials: the pads, blocks, linings, discs and adhesives used in brake &
clutch systems that create retarding or holding forces with application against
a moving part.
▪ Proppants (Frac Sand): Oil and natural gas producers improve well yields
using hydraulic fracturing fluids containing round specialty sands coated
with phenolic resin.
▪ Refractory: High carbon yield, wear resistance, and excellent particle
wetting and bonding properties make phenolic resins ideal for refractory
products.
▪ Rubber:Tires and technical rubber goods use straight phenolic novolac
resins as reinforcing agents.
▪ Substrate Saturation:Many applications use liquid phenolic resins
to saturate substrates such as paper, fabrics, and wood. 2.12.12 - Urea Formaldehyde Urea reacts with formaldehyde in alkaline medium to produce monomethylol and
dimethylol urea. These when heated under pressure with catalyst forms a cross-
linked urea formaldehyde polymer.
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Properties
High tensile strength and compressive strength, good electrical insulation. Good
resistance to heat, water and chemicals except strong acids and alkalies. Available
in liquid, waxy and rubbery form, good dielectric properties and a sustain
temperature between — 50 to 300°C.
Uses
For making laminates, insulation goods, control knobs, plates, dishes, etc. Fevicol
used as adhesive is also used in making varnishes, surface coating, light weight
foams in aeroplanes.
Questions
1. What are polymers? Describe the classification of polymers.
2. Distinguish between thermoplastics and thermosetting polymers.
89
3. Write a note on melting and glass transition phenomena. 4. What is polymerization? Explain the degree of polymerisation. 5. Discuss various types of polymerisation with suitable examples. 6. What are plastics? Explain compounding of plastics. 7. What are elastomers? 8. What is vulcanization? How drawbacks of natural rubber are rectified. 9. What are advanced polymer materials? 10. Describe conducting polymers in detail 11. What are electrical properties of polymers? 12. What is meant by fabrication of plastics? Mention various methods of
fabrication of plastics. Describe two methods of fabrication of plastics. 13. Write synthesis, properties and uses of
(i) Polyethylene (PE) (ii) Phenolformaldehyde
(iii)Kevler (iv) Buna -S 14. Write a short note on polymer composite materials.
(ii) PMMA formaldehyde resin. 15. What are the applications of kevler 16. Write down applications of polymer in medical uses
3. Lubricants
Smooth surfaces when viewed through microscope it is observed they contain
many peaks and troughs. Peaks are called asperities and contact between surfaces
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takes place through these asperities. When two surfaces are in contact and when
movement is initiated by sliding or rotation, there is considerable resistance to
such movement.
Friction is defined as the force resisting motion when two contacting surfaces are
moved with respect to each other. The frictional resistance results considerable
loss of energy and damage to the contacting surfaces. In between the molecules of
contacting surfaces there exists molecular force of cohesion, known as van der
Waals force of attraction. When such surfaces are in contact with each other, the
actual contacting points are a few asperities. The pressure at these points is very
high as they have to bear the total load and this result in interlocking or welding of
these junctions. Thus, the vander Waals forces, as well as the welded junctions
offer resistance to motion. As one surface slides over the other surface some of the
asperities break and this results in wear and tear of the surfaces. Thus, the friction
generates heat and is also associated with high pressure developed even under
small loads, which causes fusion of the material at the peaks and accounts for
formation of welded junctions. If the relative motion of the contacting surfaces is
to be maintained, an additional force is required to break these welded junctions,
which results in generation of more heat. Thus, generation of heat becomes a self-
accelerating process which may ultimately result in a grinding halt.
Fig. 3.1 Surface roughness as seen through microscope
The friction and wear can be minimised by lubrication which involves
interposition of the substance, of low shear strength between the moving surfaces,
which is adequate in preventing or at least minimizing asperity contact. It reduces
the frictional resistance and the loss of energy due to friction is considerably
reduced.
A substance which is capable of reducing the friction between two surfaces which
are sliding over each other is called ‘lubricant.’ The process of reducing friction or
introduction of lubricant between two sliding surfaces is called lubrication.
3.1 - PURPOSE OF LUBRICATION
i. It covers the surface abnormalities, prevents metal to metal contact and then
reduces wear and tear and surface deformation, by avoiding direct between
the rubbing surfaces.
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ii. To act as a coolant or heat transfer medium by absorbing the heat of friction
created by rubbing of surfaces.
iii. To increase the efficiency of the machine by reducing the consumption of
energy.
iv. To prevent entry of dust and moisture between the moving parts. This
prevents corrosion of the metal surfaces.
v. In internal combustion engines, it acts as a seal and prevents leakage of
gases at high pressure in the combustion chamber and hence reduces the
maintenance as well as running cost of the machine to a large extent. vi. It reduces frictional resistance.
3.2 CLASSIFICATION OF LUBRICANTS Lubricants are classified on the basis of physical state as follows:
i. Solid lubricants: e.g., Graphite, molybdenum disulphide, strong, PTFE,
teflon, talc, mica, metals like Ga, In, etc.
ii. Semi-solid lubricants: e.g., Soaps, greases, vaselines, etc. iii. Liquid lubricants: Vegetable oils, blended oils and silicon oils, etc. iv. Emulsions: e.g., oil in water and water in oil emulsions. v. Synthetic lubricants: Hydrocarbons, phosphates, esters, polyglycols,
silicones, chlorofluorocarbons, etc. 3.2.1 Solid Lubricants Solid lubricants are used in the form of powder or a suspension in water or oil. The coefficient of friction lies in the range 0.005 to 0.01. The solid lubricants separate two moving surfaces under boundary conditions.
They are economical, non-toxic, easy to apply, chemically inert, thermally stable
and are resistant to radiations. Solid lubricants are generally used where use of
liquid or semi-solid lubricant is not desirable and where high load, temperature,
pressure and low speeds are involved. They include materials having layer lattice structure because of which they are
soapy and can be used as lubricant, e.g., graphite, molybdenun disuiphide, talc,
mica, etc. (i) Graphite: Graphite is a black crystalline form of carbon. The layers are
arranged parallel to one another with an interlayer distance of 3.35A. The layers
are held together by weak van der Waals forces, so they can slide one over other
easily. It is soapy to touch, non-inflammable and resistant to oxidation in air
below 350°C. It can be used as a dry powder or as a paste or as a suspension.
Aqua dag and oil dag are suspensions of graphite in which it is dispersed in water
and oil respectively. Graphite greases can be used at high temperature. Graphite is
used as a lubricant in intern & combustion engines, lathes, railway track-joints,
gears, cast iron bearing, in hot industrial processes such as wire drawing, tube
drawing, forging, etc.
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Fig. 3.2 Structure of graphite (ii) Molybdenum disulphide (MoS2): It has sandwich like structure in which a layer of molybdenum atoms is between the two layers of sulphur atoms. The layers are arranged parallel to each other and interlayer distance is 3.13 A°.
Mo layer
Sulphar Layer
Mo layer
Fig. 3.2.1 Structure of molybdenum disulphide
The layers are held together by weak van der Waals forces, so they can slide one
over the other easily and account for the softness of MoS2. It can be used as a
lubricant, as a dry powder, as a paste or as suspension. It is stable in air up to
400°C. It is added to greases, which are then used in automotive and truck chassis.
It can be used in vacuum and thus can be used in spacecraft (graphite cannot be
used). Its other applications are similar to graphite.
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(iii) Mechanical lubricants: A continuous adherent film of materials like plastics
and metals is formed on moving surfaces and thus their wear is reduced. Among
all plastics, polytetrafluoroethylene (PTFE) is a superior material. It provides
lowest coefficient of friction (0.03 - 0.01). It is effective from - 200 to 250°C. It is
chemically inert. Soft metal films such a gallium, indium, thallium, lead, tin, gold
and can be used for this purpose. (iv) Chemically Active Lubricants: They include extreme pressure additives and
chemicals which react with metal surface to wide inorganic surface compounds.
One of the best known treatments for steel is phosphating which the surface is
coated with a layer of mixed zinc, iron and manganese phosphates. (v) Refractories, ceramics and glasses: They are used in defense programmes
and in rockets. Refractory materials can be used at temperature for a short period
as a lubricant. The glass is used in hydrodynamic cation as it gets softened at
operating temperature. 3.2.2 Semi-Solid Lubricants Grease and vaselines are the most important semi-solid lubricants. Grease is made
from lubricating oil thickened with metallic soaps or sometimes by adding solids
such as graphite, bentonite, silica, talc, etc. to petroleum oil. Lubricating greases
are not simply very lubricating oils. They are lubricating oils in which thickener is
dispersed to produce a colloidal structure or gel. In dispersion of solid particles in
a liquid, if solid particles prevented from sticking together and settling out by
electrical charges, such a stabilised erosion of fine solid particle in a liquid is
called a colloidal dispersion. When a dispersion whole is concentrated enough to
behave as a solid it is called a gel. The properties of grease depend upon the nature and amount of soap used and
characteristic of oil used in their preparations. Due to soap the grease sticks to the
metal surface more firmly. The nature of soap decides resistance of grease to
temperature, its consistency, its resistance to water and oxidation and its ability to
stay in place. The grease on storage separates into oil and soap. It has very little
cooling effect in bearings. Due to these properties grease has limited applications.
Nearly, all greases soften under working condition, but reharden slowly on
standing. The viscosity of the greases in high, thus they cause more friction. With
increase in speed the friction increases and m1re heat will be generated. As
greases are poor coolants they will thus get overheated. This imposes lower speed
limit on grease lubricated bearing in comparison to oil lubricated bearing.
Petroleum oil is used, in about 99% of the grease produced. On the basis of
composition or soap used and method of preparation the greases can be classified
as: a. Calcium base or cup greases: They are prepared by mixing slaked lime
(calcium hydroxide) solution to tallow oil with constant stirring, in hot
condition. After soap formation is complete hot petroleum is added and
mixed with it. Certain amount of water is usually incorporated with grease to
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obtain smooth mixture of soap and oil. They are also known as axle greases.
These greases are very economical and are widely used. As calcium soap is
insoluble in water, these greases are water resistant. They are useful in
lubricating water pumps, tractors, caterpillar threads, etc. b. Sodium base greases: They are obtained by thickening of petroleum oil with
sodium soaps. Since sodium soaps are water soluble they are not water
resistant. Since they are not stabilized with water, they can be used upto
175°C. They can be used in ball and roller bearings where lubricant gets
heated due to friction. These greases provide protection against corrosion by
absorbing moisture and forming an emulsion with it. c. Lithium soap greases: They are obtained by thickening of petroleum oil with
lithium soaps. They have combined advantages of both the calcium base and
sodium base greases and thus they are water resistant. They can be used at
high temperature. Due to these properties they have wide range of
applications. About 65% of the market is captured by lithium base greases.
They have high mechanical stability and are stable on storage. They are
expensive and thus are used for specific application such as in aircraft. d. Complex greases: The particles or fibres formed by reacting two dissimilar
acids with single alkali are used as thickener in most of the commercial
complex greases. Calcium complex grease can be made from lime, a fatty
acid and acetic acid. Similarly, grease can be made with sodium, aluminium
and lithium, they have very high melting point. They are useful in still mills
and automotive wheel hub bearing, ball and roller bearing, household
appliances, machine tools, aircraft accessories, etc. Additives such as
antioxidants, corrosion inhibitor and extreme pressure additives are added to
greases.
3.2.3 Liquid Lubricants
The Liquid lubricants have a high cooling ability when circulated through bearing
areas. So they are widely used in industry. They also 4ct as sealing agent and
prevent corrosion. Good liquid lubricant should possess the following properties. i. An adequate viscosity
ii. High boiling point iii. Low freezing point iv. Good oiliness v. Stability towards heat and oxidation
vi. Should not undergo decomposition and corrodes the machine part.
The liquid lubricants are:
a. Minerals oils b. Blended oils
Table 3.1: Some vegetable and animal oils used in lubricant
Name Use as Lubricant
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Vegetable oils:
1. Olive oil For bearing and machine parts working under low pressures
and high speed.
2. Castor oil For bearing and machinery operating at low pressure and
high speed. In medical, printing and plastic industry.
3. Palm oil For delicate instruments like watches and scientific
instruments.
4. Rapeseed oil Steam cylinders and delicate apparatus.
Animal oils:
1. Whale oil For light machinery
2. Lard oil For ordinary machine parts and as a cutting oil.
3. Tallow oil For ordinary machinery, team cylinders.
4. Neats foot oil For guns, sewing machines, watches and clocks.
a. Petroleum oils: They contain C12 to C15 hydrocarbons. These oils, are
obtained from crude petroleum on fractional distillation. They are known as
lubricating oils and aress essentially hydrocarbon oils. They are cheap and
have wide applications. Around 98% market is captured by them. The
hydrocarbons found in mineral oils are mainly straight and branched chain
paraffinic compounds, naphthenes (cycloparaffines or cycloalkanes) and
aromatics. They are very efficient in preventing corrosion.
The lubricating oil must be refined to remove wax, asphaltic matter and
aromatic constituents. These impurities if not removed, crystallise at lower
temperature and thus interfere with the flow properties of lubricating oils. The
% of wax in petroleum oil decides its pour point and cloud point. Easily
oxidisable impurities cause sludge formation during operating conditions.
Asphaltic, naphthenic and resinous impurities decompose at higher
temperature resulting in formation of carbon and sludge. These impurities are
removed by refining methods like dewaxing (to remove wax), acid- refining
(to remove unsaturated hydrocarbons) and solvent refining (to remove
aromatic compounds).
b. Blended oils: Although refined petroleum oils serve as lubricants, for
achieving satisfactory performance in a particular machinery, some of their
properties are necessary to be improved by adding chemical reagents known
as additives. These oils having improved properties are known as blended
oils. Types of additive added are as follows.
3.4 TYPES OF ADDITIVE
The common types of additive are
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Oxidation inhibitors
The most common reason of degradation of petroleum oils is their oxidation. At
high temperature the hydrocarbons in petroleum oil undergo homolytic fission to
generate free radicals. One of the reactions of these free radicals involves reaction
with oxygen which results in formation of hydroperoxides. Some hydroperoxides
decompose to form alcohols, aldehydes, ketones and organic acids which may
polymerise or break down further to viscous soluble polymers, insoluble sludge
and finally dark coloured varnish-like deposits.
The oxidation inhibitor terminates hydroperoxide chain reactions by reacting or
combining with hydroperoxide. The common oxidation inhibitors are di-tert-butyl
p-cresol, 2-naphthol, l- naphthyl (phenyl) amine, etc.
Rust inhibitors
They are surface active additives which form adsorbed film on iron and steel and
guard them from water corrosion. In shipping and storage machinery, sodium and
calcium sulphonates, organic phosphates are used as rust inhibitors. For protection
against non-ferrous and copper alloy corrosion, thiadiazole and triazole
derivatives are useful.
Antiwear and extreme pressure agents
For reducing wear in gears and high pressure hydraulic components, zinc dialkyl
dithiophosphates are used as antiwear agents. In steel -and-steel lubrication zinc
dialkyl dithiophosphate forms a brown surface film of ZnO, ZnS, FeO and some
iron and zinc organophosphates prevent the wearing of steel surface. Tricresyl
phosphate is a effective antiwear agent as it forms protective metal phosphite or
phosphate film. Under extreme rubbing conditions extreme pressure additives are
used in hypoid gears, machine tool slideways and various machine cutting
operations.
Friction modifiers
These additives are used in automotive applications as mild extreme pressure
agent in boundary lubrication condition. They prevent stickslip oscillations and
noise in automatic transmissions and also conserve energy. The fatty acids with 12 – 18 carbon atoms and fatty alcohols or esters of fatty acids. e.g., glycerides of
rapeseed and lard oil are used as friction modifiers.
Detergents and dispersants
They reduce deposition of oil insoluble sludge, varnish and carbon from fuel
combustion in internal combustion engine. Along with surface cleaning action, a
detergent also adsorbs on insoluble particles (as described above) to maintain
them as suspension in bulk oil and hence minimise deposits on rings, valves and
c1inder walls. A dispersant serves almost similar purpose.
97
Calcium, sodium and magnesium salts of alkylabenzene sulphonic acid,
carboxylic acids, alkyl phosphoric acid are commonly used detergents. Most
commonly used dispersants are polybutenyl succinic acid. Pour-point depressants The pour-point of low viscosity paraffinic oil may be lowered by 30-40°C by
pour-point depressants such as polymethyl methacrylate, styrene esters, etc. On
cooling below the normal pour-point, wax crystallises out of solution from liquid
oil. The additive molecule prevent such crystallisation as they get adsorbed on
crystal faces. Viscosity index improvers The viscosity index can be improved by addition of linear polymers such as
polyisobutylenes, polymethacrylates, polyalkylstyrene having molecular weight
ranging from 10,000 to 100,000. They function by thickening light oil to a higher
viscosity while retaining the original viscosity temperature coefficient. These
viscosity index improvers are used in multi grade automotive engine oils,
automatic transmission oils, and gear oils, in aircraft and in some industrial
hydraulic fluids. Foam inhibitors They are used to prevent foaming of oil in internal combustion engines, turbines,
gears and aircraft applications. In the absence of the foam inhibitors, severe
churning and mixing of oil with air may result in foam formation which may
overflow from lubricating system or interfere with normal oil circulation. Methyl
silicone polymers are effective foam inhibitors. They are not completely soluble
in oil, thus they form a dispersion of minute droplets of low surface tension which
help in breaking foam bubbles. Oiliness improvers They improve oiliness of lubricants by adding oiliness carriers like vegetable oils
such as castor oil, oleic acid, etc., the compounds containing strong polar group
such as dibenzyl disulphide, amyl phenyl phosphate, etc. They make oil to adhere
more strongly to metal surface. 3.5 LUBRICANT EMULSIONS A dispersion system consisting of two immiscible liquids is called an emulsion
and it is stabilized by adding a third component called emulsifying agent. The
emulsifiers or emulsifying agents are the substances which exhibit polar as well as
non-polar character, because they contain hydrophobic as well as hydrophilic end.
