Which one of the following chemicals is commonly used to control chlorine in water samples?

Sodium thiosulfate is widely used to control chlorine levels in water samples because it acts as a reducing agent. When sodium thiosulfate is added to water containing chlorine, it reacts with the chlorine to form sodium chloride and sodium sulfate, effectively neutralizing the chlorine. This is particularly important in situations where the presence of chlorine can interfere with water testing procedures or processes. Sodium thiosulfate is often utilized in laboratory settings, for example, when preparing water samples for analysis where chlorine-free samples are necessary for accurate results.

Other chemicals listed, such as calcium carbonate, sodium bicarbonate, and magnesium sulfate, do not have the same reducing properties and would not effectively neutralize chlorine in the same way. Instead, they may have different applications in water chemistry and treatment processes, but they are not typically used for dechlorination purposes.