1. PURPOSE:

The purpose of this experiment is to determine hardness of water and to familiarize the students with complex formation.

REAGENTS:

1. Buffer solution containing Mg – EDTA:

Dissolve 16.9 g NH4Cl in 143 mL concentrated NH4OH. Weigh out 0.25 g MgCl2.6H2O, transfer it to 100 mL volumetric flask, and make up to 100 mL with distilled water. Pipet 50 mL of this solution into a porcelain casserole; add a few drops of NH4OH solution (sufficient to achieve a pH of 10); and add a few Erichrome Black T indicator. Add EDTA solution slowly, with stirring until the color of solution changes from red to blue. To the 50 mL of the solution remaining in the volumetric flask add exactly the same amount of EDTA required in the above titration. Add this solution to the NH4Cl-NH4OH mixture and dilute to 250 mL with distilled water. Keep the solution in a plastic bottle and tightly stopper to prevent loss of NH3 or pick up CO2.

2. Erichrome Black T (EBT) indicator: The following two formulations can be given.

a) Liquid form: Mix 0.5 g EBT with 4.5 g hydroxylamine hydrochloride. Dissolve this mixture in 100 mL 95 % ethyl alcohol.

b) Powdered form: Mix 0.5 g EBT with 100 g NaCl to form a powder mixture.

3. Murexide indicator: Mix 200 mg Murexide with 100 g NaCl and grind the mixture to get homogeneous powder.

4. Standard EDTA solution, (1 mL = 1mg hardness as CaCO3.)

Dissolve 3.722 g EDTA in 1 liter distilled water and standardize as given below. This solution should be stored in plastic containers since EDTA might extract hardness producing cations from soft glass containers.

a) Preparation of standard calcium solution: Dry 3 grams of calcium carbonate powder at 105oC overnight. Take 1 g of it into 500 mL erlenmeyer flask. Put 1+1 HCl solution dropwise until all the CaCO3 has dissolved. Add 200 mL distilled water and boil for 5 min.to expel CO2. Cool and add 2-3 drops of methyl red indicator and adjust the color of the solution to orange by adding either NH4OH or HCl. Transfer the solution to 1 liter volumetric flask and dilute to the mark with distilled water. 1 mL of this standard solution is equivalent 1 mg CaCO3.

b) Standardization of EDTA solution with above standard calcium solution: Make the standardization just as given in procedure part.

2. PROCEDURE:

Total Hardness:

1. We put 50 mL of sample into an Erlenmeyer flask.

2. We add 2 mL buffer (NH4CI-NH4OH) solution.

3. The pH of the solution should be 10+ (or -) 0.1 at this point.

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4. We add some EBT indicator until you observe the deep purple color.

5. We titrate with EDTA until a blue color is reached.

Calcium Hardness:

1. We put 50 mL of the sample into an Erlenmeyer flask

2. We add 2 mL 1 N NaOH solution.

3. The pH of the solution should be 12+ (or -) 0.1 at this point.

4. We add some Murexide indicator until you observe the pink color.

5. We titrate with EDTA until a purple color is reached

We followed the above procedure for tap water and bottled water samples.

3. THEORY:

Water hardness is a traditional measure of the capacity of water to react with soap. Hard water requires a considerable amount of soap to produce a lather, and it also leads to scaling of hot water pipes, boilers and other household appliances. Water hardness is caused by dissolved polyvalent metallic ions. In fresh waters, theprincipal hardness-causing ions are calcium and magnesium; strontium, iron, barium and manganese ions also contribute.

Cations causing hardness AnionsCa2+ HCO3

-

Mg2+ SO42-

Sr2+ Cl-

Fe2+ NO3-

Mn2+ SiO32-

(Sawyer et al., 2003, p.565)

Hardness can be measured by the reaction of polyvalent metallic ions in a water sample with a chelating agent such as ethylenediaminetetraaceticacid (EDTA) and is commonly expressed as an equivalent concentration of calcium carbonate.

Hardness can also be estimated by determining the concentrations of the individual components of hardness and expressing their sum in terms of an equivalent quantity ofcalcium carbonate. The degree of hardness of drinking water may be classified in terms of its calcium carbonate concentration as follows:

(URL 1, 2007)

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Table 1

Hardness rating

Concentration of Calcium Carbonate (mg/L)

Concentration of Calcium Carbonate (grains/US gallon)

Soft 0 to <75 0 to <5.2

Medium hard 75 to <150 5.2 to <10.5

Hard 150 to <300 10.5 to <21

Very hard 300 and greater 21 and greater

Information for the table above http://www.gov.calgary.ab.ca/cww/water_quality/water_hardness/from

