ANALYTICAL cHEMISTRY - V

MSCM303

Experiment 5

Aim of the Experiment

To determine the total hardness of the tap water.

Principle

When water through or over deposits such as time stone level of Ca²⁺, Mg²⁺and HCO₃⁻ ions in water can greatly increases and make water hard and water of this type called as hard water. Ca²⁺and Mg²⁺ ions combine with soap molecules and reduce its foaming properties.
Here, in experiment, EDTA forms colorless stable complexes with Ca²⁺ and Mg²⁺ ions present in water at pH = 9 - 10. To maintain the pH of the solution 9 - 10, buffer solution (NH₄OH and NH₄Cl) is used, EBT is used as an indicator.
The sample of hard water must be treated with buffer solution, and EBT indicator which forms, unstable wine-red complex with Ca²⁺ and Mg²⁺ present in water.

Apparatus required

Filter paper, Beakers, Pipette, etc.

Chemicals required

Buffer solution (pH 10), Tap water, 0.01 M EDTA solution, Eriochrome Black T indicator, Patton and Reeder’s indicator, 8 M KOH solution, etc.

Preparation of reagents

1. EDTA solution

1000 ml 1 M EDTA = 372.24 g

250 ml 0.01 M EDTA = X g

X = _______ gm.

2. Buffer (pH 10) solution

Dissolve 17.5 g NH₄Cl and 142 ml liq. NH₃ and and dilute with water up to the mark of 250 ml volumetric flask.

3. 8 M KOH

1000 ml 1 M KOH = 56.10 g

50 ml 8 M KOH = X g

X = _______ gm.

Procedure

1. Total Hardness (Ca²⁺and Mg²⁺)

Take 50 ml tap water sample in flask and add 4 ml buffer pH 10 and titrate it against 0.01 M EDTA solution. Color change is wine red to blue using EBT as an indicator.

2. Permanent Hardness:

Take 100 ml water and boil for 20 min and then filter it and make filtrate up to 100 ml. Pipette out 50 ml of the solution and add 4 ml buffer and titrate it against 0.01 M EDTA using EBT indicator. Color Change is wine red to blue.