The hydrophobic end of the emulsifier molecule is wetted by oil and hydrophilic
end will be wetted by water. Hence, emulsifier molecule is adsorbed at the
interface of the two immisicible phases or liquids (oil and water), resulting in the
formation of a protective film around the dispersed droplets. A simple example of
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emulsifying agent is sodium soap, which possesses hydrophilic or water loving
group -COONa and hydrophobic or water hating end -C15H31.
Fig 3.3 Function of emulsifier molecules around the droplets in o/w and
w/o emulsions.
There are two types of emulsions.
These are oil in water emulsion and water in oil emulsions.
(A) Oil in water emulsions are obtained by adding oil to a suitable quantity of
water in the presence of 3-20% water soluble emulsifying agent, such as sodium
soap or sodium or potassium salts of sulphonic acids. These emulsions are
generally used as cooling and lubricating liquids for cutting tools. These
emulsions are also as rust preventers and as lubricant for certain heavy sliding
components such as pistons in marine diesel engines.
(B) Water in oil emulsions are prepared by mixing water containing 1-10% water-
soluble emulsifier such as alkaline earth soap (calcium stearate) to the oil. These types
of emulsions are widely used to lubricate compressors and provide cooling effect
because of evaporation of water, in addition to lubrication. These emulsions possess
higher viscosity, than that of the oil from which they are prepared.
3.6 SYNTHETIC LUBRICANTS
They are either oils or greases which are very costly. They are prepared so as to
facilitate better characteristics compared to petroleum oil for lubrication purpose.
They have a high resistance to oxidation and with thermal stability and are
resistant to hydrolysis. They have high viscosity index, high flash or firepoint and
low pourpoint.
The types, structures and properties of important synthetic lubricants can be
tabulated as Table 3.2.
Table 3.2: Types and uses of synthetic greases
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Sr. No. Type Uses
1 Synthetic hydrocarbons, (1) Auto engines
e.g., (2) Gas turbine gears for army, navy. nuclear
(1) Poly-butylenes and industrial applications
(2) Poly (a-olefins)
(3) Alkylated benzene
2 Polyalkylene glycols (1) Fire resistant hydraulic fluids in
combination with 30-60% water in foundry,
steel mills and mines.
(2) Brake fluids for automobiles.
(3) Textile fibre and textile machine Lubricants
as they are nonstaining and easily washable.
(4) Compress or lubricants for ethylene, natural
gas, helium, nitrogen.
(5) Lubrication of food processing equipment.
(6) Non-sludging lubricant for bearings and
gears in mills used by rubber, paper and plastic
industry upto 175°C.
(7) It is used as additive in water- based
synthetic cutting and grinding fluids. At high
temp, polyalkylene glycol comes out of solution
as fine droplets, which coat hot metal surface.
3 Phosphate esters (1) In air compressors
(2) In aircraft.
(3)In hydraulic control of steam turbines in
power stations.
4 Silicones (1) For lubriation of glasswares and rubber
surfaces.
(2) They can be used as extreme pressure
lubricants.
(3) They are used in aircraft and missiles.
5 Chlorouluoro ethylenes (I) Used in vacuum pump oils:
(2) They can be used with a wide range of
chemicals including O2,Cl2H2O2 and mineral
acids.
(3) They are used as lubricant in oxidiser
section of missiles.
3.7 MECHANISM OF LUBRICATION
100
The purpose of lubrication is achieved by forming a film of lubricants between the
contacting surfaces. The formation of the film of the lubricant can be achieved by
three mechanisms.
(1) Fluid film or hydrodynamic or thick film lubrication (2) Boundary or thin film lubrication (3) Extreme pressure lubrication.
3.7.1 Fluid or Hydrodynamic Lubrication (Thick Film Lubrication)
In this type of lubrication the sliding surfaces are separated completely by
applying a thin uniform film of liquid lubricants between them. The thickness of
liquid film is at least 1000 A. The liquid lubricants do not have any chemical
affinity to the metal surface and it sticks to it due to its physical property known
as viscosity or “stickness”. The liquid film covers all irregularities in the sliding
surface and thus it prohibits formation of welded junction and prevents contact
between sliding surface. Since instead of sliding surfaces the liquid film comes in
contact with each other it offers resistance to motion due to its viscosity. Thus, the
liquid lubricant must have sufficient viscosity so as to maintain the fluid film in its
place. If the lubricant viscosity is higher, a large amount of energy is required to
circulate and maintain the viscous lubricant film. The coefficient of friction under
these conditions is as low as 0.001 to 0.01. [The coefficient of friction for
unlubricated surface ranges from 0.1 to 1.5].
In such type of lubrication when the load is applied, the corresponding pressure
developed in the lubricant is sufficient to keep moving surfaces apart and
therefore, it is known as hydrodynamic lubrication.
Fig. 3.4 Fluid film lubrication (coefficient of friction 0.001 to 0.01)
Hydrodynamic lubrication occurs in journal bearing and its effectiveness depends
on the design of bearing, load and the rate of rotation of shaft.
This type of lubrication is useful in delicate machinery like watches, clocks,
sewing machine, etc. It is also used in electric motors, steam turbines, car axels,
automobile engine main bearing, and automobile connecting-rod bearings.
3.7.2 Boundary Lubrication (Thin Film Lubrication)
101
Under the condition such as high load, slow rate of rotation, very low viscosity of
oil, etc., a continuous fluid film cannot be maintained between the rubbing
surfaces. Under such condition the thickness of the fluid film should be less than
1000 A. The coefficient of friction under these conditions is 0.05 to 0.15. Such a
thin film consists of one or two molecular layers and to forth it. The lubricant has
to be adsorbed on the metal surface by physical or chemical force or by both. The
adsorption of lubricant results in the formation of an oily film by attachment of
polar molecules to the metal surface. In some cases, the lubricant may chemically
react with metal surface forming a thin film which acts as lubricant. This film is
also known as boundary film. Although this film fills the regularities in metal
surface, some peaks may have more height than the thickness of this thin film and
thus this mechanism provides partial separation of moving surfaces. They may
establish contact which further leads to formation of welded junctions and results
in friction as well as wear and tear of metal surfaces involved.
Fig. 3.5 Boundary lubrication (coefficient of friction 0.05 to 0.15) The property of oil, responsible for its adsorption to metal surface, is known as
‘oiliness’. Vegetable and animal oils, containing fatty acids in them having
general formula R—COOH, have more oiliness, e.g., saturated stearic acid
C17H35COOH, unsaturated oleic acid C17H33COOH, etc. These oils have polar
carboxylic group which react with metal surface to form a continuous
monomolecular film of adsorbed molecules. The hydrocarbon chain (R) of fatty
acid gets oriented outwards in a perpendicular direction as shown in above figure.
The petroleum oils do not contain such polar groups and thus have less oiliness. The solid lubricants separate two sliding surfaces under boundary condition. 3.7.3 Extreme Pressure Lubrication Under boundary lubrication condition, a thin film of lubricant is formed between
the two moving metal surfaces which may permit a small contact between them.
This results in friction and generation of heat. Thus, welded junction and metal
tearing do take place. Under the conditions of high load and extreme pressure, the
contact between the metal surfaces increases and more heat is generated due to
increased friction. As a result of this, the liquid lubricant may get decomposed or
evaporated and thus it becomes ineffective. For effective lubrication under these
conditions special additives known as extreme pressure additives are used along
with the lubricant. These are generally chloride, sulphur, phosphorus, oxygen and
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lead containing organic compounds. Compounds of chlorine (e.g., chlorinated
ester), sulphur (e.g., sulphurised fats and oil) and phosphorus (e.g., tricresol
phosphate) are used as additives. Under these extreme conditions, these additives
undergo a chemical reaction with metal surface and form a solid surface film of
metallic chlorides or sulphides or phosphides. Thus, now while moving, instead of
metallic surfaces, these additive films which have relatively low shear strength,
come in contact with each other and thus they protect the metal surfaces. If this
film breaks from the metal surface, extreme pressure additives further react with
the metal surface and additive film is again formed.
The extreme pressure additives are used in ‘cutting and in machining of tough
metals. Cutting fluid is a lubricant and/or cooling medium used to reduce wear
and heating of metal cutting tools. They are also used in wire drawing. Metals like
titanium, chromium can be drawn into wire in the presence of chlorine-containing
additive which react with metal surface to form a stable oxide film.
3.8 PROPERTIES OF LUBRICANTS AND THEIR SIGNIFICANCE
3.8.1 Viscosity and Viscosity Index
Viscosity is the property of lubricant by virtue of which it offers resistance to its
own flow. The viscosity of an oil decreases with increase in temperature. In
certain cases like internal combustion engine, aeroplanes, the lubricant has to
function at low starting temperature as well as at high operating temperature.
Thus, viscosity should remain constant over a wide range of temperature. A good
lubricating oil should possess moderate viscosity
Viscosity index: Rate of change of viscosity with rise in temperatures is measured
by an arbitrary scale and is known as viscosity index.
The viscosity index can be found out by comparing viscosity of oil under test at
100°F, with two standard oils. One of these reference oils is chosen from a
standard set made from Pennsylvania crude, as it exhibits relatively small
decrease in viscosity with increasing temperature, and arbitrarily assigned a V.I.
of 100; the other is chosen from a standard set made from Gulfcoast crude and
arbitrarily assigned a V.I. of 0 as it shows relatively rapid change in viscosity with
temperature.
Mathematically
Viscosity index = LL
−−
U
H ×100
where U = Viscosity at 100°F of the oil under the test.
L= Viscosity at 100°F of standard Gulf coast oil having V.I. zero
and
H= Viscosity at 100°F of std. Pennsylvanian crude oil having V.I. 100
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An oil whose viscosity changes rapidly with change in temperature has a low
V.1., while the one whose viscosity changes only slightly has a high V.1. Addition
of linear polymers increases V.I. and oils with V.I. higher than 100 have been
prepared.
Fig. 3.6 Viscosity-temperature curves for the standards (L and H) and the oil
under test (U). Significance of Viscosity Viscosity is the single most important property of the lubricating oils which
determines their performance under operating conditions. For example, it is not
possible to maintain a liquid oil film between two moving or sliding surfaces if
the viscosity of the lubricant is too low and hence excessive wear will occur.
Excessive friction will take place, if the viscosity of the lubricant is too high. A
lubricating oil should have sufficient viscosity to enable it to stay in position. On
machine parts moving at slow speeds under high pressures a heavy oil should be
used as it better resists being squeezed out from between the rubbing pans. Light
oils can be used when lower pressures and higher speeds are preferred as they do
not impose as much drag on high speed parts. Therefore, for minimum friction,
the thinnest (least viscous) oil that will stay in position should be used. Determination of Viscosity by Redwood Viscometer Redwood No. 1: It is used to find viscosity of light or thin lubricating oils and
have efflux time of 2000 seconds or less. Redwood No. 2: It is used to find out viscosity of highly viscous oil such as fuel
oils. Its jet for the outflow of the oil is of a larger diameter and efflux time is 200
seconds or less. By using Redwood viscometer we can find out relative viscosities of oils by
measuring the time of efflux of 50 ml of oil through a standard orifice of the
instrument under standard conditions.
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Redwood Viscometer No. 1
Description of Apparatus:
Redwood Viscometer No. 1 consists of the following parts.
Fig. 3.7 Redwood viscometer
(1) Oil cup: It is a standard cylindrical oil cup which is made up of brass. It is
silvered from inside. Its height is 290 mm and diameter is 46.5 mm. It is open at
upper end and at base it is fitted with an agate jet, with bore of diameter 1.62mm
and internal length 10mm. The jet is opened or closed by a ‘valve rod’ which is a
small silver plated brass ball fixed to a stout wire. The level to which the cup is to
be fixed with oil is indicated by a stout wire fixed in the side of the oil cup. The
wire is turned upwards and it is tapered to sharp point to indicate level properly.
The cup is provided with thermometer which indicates oil temperature. The lid of
cup is provided with spirit level for vertical levelling of the jet.
(2) Heating bath: The oil cup is surrounded by cylindrical copper vessel which
serves as water bath. It is provided with a tap for emptying water from it and a
long side tube projecting outwards for heating water bath by means of gas burner
or spirit lamp. The copper vessel is provided with thermometer to indicate
temperature of water.
(3) Stirrer: The water bath is provided with a stirrer having four blades to
maintain uniform temperature in the bath to facilitate uniform heating of the oil.
(4) Tripod stand: The entire apparatus rests on a sort of tripod stand provided
with leveling screws at the bottom of three legs.
Flask: It has a specific shape. It receives the oil from jet outlet. Its capacity is 50
ml. This flask is known as Kohlrauch flask.
105
Working The instrument is levelled by using levelling screws on the tripod stand. The water
is filled in the water bath upto the tip of indicator upto which the oil is to be filled
in the cylindrical cup. Agate jet is sealed by keeping brass ball valve in position.
Then the oil to be tested is filled in the oil cup upto the tip of the oil level
indicator. The Kohlrauch flask is kept exactly below the jet. The water bath is
slowly heated by heating the side tube, and thus the oil also gets heated
simultaneously. The temperature of water and oil is kept uniform by continuous
stirring. Their temperatures are recorded by the thermometers as T1 and T2. When
the desired temperature is maintained, the oil is allowed to fall through the jet in
Kohlrauch flask. A stopwatch is started simultaneously and when exactly 50 ml
oil is collected in the flask, the stopwatch is stopped and the efflux time required
is noted in seconds. The experiment is repeated and mean value of time of flow
for 50 ml oil is reported as t seconds, Redwood 1 at t°C. Usually, the viscosity is
recorded at 21.11°C (70°F), 60°C (140°F) and 93.33°C (200°F). From this value of time of efflux, the kinematic viscosity and absolute viscosity
can be determined if the density of oil is known. The absolute viscosity of liquid or oil is given by
η = × t
1
D2 , where
t2 D1
t1 = time in seconds, taken for the flow of 50 cc. of oil.
t2 = time in seconds, taken for the flow of 50 c.c. of standard oil or liquid (usually
rapeseed oil)
D1 = specific gravity of oil to be tested.
D2 = specific gravity of standard liquid. K = an arbitrary constant, which for water is equal to 1 and for rapeseed oil 100. The kinematic viscosity of oil can be calculated by the formula, if density (p) of
the oil is known.
ν = η
p
3.8.2 Neutralization Number (Acid Value) or Acid Number Acid number or value of lubricating oil is mgs of KOH, required to neutralise all
acidic constituents of I gram of the oil. Sources of Acidity and Significance Fatty oil consists mostly of glyceryl or other esters of higher fatty acids and in
some cases notable amount of free acids are present. The acid count increases with
time due to hydrolysis with moisture and is therefore, a rough indicator of the age
of the oil; i.e., it gives an idea of how old is a fatty oil. Periodic determination of
acid number is useful to indicate the progress of oxidation of lubricant and with
106
this information we can predict the stage at which the lubricating oil should be
replaced. The acid value of lubricating oil should be less than 0.1
New, unblended petroleum oils should have very low neutralisation values usually
ranging from 0.02 to 0.1. Values higher than this indicate faulty refining. Blended
or compounded oils may have higher values of neutraliiation number because of
the presence of additives such as oiliness carriers, oxidation and corrosion
inhibitors, etc.
As the oil is used, the neutralisation number may increase due to contamination
(e.g., SO2 from combustion of sulphur in the fuel, CO2 from combustion or that
present in atmosphere) and/or oxidation of the oil. The oxidation of the oil results
in the formation of all soluble alcohols, ketones, acids and peroxides thereby
increasing the acid number, viscosity and darkening of the oil colour.
Although the neutralisation number gives the amount of acid or base present in the
lubricating oil, it gives no information about their source and corrosive nature.
Determination of Total Acid Number of Oil
10 gm of oil and 50 ml of alcohol are mixed in a flask. The flask is heated on a
water bath for half an hour. Flask is cooled and contents titrated against 0.1 N
KOI-1 using phenolphthalein as an indicator
Acide value = Volume of 0.1 N KON used x 5.6
or Vol. KOH x N KOH x 56
Weight of the oil taken Weight of oil
3.8.3 Saponification Value or Number
Saponification number of value of an oil is defined as the number of mgs of KOH
required to saponify fatty material present in 1 gm of the oil. It is alkaline
hydrolysis of fatty oils which led to formation of soaps.
Mineral oils, being mixtures of hydrocarbons do not react with KOH and so are
not saponifiable. Vegetable and animal oils, however, are mixtures of glyceryl
esters of fatty acids and hence require large amounts of alkali to get hydrolysed.
Their saponification values are very high and each fatty oil has its characteristic
value.
Sap Value = Volume of KOH x N KOH x 56
Weight of the oil taken
Sr. No. Name of oil Acid value Sap value
1. Groundnut oil 0.2 – 0.8 194— 196
2. Castor oil 0.4 – 0.8 201 — 203
3. Coconut oil 10.0 – 35.2 253 — 260
4. Cottonseed oil 0.4 – 2.2 194— 195
5. Whale oil 0.3 – 51.4 190— 191
6. Rapeseed oil 1.4 – 4 .0 177 — 199
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Significance In distinguishing fatty and mineral oil
a) To identify a given fatty oil. b) To determine the extent of adulteration, if any, in a given oil. c) To determine the extent of compounding in a lubricant. When the type of
fatty ingredient in a compounded oil is known, its exact amount is given
by
Percentage of fatty oil = C
F × 100 Where, C = Saponification value of the compounded or lubricating oil
F = Saponification value of the fatty oil Determination of saponification value of an oil Theory: A known weight of the sample is mixed with a known excess of standard
alcoholic KOH solution and refluxed for 1 hr
CH2COOR CH2OH
CHOH + RCOOK + R2COOK + R1COOK
CHCOOR1 + 3KOH ⟶
CH2COOR2 CH2OH
Triglyceride Glycerol Potassium of fatty acid (acid)
Where R, R1 and R2 are alkyl groups The unreacted KOH is titrated back with standard acid using phenolphthalein as
an indicator.
H + OH − → H 2O unreacted
Procedure Transfer about 1 g of accurately weighed oil sample to 500 ml round bottom flask.
