To remove chlorine interference in an acidity measurement, which reagent is used?

To remove chlorine interference in an acidity measurement, which reagent is used?

• A. Sodium thiosulfate (Na2S2O3)
• B. Sodium hypochlorite
• C. Hydrochloric acid
• D. Potassium permanganate

Answer: (A)

Chlorine species in a sample can act as oxidizers and interfere with acidity measurements by reacting with reagents or indicators, giving falsely altered results. The way to prevent this is to chemically neutralize the chlorine before measuring acidity. Sodium thiosulfate serves as a reducing agent that reacts with chlorine and hypochlorite to convert them into non-oxidizing forms, typically producing chloride ions. A representative reaction is Cl2 reacting with thiosulfate to yield chloride and dithionite-related species. This “dechlorination” step removes the interfering oxidants, allowing the acidity to be measured accurately. The other options don’t address the interference: sodium hypochlorite adds chlorine species rather than removing them, hydrochloric acid changes the acidity without removing oxidants, and potassium permanganate is an oxidizer that would introduce more interference rather than eliminate it.

Chlorine species in a sample can act as oxidizers and interfere with acidity measurements by reacting with reagents or indicators, giving falsely altered results. The way to prevent this is to chemically neutralize the chlorine before measuring acidity. Sodium thiosulfate serves as a reducing agent that reacts with chlorine and hypochlorite to convert them into non-oxidizing forms, typically producing chloride ions. A representative reaction is Cl2 reacting with thiosulfate to yield chloride and dithionite-related species. This “dechlorination” step removes the interfering oxidants, allowing the acidity to be measured accurately.

The other options don’t address the interference: sodium hypochlorite adds chlorine species rather than removing them, hydrochloric acid changes the acidity without removing oxidants, and potassium permanganate is an oxidizer that would introduce more interference rather than eliminate it.