A public health engineer tests treated piped water using the orthotolidine (OT) test. The sample turns yellow immediately on adding the reagent, before any waiting period. This indicates the presence of:
- A Combined available chlorine only
- B Both free and combined residual chlorine
- C Free residual chlorine only ✓
- D Absence of any residual chlorine
Correct answer: C. Free residual chlorine only
Explanation
In the orthotolidine test, free residual chlorine reacts instantly with orthotolidine to give yellow colour, while combined (chloramine) chlorine develops colour slowly over several minutes. An immediate flash of yellow therefore indicates free residual chlorine. Reading the colour after about 5 minutes gives total chlorine, and subtracting gives combined chlorine. The DPD method has now largely replaced OT because orthotolidine is carcinogenic, but the timing distinction remains examinable.
Reference: Park's Textbook of Preventive and Social Medicine, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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