A health inspector collects a water sample from a public tap for bacteriological examination. A pinch of sodium thiosulphate is added to the sample bottle before collection. The purpose of this is to:

• A Prevent sedimentation of particulate matter
• B Preserve the coliform bacteria for 48 hours
• C Lower the pH to favour pathogen survival
• D Neutralise any residual chlorine so it does not suppress bacterial growth during transport ✓

Explanation

Residual chlorine in the sample would continue its bactericidal action inside the bottle and give falsely low bacterial counts. Sodium thiosulphate neutralises residual chlorine immediately on contact, freezing the bacteriological status at the time of sampling. It does not preserve organisms or alter pH, so options B and C are incorrect mechanisms attributed to an unrelated reagent.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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