Water Quality Parameters MCQ with answer and solution

Concept

Total hardness is found by titrating with EDTA, using Eriochrome Black T as the indicator.

Formula

Hardness (mg/L as CaCO₃) = (V_EDTA × 1000) / V_sample (for 0.01 M EDTA)

Step-by-step solution

1. EDTA forms complexes with Ca²⁺ and Mg²⁺ ions. 2. Use Eriochrome Black T indicator at pH 10. 3. The colour changes from wine red to blue at the end point. 4. This measures hardness, so the answer is (C).

Explanation

EDTA binds the calcium and magnesium ions that cause hardness. The end point shows a colour change from wine red to blue. So EDTA titration is the standard test for hardness.

Why the other options are wrong

A: Chlorides are tested by Mohr's method with silver nitrate. B: Dissolved oxygen is tested by Winkler's method with sodium thiosulphate. D: Residual chlorine is tested by the orthotolidine or DPD method.

Shortcut

EDTA = hardness, AgNO₃ = chloride, Winkler = DO, orthotolidine = chlorine.

Common mistake

Mixing up the titrants for chloride and hardness.