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INORGANIC TECHNOLOGY

Prof. Dr. Levent Ballice

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In order to treat water, we must understand it and be able to describe it as much detail as possible.

Certain expressions used in water treatment are far removed from usual scientific terminology.

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The most comman of these are listed below.

Turbidity

Together with the measurement of suspended solids, this gives an initial indication of the level of colioidal matter of inorganic or organic origin. Turbidity is judged either by comparing the specimen with reference opalescent solutions (formazin, etc.) or by measuring the limit of visibility using a well-defined object (a platinum wire or a Secehi disc).

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The Fouling indexThe Fouling index

It is a measure of the fouling potential of water. It is also linked with suspended solids and is involved in water treatment by membranes.

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Suspended solids (SS)

This includes all matter suspended in water that is large enough to be retained on a filter with a given porosity

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Colour

True colour after filtration is due most often to the presence of dissolved or colioidal organic marter.

There is no relationship between the colour and the quantity of organic matter.

Colour is measured by comparing the specimen with a reference solution (Platinum-Cobalt method).

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Concentration (by volume)

This is a measure of the quatity of matter dissolved or dispersed in a given volume of water.

As an example, it is mg/L, g/m3, g/L

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Gramme-equivalent

The gramme-equivalent is equal to the molar weight of a substance divided by the number of charges of the same sign carried by the ions released by a molecule of that substance in an aqueous solution.

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For instance,

a molecule of orthophosphoric acid, H3P04, releases three positive charges and three negative charges. One gramme-equivalent of H3PO4 is therefore equal to one third of the weight of one mole of H3PO4 .

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Normality

A normal solution is one comaining one gramme-equivalent of the relevant substance per litre. Multiples and submultiples of the normal solution are also used (2N, N/10, N/25, N/50, N/100, etc. solutions).

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In general, when a volume V1 of an electrolyte of normality N2 is acted upon another electrolyte of normality N2, the volumeV2 is determined from the relation:

N1V1= N2V2

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Milliequivalent per liter

The unit often used in practice is the milliequivalent per liter (meq./L) which is obtained by dissolving a quantity of the electrolyte equal to one thousandth of its gramme-equivalent in one liter of water. This is the concentration of an N/1000 solution.

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The French degree

The unit used in practical water treatment is the French degree which corresponds to the concentration of an N/5000 solution.

1 meq/L = 5 French degrees

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Examples:

A solution of a calcium salt at 25 French degrees contains in Calcium (of molar weight 40 g an valency 2):

(40 x 5) / 2x 1000 = 0.1 g per liter

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The equivalance of various degrees

1 French degree = 0.56 German degree = 0.7 English degree = 10 ppm CaCO3

1 German degree = 1.786 French degree = 0.25 English degree = 17.86 ppm CaCO3

1 ppm CaCO3 = 0.1 French degree = 0.056 German degree = 0.07 English degree

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Titration for hardness (TH):

The titration for hardness indicates the concentration of alkaline-earth ions in water.

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The various types:

(A) Total TH: content of Ca and Mg.

(B) Calcium TH: content of Ca.

(C)Carbonate hardness: content of bicarbonates and carbonates of Ca and Mg.

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(D) Noncarbonate hardness (permanent hardness):

Indicates the content of Ca and Mg corresponding to strong anions. It is equal to the difference between A and C.

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Phenolphthalein alkalinity (P alk.) and methyl orange alkalinity (M alk.)

The relative values of P alk. and M alk. indicate the quantities of alkaline or alkaline earth hydroxides, carbonates or bicarbonates in water.

The P alk. value therefore includes all the hydroxide content but only half of the carbonate content.

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The M alk. therefore incidate the bicarbonate content.

In some very polluted waters, the M alk also covers organic acids.

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Measurements of salts of strong acids (SSA):

Natural waters contains no free strong acids but only their salts, in particular the sulphates and chlorides of calcium, magnesium and sodium. The SSA expresses the total content of these salts of strong acids.

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Salinity

The overall salinity of water corresponds to the total number of cations and anions as expressed in mg/L.

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Permanganate value:

All substances susceptible to oxidation by potassium permanganate (KMnO4).

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Chemical oxygen demand (COD)

The COD indicates the total hot oxidation by potassium dichromate and covers the majority of organic compounds as well as oxidizable mineral salts.

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Biochemical oxygen demand (BOD)

This is the quantity of oxygen consumed at 20 oC and in darkness during a given period to produce by biological means oxidation of the biodegradable organic matter present in water.

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By convention, BOD5 is used, which is the quantity of quantity of oxygen consumed after five days’ incubation. BOD5 normally represents only the biodegradable carbonaceous organic pollution.

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Total organic carbon (TOC)

This indicate the content of carbon linked to organic material by measuring the CO2 after complete oxidation.

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