Add 25 ml of 1 N alcoholic KOH and 20 ml of alcohol. Fit the flask with water
condenser and reflux the contents on a water bath for one hour. Cool the content
and disconnect the condenser dilute to 250 ml in standard measuring flask. Pipette
out 25 ml in 250 ml conical flask. Add few drops of phenolphthalein indicator and
titrate the 0.1 N HC1 until pink colour has just disappeared. (a) Pipette out 25 ml of 1 N alcoholic KOH in 250 ml standard measuring flask
and dilute upto the mark. Pipette out 250 ml diluted solution in 250 ml conical
flask and titrate against 0.1 N HCl using phenolphthalein as an indicator. Example 3.1: In determination of saponfication value of vegetable oil, 2.5 gm of
oil sample was refluxed with 50 ml of alcohol and 50 ml of 0.5 N alcoholic KOH.
To get the endpoint 10 ml of 0.5 N HCl was required. The blank reading obtained
was 26 ml. Find saponification value of an oil.
108
Solution:
1. Weight of oil, w = 2.5 gm 2. Volume of 0.5 N HCI required for main titration, A = 10 ml. 3. Blank titration reading. B = 26 ml
As saponification value of sample is given by (B
−
A)
×
56
×
0.5
w
= 26 − 10 × 28 = 179.2 2.5
Saponification value is 179.2
Example 3.2 2.0 gm of oil was saponified by using 0.5 N alcoholic KOH. The
mixture required 6.0 ml of 0.5N HCI. The blank titration reading 18.0 ml of same
HCl. Calculate saponification value of the oil Wt. of the oil = 2.0 gm, Back
titration reading 6.0 ml.
Solution:
Blank titration reading 18.0 ml.
Sap. value = vol. of KOH × N KOH × 56 wt. of the oil in gm
= 12 × 0.5× 56 1.0
= l68 mg of KOH.
Example 3.3: l.0 gm of an oil sample required 1.0 ml of 0.01 N KOH for
neutralisation. Find acid value of the oil.
Solution: Wt. of the oil = 1.0 gm, vol of 0.01 N KOH = 1.0 ml
Acid value = vol. of KOH × N KOH × 56
wt. of the oil in gm
= 1.0 × 0.01× 56 1.0
= 0.56 mg/gm.
Example 3.4 Find the acid value of a vegetable oil whose 10 ml required 4.0 m of
0.01 N KOM during titration, (d = 0.92). Solution: Wt. of the Oil (mass) = Density × volume
= 0.92 × 10
= 9.2gm
Acid value = vol. of KOH required × N KOH × 56 wt. of the oil in gm
= 4.0 × 0.01× 56 = 0.243mg 9.2
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3.8.4 Cloud Point and Pour Point Definition of Cloud Points It is that temperature, expressed as multiple of 1°C, at which a cloud or haze of
wax crystal appears at the test jar when the oil is cooled under prescribed
condition. Significance The cloud point of petroleum oil is an index of the lowest temperature of its utility
for certain applications. Definition of Pour Point The pour point is the lowest temperature, expressed as a multiple of 3°C at which
the oil is observed to flow when cooled and examined under prescribed condition. Significance The pour point of petroleum oil is an index of lowest temperature limit for utility
of lubricating oil and it also indicates dissolved wax concentration of lubricating Apparatus Test jar: A cylindrical test jar of clear glass, flat bottom, approximately 3.0 to
3.35 cm in inside diameter and 11.5 to 12.5 cm in height. There is mark upto
which the sample must be taken. The cork should fit the test jar, and bored
centrally to insert the test thermometer
Fig. 3.8 Determination of pour point Jacket: A watertight cylindrical jacket of glass or metal with flat bottom, about
11.5 cm in depth is used. The jacket is fitted with a disk. It is provided with a
gasket which prevents test jar from touching the jacket. Cooling Bath: It contains suitable freezing mixture for cooling.
110
Procedure
Pour the oil into test jar to the level mark. Cork it tightly. Insert the test jar in the
jacket. Start cooling. After every 1°C remove the test jar from jacket quickly but
without disturbing the oil, inspect for cloudiness. If the oil does not show cloud
replace the test jar in jacket and repeat the same procedure. When such inspection
first reveals a distinct cloudiness or haze in the oil at the bottom of the test jar,
record the reading of thermometer as cloud point.
For finding out pour point continue coding, after cooling by 3°C remove test jar
from the jacket, tilt the jar to horizontal position for exactly 5 seconds. The
temperature at which it does not flow is taken as pour point of the oil.
3.8.5 Oxidation Stability
Rate of oxidation in the petroleum oils proceeds slowly at room temperature but at
elevated temperature (above 200°C), the rate is high. The factors which increase
the oxidation are moisture in the environment and the presence of oxidation
catalyst like iron, aluminium and copper. In most commercial oils the rate of
oxidation is retarded by adding sacrificial oxidation inhibitors like phenyl -
naphthylamine.
Oxidation in lubricants is undesirable because the insoluble product or sludge may
clog oil holes, oil pipe lines, filters and other parts of the lubricating system. If the
oxidation product is soluble it circulates with the oil and may corrode or pit
bearing surfaces or may form vanish-like deposits and gums.
3.8.6 Aniline Number
Definition
Aniline number is thus minimum equilibrium solution temperature for equal
volume of aniline and lubricating oil sample.
Significance
The tendency of a lubricant to mix with aniline is expressed in terms of aniline
point of the sample. As like dissolves like, aniline being an aromatic compound, it
is miscible with oil having high percentage of aromatic hydrocarbons at lower
temperature. A high aniline point thus indicates lower percentage of aromatic
hydrocarbons, and thus higher percentage of parafinic hydrocarbons in oil and
vice-a-versa.
Aniline point of lubricant is thus measure of its aromatic content. A lubricant with
high aniline point is recommended for systems in which rubber seals, gaskets and
packing materials are involved. A lubricating oil with low aniline point will tend
to attack these rubber parts, used in system to prevent leakage. This results in
deterioration of rubber parts and leakage may take place.
Aniline point of an oil is indication of tendency of deterioration of an oil when it
comes in contact with packing, rubber sealing, etc. Usually, the aromatic
111
hydrocarbons have tendency to dissolve natural as well as some synthetic rubbers. Thus, presence of aromatic hydrocarbons in oil is not desirable. Determination of Aniline Point Description of Apparatus (1) Test tube: It is made up of heat resistant glass, approximately 25 mm in
diameter and 150 mm in length. The cork of test tube is fitted with stirrer and
thermometer (2) Jacket tube: It is made up of heat resistant glass, approximately 37 to 42 mm
in diameter and 175 mm in length. Cork of jacket is fitted with test tube. It
provides air jacket. (3) Stirrer: It is a metal stirrer with a concentric ring at the bottom. A glass sleeve
is usually used as a guide for the stirrer. (4) Heating bath: A suitable non-aqueous, non-volatile, transparent liquid bath is
used (usually a paraffin bath).
Fig. 3.9 Aniline point
apparatus Working of aniline point Clean and dry the apparatus perfectly as aniline is hygroscopic. Since aniline is
highly toxic pipette out 10 ml of aniline by using aspirator in the test tube. Add
exactly same amount of dry sample of an oil to it. Fit the test tube with cork fitted
with stirrer and thermometer. See that they do not touch the bottom of tube. Fit the
tube in air jacket. Heat it in paraffin bath and simultaneously stir the mixture
rapidly till a homogeneous liquid is obtained. Remove the jacket from hot paraffin
bath and cool it with constant stirring till two distinct phases separate (appearance
of cloudiness throughout). This is called aniline point of given oil.
112
3.8.7 Flash and Fire Point
Definition
‘Flash -point’ is the lowest temperature at which an oil gives off enough vapour,
which give momentary flash of light when a flame is brought near it. ‘Firepoint”
is the lowest temperature at which the vapours of oil burn continuously for at least
5 seconds when a small flame is brought near it. Usually, the fire-point of an oil is
about 5 to 40°C higher than its flash-point. Significance
Many times the lubricant under use has to face high temperature. A good lubncant
should not volatilise under the working conditions and even if it volatilises, the
vapour formed should not catch fire under the working temperature conditions.
Thus, the lubricant used must have reasonably high flash-point than the working
temperature so as to insure safety. Pensky - Marten’s Apparatus
The apparatus is useful for determination of flashpoint between 80°C and 370°C.
Description of Apparatus
(1) Oil cup: It is made up of brass or other non-rusting metal of equivalent heat
conductivity. Its diameter is 5 cm and depth is 5.5 cm. The level upto which the
oil has to be filled is shown by oil level mark. The lid is made up of brass. The lid
of the cup is provided with four openings for stirrer, standard thermometer, for
introducing flame and air inlet. The cup is supported by its flange over a heating
vessel in such a way that there is clearance between them. The flange is equipped
with device for locating the position of the cup in the air bath.
Fig. 3.10 Pensky-Marten’s flash point apparatus
113
(2) Shutter: it is made up of brass and is provided at the top of lid of cup. It has
lever mechanism; the shutter can be moved to open the opening for air and
opening for introducing flame exposure device which carries the flame. (3) Flame exposure device: It is connected with the shutter in such a way that
when the shutter is moved to open the opening in it, the flame exposure device is
dipped into the opening. (4) Air Bath: The oil cup is supported by flange over an air bath which is heated
by gas burner. (5) Pilot burner: When the test flame is introduced in the opening, it gets
extinguished, but when it returns to its original position, it is automatically
relighted by the pilot burner. Working The oil to be tested is filled upto the oil level mark in the oil cup and it is heated
by heating the air bath by a burner. The oil is stirred continuously at a rate of
about 1 to 2 revolutions per second. The heating is done in such a way that the
temperature rises by 5C per minute. Shutter is moved and oil is exposed to flame
at every 1°C rise in temperature. While applying the test flame, the stirring is
simultaneously interrupted. The temperature at which a distinct flash appears
inside the cup is recorded as flashpoint. The heating is further continued and the
test flame is introduced after every 1°C rise in temperature; in similar way. The
temperature at which oil ignites and continues burn for 5 seconds is recorded as
fire point. 3.8.8 Oiliness Is the property of lubricant to stick on the surface of the machine parts. Vegetable
and animal oils have good oiliness but mineral oils have poor oiliness. Oiliness of
mineral oils can be improved by adding certain additives. 3.9 SELECTION OF LUBRICANTS At a high temperature lubricant used may undergo volatilisation or decompose
leaving a residual oil, which will have different lubricating properties. So a careful
study of various properties and their correct interpretation is necessary for the
selection of lubricants. Selection of a lubricant for typical job is illustrated as : 3.9.1 Lubricants for Cutting Tools In these operations, a metal is continuously removed from the surface and fresh
metal surface is continuously exposed to the tool used. For heavy cutting: the most effective lubricants are cutting oils. The cutting oils
are, essentially, mineral oils of flow viscosities containing additives like fatty oils,
sulphurized fatty oils and chlorinated compounds, which by virtue of their polar
groups attached themselves to the surface of continuously exposed fresh metals.
As the shear strength of such an oil film is much less than that of the metal, a
considerable reduction in friction results, thereby decreasing both the power
consumption and the extent of heat generation.
114
In light cuttings: The most effective lubricants are oil-emulsions. Oil- emulsions
have somewhat smaller lubricating effects than cutting oils, but they are more
efficient as cooling media, due to the high heat capacity of water, which is present
in them as an external phase.
3.9.2 Lubricants for Internal Combustion Engines
The lubricant is to be exposed to high temperatures in an internal combustion
engine. Therefore, the lubricant should possess high viscosity-index and high
thermal stability, petroleum oils containing additives, which impart high viscosity-
index and oxidation stability to them, are used as lubricants for internal
combustion engines.
3.9.3 Lubricants for Gears
Subjected to extreme pressures lubricants for gears should: (i) possess good
oiliness, (ii) not to be removed by centrifugal force from the place of application, (iii) possess high resistance to oxidation, and (iv) have high load-carrying
capacity. Usually, thick mineral lubricating oils, containing extreme-pressure
additives are employed.
3.9.4 Lubricants for Delicate Instruments
Watches, clocks, scientific equipment, sewing machines, etc. are not exposed to
high temperature or to water or to extreme load. etc., so properties relating to
these conditions are not considered for such uses. Consequently, for such
purposes, thin vegetable and animal oils like palm oil, neat oil, etc. are employed.
3.9.5 Lubricants for Very High Pressure and Low Speeds
Such as for tractor rollers, concrete mixers, lathes, railway track joints, etc. Under
these conditions, oil/grease films cannot be maintained, so solid lubricants MoS2
like graphite, soapstone, mica, etc. are employed either in dry powder form or as
emulsion in oil or water.
3.9.6 Lubricants for High Pressure and Low Speeds
Rail axle boxes, wire ropes, tractor rollers, etc., are greases and blended thick oils.
3.9.7 Lubricants for Transformers
Transformer oils should be properly filtered and dried, before being put into use. They must possess good dielectric strength.
3.9.8 Lubricants for Spindles in Textile Industry
For spindles moving at very high speeds, thin oils are used. For better results,
oxidation and rust inhibitors are added to the oil.
3.9.9 Lubricants for Refrigeration System
Oil with low pour-point, low viscosity and low cloud-point is needed. So
naphthalene-base oils, possessing such characteristics, are accordingly employed
mostly. The pour-point requirements are - 40°F (maximum) for the lightest grade
and - 13°F for the heaviest grade oils. Their viscosity range is 85 to 325 SUS
(Seybold Universal Seconds) at 100°F
115
Questions 1. What is a lubricant? How they classified? 2. Explain the following properties of lubricants with their significance.
a. Viscosity and viscosity index c. Flashpoint and firepoint b.
Saponification number d. Neutralization number 3. Discuss the condition for which solid lubricants are used. Explain the use of
graphite as lubricant. 4. Distinguish between third film and boundary lubrication. 5. How would you determine viscosity of lubricating oil by using Redwood
viscometer? 6. Write short notes on:
a. semi solid lubricants c. synthetic lubricants b. selection of lubricant d. extreme pressure lubrication
7. A sample of vegetable oil was tested for acid value. 10 gm of oil was titrated
against N/40 KOH and burette reading was found to 2.6 ml. state whether oil
is a proper lubricant.
8. Find acid value of the used oil whose 10 ml require 3.5 ml of N/50 KOH
during titration (density of the oil 0.81)
9. 1.532 gm of cottonseed oil was refluxed with 25 ml of 0.5N al. KOH. The
back titration reading was 15.7 ml of 0.5 N HCl and standardization readings
26 ml. calculate sap value of the oil.
10. 5 gm of cod-liver oil sample require 11.3 ml of N/50 KOH. Find acid value of
the oil (6.3 gm.). 11. 2.5 gm of oil was saponified with al. KHO (0.25N). The blank titration
reading with 0.5 N HCl was 40 ml while back titration reading was 20ml with
same HCl, find sap value of oil.
12. 1.55 gm of oil is saponified with 26 ml of N/2 alcoholic KOH, after refluxing
the mixture it require 15 ml of N/2 HCl. Find saponification value of oil. 13. In determination of saponification value of vegetable oil 5 gm of oil sample
was refluxed with 50 ml of alcohol and 50 ml of 0.5 N KOH solution. To get
end point 20 ml of 0.5N HCl was required the blank reading was 52 ml. find
saponification value of oil.
14. Find acid value of 3 gm of an oil which required 0.2 ml of 0.025 N KOH to
neutralize free acid present. 15. 16 gm of blended oil was heated with 50 ml KOH. The mixture than require
31.5 ml of 0.5N HCl. 50 ml of KOH required 45 ml of 0.5 N HCl. Find % of
Cottonseed oil, is saponification value is 192mg. 16. Find acid value of a used oil sample whose 7.0 ml required 3.8 ml of N/20
KOH during titration (density of oil = 0.88). state whether oil is proper
lubricant or not from acid value.
116
4. Phase Rule
4.1 GIBB’S PHASE RULE
The equilibrium conditions of a heterogeneous system involving number of
components, is influenced by temperature, pressure and concentrations of
reactants. Under such conditions, there exists a relationship between the phases
present, components taking part in equilibrium and degrees of freedom available.
This relationship was first proposed by Willard Gibb’s and is known as Gibb’s
Phase Rule. It is represented as follows.
F + P = C + 2 or F = C – P + 2 where P = phase, C = component, F = degree of freedom.
It is assumed that the equilibrium is not affected by gravitational, electrical or
magnetic forces, or by surface tension.
4.2 DEFINITION OF VARIOUS TERMS
4.2.1 Phase
It is defined as “any homogeneous and physically distinct part of system which is
separated from such other parts of system by well-defined boundary surfaces”.
(a) Air is a mixture of various gases such as oxygen, nitrogen, carbon dioxide,
argon, etc. constitute a single phase. This is because, air is homogeneous, the
different gases are uniformly distributed and there is no definite boundary
surfaces separating them. Hence, air, a mixture of gases, constitutes a single
phase. (b) Two liquids like alcohol and water which are miscible in all proportions,
constitute a single phase, since there is no separating boundary surfaces
between alcohol and water. other examples are acetone and water, ethanol
and water, etc. (c) When two liquids are immiscible, they constitute two phases, e.g., benzene or
chloroform and water.
Two Phase
(d) A solution of salt constitutes a single phase even though salt may be complex
salt. For instant Mohrs salt, FeSO4 .(NH4)2 SO4.6H2O containing FeSO4, and
(NH4)2 SO4 constitute a single phase solution.
(e) Each solid constitutes separate phase. For instance, different forms of sulphur
when present in equilibrium, each form constitutes separate phase.
117
(f) Homogeneous solid solution constitutes a single phase. (g) Water can exist in solid form as ice, in liquid form as water and in vapour
form as steam. Each is a phase and can be represented as:
Ice ⇌ water ⇌ steam
(Solid) (liquid) (vapour) 4.2.2 Component The number of components of a system at equilibrium is the smallest number of
independently variable constituents by means of which, the composition of each
phase, can be directly expressed or represented by chemical equation. Consider
the decomposition of CaCO3 into CaO and CO2.
CaCO3 ⇌ CaO + CO2
100 56 44 Since each solid constitutes a single phase, this system has three phases, 2 solids
and one gaseous, namely, CO2. Since the reaction is represented by chemical
equation, by knowing any two the third one can be calculated. The composition of
each phase can be represented as follows. Phase 1 due to CaCO3 is represented as
CaO + CO2 ⇌ CaCO3
Phase 2 due to CaO is represented as
CaO ⇌ CaO + 0.CO2 (here CO2 is Zero)
Phase 3 due to CO2 is represented as CO2 ⇌ CO2 + 0.CaO (Here CaO is Zero)
Thus, composition of each can be represented by knowing CaCO3, CaO or
CaCO3, CO2 or by CaO and CO2. This means only two constituents are sufficient
to represent each phase in the system and hence it is a two-component system. Considering water system, though water is present as solid, liquid and gaseous
phases, it is the same substance chemically. Hence, it is a single component
system. In a similar way all the different forms of sulphur can only represent sulphur It is
once again a single component system.
