Page 93 - Chemistry - XI

Materials Required

250 mL conical fl ask, funnel, beaker, burette, pipette, standard EDTA, (Na H Y) solution, buff er
2
2
solution (pH = 10), and Erichrome Black T (EBT) indicator
Procedure
1. Take 100 mL of water sample in a conical fl ask. If there is any suspended impurity, remove by fi ltration.

2. If water solution is acidic to litmus or pH paper, add 1 M solution of NH until it becomes basic.
3
3. Rinse the burette with standard Na H Y (Sodium ethylene diamine tetra acetate) solution and fi ll the
2
2
burette and note down the initial reading.
4. Rinse the pipette with the given sample of water. Take 20 mL of the given sample of water in a conical
fl ask with the help of a pipette.
5. Add 1 mL of buff er (pH = 10) and 2 drops of EBT indicator. The colour of solution will become

wine red.
6. Titrate the above solution with Na H Y solution. Note the fi nal reading when solution becomes sky
2
2
blue or purple in colour.
7. Repeat the procedure so as to get concordant reading.

Observations

S. No. Initial burette reading Final burette reading Volume of EDTA used

1.
2.
3.

Calculations
Molarity of EDTA solution = 0.01 M (say)

Volume of water taken in each titration = 20 mL
Let the volume of EDTA used = x mL

0.01 × x
Moles of Na H Y (EDTA) used =
2 2 1000
0.01 × x
Moles of EDTA (Na H Y) = Moles of Ca =
2+
2 2 1000
0.01 × x
Mass of calcium carbonate = × 100 = x mg
1000
x
Mass of CaCO per litre = 1000 50 x mg/L
3
20
∴
Hardness = 50 x ppm.

Result

The hardness of water is 50x ppm.

91