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Hardness of Water Experiment

Investigatory Project and Viva Questions on Hardness of Water Experiment

Principle


Hardness of water is due to the existence of bicarbonates, chlorides and sulphates of calcium and magnesium. Temporary hardness of water is due to bicarbonates and it can be got rid off by boiling water. The bicarbonates will decompose giving insoluble carbonates which can be removed by decantation or filtration.


Ca(HCO3)2 --> CaCO3 + H2O + CO2


Hardness of water can be easily estimated by direct titration with ethylene diamine tetra acetic acid (EDTA) using the metal ion indicator eriochrome black – T


(HOOC.CH2)2.N.CH2.CH2N.(CH2.COOH)2   EDTA


EDTA is an excellent hexadentate ligand which can form very stable complexes with metallic ions by closing five rings with the metallic ion. The complex ion is formed instantaneously and so the end point of the reaction can be detected with suitable metal ion indicator. Since EDTA itself is insoluble in water, its disodium salt is usually used for titration. Its reaction with the metallic ion like Ca2+ ang Mg2+ can be represented as follows.


Na2H2Y + M2+ --> MY2-+2Na++2H+

Where, Y = (ŌOC.CH2)2.N.CH2.CH2.N.(CH2.COŌ)2


The best metal ion indicator is eriochrome black – T which gives a wine-red colour with the metallic ion. The titration is carried out at a pH = 10 and at this pH the free indicator has a deep blue colour. So the end-point of the titration is the colour change from blue to red to blue.


Reagents


1. 0.01M EDTA solution. Dissolve 3.723g of A.R. disodium ethylene diamine tetraacetate in distilled or deionised water and make it up in a 1 litre standard flask.

2. Eriochrome Black — T. Dissolve 0.2g of the substance in 15mL of triethanolamine and 5mL of absolute alcohol.

3. Buffer solution (pH = 10). Add 142mL of concentrated ammonia solution (sp.gr. 0.88 to 0.9) to 17.5g of A.R. NH4CI and dilute to 250mL.

4. Standard ZnSO4.7H20 solution. A standard 0.01M ZnSO4.7H20 solution is prepared by dissolving 2.87g of A.R. ZnSO4.7H20 in 1 litre of distilled or deionised water. This is used to standardise EDTA solution.


Objective of the Experiment: 


The objective of the Experiment is to determine the hardness of water from different sources


Procedure


Collect water from different sources like tap—water, wells in different localities, from different rivers, lakes etc. 50mL portion of water from each sample is pipetted out into a clean beaker. Add 1 mL of the buffer solution and 3-4 drops of Eriochrome Black T indicator. Titrate with 0.01 M EDTA solution taken in a burette until the colour of the solution in the beaker changes form wine-red to pure blue with no reddish—tinge. Repeat the experiment with other samples of water. Total hardness can be calculated from these values.


Temporary and Permanent Hardness


Pipette out 250mL of a sample of water into a clean beaker. Boil it for about 30 minutes, cool it and filter it into a 250mL flask. After filtration, make it upto 250mL. Take 50mL from it and titrate with EDTA solution as before. From this permanent hardness can be calculated.


Calculation


Usually hardness is expressed in gm of CaCO3 in million parts of water. ie, CaCO3 in ppm. 1mL of 0.01 M EDTA = 0.001g of CaCO3. Suppose a 50mL sample of water requires Vcm3 of 0.01 M EDTA. Then the hardness of water is given by


(V x .001 x 106/50) ppm of CaCO3 or (V x 103/50) ppm of CaCO3


The values obtained from different sources of water are tabulated as follows to compare the permanent and temporary hardness in these samples.

Sample of water

Volume of water taken (mL)

Volume of 0.01M EDTA required (VmL)

Hardness of water in CaCO3 ppm

Vx103/50

Volume of EDTA required after boiling (V1mL)

Permanent hardness in CaCO3 ppm V1x103/50

Temporary hardness in CaCO3 ppm (V-V1) x 103 / 50

1

2

3

4

5

6

50

50

50

50

50

50

 

 

 

 

 

 

HARDNESS OF WATER EXPERIMENT VIVA QUESTIONS WITH ANSWERS


1. What is hard water?


Ans: Water that does not lather easily with soap is called hard water.


2. Which are the substances that make water hard?


Ans: Bicarbonates, chlorides and sulphates of calcuim and magnesium make water hard.


3. How is hardness of water classified?


Ans: Hardness of water is classified into temporary hardness and permanent hardness.


4. What is temporary hardness due to?


Ans: It is due to the presence of bicarbonates of Ca and Mg. These, on boiling decompose and precipitate as their carbonates.


5. How is hardness of water estimated?


Ans: Hardness of water is estimated by E.D.T.A. titration.


6. What is E.D.TA?


Ans: The chemical name of E.D.T.A. is ethylene diamine tetraacetic acid


7. What is co-ordination number of E.D.T.A.?


Ans: Co-ordination number of E.D.T.A. is six.


8. Give the name of the metal indicator used in the E.D.T.A titration of water?


Ans: The metal indicator used is eriochrome black —T


9. What is the colour change expected at the equivalence point in the E.D.TA. titration using eriochrome black — T?


Ans: The colour change is from red to blue.


10. What is the red colour due to?


Ans: It is due to the complex formed between the metal and eriochrome black—T


11. What is the blue colour due to?


Ans: It is due to free E.D.T.A. at pH =10


12. What is the buffer used to get pH =10


Ans: NH4OH/NH4Cl mixture is used to get pH= 10


13. How is hardness of water usually expressed?


Ans: It is expressed in parts/weight of CaCO3 in million parts of water ie, in ppm.

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