The decomposition of CuSO4.5H2O into CuSO4.3H2O + 2H2O is a two-
component system. The decomposition of steam by iron can be shown as three-component system. Fe (s) + H2O (g) ⇌FeO (s) + H2O(g)
118
4.2.3 Degree or Freedom or Variance
The number of degrees of freedom of a system is the number of variable factors
like temperature, pressure and concentration which must be specified so that the
system can be defined completely In order to understand this aspect, consider a single-component system like water
system. If we consider any one phase individually only, then by applying the
phase rule, F = C – P + 2,
We have
C = 1, P = 1
Substituting,
F = 1 – 1 + 2 = 2
The degree of freedom is two. The system is bivariant. This means to define the
system consisting of a single phase, two variables, namely, temperature and
pressure should be known.
When two phases are in equilibrium, say liquid water and water vapour, and then
the number of phases is two. By applying the phase rule, F = C – P + 2,
We have C = 1, P = 2
∴ F = 1 – 2 + 2 = 1
That means, the degree of freedom is one. The system is univariant. Consider
water boiling at 100°C. It is a case of liquid water being in equilibrium with water
vapour. Two phases are in equilibrium at 100°C and 1 atmospheric pressure. In
other words, two phases, liquid water and water vapour can be in equilibrium at
100°C, only at 1 atmospheric pressure or at 1 atmospheric pressure, the two
phases can coexist at 100°C. That is either pressure or temperature alone can
define the system.
When we consider all the three phases to be present in equilibrium, the number of
phases is three and by applying the phase rule, F = C – P + 2,
We have C = 1, P = 3
∴ F= 1 – 3 + 2 = 0
The degree of freedom is zero, the system is invariant. That means, all three
phases can coexist only at an unique temperature and pressure. When one of them
is altered, the number of phases will not remain three, one of the phases will
disappear.
4.3 ONE-COMPONENT SYSTEM WATER
It is a single -component system and water can exist in different phases—solid
phase as ice, liquid phase as water and gaseous phase as steam or water vapour.
These three phases can exist as three two-phase equilibrium and one three-phase
equilibrium. They are:
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Two-phase equilibria
Three-phase equilibriim
Solid
⇌ Liquid Solid
⇌ Liquid
⇌ Vapour
Liquid
⇌ Vapour
Vapour
⇌ Solid
The curve OB represents the equilibrium between liquid w er and water vapours.
Water boils at 100°C to get converted into vapour at atmospheric pressure, i.e. if
the pressure is reduced, water will vaporise at lower temperature and if pressure is
increased water will vaporise at higher temperature. That means water can remain
in liquid form even at high temperatures provided, the pressure is high. The upper
limit is 374°C at 220 atmospheric pressure. Above this temperature water cannot
exist in liquid state. Again along the curve for every temperature there is a
corresponding pressure when both liquid water and water vapour can coexist. By
applying the phase rule, F = CV – P + 2
We have C = 1, P = 2
∴ F = 1 – 2 + 2 = 1 The system is univariant, the degree of freedom is one. That means by knowing
the pressure or temperature, the system can be completely defined. Similarly, the curve OC represents the equilibrium between solid ice and liquid
water. Here again for every pressure, there is a corresponding temperature when
solid ice is in equilibrium with liquid water. This curve is also known as freezing
point curve and degree of freedom along this line is one. The system is univariant.
Since by applying the phase rule, F = C – P + 2
We have C = 1, P = 2
∴ F = 1 – 2 + 2 = 1 As the curve slopes towards the pressure axis, it can be inferred that freezing point
of water is lowered as the pressure increases. The following figure describes the equilibrium conditions between various phases.
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Now consider the existence of all the three phases in equilibrium. The point where
the curves OA, OB, OC meet, the point 0 is called the triple point, signifies an
unique situation. Such an equilibrium is only possible at a particular temperature
0.0075°C and pressure 4.58 mm. Any change in temperature or pressure will
disturb the equilibrium between the three phases, resulting in the disappearance of
one of the phases.
The three curves OA, OB, OC represent the equilibrium conditions between two
phases solid with vapour, vapour with liquid and liquid with solid phase of water.
These curves divide the diagram into three areas representing single phase system,
i.e., AOB representing vapour, BOC representing liquid and COA representing
solid phase. Within these regions of single phase, both temperature and pressure
have to be stated to define the system completely
Taking individually the two-phase equilibria one by one, curve OA represents the
equilibrium between solid and vapour phases of water. This curve is also known
as vapour pressure curve or sublimation curve. Along this line OA, for every
temperature, there is a corresponding pressure at which both solid ice and water
vapour coexist in equilibrium. Applying the phase rule, F = C – P + 2,
We have C = l, P =2
∴ F= 1 – 2 + 2 =1
The degree of freedom is 1, the system is univariant and by knowing either the
temperature or the pressure, the system can be completely defined.
An unstable equilibrium can exist when cooling is carried under careful conditions
without the separation of solid phase. This is represented by curve 00’. Under
these conditions equilibrium can exist even at lower temperatures. However, such
equilibrium is very unstable and can easily be disturbed by providing small
amounts of nucleating substances. This will immediately result in solidification of
super cooled liquid to solid ice.
4.4 TWO-COMPONENT SYSTEM
For a two-component system, the phase equation becomes:
F= 2 – P + 2 = 4 – P
Since in any equilibrium, at least one phase must be present, F= 4 – 1 = 3, i.e., the
degrees of freedom for a two-component system becomes three. This means, all
the three factors like temperature, pressure and concentrations have to be specified
in order to define the system completely. it is only possible to represent such
equilibria by three-dimensional diagram. To depict such diagram on paper, will be
a difficult proposition. If one of the variables can be kept constant, the other two
variables can be used for representing the equilibrium. It is also possible that one
of the phases may have little effect on the equilibrium condition and can thus be
ignored.
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Examples of two-component system can be made up of the following types:
1. liquid – solid equilibria
2. liquid – liquid equilibria
3. liquid – gas equilibria
4. solid – gas equilibria Of these four, liquid-solid equilibria is of practical importance and will be studied.
Further such systems do not have gas phase and effect of pressure under such
conditions is negligible. These equilibria exist under atmospheric pressure so that
the degree of freedom is reduced by one. The phase equation can be written as:
F = C – P + 1 This is known as condensed (reduced) phase equation and is used to represent
equilibrium with only two variables, namely, temperature and concentrations. Let us study a two-component system with the two components completely
miscible in liquid state. The temperature composition curve for such a system can
be represented as follows
Fig. 4.2 The two components of the system are represented by A and B, and they are
miscible in molten state completely. In the figure point A and point B represent the melting points of pure A and pure
B. If B is added to A in molten state, the melting point decreases along the line
AC. Similarly, point B represents the melting of it of pure B. Any addition of A,
lowers its melting point as represented by line B
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The curve A C is also called freezing point curve of A and represents the
composition of the solutions saturated with component A in the range of
temperatures from A to E. Similarly, curve BC is known as freezing point of
component B and represents the composition of solution saturated with B and at
temperature range from B to F
Since two phases are present, applying the reduced phase rule to the system, we
have F = C – P + 1,
We have C = 2, P = 2
∴ F = 2 – 2 + 1 = 1
The system is univariant along the lines AC and BC.
The two curves intersect at C, where both solids A and B are in equilibrium with
the liquid phase. There are three phases in equilibrium at C and applying the
reduced phase rule, F = C – P + 1
We have C = 2, P = 3
∴ F = 2 – 3 + l = 0
The system is invariant and thus at C the temperature and composition remains
constant as long as all the three phases coexist. If either the temperature or
composition is changed the equilibrium will be changed. Further, C represents the
lowest temperature at which any liquid state can exist since below this
temperature, the liquid phase completely disappears. Point C is called as eutectic
point, temperature corresponding to this state is called as eutectic temperature and
the composition of solids is called eutectic composition.
As an example of such liquid-solid system the extraction of silver from lead ores
is described below. This is also referred to as the process of desilverisation of
lead. The fig. shows Lead – Silver system
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The lead ores contain 0.1% of silver and it is difficult to extract such small
quantities. The silver content can be increased by adopting the following
procedure. The ore is melted and cooled. As the cooling takes place, lead begins
to separate out and is removed from the molten liquid. Further cooling separates
out more lead, so that the resulting mixture is more of silver. The maximum
concentration of silver that can be obtained by this method is 2.6% Ag with 97.4%
lead to 303°C. 4.5 APPLICATIONS OF PHASE RULE ▪ It indicates the behaviour of a system under a particular set of conditions.
Different systems with the same degree of freedom behave in a similar
manner.
▪ It helps to find out, under a set of conditions whether all substances involved
in an equilibrium can exist or whether a particular phase ceases to exist or
whether any transformation has taken place.
▪ It does not require any knowledge of molecular or microstructure and it does
not take into consideration nature and quantities of components present in
equilibrium.
▪ Phase rule facilitates study of different equilibria and classify them
accordingly. Limitations of Phase Rule ▪ Phase rule deals with systems in equilibrium and is not of much help in study
for systems which attain equilibrium slowly.
▪ Since no quantitative analysis is done, it is necessary to determine exactly the
number of phases present under equilibrium conditions.
▪ Though each solid is supposed to constitute a single phase, phase rule cannot
be applied to solids in finely divided state.
▪ It does not furnish enough information regarding the extent of changes that
take place when the system shifts from one equilibrium to another.
QUESTIONS 1. State Gibb’s phase rule. Explain various terms involved in it. 2. Explain the terms (I) phase (ii) component (iii) degree of freedom. 3. Explain the applications of phase rule to one-component system. 4. Explain the term condensed phase rule. 5. State and explain limitations of phase rule. 6. Discuss in brief lead – silver equilibrium with diagram. 7. What do you mean by reduced phase rule?
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5. Important Engineering Materials
5.1 Cement
Cement is a powdered material initially have plastic how when mixed with water
and which possessing adhesive and cohesive Properties which is capable of
binding materials like bricks, stones and building blocks of concrete. In the
presence of water, cement form a pasty plastic mass, which after a time, sets to a
hard rock like material, due to reactions of various constituents of cement with
water. These reactions help to bind the various materials like sand, bricks, stones
etc. in a firm manner. Concrete is a mixture of cement, stone aggregate, sand and
water mixed in definite proportion and this mixture sets to become hard and
durable material. Modern cement was discovered by Joseph Asphidin, an English
brick layer in 1924. But cement like materials have been in use from earlier times.
5.1.1 - Types of cements
There are many types of cements are available depends on their composition and
properties. Some of these cements are given as follows
Natural Cements: When lime stone which contains impurities of silica, alumina
and iron oxide to the extent of 20 - 40%, is calcined and powdered, natural cement
is obtained. In the calcination process, the impurities present react with lime to
form calcium silicates and aluminates. However the proportion of these
constituents varies depending upon the initial proportion of these impurities.
Because of these reasons, such cement never possessed uniform properties when
mixed with water and allowed to set. However since they were able to bind bricks
and stones they were used for construction purposes during the early days.
Portland cement:It is obtained by grinding a mixture of lime and clay that has
been burnt. Clay supplies the silicate and aluminate portion whose proportion
determines the setting strength and durability of cement.
Pozzolona Cements:Romans were the first to use cements of different kind. They
used a mixture of volcanic ash and lime, ground to a fine powder. The volcanic
ash contained the silicates and aluminates of calcium and with lime, readily
formed a cement like material capable of setting even under water. Once again
because of their non uniform properties, they were discarded eventually.
Super sulphate cement: is available and manufactured by fusing together blast
furnace slag small amount of lime and large amount of gypsum in a kiln. It has
greater resistance to the sulphate water.
White cement: it is ordinary portland cement containing Fe203 as one of the
consistent Mainly used for decorative constructions.
Acid resisting cement: special type of cement is prepared by taking into
consideration it capability if resisting corrosion by acids. It is used in concrete and
concrete reinforced structures.
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Slag Cements With development of steel industry, another material became available as a good
binding material. This is the blast furnace slag which is made up of silicates of
calcium and aluminum. The chemical composition of blast furnace slag resembles
that of Portland cement. CaO, SiO2, and Al2O3 constitute 90 - 95% of slag. The
quality of slag is determined by relative proportion of oxides which in turn
determine its basicity and activity of material. If the ratio of sum of percentages of
CaO and MgO is greater than sum of percentages of SiO2 and Al2O3, the material
is basic in nature and if the ratio of M203 to SiO, is greater than 1, slag cement
has good setting property even under water and setting is also faster. The binding
and setting properties are also dependent upon the manner in which the stag is
cooled. Rapid cooling results in the formation of amorphous material which has
better binding properties whereas slow cooling results in more crystalline product
which has less binding property. 5.1.2 - Portland cement It is the mostly widely used reliable cementing material used for construction
purposes. It was discovered in 1924 by English brick layer Joseph Aspidin. After
setting the stone like mass resembled famous Portland rock (stone) of England
hence it was named Portland and cement depending upon the rate of setting heat
evaluation and strength characteristics. Portland cement is of following types Type - 1 Regular Portland cement Type - 2 Modified Portland Cement Type - 3 High Cary Strength Type - 4 Low heat Portland cement Type - 5 Sulphate resisting Portland cement Raw materials required for the manufacture of Portland cement Raw materials required for the manufacture of portland are broadly classified as
follows: 1. Calcarius Raw Materials These supply the calcium part required in cement. These are calcium compounds
like calcium carbonate - lime, stone, marble marl, shells, calcium sulphate, lime
etc. 2. Argillaceous Raw Materials These supply silica and alumina part of cement. These are various types of clays,
shale, cement rock, blast furnace slag etc.
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Table 5.1 - Sources of Various Raw Materials
Lime Silica Alumina Iron Oxide
Lime Stone Sand Alumina Iron Ore
Chalk Calcium Silicate Clay, Shale Iron Dust
Sea Shells Quartzite Slags, Kaolin Iron Oxide
Marl Fullers earth Aluminium Ore Iron Sinters
Cement Rock Refuse
Marble
Alkali Waste
In modem manufacturing processes, the requirement for producing cement of
reliable and consistent quality are more exacting. Variations in quality of raw
materials should be taken care of for producing good quality cement. For instance,
if the proportion of CaO is less in raw material mix, it may result in deficiency of
constituents responsible for strength and durability. Higher than the required
amount of CaO, also leads to expansion of hardened cement on setting which is
also not a desirable property in good quality cement. Hence it is essential that raw
materials are mixed in proper proportion to ensure that quality of cement
produced is good and consistent. This is achieved by specifying the range of
various compositions in the mixture as follows.
Composition of the Various Raw Materials in the Mixture
Lime as CaO 60-68%
Silica as SiO2 20-25%
Alumina as Al203 4-11%
Iron oxide as Fe2O3 0-4%
Magnesia as MgO 0-5%
Sulphur trioxide as SO3 0-3%
Alkali Oxides as (Na2O + K20) 0.3-1.5%
Lime Saturation Factor
Lime saturation factor is calculated from the following ratio:
CaO − 0.7SO3
2.8 SiO2 +1.2 Al2O3 + 0.65Fe2O3
It should be between 0.66 to 1.02 to ensure the four main constituents of cement
are in proper proportion.
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Modulus of Silica (n)
Modulus of silica is the ratio of Si02 to sum of A12O3 and Fe2O3 present in raw
material mix. i.e.
n = SiO2
This ratio should be between 2.2 to 3.5 Al O + Fe O 2 3 2 3
Alumina Module
p = %Al2O3
The ratio should not less than 0.66 %Fe O
2 3 The Concentration of MgO should be below 5% All these specifications would eventually ensure that cement produced would
have desirable properties of setting and hardening. Further such cement retains its
properties for a long time. Functions of Various Ingredients of Raw Material Mix The various raw materials present in the mixture when calcined gives good quality
cement. Each of the ingredients react with other ingredients to form the various
constituents of cement namely di calcium silicate, tri calcium silicate which are
responsible for early and final strength of cement and tri calcium aluminates and
tetra calcium aluminofurite which are responsible for the setting quality of
cement. The proportion of various ingredients ensures that final proportion of four
constituents in cement is as required for good quality cement. 1. Lime: It is the principal ingredient, contributes towards setting and strength of
cement, if lime is present in excess, cement formed has lower strength, since the
cement expands on setting and disintegrates. If the lime content is below the
desired level, the proportion of four constituents namely di calcium silicate, tri
calcium silicate, tri calcium aluminate and tetra calcium aluminofurite will
change. This will alter the properties like setting and hardening of cement. Such
cements have poor strength. . 2. Silica: Lime reacts with silica to form both di calcium and tricalcium silicate.
They are responsible for early and final strength of cement. 3. Alumina: Alumina reacts with lime to form tricalcium aluminate. The setting
quality of cement depends upon the proportion of tricalcium aluminate. If cement
were to set very fast it may not be possible to complete all the operations like
mixing, pouring, leveling etc. This could lead to imbalance in structures. If the
setting is too slow, heavy loads can result in sagging. Hence the right proportion
of alumina in raw material would ensure the right quality of setting in cement. 4. Iron Oxide: Iron oxide is responsible for the peculiar colour of cement. It also
contributes towards strength and hardness.
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Manufacture of Portland Cement
There are three processes employed for the manufacture of cement. They are:
1. Dry process - 2. Wet process - 3. Semi dry process -
The selection of process for the manufacture is very much dependent upon the
availability and nature of raw material, the climate of the place and cost of fuel.