Types of Hardness:Carbonate and Noncarbonate Hardness:The part of the total hardness that is chemically equivalent to bicarbonate plus carbonate alkalinities present in water is considered to be carbonate hardness. Since alkalinity and hardness are both expressed in terms of CaCO3, the carbonate hardness can be found as follows:

When alkalinity < Total hardness,Carbonate Hardness in mg/L = alkalinity in mg/L

When alkalinity >=Total hardness,

Carbonate Hardness in mg/L = Total hardness in mg/L

Carbonate hardness is singled out for special recognition because the bicarbonate andcarbonate ions with which it is associated tend to precipitate this portion of the hardness at elevated temperatures such as occur in boilers or during the softening process with lime.It may also be considered as that part of the total hardness that originates from the action of carbonic acid on limestone. Carbonate hardness was formerly called temporary hardness because it can be caused to precipitate by prolonged boiling.

The amount of hardness that is in excess of the carbonate hardness is called noncarbonate hardness and can be estimated as follows:

Noncarbonate hardness = Total hardness – Carbonate hardness

Noncarbonate hardness was formerly called permanent hardness because it cannot be removed or precipitated by boiling. Noncarbonate hardness cations are associated with sulfate, chloride, and nitrate anions.(Sawyer et al., 2003, pp.568-569)

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In this experiment hardness is measured by the use of chealating principle.This method is based on a titration of the water sample with a solution of acomplexing agent, named ethylenediaminetetraacetate (EDTA). The common form of the agent is disodium salt Na2H2EDTA.

It is colorless. It can be weighed and dissolve in water to form a stable solution. At high pH (> 10) the remaining protons leave EDTA forming EDTA4- anion:

This ion readily reacts with Mg2+ (and Ca2+) forming colorless complex:

Mg2+ + EDTA4- MgEDTA2-

Ca2+ + EDTA4-CaEDTA2-

All reactants are colorless as well. Therefore, to detect the end point of this reaction we need an indicator. For this purpose we will use an organic dye named Eriochrome Black T . This compound forms purple complexes with Mg2+ ions and with Ca2+ ions. Aqueous solutions of the indicator at pH 10 are blue. Thus when purple complex MgIn- reacts with EDTA, it results in the blue solution of indicator:

HIn-2 + Mg2+ H+ + MgIn- (blue) (purple)

The reaction occurs because the EDTA forms much more stable complex than the indicator. A similar equation can be written for calcium. (URL 2, 2004)

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Public Health Significance:Hard waters are as satisfactory for human consumption as soft waters. There is some evidence that Ca2+ and Mg2+ are protective against heart disease. Because of their adverse action with soap, however, their use for cleansing purposes is quite unsatisfactory, unless soap costs are disregarded. (Sawyer et al., 2003, p.566)

4.DATA ANALYSIS AND CALCULATIONS:

For tap water: For bottled water:Total Hardness: 16 mL Total Hardness: 4.6 mLCalcium Hardness: 11.5 mL Calcium Hardness: 3.2 mL

1.Calculate the hardness values of your samples as follows:

Total Hardness,TH:

1 mL EDTA solution = 1 mL hardness as CaCO3

TH as mg/L CaCO3 = mL EDTA used*1000/mL sample

Calcium Hardness, CH:

CH as mg/L CaCO3 = A*B*1000/mL sample

mg/L Ca ion = A*B*400.8/mL sample

Where, A= mL EDTA used in titration for sample

B = mg CaCO3 equivalent to 1 mL EDTA titrant (1mg CaCO3 = 1 mL EDTA)

For tap water:

TH= 16*1000/50 = 320 mg/L as CaCO3

CH= 11.5*1*1000/50 = 230 mg/L as CaCO3

Ca2+ = 11.5*1*400.8/50 = 92.2 mg/L

For bottled water:

TH= 4.6*1000/50 = 92 mg/L as CaCO3

CH= 3.2*1*1000/50 = 64 mg/L as CaCO3

Ca2+ = 3.2*1*400.8/50 = 25.6 mg/L

2. Express your results in German and French hardness.

1 Degree German Hardness is equivalent to 10 mg/L CaO or 17.85 mg/L CaCO3

1 Degree French Hardness is equivalent to 10 mg/L CaCO3

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For tap water:

TH= 320 mg/L as CaCO3

CH= 230 mg/L as CaCO3

THGerman Hardness=320/17.85 =17.92 German degrees French Hardness =320/10 = 32 French degreesCH German Hardness=230/17.85 =12.88 German degrees French Hardness =230/10 = 23 French degrees

For bottled water:

TH= 92 mg/L as CaCO3

CH= 64 mg/L as CaCO3

THGerman Hardness=92/17.85 =5.15 German degrees French Hardness =92/10 = 9.2 French degreesCH German Hardness=64/17.85 =3.58 German degrees French Hardness =64/10 = 6.4 French degrees

