pHYSICAL cHEMISTRY - v

Experiment 3

Aim of the Experiment

To estimate the amount of total, permanent and temporary hardness in the collected sample of water by complexometric method.

Principle

Hardness in water is due to the presence of dissolved salts of calcium and magnesium. It is unfit for drinking, bathing, washing and it also forms scales in boilers. Hence it is necessary to estimate the amount of hardness producing substances present in the water sample. Once it is estimated, the amount of chemicals required for the treatment of water can be calculated. The estimation of hardness is based on complexometric titration. Hardness of water is determined by titrating with a standard solution of ethylene diamine tetra acetic acid (EDTA) which is a complexing agent. Since EDTA is insoluble in water, the disodium salt of EDTA is taken for this experiment. EDTA can form four or six coordination bonds with a metal ion.

1. Total hardness

Total hardness is due to the presence of bicarbonates, chlorides and sulphates of calcium and magnesium ions. The total hardness of water is estimated by titrating the water sample against EDTA using Eriochrome Black-T (EBT) indicator. Initially EBT forms a weak EBT-Ca2+/Mg2+ wine red coloured complex with Ca2+/Mg2+ ions present in the hard water. On addition of EDTA solution, Ca2+/Mg2+ ions preferably forms a stable EDTA-Ca2+/Mg2+ complex with EDTA leaving the free EBT indicator in solution which is steel blue in colour in the presence of ammonia buffer (mixture of ammonium chloride and ammonium hydroxide, pH 10)

Eriochrome Black-T + Ca 2+ /Mg 2+ ------> Eriochrome Black-T-Ca 2+ /Mg 2+

(Wine red)

Eriochrome Black-T-Ca 2+ /Mg 2+ + EDTA -------> EDTA-Ca 2+ /Mg 2+ + Eriochrome Black-T

(Wine red) (Steel blue)

2. Temporary hardness

Temporary hardness is due to the presence of bicarbonates of calcium and magnesium ions. It can be easily removed by boiling.

When water is boiled, temporary hardness producing substances (bicarbonates) are precipitated as insoluble carbonates or hydroxides. This precipitate can be removed by filtration. (The filtrate is used in the next step)

Eriochrome Black-T + Ca 2+ /Mg 2+ ------> Eriochrome Black-T-Ca 2+ /Mg 2+

(Wine red)

Eriochrome Black-T-Ca 2+ /Mg 2+ + EDTA -------> EDTA-Ca 2+ /Mg 2+ + Eriochrome Black-T

(Wine red) (Steel blue)

3. Permanent hardness

Permanent hardness is due to the presence of chlorides and sulphates of calcium and magnesium ions. This type of hardness cannot be removed by boiling. The filtrate obtained from the above step contains permanent hardness producing substances and is estimated against EDTA using EBT indicator.

Procedure:

The burette is filled with standard EDTA solution to the zero level, following usual precautions.

1. Estimation of Total Hardness

20 ml of the given water sample is pipetted out into a clean conical flask. 5 ml ammonia buffer and 2 drops of EBT indicator are added and titrated against EDTA from the burette. The end point is the change of colour from wine red to steel blue. The titration is repeated to get concordant titre value.

2. Estimation of Permanent Hardness

100 ml of the given sample of water is pipetted out into a clean beaker and boiled for 20-30 minutes. It is then filtered to remove the precipitate formed due to the decomposition of temporary hardness producing salts. The filtrate is made up to 100 ml in standard measuring flask (SMF) using distilled water.

20 ml of the made up solution is pipetted out into a conical flask, 5 ml ammonia buffer and 2 drops of EBT indicator are added and titrated against the EDTA. The end point is the change of colour from wine red to steel blue. The titration is repeated to get concordant titre value .

3. Temporary Hardness

The temporary hardness is calculated from the total and permanent hardness.

