What is used to remove residual chlorine from a sample?

• A. Sodium thiosulfate
• B. Sodium sulfite
• C. Hydrogen peroxide
• D. Sodium bisulfite

Answer: (A)

When a water sample contains residual chlorine, you need a dechlorinating agent to stop chlorine from reacting further before analysis. Sodium thiosulfate is the standard choice because it quickly neutralizes free chlorine (hypochlorite or chlorine species) and converts it to chloride, effectively quenching the oxidizing agent without significantly affecting other properties of the sample. The reaction neutralizes chlorine in a way that keeps the sample stable for subsequent measurements, which is why this reagent is preferred in routine testing. Other options can dechlorinate, but they’re not as reliable or convenient for standard analyses—sodium sulfite and bisulfite can sometimes interfere with certain tests, and hydrogen peroxide, while capable of oxidizing chlorine, can alter the sample chemistry and testing results, so it’s not the typical dechlorant used in this context.