Where the raw materials are hard and dry and the place is dry, dry process is
preferred as it will consume less fuel. On the other band where raw materials are
soft or obtained in wet condition from processes, the climate humid, wet process
is preferred. Another main consideration is fuel cost as it is directly related to
manufacturing cost. In many countries the trend is towards adopting dry process
in view of escalating fuel cost. Basically the manufacturing process adopted in the
processes is the same. It involves mixing the raw materials in proper proportion,
calcining them in kiln and powdering the clinkers formed. The sequences of
operations in details are as follows 1. Selection of Raw Materials
Lime stone is available in various forms and the quality also varies. Low grade
lime stones are concentrated by froth floatation process. Very high grade materials
like marble and sea shell are available only in limited quantities. Clay, bauxite,
shale, blast furnace slag are the other raw materials. These are selected on the
basis of uniformity in composition and availability on regular basis. Large
variation in composition would entail frequent checking to ensure proper
proportioning of various ingredients. Water is another raw material used in mixing
in wet process. Coal in powder form, furnace oil, natural gas is the fuels generally
used for heating the kiln to the desired temperature. 2. Crushing and Grinding
The raw materials are first broken down to smaller pieces by jaw crushers and
then pulverised in ball mills. In dry, process crushing is carried out after
subjecting the raw materials to initial drying to remove the inherent moisture
present by making use of available waste heat. In the wet process, wet grinding is
carried out using water to make slurry of raw materials. This not only makes the
mixture uniform, but also makes it easier to adjust the composition to the desired
temperature. 3. Storage and Proportioning
The raw materials are crushed to fine powder and stored separately in big storage
tanks. In the case of wet process, the prepared slurry is stored and then led to
correction tank where the final adjustment of composition is done after analysing
the sample from the storage tank. From the correction tank, the slurry is led into
the rotary kiln. In the case of dry process, the dry powders from the bins are
transferred to mixing tanks provided with a stirrer. The dry powders are mixed
together, analysed and correction made so as to get a proper composition.
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4. Rotary Kiln
The rotary kiln constitutes that part of the assembly where the different raw
materials react with one another at high temperature to form cement clinkers. The
rotary kiln consists of a long cylindrical tube, lined inside with refractory and
rotating at a speed of 0.5 to 2 rpm. The tube is slightly inclined to facilitate
movement of feed through the cylinder. The fuel either powdered coal or furnace
oil sprayed into the combustion chamber, the burning hat gases are forced through
the kiln by means of a blower. The hot gases travel along the length of the tube
mixing and heating the raw material while flowing down. The temperature near
the combustion chamber is highest whereas the temperature at the end where
slurry enters the kiln, is lowest. The burnt gases along with particles of cement,
raw materials etc. passes through cyclones and dust chambers because of exhaust
system before they are led into the atmosphere through tall chimney. The
temperature profile of kiln is such that the various chemical reactions get
completed by the time the raw material mix, fed at the top of the kiln travels to the
other end of the kiln. Broadly the entire length is divided-into three zones. They
are (a) Evaporation zone, (b) Calcining zone and (c) Clinker formation zone. (a) Evaporation zone: The inherent moisture present in raw material, water
added to form a slurry, are evaporated in this zone. The temperature varies from
400 - 700°C. All moisture (including water of crystallisation) is completely
removed and the material is absolutely dry as it enters the calcination zone.
(b) Calcination zone: The temperature varies from 700 - 1100°C. In this zone
CaCO3 undergoes decomposition forming CaO and CO2. CaO formed
immediately start reacting with other constituents like Si02, Al203, Fe203 etc. to
form various constituents of cement. A number of intermediate compounds are first formed in this zone which later decomposes at higher temperature to form cement clinkers.
(c) Clinker formation zone: The various chemical reactions which begin at the
calcining zone go to completion in this zone because of higher temperature. The
temperature varies from 1200 - 1500°C.
The various reactions taking place in the three zones can be summarised as below
Temperature Reactions and Formation
~800°C Formation of CaO Al2O3, 2CaO Fe2O3 2CaO SiO2 begins
800- 900°C Formation of 12CaO. 7Al2O3 begins
900-11000C 2CaO SiO2. Al2O3 forms and decomposes formation of
3CaO.Al2O3, 4CaO.Al2O3 starts. All CaCO3 undergoes
decomposition, CaO content Maximum
1100-12000C Formation of major part of 3CaO Al2O3, 4CaO A12O3
Fe2O2CaO SiO2 content reaches maximum
12600C First liquid formation
1200-14500C All reactions go to completion. No free CaO is present.
Fusion of compounds results in clinker formation
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The final products formed are:
2 CaO + SiO2 → 2 CaO.SiO2 dicalcium silicate
3 CaO + SiO2 → 3 CaO.SiO2 tricalcium silicate
3 CaO + A12O3 → 3 CaO.Al2O3 Tricalcium aluminate
4CaO + Al2O3 + Fe2O3 → 4CaO.Al2O3.Fe2O3 tetra calcium alumino ferrite
The clinkers formed are discharged at the lower end of kiln where they are cooled
by blast of air. The resulting hot air is used for preheating the fuel.
5. Grinding of Clinkers
The clinkers are ground into fine powder after mixing with 5% of gypsum.
Without gypsum the setting of cement is very fast. Inclusion of gypsum in final
stage of grinding of clinkers helps to retard the setting of cement paste.
6. Storage and Packing
Cement in the fine powdery form is transferred to concrete silos and kept agitated
by means of compressed air. ft is packed in jute bags lined inside with polythene
or laminated woven bags. Cement absorbs moisture rapidly and is always kept in
dry place. Otherwise it will absorb moisture and set to become hard rock like
material.
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Constituents of Cement
The proportion of various constituents in cement determines its properties. The
following table gives the relevant information.
Name of Constituent Chemical Abbreviated % in Setting time
Formula Form Cement in days
Tricalcium silicate 3CaO.SiO2 C3S 48 7
Diacalcium silicate 2CaO.SiO2 C2S 27 28
Tricaclium Aluminate 3CaO. Al2O3 C3A 10 1
Ferrite FeO3 C4AF 9 1
Calcium Sulphate CaSO4 5
Free CaO CaO 1
MgO MgO 4
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1. Tricalcium Silicate 3CaO.SiO2 (C3S): Tribalism silicate undergoes hydration
and develops good strength quite early in setting. Its ultimate strength is highest of
the four constituents. Its heat of hydration is around 120 cal/gm.
2. Di Calcium Silicate 2CaO.SiO 2 (C2S): Di Calcium silicate undergoes
hydration slowly developing strength over longer period of time. Its ultimate
strength is comparable to that of tricalcium silicate. Its heat of hydration is also
lowest at 60 cal/gm.
3. Tricalcium Aluminate: Tricalcium aluminate hydrates very fast so much so it
actually prevents hydration of other constituents in cement. Hence to retard this
rapid hydration gypsum is added. Its early strength is good but its contribution to
final strength is low. Its heat of hydration is highest at 210 cal/gm.
4. Tetra Calcium Aluminoferrite: It hydrates slowly developing strength over a
period of time. However its ultimate strength is lowest of the four constituents. Its
heat of hydration is 100 cal/gm.
Setting and Hardening of cements: The usefulness of cement arises out of the fact
that it forms a pasty mass, when mixed with water and the pasty mass helps to bind
various materials like sand, bricks, stones and hold them together strongly for long
duration. All the constituents of cement undergo hydration forming hydrated
compounds. However the rate of hydration is not the same for all the constituents. The
first compound to hydrate when cement comes in contact with water is C3A. Its rapid
hydration leaves little water for hydration of other constituents. With enough water all
the constituents get sufficiently hydrated to form hydrated
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compounds. The solubilities of these hydrates are lower and hence they get
precipitated out in the form of gels and crystals. It is these gels and crystals which
help to bind the different materials like sand, bricks and stone. The paste formed
when cement is mixed with water does not remain plastic for ever but becomes
quite rigid after some time. It is called as initial set and depending upon cement,
the time can vary from 30 minutes to 1 hour. This initial set is due to hydration of
tricalcium aluminate C3A.
3CaO Al2O3 + 6H2O → 3CaO Al2O3 .6H2O As stated earlier, this reaction is so fast that it prevents hydration of other
constituents of cement. Hence gypsum is added to cement clinkers in the final
grinding stage. Gypsum helps to retard the setting of cement by forming complex
hydrates which as slower hydration rates:
3CaO Al2O3 + x H20 + y CaSO4 2H2O Water
→ gypsum
3CaO . A12O3 .y CaSO4 . Z H2O (Insoluble complex calcium sulpho aluminate) The complex formed does not undergo rapid hydration and its takes more time for
the cement paste to become rigid. This is of practical importance since many of
the operations like mixing, laying, leveling, compacting require rime and if
cement paste were to become rigid, many of these operations cannot be done
smoothly. Like C3A, C4AF undergoes rapid hydration forming both gels and crystals
4CaO. Al2O3. Fe2O3 + 7H2O→ 3CaO. Al2O3. 6H2O + CaO. Fe2O3. H2O
gel crystals Both dicalcium silicate and tricalcium silicate hydrate to form gels and crystals.
As the time passes the gels shrink and form capillaries through which water
continues to seep to allow for completion of hydration and hydrolysis processes.
Hence the final strength of concrete structures are only realised at the end of the
year.
3CaO.SiO2 + xH2O→, 2CaO. SiO2(x—1) H2O + Ca (OH)2
gel crystals
2CaO. SiO2 + x H2O → 2CaO. SiO2. xH2O
gel Setting and hardening of cement paste has been explained on the basis of colloidal
theory of Michaelis and crystalline theory of Le Chattier. According to Michaelis,
the silicate gels, undergo hardening and bind the various materials with which it is
in contact. On the other hand, Le Chatlier’s explanation rests on the formation of
crystals which interlock as they grow to hind various materials. In fact both these
reactions contribute towards hardening of set cement paste.
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This is represented as follows
5.1.3 - Concrete: A mixture of cement, sand (below 3/16 inch mesh size) with
calculated amount of water. The size of gravel or coarse aggregates varies with the
purpose for which the concrete is required. Common maximum sizes of coarse
aggregates are 0.75 inch or 1.5 inch, but coarse aggregate of even 0.5 inch have
also been used for some purposes. In case of heavy mass concrete the size may be
even 6 inch or more. The common proportions of cement, sand and coarse gravel
may be in ratios
(a) 1 : 1.5 : 3
(b) l : 2: 4
(c) l : 3 : 6
When the cement concrete is filled in and around a wire netting of iron rods and
allowed to set, the resulting structure. Concrete has high compressive strength and
relatively low tensile strength. So in order to impart high strength as high tensile
strength so that it can rest loads which lend to crush concrete, another form of
concrete, known as reinforced concrete is used. The reinforced concrete can
withstand not only high tensile strength but also the compressive stresses. The
combination of steel and concrete produces a structure known as reinforced
concrete construction (RCC), which is capable of bearing all types of loads RCC
possesses greater rigidity, moisture and fire resistance than plane concrete. RCC is
easier to make and cast into any desired shape, which can withstand all types of
loads.
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Curing of concrete Setting and hardening of concrete is due to hydration of the cement constituents.
The process continues indefinitely but maximum amount of strength as well as
hardness is developed during few early days after placement. It is, therefore,
necessary to keep the concrete damp for about 7 days in order to enable hydration
reactions to go to completion. The chemical reactions taking place between cement and water occur only under
favourable conditions of temperature. At low temperature, the rate of reactions in
concrete is slow but completely stops, when water in concrete is frozen. The
process of dampening concrete by spraying water is known as curing of concrete.
Hence curing may be regarded as the process of maintaining a satisfactory
moisture content and favourable temperature in concrete during the period
immediately following placement, in order to allow the process of hydration to
continue, until the desired properties such as strength are developed to a sufficient
extent. Decay of concrete The cement concrete is mechanically very strong, but because of the presence of
some free lime (CaO), it becomes highly susceptible to chemical attack, especially
in acidic water (pH > 7). In acidic water, the lime present in flue concrete
dissolves and makes the concrete weak in strength. Alkaline waters (pH > 7) do
not have any marked effect on the strength of concrete. Moreover lime is more soluble in soft water than hard water. Consequently
concrete undergoes decay or deterioration in contact with acidic water and soft
water. Hence decay is quicker as the pH of water decreases and softness of water
increases from hard to soft. Sulphates and chlorides present in hard water also
remove lime present in concrete. The resistance of abrasion decreases when
concrete is soaked in mineral oils. Even minute amount of sugar present in concrete has been found to increase the
setting time of concrete and as a result, strength of concrete is also reduced
considerably, especially during first 30 days. Sulphates cause maximum damage
to concrete, because they react with tricalcium aluminate to form
sulphoaluminates, which occupy more volume and hence undergo expansion. As a
result, life of concrete is greatly reduced. This can be prevented by eliminating tricalcium aluminate from the cement
composition and using cement manufactured from tetra calcium alumina ferrite in
place of aluminate. In general, concrete can be protected by giving a coating of bituminous material
which is capable of preventing direct contact between concrete and water. Decay
of concrete can also be prevented by coating the surface with silicon fluorides
(soluble) together with ZnO, MgO or Al2O3. The CaF2 so formed in the
capillaries prevents the dissolution of lime in concrete.
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5.1.4 - Corrosion of concrete or cement stone
In addition to the external mechanical loads acting on concrete and allowed for in
the design of structures concrete is acted upon physically and chemically by the
environments. Concrete is said to be exposed to physical and chemical processes
of weathering. The failure of concrete is usually related to the action of water on
it. If concrete is not saturated with water systematically, its failure at temperatures
below zero is precluded, because the concrete will not suffer multiple freezing and
thawing, no chemical corrosion of the fabricated stone will occur due to lime
being washed out of it, or, for instance, decay due to the formation of new
chemical compounds which lack cementing properties, the formation of chemical
compounds of a volume exceeding that of its components etc. Besides water,
concrete and stone are also acted upon by other weathering agents.
The corrosion of cement stone is as if identical to the corrosion (weathering) of
rock and metal (rusting). It results in a loss of bond between newly formed
particles of the cement stone and between the stone materials- aggregates (sand.
crushed stone, gravel) in concrete. Since any of the weathering (corrosion) cases
leads to the destruction of concrete, none of them is tolerated.
Leaching corrosion manifests itself only when the hydrocarbonate alkalinity of
water is insufficient to ensure stable existence of lime in the cement stone,
separating by the reaction
3CaO. SiO2+ aqueous → 2CaO.SiO2 aqueous +
Ca(OH)2 and of other hydrated compounds.
The, rate of corrosion depends on: density of concrete , water pressure, flow
velocity etc.
Soft water dissolves Ca(OH)2, resulting in that the cement stone loses strength. It
is known that when concrete sets and hardens under optimum conditions, at the
end of 90 days upto 15% of free lime, expressed as Ca(OH)2 by mass, is separated
from the cement stone.
In the presence of soft water near nondense concrete conditions are created for the
physical decay of concrete, irrespective of the kind of portland cement and its
compressive strength, because the lime forming in the course of cement hydrolysis
will be removed from the concrete by leaching due to its good solubility in water. This
is the manner in which conditions for further decay of other hydrated newly formed
minerals are created. The removal of 30% of lime due to its dissolution in water
(leaching) reduces the strength of concrete by more than 50%. Concrete undergoing
this kind of weathering loses its other engineering properties, such as water
impermeability, resistance to frost, salt resistance, abrasion resistance, deformability
etc. The density of concrete is of great importance to the rate of lime removal by
dissolution (leaching). Concretes characterized by high water tightness reliably serve
in soft water because of considerable retardation of lime diffusion into the surrounding
medium (water basin, water saturated soil etc.)
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Acid corrosion is provoked by any acid. The possible destruction of concrete in an
aqueous medium is determined by the magnitude of the pH value. The final decay
products of the cement -stone constituents of concrete are silicic acid gel, calcium
and aluminium salts of the acid attacking the cement stone or when a weak acid is
involved-the gel of aluminium hydroxide.
mCaO.SiO2. aqueous + nH2O →SiO2.aqueous +
mCa(OH)2 Silicic acid gel.
qCaO.Al2O3 aqueous + pH2O → 2Al(OH)3 +
qCa(OH)2 Aluminium hydroxide gel. The corrosiveness of free acids is often but little responsible for the corrosion of
concrete. However, these acids contribute to dissolution of the carbonate film on
the surface of concrete and prevent the possible formation of a new carbonate
film. The action of these acids creates favourable conditions for the removal of
lime by the process of leaching. Carbon dioxide corrosion resembles in many aspects to the acid and magnesia kinds of corrosion, because the action they produce may be reduced by the action of f ions on the cement stone or concrete. When water contains magnesia salts,
MgCl2 and MgSO4, the hydrogen ions are formed due to the hydrolysis of these
salts. The free CO 2 contained in natural water may not corrode concrete, corrode
it partly or act as a fully corrosive agent. Let us consider what causes the various
corrosive actions of free CO2.
The action of H2O and CO2 on carbonate rocks results in the formation of
bicarbonates.
CaCO3 + CO2 + H2O→Ca(HCO3) 2
MgCO3 + CO2 + H2O → Mg(HCO3) 2 This process, turning insoluble carbonates into the soluble bicarbonates, only
develops in a definite time and its reversibility can be expressed as follows.
CaCO3 (in solution) + CO2 + H2O Ca(HCO3) 2
CaCO3 (solid)
Only a fraction of CO2 dissolved in the layers of water adjoining the solid carbonate,
reacts with the latter (having reached a definite concentration of Ca(HCO3) 2. The
rates of forward and backward reactions become equal, that is, an ordinary chemical
equilibrium sets in. The non-reacting fraction of free CO2 is called as equilibrium
carbon dioxide. The formation of Ca(HCO 3)2 or dissolution of solid bicarbonate will
continue if the concentration of CO2 increases or the concentration of Ca(HCO3)2
diminishes. Even a small amount of corrosive CO2 in
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water is sufficient to provoke dissolution of the solid film of CaCO3 on the
surface of concrete.
Ca(OH) 2 + CO2 → CaCO3 + H 2 O
lime in concrete. CaCO3+CO2+H2O
→Ca(HCO3)2 hardened film in concrete.
The surface of concrete may sometimes be strengthened by the corrosive CO2
provided this strengthening process proceeds till the entire corrosive CO2 is
bonded (only until the formation of CaCO3) in accordance with the reactions,
Ca(OH)2 + CO2 → CaCO3 + H2O
Ca(OH) 2 + Ca(HCO3) 2→2CaCO3 + 2H2O
The destruction of concrete due to soleplate Carrolton is associated with the
formation of a stable complex compound (hydrogen cement, from hydrated
tricalcium aluminate and gypsum under certain conditions.
