Which chemical neutralizes residual chlorine in a water sample?

• A. Sodium Hypochlorite
• B. Sodium Thiosulfate
• C. Sodium Chloride
• D. Sodium Sulfate

Answer: (B)

Neutralizing residual chlorine is about stopping the oxidizing chlorine species from reacting further with the test reagents. Sodium thiosulfate acts as a reducing agent that quickly converts chlorine species into harmless forms, typically producing chloride ions and sulfur-containing products. This “quenching” prevents chlorine from continuing to react during measurement, which is essential for accurate results in chlorine tests like the DPD method.

The other chemicals don’t serve this purpose. Sodium hypochlorite adds chlorine rather than removes it, which would skew results. Sodium chloride and sodium sulfate are inert salts in this context and do not neutralize residual chlorine.