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ESTIMATION OF TOTAL HARDNESS OF WATER BY EDTA METHOD

AIM

To estimate the total hardness present in the given water sample using approximately 0.01 M ethylenediamine tetraacetic acid (EDTA) solution.

PRINCIPLE

The estimation of hardness is based on complexometric titration. These titrations are particularly useful to determination of a mixture of different metal ions in solution. Calcium or magnesium ions present in the water sample in the presence of ammonical buffer solution form an unstable wine red colour complex with Eriochrome Black T (EBT) indicator. When the EDTA solution is added, it reacts with the calcium or magnesium ions present in water to form a stable deep blue colour (M–EDTA) complex and releases the free indicator.

EDTA metal ion complex is stable at pH = 10.

Part A: Preparation of Standard Magnesium Sulphate Solution

Take approximately 0.25 g of magnesium sulphate crystals in a clean and dry weighing bottle. Weigh the bottle along with the crystals accurately up to the fourth decimal place using the rider in a simple balance. Then, transfer the chemical substance into a clean 100 ml standard flask with the help of a glass funnel. After the transfer, weigh the empty bottle accurately. The difference between these two values gives the weight of magnesium sulphate. Dissolve the magnesium sulphate crystals in a minimum quantity of distilled water and make it up to the mark. Shake the standard flask well to obtain uniform concentration and keep aside the solution for further use.

Weight of magnesium sulphate + weighing bottle (W₁) = _____ g

Weight of empty weighing bottle (W₂) = _____ g

Weight of magnesium sulphate (W) = (W₁ − W₂) = _____ g

Gram molecular weight (GMW) of MgSO₄ · 7H₂O = 246.47

Part B: Standardisation of EDTA Solution

Clean a burette with tap water and then rinse it with distilled water as well as the EDTA solution. Then, fill it with the EDTA solution and note down the initial reading. Pipette out 20 ml of the standard magnesium sulphate solution into a clean conical flask. Add 20 ml of distilled water, 5 ml of ammonical buffer solution and two to three drops of the EBT indicator. The solution turns to wine red colour. Titrate this solution mixture against the EDTA solution taken in the burette until the colour changes from wine red to blue. Note down the volume of EDTA run down. Repeat the titration until two consecutive concordant values are obtained. Note down the values in a tabular column. Calculate the strength of EDTA based on the titre values.

Part C: Estimation of Total Hardness of Given Water Sample

Pipette out 40 ml of hard water into a clean conical flask and add 20 ml of ammonical buffer solution to maintain the pH as 10. Then, add two to three drops of the EBT indicator and titrate the solution mixture against the EDTA solution taken in the burette. The end point of titration is the change of colour from wine red to deep blue. Repeat the titration for two consecutive concordant values. Note down the values in a tabular column. Calculate the molarity of the given hard water and its total hardness from the titre values.

RESULT

Molarity of the magnesium sulphate solution (M₁) = _____ M

Molarity of the EDTA solution (M₂) = _____ M

Molarity of the given water sample (M₃) = _____ M

Total hardness of the given water sample (W) = _____ g

VIVA QUESTIONS

1. What is hardness?
2. Define temporary hardness.
3. In this experiment, which solution maintains the pH?
4. What is the importance of pH balance in this experiment?
5. What are the units of hardness?
6. What does the wine red colour indicate?
7. What is the relation between temporary and permanent hardness?
8. How can you remove temporary hardness?
9. What is buffer?
10. What is the colour of the EBT indicator?