3. Tabulate all of your findings along with data from the bottled water labels.Tap water Bottled water Bottled

water(label)TH mg/L as CaCO3 320 92 90CH mg/L as CaCO3

230 64 45

Ca2+ mg/L 92.2 25.6 17.5

5. DISCUSSION AND CONCLUSIONS:1. Total Hardness = Calcium Hardness + Magnesium Hardness (mg/L as CaCO3) (mg/L as CaCO3) (mg/L as CaCO3) OR Total hardness = Carbonate hardness + Non-carbonate hardness (mg/L as CaCO3) (mg/L as CaCO3) (mg/L as CaCO3)

For tap water:

TH= 320 mg/L as CaCO3 TH > CH

CH= 230 mg/L as CaCO3

For bottled water:

TH= 92 mg/L as CaCO3 TH >CH

CH= 64 mg/L as CaCO3

Therefore, these results are consistent.2. For tap water: For bottled water:

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TH= 320 mg/L as CaCO3 TH= 92 mg/L as CaCO3

CH= 230 mg/L as CaCO3 CH= 64 mg/L as CaCO3

Tap water Bottled waterTH Very hard Moderately hardCH hard soft

3. The successful use of EDTA for determining hardness depends upon having an indicator present to show when EDTA is present in excess, or when all the ions causing hardness have been complexed. EBT or Calmagite serve as excellent indicators to show when all the hardness ions have been complexed. When a small amount of either, which alone have a blue color, is added to a hard water with a pH of about 10 it combines with a few of the Ca2+ and Mg2+ ions to form a weak complex ion that is wine red color. (Sawyer et al., 2003, pp.567-568)

Therefore, we make the titration at pH 10 because EDTA and EBT work at this pH.

4. Environmental significance:

Scales are formed as inner coating of the pipelines prevents corrosion. Absolutely soft waters are corrosive and dissolve the metals. More cases of cardio vascular diseases are reported in soft water areas. Hard water is useful to growth of children due to the presence of calcium. Hard waters cause excessive consumption of soap used for cleaning purpose.

Sodium soaps react with multivalent metallic cations to form a precipitate, thereby lose their surfactant properties. Lathering does not take place until all hardness ions precipitate out.

This precipitates adheres to surfaces of tubes, sinks, dish water and may stain clothing.

Scales formed mainly due to carbonate hardness act as insulations and cause enormous loss of fuel in boiler.

Scales deposited mainly due to increase in pH to 9 at which bicarbonates are converted as carbonates are formed in distribution mains reducing their carrying capacity.

(Experiment on Determination of Total Hardness,

http://nitttrc.ac.in/Four%20quadrant/eel/Quadrant%20-%201/exp5_pdf.pdf)

5. Chelating agents:

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DTPA ACID EDTA ACID NTA ACID

FORMULA: C14H23N3O10 FORMULA: C10H16O8N FORMULA: C6H9NO6

(Chelating Agents,

www.chemicalland21.com/specialtychem/perchem/CHELATING%20AGENTS.htm)

porphine heme

(Chelates and Chelating Agents,

http://scifun.chem.wisc.edu/chemweek/chelates/chelates.html)

6. For bottled water: For bottled water (label):

TH= 92 mg/L as CaCO3 TH= 90 mg/L as CaCO3

CH= 64 mg/L as CaCO3 CH= 45 mg/L as CaCO3

Total hardness values are really close to each other. However, carbonate hardness values are really different.

Error =64-45/64*100 = 29% error

This error maybe originated from followings:

pH value didn’t increase at 12-13. Hence, Mg2+ didn’t precipitate exactly.

Murexide indicator did not work very well.

6. REFERENCES

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1. URL, 2007. Environmental and Work Place Health, web page: http://www.hc-sc.gc.ca

2. URL 2004. Determination of Water Hardness by complexomatic titration, web page: http://homepages.ius.edu.tr

3. Sawyer ,C.N., McCarty, P.L., Parkin , G.F., Chemistry for Environmental Engineering and Science 5th edition, The McGraw Hill Companies, New York, 2003 pp. 563-570

4. Experiment on Determination of Total Hardness, retrieved 19 November 2012 from,

http://nitttrc.ac.in/Four%20quadrant/eel/Quadrant%20-%201/exp5_pdf.pdf

5. Chelating Agents, retrieved 19 November 2012 from,

http://www.chemicalland21.com/specialtychem/perchem/CHELATING%20AGENTS.htm

6. Chelates and Chelating Agents, retrieved 19 November 2012 from,

http://scifun.chem.wisc.edu/chemweek/chelates/chelates.html

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