What chemical is added to a sample bottle to remove residual chlorine?

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Multiple Choice

What chemical is added to a sample bottle to remove residual chlorine?

Explanation:
Residual chlorine must be quenched because it can react with analytes and skew results. Sodium thiosulfate is a reducing agent that neutralizes chlorine species such as Cl2, HOCl, and OCl−, converting them to chloride and water while the thiosulfate is oxidized further to sulfate. A typical dechlorination reaction is Na2S2O3 + 4 HOCl → Na2SO4 + 2 Cl− + 2 H2O. This effectively removes oxidizing chlorine from the sample. Other options wouldn’t achieve this dechlorination: sodium chloride is just a salt and doesn’t remove chlorine; sodium hypochlorite introduces chlorine; and sodium sulfate is a salt with no dechlorinating capability.

Residual chlorine must be quenched because it can react with analytes and skew results. Sodium thiosulfate is a reducing agent that neutralizes chlorine species such as Cl2, HOCl, and OCl−, converting them to chloride and water while the thiosulfate is oxidized further to sulfate. A typical dechlorination reaction is Na2S2O3 + 4 HOCl → Na2SO4 + 2 Cl− + 2 H2O. This effectively removes oxidizing chlorine from the sample.

Other options wouldn’t achieve this dechlorination: sodium chloride is just a salt and doesn’t remove chlorine; sodium hypochlorite introduces chlorine; and sodium sulfate is a salt with no dechlorinating capability.

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