3CaSO4 + 3CaO.Al2O3.6H2O + 25H2O→ 3CaO.A12O3.3CaSO4.31 H2O
The volume of this compound considerably exceeds the sum of the initial volumes
of its constituents and this causes failure of concrete, resulting from the internal
stresses originating in the cement stone. This compound, however, lacks stability
with a change in the humidity of surroundings.
Gypsum corrosion manifests itself in the formation of gypsum crystals. It may be
provoked by an aqueous medium containing a large amount of Na2SO4 or K2SO4
and cause destruction of concrete.
Sulphate and gypsum corrosions may attack concrete structures simultaneously.
Magnesia and sulphate magnesia corrosion may also occur. There are Ca2+
ions in
the pores and capillaries of concrete. Stable composition of the liquid phase filling
the pores and capillaries of the cement stone ensures stability of the solid phase,
ie, of the newly formed mineral components of the cement stone. This stability
changes when water contains magnesia sails of definite concentrations. Then the
following reactions proceed in hydrated portland cement (mortars and concretes). 3MgSO4 ⇌3Mg2+ + 3SO4
2- Complete dissolution of magnesium sulphate; formation from OH
- ions of
hydrated cement lime and Mg2+
ions that diffuse into the cement stone, of
insoluble Mg(OH)2 having no cementing properties. 3Mg2+ + 6OH- ⇌ 3Mg(OH)2
3Ca(OH)2+ 3 MgSO4 3 Mg(OH)2 + 3Ca2+
+3 SO42-
4CaO.A12O3.12H2O + 3Ca2+
+ 3SO42-
+ 20H20 3CaO.Al2O3.3CaSO4.31H2O
+ Ca(OH)2
139 Ca2+ + SO4
2- ⇌ [Ca2SO4 + 2H2O CaSO4.2H2O] In addition to the destruction of concrete due to chemical weathering or varios
kind of corroision failure of concrete as a result of the repeated combined attack of
water and froast must also be taken into consideration. Such failure of concrete is
due to : (a). Systematic thawing of the water frozen in to pores and capillaries. (b). Repeated filling of the pores and capilliaries with water and its freezing in
concrete. The frozen water gradually expands in concrete on cooling.
The formation of a complex salt in hardening cement containing alkalies (K2O
and Na2O ) and Amorphous sillica (SiO2.nH2O) of the stone material (sand,
gravel, crushed stone) Proceeds with a considerable increase in its volume
resulting in concrete cracking. This kind of corrosion may occure even at a slight
content of sillica ( less than 0.6% of the Mass of Cement). 5.2 REFRACTORIES Word refractory implies resistant to melting or fusion. In technology refractory are
materials which can withstand very high temperatures without softening melting
or deformation. They are mainly inorganic materials or ceramic materials
possessing high thermal stability, resistance to abrasion and corrosion. They are
essential structural materials used where resistance to both high temperature and
oxidation are needed. They are used widely in the construction of steel making
and glass making furnaces. They are also used in the lining of furnaces used for
metallurgical purposes and in cement manufacturing kilns. Refractory materials
are used in the form of bricks, crucibles, ladles, etc. in ferrous and nonferrous
industries. Special kinds of refractory materials are used in rockets, lets and
nuclear power plants. Refractory materials are generally constituted of oxides having high melting
points such as SiO2, Al2O 3 and MgO. Refractories having very high melting
points are made from oxides such as ZrO2, BeO etc. Apart from these oxides,
carbon, carbides, borides, nitrides may also be used as good refractory materials.
The purpose of refractory material in a furnace may be to confine heat within the
furnace or to transmit heat from one surface to the other surface as in the case of
recuperators and retorts or to store heat as is needed in the case of regenerators
used along with furnaces. 5.2.1 REQUIRMENTS OF GOOD REFRACTORIES A refractory material selected for structural purposes should possess the following
characteristics. - 1. The refractory materials should possess proper refractoriness. It should retain
its structure without undergoing any deformation at the operating temperature
of the furnace i.e. It should not begin to fuse at the temperature to which it is
exposed.
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2. It should have good spalling resistance i.e. when the refractory material is
exposed to sudden changes in temperature, it should be able to maintain its
original form without cracking, splitting or flaking. Spalling may occur due to
the compressive stresses caused by the surrounding furnace structure and the
charge inside the furnace. 3. It should be capable of withstanding the load put into the furnace at the
operating temperature and also at other service conditions of the furnace.
4. A refractory material used in a furnace should possess uniform rate of
expansion and contraction when the temperature changes in the furnace are
uniform. 5. It should possess good resistance to abrasion caused due to the movement of
the solid particles of the charge, molten slags, molten metals, and gases like
CO2, CO, SO2 etc evolved during the process.
6. It is also necessary that a good refractory material resists chemically the
action of gases evolved and slags produced in the furnace. Chemical reactions
of the refractory materials with substances in the furnace may result on
account of the dissimilarity in the nature of the two materials. Thus in a
furnace where acidic gases or slags are produced, only acidic refractory
materials should be used for lining.
5.2.2 CLASSIFICATION
Refractory materials are classified into three kinds depending upon their
constituents:
1. Acidic Refractories
They are constituted of acidic substances like A12O3, SiO2 etc. These materials
are not affected by acidic slags and gases produced in the furnace but are attacked
by basic substances in the furnace. Example: Silica and Fire clay refractories.
2. Basic Refractories
They are composed of basic materials which are easily attacked by acidic
substances but not attacked by basic substances in the furnace. Example:
Magnesite, Chrome-magnesite, dolomite.
3. Neutral Refractories
They contain mildly acidic or mildly basic substances. They can withstand the
action of acidic and basic substances, Examples Graphite, Silicon Carbide,
Chromite etc.
Apart from these refractories, there are refractory materials which are used for
special purposes and have superior properties. They are the refractories like
Alumina, Zirconia, Magnesia, Carbides, Silicides, and Borides etc.
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5.2.3 SCLECT1ON OF REFRACTORY MATERIALS A refractory material for a particular purpose is decided by various conditions
existing in the operation of furnace. The important aspects to be taken into
consideration are (i) the operating temperature of the furnace and the nature of
variations in temperature. (ii) the type of materials loaded into the furnace and the
possible chemical reactions with the surrounding substances. Since a single refractory does not serve the purpose of withstanding different
conditions existing in different parts of the furnace, a combination of different
refractories is used for constructing furnaces. 5.2.4 PROPERTIES OF REFRACTORY MATERIALS 1. Refractoriness It is the ability of the material to withstand high temperature without significant
softening or deformation in shape and size under normal working conditions of
the furnace. It is measured in terms of the temperature upto which there is no
softening or fusion of the refractory material. Normally, the softening temperature
of the material used in a furnace is higher than the operating temperature of the
furnace. However a refractory material may be used to withstand i slightly higher
temperature above its softening temperature as the outer portion of the brick is
normally at a much lesser temperature than the inner portion and as the brick
being in solid condition gives strength to the refractory lining - The softening
temperature of a refractory is measured in terms of pyrometric cone equivalent or
P.C.E. value. 2. Strength or Refractoriness under Load R.U.L. A refractory material should be strong enough to withstand the load put into the
furnace and should not be worn out due to physical impacts. It should possess
proper mechanical strength even at high temperatures, so that it can withstand
varying loads without cracking. Refractories like fire clays, alumina, soften under
the effect of heavy load much below their real fusion points, though they undergo
softening gradually over a wide range of temperatures under normal
circumstances. Silica bricks, though soften in a short range of temperature are able
to show good load bearing capacity even close to their true fusion points. The load
bearing characteristics at high temperature of a refractory material is given by
refractoriness under load or R.U.L. - 3. Thermal Conductivity Thermal conductivity of the refractories depends on chemical composition and
porosity. Thermal conductivity decreases with porosity due to the insulating effect
of air in the pores. In general, thermal conductivity increases with rise in
temperature. Fire clay, Silica, Magnesite etc. possess low thermal conductivity
whereas Carbon, Silicon, Carbide are fairly good conductors. Furnaces like blast
furnace, open-hearth furnace etc. are lined with materials of low thermal
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conductivity to prevent heat losses whereas recuperators, muffle furnaces, retorts
etc. need to be lined with refractories of high thermal conductivity.
4. Thermal Expansion and Contraction
Refractory materials undergo expansion or contraction when heated and thereby
cause change in the volume. If there are large changes in the volume, cracks may
appear, joints may get damaged and the furnace lining may undergo destruction.
Fireclay bricks, magnesite bricks and chrome-magnesites are found shrink
whereas silica bricks are found to show expansion due to allotropic
transformations.
When the refractory material shows resistance to changes in volume when
exposed continuously to high temperature, it is. said to possess dimensional
stability. Refractory bricks when subjected to rapid heating or cooling show
uneven expansion, or contraction due to temperature gradients, resulting into
thermal spalling. Spalling is the cracking of the material in such a manner as to
expose a fresh surface to the action of the environment.
5. Porosity and Permeability
The refractory material may contain pores which may be open or almost closed
depending upon the method of manufacture. When the refractory brick is more
porous, molten charges, gases etc. enter the pores and cause changes in the
properties of the brick which may lead to internal stresses during heating.
A porous brick also has low thermal conductivity due to the insulating effect of
the air entrapped in the pores. Less porous and dense bricks have better thermal
conductivity. A porous brick has less strength and abrasion resistance and is also
subjected to corrosion by slags.
6. Electrical Conductivity
Almost all refractories except graphite are bad conductors of electricity.
7. Specific Gravity
It is an important property which decides the cost of the material. Materials of
high specific gravity yield fewer bricks compared to those of lower specific
gravity. Appropriate materials to suit the different parts in furnace may be used.
Portions of the furnace which are not subjected to heavy loads may be lined with
materials of low specific gravity.
8. Chemical Composition
The refractory material used in a furnace should not be subjected to corrosion,
abrasion and erosion due to the action of slags, molten metal, and gases.
Refractory material of proper composition should be chosen so that there is no
possibility of any chemical reaction with the substances in the furnace.
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5.2.5 MANUFACTURE OF REFRACTORIES The manufacture of refractory bricks involves several preliminary treatments of
the raw materials such as calcination, grinding, sizing etc. The method of
manufacture of the bricks is nearly same for all kinds of refractories. The
following procedure is used in making the bricks. 1. Grinding The necessary raw materials are crushed in jaw crushers or gyratory crushers and
finely ground in ball mills or tube mills. Generally coarse and fine particles
together are used in the manufacture. The impurities from the raw material may be
separated by magnetic separation, electrostatic separation and froth flotation. 2. Mixing and Blending The ground raw materials are mixed with suitable binding agent and the required
quantity of water. The size of the particles, the quantity of water used depend on
the porosity of the refractory brick and also the type of the molding to be
followed. Mixing in pug mills or paddle mills enables the distribution of the
plastic constituent throughout the mass thereby facilitating easy mounding.
Meting and blending eliminates the entrapped air in the mass and improves the
density and strength. It also prevents laminations and cracking. 3. Moulding Moulding may be done by mechanical methods such as hand moulding using high
pressure or it may be done by methods such as extrusion, casting etc. 4. Drying Drying is done to remove moisture from the refractory material. It is done slowly
and under selected conditions of temperature and humidity. It may be done in
different types of dryers such as hot floor dryers or tunnel dryers. It may be done
by placing the bricks in sunlight. 5. Fixing and Burning The dried bricks are burnt in down draught or continuous tunnel kiln. By burning
there is formation of permanent bond and the material develops into stable
mineral forms which will not undergo any change in dimensions on further
heating in furnaces. During burning, there is elimination of water, calcination of
carbonates to form oxides, oxidation of metals to their higher oxidation states.
These chemical changes result in the shrinkage in volume which may produce
stresses in the material. Excessive shrinkage can be prevented by using
prestabilised raw materials of appropriate size and proper pressing. 5.2.6 - Silica Refractories Silica refractories are one of the important acidic refractories used extensively in
the construction of furnaces.
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Raw materials
i. Natural deposits of silica in the crystalline form such as quartz, quartzite,
sand, sandstone etc. are the principal raw materials which contain very little
alkaline oxides as impurities. Presence of excess of alkaline oxides may lower
the temperature of fusion. ii. Binding materials such as lime, clay, magnesia, silicates of magnesium,
sodium, aluminium and waste products of petroleum industries such as tar,
heavy mineral oil etc.
Manufacture
The raw materials are subjected to grinding in gyratory crushers and stored
separately. The materials are then ground to fine powder in ball mills or edge
runners. Powdered silica is mixed with the binding material like lime (2%) and
water to a paste of proper consistency in an edge runner. This process of mixing is
called tempering. When the mixture has attained enough plasticity, it is made into
brick by machine pressing or by hand moulding. The moulded bricks are dried in
air, heated rooms or in the sun and drying is completed normally in 18 to 24 hrs.
The dried bricks are transferred to kilns and heated. The temperature of the kiln is
slowly increased to 1500°C in about 24 hrs. They are kept at this temperature for
nearly 15 hrs for the conversion of quartzite to a more stable form like tridymite
or crystoballite. The kiln is then slowly cooled and it requires nearly 1 to 2 weeks
for cooling.
The following changes occur during the burning of the bricks.
a. Elimination of water : mechanically and chemically held water gets
eliminated first.
b. Lime reacts with silica to form calcium silicate which binds the various
particles by fusion.
c. Conversion of quartz into more stable cristobalite or tridymite resulting
into changes in. the volume of the brick. Tridymite and cristobalite possess different specific gravities and they exist in and
β forms. The conversion can be represented as below.
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On account of such conversions, the overall linear expansion of the silica brick is
by 3.5%. Tridymite and cristobalite are the stable forms which should be produced while
making the bricks. If the conversion of quartz to tridymite and.cristobalite does not occur due to
improper firing, the changes in the forms may occur during its use in the furnace
and may cause weakness in the structure.. Properties 1. The colour of the bricks vary from yellowish to brownish colour. 2. The specific gravity of the brick varies from 2.3 to 2.6. 3. Porosity is roughly 25%. 4. The refractoriness of good quality bricks is 1750° and their softening
temperature depends on quantities of lime, and other alkaline oxides, 5. The refractories under load (R.U. L.) can be as high as 1680°C for bricks free
from alumina. 6. Even though the bricks are light, they possess good mechanical strength and
good rigidity. 7. These bricks cannot withstand thermal shocks below 600°C. 8. The thermal conductivity of these bricks is more than fire-clay bricks. A good silica brick should withstand up to 1690°C and should possess good
crushing strength. When used in furnace should not show any volume change and
should not have tendency to spalling when subjected to sudden changes in
temperature. Uses Silica bricks are used where high resistance to temperature is needed and where
there are no rapid temperature changes. They cannot be used in conjunction with
basic slags or basic fluxes. Silica refractories are used to the largest extent by iron
and steel industries for constructing steel making furnaces. It is used in coke
ovens, glass furnaces, roofs of electrical furnaces, linings of acid converters. 5.2.7 - Dolomite bricks Manufacture These are prepared by mixing CaO and MgO mixture in equimolecular proportions
with silica as a binding material for magnesium silicate basic slags quick lime
hematite from oxide Fe2O 3 clays etc are also used as binding materials.
Dolomite of Composition CaMg(CO3)2 is calcined and mixed with binding agent
and water in edge runner or pug mill then the mixture is allowed to age by storing
in wet conditions By hand moulds or pressing finally it is moulded into a bricks.
These bricks are air dried and fixed at 1500C for about 24 hours.
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Properties of Dolomite Bricks
1. These are less strong. 2. They have great volume shrinkage then magnesia bricks. 3. These are more resistant to slags and spalling than margining bricks. 4. Porous, soft and wear away quickly.
Properties of dolomite bricks are further modified by stabilization in which
dolomite is mixed with serpentine (MgO.SiO2) and mixture is calcined these are
more resistant towards basic slags.
Uses
1. It is rarely used as a direct refractory mainly useful as repair material 2. Stabilized dolomite bricks are used for basic electric furnace linings ladhe
linings bessemer converters open hearth furnaces.
5.2.8 - SILLICON CARBIDE REFRACTORY
These are also known as carborundum refractories. Silicon carbide comes under
the class of super refractories which are capable of resisting the chemical changes
in contact with stags, fluxes, molten metals etc. They also withstand thermal
shocks.
Manufacture
They are manufactured by mixing silicon carbide and clay as the binding agent. The moulded bricks are fired in the furnace at a temperature of 1400° - 1600°C. The firing is done in a reducing atmosphere.
They are also manufactured by heating sand 50 - 52%, coke 35% saw dust 8 - 11% and a little amount of salt, 1 - 3% in an electric furnace at 1300° - 2200℃ when silica combines with carbon to form silicon carbide.
SiO2 +3C — SiC + 2CO ↑
The material obtained from the furnace is finely ground and mixed with binding
agents like clay, molasses, tar, lime, resin or plaster of paris and the mixture so
obtained is moulded into bricks and fired in an electric furnace at temperature
around 2000°C.
Properties
1. They are dark grey or blackish blue possesing high hardness. 2. Although it can withstand temperatures up to 2500°C it may undergo
decomposition at around 2200°C.
3. It possesses very low thermal expansion but possesses a high thermal
conductivity.
4. It possesses very good mechanical strength and good abrasion resistance and
resistance to spalling.
5: It is not affected by reducing agents but may get oxidised above 1750°C.
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6. It gets oxidised to silica when heated in air to a temperature around 1000°C. Uses I. As it possesses high thermal conductivity it is used to a larger extent in muffle
furnace, in the form of rods and bars. 2. It is used for partition walls of chamber kilns, coke ovens, recuperators etc. 5.3 NANO MATERIALS Nanotechnology (sometimes shortened to "nanotech") is the study of
manipulating matter on an atomic and molecular scale. Generally, nanotechnology
deals with developing materials, devices, or other structures with at least one
dimension sized from 1 to 100 nanometres. Quantum mechanical effects are
important at this quantum-realm scale. Nanotechnology is considered a key
technology for the future. Consequently, various governments have invested
billions of dollars in its future. The Nanoscience and Nanotechnology market
expected to be 350 billion dollors.
