Sodium thiosulfate is used to

• A. buffer chlorine solutions
• B. neutralize chlorine residuals
• C. detect chlorine leaks
• D. sterilize sample bottles

Answer: (B)

Sodium thiosulfate is a dechlorinating agent: it reacts with chlorine residuals to neutralize them so disinfection stops in the sample or test setup. In water testing and sampling, you don’t want the chlorine that’s already present to keep reacting and alter the measurement or affect the sample’s quality. By neutralizing chlorine quickly, thiosulfate allows accurate analysis or safe handling of the sample. It’s not used to buffer chlorine solutions, detect leaks, or sterilize bottles, which is why neutralizing chlorine residuals is the correct purpose.