Which field test is recommended for measuring FREE residual chlorine specifically in drinking water, giving an immediate colour reaction?
- A Orthotolidine test
- B Membrane filtration technique
- C DPD (N,N-diethyl-p-phenylenediamine) test ✓
- D Presumptive coliform count
Correct answer: C. DPD (N,N-diethyl-p-phenylenediamine) test
Explanation
The DPD test reacts immediately with free residual chlorine to give a pink colour, read against a comparator disc, and is the current WHO-recommended field method. The older orthotolidine test produces a yellow colour with both free and combined chlorine, so it cannot distinguish the two without a delayed reading at 5 minutes, and it is now discouraged because orthotolidine is a suspected carcinogen. Membrane filtration and coliform counts measure bacteriological quality, not chlorine.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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