Nanotechnology is very diverse, ranging from extensions of conventional device
physics to completely new approaches based upon molecular self-assembly, from
developing new materials with dimensions on the nanoscale to direct control of
matter on the atomic scale. Nanotechnology entails the application of fields of
science as diverse as surface science, organic chemistry, molecular biology,
semiconductor physics, micro fabrication, etc. Nanotechnology may be able to create many new materials and devices with a
vast range of applications, such as in medicine, electronics, bio - materials and
energy production. On the other hand, nanotechnology raises many of the
same issues as any new technology, including concerns about the toxicity and
environmental impact of nanomaterials, and their potential effects on global
economics, as well as speculation about various doomsday scenarios. Nanomaterials The nanomaterials field includes subfields which develop or study materials
having unique properties arising from their nanoscale dimensions. ▪ Interface and colloid science has given rise to many materials which may be
useful in nanotechnology, such as carbon nanotubes and other fullerenes, and
various nanoparticles and nanorods. Nanomaterials with fast ion transport are
related also to nanoionics and nanoelectronics.
▪ Nanoscale materials can also be used for bulk applications; most present
commercial applications of nanotechnology are of this flavor.
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▪ Progress has been made in using these materials for medical applications, the
study under field of Nanomedicine.
▪ Nanoscale materials are sometimes used in solar cells which combats the cost
of traditional Silicon solar cells
▪ Development of applications incorporating semiconductor nano -particles to
be used in the next generation of products, such as display technology,
lighting, solar cells and biological imaging.
5.3 - CARBON NANOTUBES
Carbon nanotubes (CNTs) are allotropes of carbon with a cylindrical
nanostructure. Nanotubes have been constructed with length-to-diameter ratio of
up to 132,000,000:1, significantly larger than for any other material. These
cylindrical carbon molecules have unusual properties, which are valuable for
nanotechnology, electronics, optics and other fields of materials science and
technology. In particular, owing to their extra -ordinary thermal conductivity and
mechanical and electrical properties, carbon nanotubes find applications as
additives to various structural materials. For instance, nanotubes form only a tiny
portion of the material(s) in (primarily carbon fiber) baseball bats, golf clubs, or
car parts.
Nanotubes are members of the fullerene structural family, which also includes the
spherical buckyballs, and the ends of a nanotube may be capped with a
hemisphere of the buckyball structure. Their name is derived from their long,
hollow structure with the walls formed by one-atom-thick sheets of carbon,
calledgraphene. These sheets are rolled at specific and discrete ("chiral") angles,
and the combination of the rolling angle and radius decides the nanotube
properties; for example, whether the individual nanotube shell is a metal or
semiconductor.
Nanotubes are categorized as single-walled nanotubes (SWNTs) and multi-walled
nanotubes (MWNTs) . Individual nanotubes naturally align themselves into
"ropes" held together by van der Waals forces, more specifically, pi-stacking.
Applied quantum chemistry, specifically, orbital hybridization best describes chemical bonding in nanotubes. The chemical bonding of nanotubes is composed
entirely of sp2 bonds, similar to those of graphite. These bonds, which are stronger
than the sp3 bonds founding alkanes and diamond, provide nanotubes with their
unique strength.
These are one of the most commonly mentioned building blocks of nano –
technology, with one hundred times the tensile strength of steel. When graphite
sheets are coiled, and then form nanotubes. These sheets come in a variety of
forms and have different properties. They may be valueable component for
nanoelectronics or as storage devices.
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Nanotubes come in a variety of diameters and length. They may have different
sized internal cylindrical cavities and may have more than one sheath. The end
caps are half fullerene balls and these can differ. There are also possibilities in the
arrangement of hexagonal sheet, leading to left and right spiraled forms (chirality)
and also folds and indentation in sheets. The CNT growth is so huge that is hard to locate a zeolite particle in the as-grown
CNT bunches. Careful diameter measurement from several TEM image showed a
diameter distribution from 5nm to 15 nm with a peak at ~ 10 nm. Presence of
amorphous carbon or graphite particles was negligible. However, the CNTs were
not very straight or high crystallinity like those grown at high temperature; typical
surface defects of low temperature CVD – grown CNTs were prevalent. The CNT purity was determined by thermogravimetric analysis. Such a CNT
specimen may directly be used for many applications without further purification. Nevertheless, for high purity applications, the zeolite content of the as- grown specimen can easily removed by 6M NaOH treatment and the CNT purity can be
increased over 99 percent. Typical TEM micrograph of octylamine capped Fe2O3,
Co3O4, Fe3O4 and dodecylamine capped Fe3O4 nanoparticle is shown in fig. ZnO
and ZnS nanoparticles obtained by thermal decomposition of Zn cupferron complex under argon and H2S atmosphere respectively such as magnetic M
Fe2O4 (M represents Fe, Co, Mg, Zn of Mn) could effectively prepared.
Micrographs of CoFe2O4 and MnFe2O4 spinel ferrite nanoparticles with dia ~ 10
nm, showed good uniformity. The details of structural and magnetic properties of nanoparticles prepared by this technique are described by Saravanan. 5.3.1 - Types of Carbon Nanotubes Carbon nanotubes come in various forms – chiral, zigzag and armchair. A
nanotube may consist of one tube of graphite (single walled nanotubes, SWNT) or
a number of concentric tubes, called multi walled nanotubes (MWNT) when
viewed by transmission electron microscopy these tubes appear as planes.
Whereas in SWNT two planes are observed, representing the edge in MWNTs
more than two planes are observed, and these can be seen as a series of parallel
lines. 5.3.2 - Formation of Nanotubes: Following are preparation methods of carbon nanotubes. i. Laser Method: In 1996 a dual pulsed laser vaporization technique was used to optimize the laser
method to produce SWNT in gram quantities and yield of >70 %. Samples were
prepared by laser vaporization of graphite rods with 50 : 50 mixture of CO and Ni
powder (particle size 1 mm) at 1200 in flowing organ, followed by heat treatment in vacuum at 1000 to remove the C60 and fullerenes.
150
The initial laser vaporization pulse was followed by a second pulse to vaporize the
target more uniformly.
The use of two successive laser pulses to minimize the amount of carbon
deposited as soot. The second laser pulse breaks up the larger particles ablated by
the first one, and feeds them in to the growing nanotubes structure.
The material thus produced appears as a mat of ‘ropes’ 10 – 20 nm in diameter
and up to 100 or more in length. Each rope is found to consist primarily of bundle
of SWNTs aligned along a common axis. By varying the growth, the temperature,
the catalyst composition and other process parameters, the average nanotube
diameter and distribution can varied.
ii. Chemical Vapour Deposition Method (CVD)
Discharge and laser vaporization are currently the principal methods for obtaining
quantities of high quality carbon nanotubes. However, both methods suffer from
some drawback. The first is that both methods involve evaporating the carbon
source so that is has been nuclear how to scale up nanotubes production to the
industrial level using these approaches. The second issue relates to the fact that
vaporization method grow nanotubes in highly tangled forms mixed with
unwanted forms of carbon or metal species. The nanotubes are difficult to purify
manipulate and assemble for building nanotube device architecture.
In this method an organometallic precursor is mixed with carbon containing feed
gas, it is polymerized in a quartz tube and nanotubes are collected from the cooler
end of the reaction vessel. The feed gas may contain several species and is often
mixed with an inert gas.
151
Nanotubes are also grown on solid catalytic substrates such as SiO2, quartz,
alumina, etc. which contain transition metal precursors. Such approaches are
important for making supported MWNT assemblies for specific application.
5.3.3 - Properties of Nanotubes: i. Electronic properties: Conductivity in multi walled nanotubes is quite
complex. The conductances of MWNTs jumped by increments as additional nanotubes were touched to the mercury surface. This quantized conductance was found in all sizes of nanotubes and is also observed in metal nanowires. Some types of armchair nanotubes appear to conduct better than other metallic nanotubes. The conductivity and resistivity of rope SWNTs has been measured directly with a technique in which four electrodes have been placed at different part of the nanotubes. The resistivity of those SWNT ropes was in
order of 10−4 ohms per cm at 27 . This means that the ropes are the most highly conductive carbon fibers known. Individual SWNTs may contain defects. These defects allow the SWNTs to act as transistors. Likewise, joining nanotubes together may form transistor like devices. A single nanotube with a natural junction (that is, where a straight section is joined to a chiral section) behaves as rectifying diode – a half transistor in a single molecule.
152
ii. Mechanical Properties: SWNTs are stiffer than steel and are resistant to
damage from physical forces. Pressing on the tip of the nanotube will cause it
to bend without damage to tip. When the force is removed the tip of nanotube
will recover to its original state. Young’s Modulus value (which describes
elasticity, hardness, ease of fracture and conduct, are all important properties)
of SWNTs is 1.8 TPa. Armchair nanotube had a Young’s Modulus of 640.30
CPa.
5.3.4 - Uses of Carbon Nanotubes:
Carbon nanotubes behave as transistor. They can conduct electricity and have
been made into simple logic circuits. CNTs can store hydrogen and may also be
useful with lithium as batteries. They have unusual tensile strength and can be
used in making valuable building materials if manufacture cheaply in quantity.
Questions
1. What are the raw materials used in the manufacture of Portland cement
what are their functions. ?
2. What are the criteria adopted to ensure that the quality of cement
produced is good. ?
3. Describe the manufacture of Portland cement with neat flow diagram. 4. What are the sources of calcarius and argillaceous raw materials? 5. Describe the properties of various constitutes of cement explain
significance of gypsum.
6. Write note on setting and hardening of cement 7. What substances are added to cement to enhance its properties? Describe
any one with its applications.
8. Explain the following
a) RCC c) RUL b) Decay of Concrete d) CNTs
9. What are refectories explain their general properties mention the various
raw materials used in the manufacture of silica bricks outline the
manufacture of silica bricks 10. Explain manufacture properties and uses of Dolomite bricks.
153
Experiments EXPERIMENT NO. 1 Aim: To determine hardness of water by EDTA method.
Requirement: Std. hard water (1mg CaCO3/ml), 0.01 M EDTA EBT, buffer pH
= 10. Theory: Estimation of water hardness as applied to boiler water is of great
importance for the chemical industries in general. It is an important factor in the
manufacturing of sugar, dyes, pharmaceuticals, food processing and textile
industries, etc. Hardness in water prevents lathering of soap due to the presence of dissolved salts
of calcium magnesium, etc.
2C17H35COONa + Ca+2
or Mg+2
→ (C17H35COO)2 CaOOMg + NaCl) This method is based on the fact that ethylene diamine tetra acetic acid (EDTA)
forms stable complexes with di and polyvalent metal ions. The indicator
Eriochrome Black T used in the estimation also forms complexes with metal ions
but they are unstable and can be easily broken down. Erichrome Black T is a fri
basic acid dye which dissociates at different pH to form different coloured ions as
shown below
Eriochrome Black T ←→ ‘ Eriochrome Black T ←→ Eriochrome Black T
Red < pH 6 > Blue < pH 12 > Orange
At pH 10, in the presence of Ca2+
or Mg2+
ions Erichrome Black forms an
unstable wine red complex. Erichrome Black T
+ Ca+2
→ EBT- Ca or EBT - Mg
Blue
wine-red in colour unstable complex
When EDTA is added to water, EDTA combines with all Ca++
ions and Mg++
present in water, as the stability of EDTA complexes is greater.
If we represent EDTA disodium salt as Na2H2Y, it dissociates as follows:
Na2 H 2Y ←→ 2Na+ + H 2Y −
Eriochrome Black T Ca complex + H2Y ←→ Ca H2 Y + Erichrome Black T
unstable complex Stable complex Blue Preparation of Solutions
1. Standard Hard Water: Accurately weighed 1 gm pure CaCO3 dissolved in
minimum amount of HCl and evaporated to dryness. The residue is dissolved
in distilled water and diluted to one litre.
154
2. Standard 0.01 M EDTA Solution: 3.722 gm of disodium ethylene diamine
tetra acetate dihydrate dissolved in one litre of distilled water.
3. pH 10.0 buffer: Dissolve 70 gm of pure ammonium chloride with 570 ml of
liquor ammonia and dilute to litre with distilled water.
4. Eriochrome Black T Indicator Solution: Dissolve 0.5 gm of Eriochrome
Black T a indicator in 100 ml of alcohol 3-4 drops of this solution are used as
indicator during estimation.
Procedure
1. Standardisation of EDTA Solution: Pipette out 25 ml of standard hard
water in a 250 ml conical flask. Add half test tube buffer solution, 3- 4 drops
of indicator. Titrate against EDTA solution from burette. The end point is
from wine red to blue. Repeat the titration with another 25 ml of standard
hard water. Note down the volume of EDTA solution consumed. Let this
reading be ‘x’ ml. 2. Determination of Total Hardness of Unknown Solution: Pipette out 25 ml
of solution in flask. Add half test tube of buffer solution, 3 - 4 drops of
indicator solution. Titrate against EDTA solution from burette. Repeat the
titration with another 25 ml of the same solution. Note down the reading. Let
this reading be ‘Y’ ml. 3. Determination of Permanent Hardness of Water: Pipette out 25 ml of
unknown water in a flask, boil for 5 minutes cool and add half test tube of pH
10.0 buffer, 3 - 4 drops of indicator solution and titrate against EDTA
solution. Repeat the titration with another 25 ml of tap water sample. Note the
volume of EDTA consumed. Let this reading be ‘Z’ ml.
Observations Standardization of EDTA
1. Burette EDTA solution 0.01 M: Solution in flask 25 ml of standard hard
water + half test tube of buffer solution pH 10 Indicator
Indicator 3- 4 drops of Eriochrome Black T
End point Wine red to blue
Sr. No. EDTA solution in ml 1 2 3 Constant burette reading
1 Initial burette reading
2 Final burette reading
3 Difference
2. Determination of total hardness of water: Burette EDTA Solution 0.01 M
Flask 25 ml f unknown solution + 5 ml ammonia buffer pH 10.0
Indicator 3 - 4 drops of Eriochrome Black T
End point
Wine red to blue
155
Sr. No. EDTA solution in ml 1 2 3 Constant burette reading
1 Initial burette reading
2 Final burette reading
3 Difference
3. Determination of permanent hardness: Burette EDTA Solution 0.01 M
Flask 25 ml tap water + 5 ml pH 10.0 ammonia buffer
Indicator 3 - 4 drops of Eriochrome Black T
End point Wine red to blue
Sr. No. EDTA solution in ml 1 2 3 Constant burette reading
1 Initial burette reading
2 Final burette reading
3 Difference
Calculation
In order to understand the calculation better, let us assume the following data:
1. 25 ml of standard hard water requires 25 ml of EDTA 0.01 M EDTA
solution.
2. 25 ml of unknown solution requires 12.5 ml of EDTA 0.01 M EDTA
solution.
3. 25 ml of water after boiling requires 6.3 ml of EDTA 0.01 M EDTA solution.
We know standard hard water contains l mg/ml of CaCO3 hardness.
Hence, 25 ml of standard hard water has 25 mg of CaCO3 hardness.
Since 25 ml of standard hard water requires 25 ml EDTA solution.
∴ 25 ml of EDTA solution = 25 mg CaCO3 hardness.
∴ each ml of EDTA solution = 25
mg CaCO3, hardness 25
= 1 mg CaCO3 hardness
1. Hardness equivalence of EDTA solution
1 ml of 0.01 M EDTA = 1 mg CaCO3.
l x 1000 = 1000 ppm
156
2. 25 ml of unknown solution requires 12.5 ml of EDTA solution
∴ 25 ml of unknown solution = 12.5 x 1 mg CaCO3 hardness
∴ 1000 ml of unknown solution = 12.5 x 1000
mg CaCO3 hardness 25
∴ Hardness of unknown solution = 500 ppm
3. 25 ml water after boiling requires 6.3 ml of EDTA
∴ 25 ml water = 6.3 x 1 mg of CaCO3 hardness.
∴ 1000 ml of tap water = 6.3 x 1000
m of CaCO3 hardness 25
∴ Hardness of tap water = 252 ppm
Temporary hardness = Total hardness - Permanent hardness
Results
1. Hardness equivalence of EDTA solution = 1000 ppm 2. Total hardness of water = 500 ppm 3. Permanent hardness of water = 252 ppm 4. Temporary hardness of water = 248 ppm
EXPERIMENT NO. 2
Aim: Removal of hardness by using ion exchange resin.
Requirement: Cation exchange resin, hard water sample, 0.01 M EDTA, buffer
solution, Eriochrome Black T indicator, etc.
Theory: When hard water is passed through cation exchanger resin in the form of
H ions the Ca+2
, Mg+2
and metal ions present in the hard water are absorbed by
resin and H ions become free.
R- H2 + Ca+2
→ R – Ca + 2H+
R- H2 + Mg +2
→ R – Mg + 2H +
Procedure
Part I. Pipette out 25 ml of hard water in 250 ml conical flask add half test tube of
buffer pH = 10 and titrate against 0.01 M EDTA using EBT indicator, till it
becomes wine red to blue.
Part II. Prepare 25 cm cation exchange column and pass approx. 100 ml of 0.5 N
HCI at the rate of 2 ml per minute then wash the column with distilled water till it
is free from acidity (test by using litmus paper). Then pass 25 ml of hard water
sample through the column with adjustment of above-mentioned rate and collect
157
the eluate in a 250 ml standard measuring flask then wash the column with 100 ml
distilled water collect the washings and dilute to 250 ml. Titrate 50 ml of diluted
solution with 0.01 M EDTA by using Eriochrome Black T indicator till it
becomes wine red to blue. Calculation
25 ml of hard water required = ‘X’ ml of 0.01 M EDTA
50 ml of diluted eluate required
= ‘Y’ ml of 0.01 M EDTA
∴ 250 ml of
“
“
“
=Y x 5 ml of 0.0l MEDTA
Hence, 25 ml of hard water after passing through the cation exchanger required
= Y x 5 ml 0.01 M EDTA % Efficiency of the operation If the ‘Y’ is zero, then efficiency of operation is
100% X – 5Y = Z’ ml of 0.01 M EDTA
% Efficiency = Z x 100 X
Result % Efficiency =_______
EXPERIMENT NO. 3 Aim: To determine saponification value of the given sample of oil. Requirement: 1N alcoholic KOH, 0.1 N HC1, phenolphthalein, etc. Theory: Oils are triesters of glycerol when treated with excess of alcoholic KOH
solution gets hydrolysed into free glycerol potassium salt of fatty acid. When hydrolysis is completed the excess of alkali is back titrated against a
standard acid.