Temporary Hardness = Total Hardness - Permanent Hardness

Materials required

0.01 N Calcium Chloride (CaCl₂) solution, 0.01 M Ethylene Diamine Tetra Acetic Acid (EDTA) Solution, Buffer Solution of pH 10.0, Eriochrome Black – T (EBT) Indicator Solution, sample water

Procedure

Part I: Standardization of EDTA solution

Take 20 mL of 0.01 M CaCl₂ solution in a conical flask and add 5 mL of 10 pH buffer solution. Then add 2 – 3 drops / a pinch of indicator. Titrate carefully against EDTA solution from the burette up to the end point where the color changes from wine – red to pure blue. The titration should be carried out slowly near the end point with constant stirring till no ting of red colour remains in the solution. Repeat the titration with four other aliquots of the Ca²⁺ solution. Calculate the molarity of the EDTA solution.

Part II:

(1)Estimation of hardness of water

Rinse a conical flask with distilled water and take 20 mL of given water sample in it. To this sample, add 5 mL of 10 pH buffer solution and 2 – 3 drops / a pinch of EBT indicator. Titrate carefully against EDTA solution from burette up to the end point where the color changes from wine – red to pure blue. The titration should be carried out slowly near the end point with constant stirring till no ting of red colour remains in the solution. Repeat the titration with four other aliquots of water sample. Calculate the total hardness of the water as ppm (Parts per Million) of CaCO₃.

(2)Estimation of Permanent Hardness

100 ml of the given sample of water is pipetted out into a clean beaker and boiled for 20 to 30 minutes. It is then filtered to remove the precipitate formed due to the decomposition of temporary hardness producing salts. The filtrate is made up to 100 ml in standard measuring flask (SMF) using distilled water. 20 ml of the made up solution is pipetted out into a conical flask, 5 ml ammonia buffer and 2 drops of EBT indicator are added and titrated against the EDTA. The end point is the change of colour from wine red to steel blue. The titration is repeated to get concordant titre value.

Observation Table

Part – I

Burette : EDTA solution

Flask : 20mL of CaCl₂ + 5 mL of Buffer Solution.

Indicators : 2 – 3 drops/a pinch of Eriochrome Black –T

End point : Wine Red to steel Blue

Part – II (Titration-1) :Estimation of total hardness

Burette : EDTA solution.

Flask : 20 mL water sample + 5 mL of Buffer Solution.

Indicators : 2 – 3 drops /a pinch of Eriochrome Black –T

End point : Wine Red to steel Blue.

Part – II (Titration-2) : Estimation of permanent hardness

Burette : EDTA solution.

Flask : 20 mL of boiled water sample + 5 mL of Buffer Solution.

Indicators : 2 – 3 drops /a pinch of Eriochrome Black –T

End point : Wine Red to steel Blue.

Calculations:

1 ml of 0.01 M EDTA ≡ 1 mg of CaCO3

V1 ml of EDTA ≡ V1 mg of CaCO3

Calculation of Part – II (Titration-1) :Estimation of total hardness

Volume of EDTA solution consumed = ………… ml

Volume of hard water taken =…………. ml

Total hardness = Volume of EDTA solution consumed X1000 ppm

Volume of the hard water taken

= ………………….. ppm

Calculation of Part – II (Titration-2) : Estimation of permanent hardness

Volume of EDTA solution consumed = ………… ml

Volume of boiled water taken =…………. ml

Total hardness = Volume of EDTA solution consumed X1000 ppm

Volume of the boiled water taken

= ………………….. ppm

Calculation of temporary hardness

Temporary Hardness of the given sample water = Total Hardness - Permanent Hardness

= ………………….. ppm

Result

The collected water sample contains

Total hardness = ppm

Permanent hardness = ppm

Temporary hardness = ppm

Questions

1. Which causes hardness to water?

2. How is temporary hardness removed?

3. Why is disodium salt of EDTA preferred to EDTA for estimation of hardness?

4. Mention the disadvantages of hard water for industrial purpose.

Reference

Practical Physical Chemistry by J.B. Yadav