C17H35 COOH + KOH → C17H35 COOK + H2O. Saponification number is the amount of KOH in milligrams required to saponify
fatty acid present in 1 gram of oil. Procedure Part I Standardization of KOH solution Pipette out 10 ml supplied 1N alcoholic KOH (approx) in a 100 ml standard
measuring flask and dilute upto the mark with distilled water. Then pipette out
diluted 25 ml of KOH in a 250 ml conical flask add 1 - 2 drops of phenolphthalein
indicator and titrate against standard 0.1 N HCl from the burette. The end point
will be from pink to colourless. Take three constant readings.
158
Part II Determination of saponification value of the oil
Add 25 ml of supplied in alcoholic KOH to the accurately weighed I gm of oil
sample in a 250 ml round bottom flask attach it with water condenser. Reflux the
mixture on water bath for about 20 minutes till the hydrolysis is completed. Cool
the flask and dilute the contents to 250 nil with distilled water in a standard
measuring flask. Pipette out 25 ml of it in 25 ml conical flask; add 1 - 2 drops of
phenolphthalein and titrate against 0.1 N HCl. From the burette, end point will be
from pink to colourless. Take three constant readings.
(I) Observation and Calculation Part
Solution in burette 0.1 N HCl
Solution in conical flask 25 ml of KOH + 1-2 drops phenolphthalein
Indicator Phenolphthalein
Change in colour Pink to colourless
Sr No. 0.01 N HCl I II III Constant reading
1 Initial reading
3 Final reading
2 Difference Xml.
Calculation
25 ml of diluted KOH required “X” mil of 0.1 NHCl solution.
∴ NKOH = N1V1 = N2V2
NKOH
×
VKOH
=
NHCI
×
VHCI
NKOH × 25 = 0.1 × X
NKOH
= 0.1X
25 = AN
(II) Observations and calculations (Part II)
Solution in burette 0.1 N HCl
Solution in conical flask 25 ml of saponified solution and 1 - 2 drops of
phenolphthalein
Indicator Phenolphthalein
Change in colour Pink to colourless
159
Sr No. 0.01 NHCl I II III Constant reading
1 Initial reading
3 Final reading
2 Difference Xml.
Calculation
Amount of KOH added in terms of 0.1 N HCl from burette is 25 ml of AN.
∴ Volume of HCl = 25 x A/0.1 = B mg of KOH
Hence, 250 mg will be ≡ 10 B mg of KOH
Amount of KOH unused in terms of 25 ml of dilute solution = “Y” ml. ∴ 250 ml of diluted KOH solution = Y X 10 = 10 Y ml of 0.1 N HCL.
Hence amount of KOH used up for saponification = (10x – 10y) = “Z” ml.
Now 10ml of 1 N HCl = 56 gm of KOH
∴ Yml of AN HCl = 56 × A× Z
= C gm 1000
Saponification value of the oil = C/ weight of oil = ….. gm = …. mg
Result
Saponification value of the oil = ….. mg
EXPERIMENT NO. 4
Aim: To determine acid value or neutralisation number of the oil.
Requirement: 0.01N KOH, 0.01N HCl, Phenolphthalein indicator, distilled
water, etc.
Theory: Acid value is defined as the milligrams of potassium hydroxide required
to neutralise free acid present in 1 gm of oil sample. Most of the fatty acids
contain free acid. Higher acidity indicates oil has been oxidised and hence roughly
it is an indicator for the age of the oil or it gives an idea how old the fatty oil is.
Preparation of Solutions
1. O.1 N KOH: Weigh 5.6 gm of A.R. potassium hydroxide and dissolve in
distilled water dilute up to a litre in a standard measuring flask.
2. 0.1 N Oxalic Acid: Weigh 6.3 gm of A.R. oxalic acid and dissolved in
distilled water dilute to a litre.
160
3. Neutral 95% Alcohol: A drop of phenolphthalein solution is added to 95%
alcohol and neutralised with just enough KOH solution to give faint colour.
This is to ensure that reagent used for experiment does not contribute to the
acidity. 4. Phenolphthalein Indicator: 1gm of phenolphthalein dissolve in 100 ml of pure
alcohol.
5. Methyl Orange Indicator: 1 gm of methyl orange dissolved in 100 ml of 50%
alcohol.
Procedure
Part I Standardisation of KOH solution
Pipette out 25 ml of 0.01 N KOH (approx) in a 250 ml Conical flask add 4 -5
drops of phenolphthalein indicator and titrate against 0.01 N HCl from the burette
till it becomes pink to colourless. Take three constant readings.
Part II Determination of acid value
Take clean and dry 100 ml conical flask weigh it accurately then add 5 ml of oil
sample weigh it again from the difference in weight, Note down the actual weight
of the lubricating oil. With the help of pipette add 25 ml of 0.01 N KOH to the
conical flak and shake it vigorously to dissolve the oil, add few drops of
phenolphthalein indicator and titrate against 0.01 N HCl from the burette till it
becomes pink to colourless.
Observation and Calculation
I. Standardization of KOH
Solution in burette: 0.01 N HCl
Solution in conical flask: 25 ml 0.01 N KOH and phenolphthalein.
Indicator: Phenolphthalein.
Change in colour: Pink to colourless
Reaction : KOH + HCl → KCl + H2O
Sr. No. 0.01 NHCI I II II Constant reading
1. Initial reading
2. Final reading
3. Difference X ml.
161
Calculations
N1V1 = N2V2
NHCI
×
VHCI
=
NKOH
×
VKOH
0.01× x = NKOH × 25
0.01× x =
NKOH
25
II. Determination of acid value
Solution in burette 0.01 N HCl
Solution in conical flask Oil + 25 ml of 0.01 KOH + Phenolphthalein
Change in colour Pink to colourless
Sr. No. 0.01 NHCI I II II Constant reading
1. Initial reading
2. Final reading
3. Difference Y ml.
Calculation
Volume of 0.01 N KOH required = ‘Y’ ml
Volume of 0.01N KOH used against HCI with respect to the lubricant or
oil = (x- y) ml.
Acid value of the oil = KOH used(x - y)x 56 x
NKOH wt of oil
Result
1. Normality of KOH =... N
2. Acid value of the oil = ... mg
162
Experiment No. - 5
Chemical oxygen Demand
Chemical oxygen Demand is related to biochemical oxygen and can be carried in
3 hours as against 5 days for BOD. COD is defined as amount of oxygen used
oxidizing organic matter by means of a strong oxidizing agent. Both biologically
oxidisable organic matter like starch and sugar, inert material such as cellulose,
etc. are oxidised and hence COD values are always higher than those of BOD.
COD determination being quicker offers a means of taking corrective steps in
treatment process without waiting for ultimate BOD results.
Reagents Required
1. 0.25 N potassium dichromate 2. 0.25 N ferrous ammonium sulphate 3. Silver sulphate-sulphuric acid reagent 4. Mercuric sulphate 5. Ferroin indicator
Procedure
50 ml of sample of sewage taken in a 50 ml round bottom flask, add 1 gm of
H2SO4 and pour in 75 ml of silver sulphate - sulphuric acid reagent slowly,
cooling the contents. Add 25 ml of 0.25 ml 0.25 N K2Cr2O7 and mix well. Attach
a water condenser to the flask and reflux for 2 hours. Wash the condenser with little water, cool the contents in the flask and add few drops of ferroin indicator Titrate against 0.25 N ferrous ammonium sulphate standard solution. A blank
estimation is carried out with 25 ml 0.25 N K2Cr2O7 solution adding the reagents
used in identical manner and refluxing the contents for 2 hours.
Observations and Calculation
Volume of sample taken = 50 ml
Volume of 0.25 N ferrous amm. sulphate used in blank titration = (x) ml
Volume of 0.25 N ferrous amm. sulphate used in test titration = (y) ml
∴ Volume of 0.25 N ferrous amm. sulphate consumed. = (x — y) ml
∴ Chemical oxygen demand = (x
−
y)×
8
×1000
mg / liter 50 × 4
= (x − y)× 40mg / litre
Precaution to be taken while adding silver sulphate H2S04 reagent, as this addition
liberates large amount of heat and hence it is better to cool the mixture
thoroughly.
Result : Chemical oxygen demand =...
163
EXPERIMENT NO. 6 Aim: To determine the melting point/glass transition temperature of a polymer. Requirement: Thermometer, oil bath, burner, dilatometer, etc. Theory: The temperature of which the polymer or any substance changes from
solid to liquid state at NTP is called its melting point. Glass transition temperature (Tg) is conveniently measured in the laboratory by
dilatometry. Amorphous polymer when cooled below a certain temperature
becomes hard, brittle and glassy, but above this they are soft, flexible and rubbery.
This transition temperature of polymer is called glass transition temperature (Tg). Procedure: Determination of glass transition temperature: The polymer
appropriately confined in the bulb at the bottom is kept immersed in a suitable
liquid, usually mercury so as to give a column of the liquid in the capillary up to a
convenient height for measurement. The positioning of the glass plug as shown,
enables heating the test specimen’s avoiding overheating. The dilatometer placed
in an outer bath may be heated at a present rate and pattern. From the rise of the
liquid in the capillary on heating and consequent rise in temperatures, the change
in the volume of the specimen may be conveniently, obtained.
Result (i) M.P. of given polymer = ….. °C (ii) Glass transition temperature of given polymer = …. °C
164
EXPERIMENT NO. 7
Aim: To determine flash point and fire point of the lubricant.
Requirement: Oil sample, Abel’s flash point apparatus, Pensky and Martens
apparatus.
Theory: Flash point of the lubricant is defined as the lowest temperature at which
a lubricant gives off enough vapours that ignite when a small flame is brought
near to it. The fire point is defined as the ionest temperature of which the oil
vapours turns continuously at least for seconds. The flash points are determined
by using (1) Abel’s apparatus (ii) Pensky and Martens apparatus.
(a) Abel’s flash point apparatus
As water bath is used in Abel’s flash point apparatus, it is used to determine flash
point upto 90°C. 1. Oil Cup
Oil cup consists of a flanged cylindrical brass up, placed on another copper cup
separated by means of air gap. The oil cup is covered by means of tightly fitting
covet Attached to cover is a knob which rotates a stirrer attached to it. There is a
point for inserting a thermometer inside the cup. The cover has a rectangular
opening which is covered by means of shutter device which can be moved so as to
open and close the opening in the cup covet Attached to the shutter device is
arrangement for providing a small flame. The arrangement consists of a small oil
reservoir with a small protruding pipe. A piece of cotton thread passes through the
pipe and dips in small reservoir containing some oil. The shuttle mechanism
operates in such a way that when the opening is made, the flame automatically
dips inside the cup. The copper cup is placed in the water bath and the whole
assembly is completely covered.
The oil cup is 5 cm in diameter and 5 cm in depth. It carries a L-shaped pointer
attached to it to indicate the required level of oil in cup.
Abel’s flash point apparatus
165
Working The cup is filled with lubricant to the desired level and placed in the apparatus.
The cover placed on the top and secured. The cotton thread dipped in oil is passed
through the small tube. Thermometer inserted into port and secured. The shutter
mechanism is in closed position. The water bath filled with water and heating is
started. The stirrer is rotated manually to ensure uniform heating of lubricant in
cup. The thread is lighted to provide a small flame. As heating continues, the
shutter mechanism is operated intermittently so that the flame dips inside the cup.
If the oil is sufficiently heated it gives off vapours which will suddenly burn in a
flash when flame dips inside the cup. The temperature at which this flash is
observed is noted as the flash point of lubricant. Result The flash point of given lubricant is °C Heating: Since water bath is used for Abel’s apparatus allows flash points
deternination of lubricant upto 90°C for heating (b) Pensky Martens Apparatus
Description of Apparatus
There is flanged brass oil cup 5 cm in diameter and 5.5 cm in depth resting in
another cup and separated from it by air gap. The outer cup forms part of
assembly which can be heated directly by gas or heater. The cover of oil cup has
four openings, one for stirrer, one for inserting thermometer, air inlet and a device
for inserting the flame. When the mechanism is operated, a small gap is made in
the cover and at the same time flame device allows the flame to dip inside the cup.
By operating the mechanism flame can be dipped and opening closed as desired.
A pilot lamp attached to mechanism allows the standard flame to be lighted again
if it gets extinguished in the process of dipping.
Pensky Martens Apparatus
166
Working
Oil cup is filled with oil to the desired level and the thermometer is placed in it.
The assembly is slowly heated and stirred to heat it uniformly. The standard flame
and pilot lamp are lighted. Shutter mechanism operated periodically to allow the
frame to dip and come in contact with the vapours evolved. The temperature at
which the vapour of the oil burns suddenly with flash is noted down as the flash
point of lubricant.
Flash point of some solvents.
Sr.No. Solvent Flash point
1 Terpentine 35
2 Diesel 31
3 Petrol 42
4 Kerosene 43
5 Decane 63
6 Ethylene glycol 113
7 Transformer oil 171
Result
(i) The flash point of the given lubricant = ... °C
(ii) The fire point of the given lubricant = ... °C
EXPERIMENT NO. 8
Aim: To estimate the amount of chloride present in the given sample of water by
Mohr’s method.
Requirement: 0.025 N NaCl, 0.025 N AgNO3, 5% K2CrO4 solution.
Theory: Natural water contains small amounts of chlorides of calcium,
magnesium and sodium. When water containing chlorides are used in boilers,
some of the chlorides undergo hydrolysis at high temperature to produce corrosive
acid HCI.
MgCl2 + 2H2O → Mg (OH)2 ↓ + 2HCl
CaCl2 also undergoes hydrolysis though to smaller extent producing corrosive
hydrochloric acid. It corrodes boiler plates and can cause serious damage to boiler.
Hence, it is essential to estimate amount of chloride present in water.
167
Mohr’s method of estimation of chloride involves titration against standard
AgNO3 solution using K2CrO4 solution as indicator.
Ag + + NO 3 + Cl → AgCl ↓ + NO 3
2 Ag + + 2NO3 + CrO42− → AgCrO4 ↓ + 2NO3
In the presence of both chloride and chromate ions, silver ions react with chloride ions to form silver chloride precipitated since its solubility product is reached
earlier. As long as chloride ions are present, precipitate of Ag2CrO 4 will not be
formed. This is the basis of chloride estimation. Further colour of Ag2CrO4
precipitate is brick red and that of AgCl is white and hence the end point can be easily detected. The titration should be carried out at slightly alkaline medium, since under acidic
condition chromate ions get converted into dichromate ions and silver dichromate
is soluble and the end point cannot be so easily followed. Hence, solution should
be made slightly alkaline by adding small amounts of Na2CO3 or CaCO3. Preparation of Solutions
Approximate 0.025 N AgNO3 Solution Weigh accurately 4.250 gm of pure silver nitrate, dissolve in distilled water and
dilute to one litre in a standard measuring flask. AgNO3 is Store in dark brown
bottle. Standard 0.025 M NaCl solution This primary standard is prepared by 1.46 gm of A.R. NaCI is dissolved in
distilled water and diluted to one litre with distilled water. 0.5% Potassium Chromate Indicator Solution
Weigh accurately 5.0 gm of pure K2CrO4 and dissolve in distilled water and dilute
to 100 ml with distilled water. Procedure 1. Standardisation of AgNO3 Solution: Pipette out 25 ml of standard 0.025 N
NaCl solution in a 250 ml conical flask. Then add pinch of Na2CO3 or
CaCO3 to ensure the slightly alkaline medium, add 1 to 2 ml of freshly
prepared 5% K2CrO4 indicator and titrate against std 0.025 N AgNO3. From the burette till the formation of slight reddish brown (chocolate coloured)
precipitate of Ag2CrO4. (Supernatant solution remains yellow in colour).
Note down the reading and calculate normality of AgNO3 solution.
Solution in Burette AgNO3 solution (0.02 N approx)
Solution in Conical Flask 25 ml of standard 0.1 N NaCl sol + a pinch of
Na2CO3 as CaCO3
168
Indicator 5% K2CrO4 solution 3-4 drops
Change in colour White ppt. to brick red
Sr. No. I II II Constant reading
1. Initial reading
2. Final reading
3. Difference X ml.
2. Estimation of Chloride Content of Water sample: Pipette out 25 ml of given
water sample in a 250 ml conical flask and titrate against 0.025 N AgNO3 as
above. Note the reading and calculate the amount of chloride present.
Solution in Burette AgNO3 solution
Solution in Conical Flask 25 ml of diluted chloride solution + a pinch of
Na2CO3 as CaCO3 +3 - 4 alcoholic KOH
Indicator 3- 4 drops of K2CrO4 solution
Change in Colour White ppt to brick red ppt.
Sr. No. I II II Constant reading
1. Initial reading
2. Final reading
3. Difference X ml.
Calculation
Normality of AgNO3 Solution
using N1 V1 = N2 V2 where
N1 = Normality of AgNO3 solution
V1 = Volume of AgNO3 solution
N2 = Normality of standard NaCI solution [0.025 N known]
V2 = Volume of standard solution taken [25ml]
Let the normality as determined by the observations be denoted by N
Now AgNO3 + NaCl → AgCl + NaNO3
1000 ml 1 N AgNO3 = 35.5gm of chloride
169
∴ 1000 ml 0.1 N AgNO3 = 3.55gm of chloride
∴ 1 ml of 0.1 N AgNO3 = 0.00355 gm of chloride
∴ 1ml of 0.025 N AgNO3 = gm of chloride
∴ Actual normality of AgNO3 as found out is N
1 ml of AgNO3 of N normality = 0.00355× N gm of chloride
0.1 Estimation of chloride content 25 ml of diluted chloride solution required
= B ml of AgNO3 solution ∴ 25 ml of diluted chloride solution contains
= B
×
0.00355
×
N
gm of chloride 0.1
∴ 1000 ml solution contains = B
×
0.00355
×
N
× 1000
gm of chloride 0.125
Chloride content of given water sample = gm/litre Results Amount of chloride present in the given sample of water
= ....... gm/litre = ... …. mg/lit = ... …. ppm.