What substance is used to neutralize chlorine residual in a water sample?

Sodium thiosulphate is the correct substance used to neutralize chlorine residual in a water sample. This chemical reacts with chlorine to form non-toxic products, effectively deactivating the residual chlorine presence. This is particularly important in water sampling processes where the presence of chlorine can interfere with the accuracy of analytical results for certain contaminants.

Using sodium thiosulphate is a common practice because it not only neutralizes free chlorine but also provides a straightforward method for ensuring that the sample composition does not change due to the influence of chlorine. This is essential for obtaining reliable data that reflects the actual quality of the water.

Other options may not serve the purpose of neutralizing chlorine effectively or could introduce additional complexities or interferences in the water sample. For example, sodium chloride simply adds more chloride ions to the sample without neutralizing the chlorine, while chlorine dioxide is itself a disinfectant and would not help in neutralizing chlorine. Hydrochloric acid, being an acid, would not directly neutralize chlorine either and could lead to other reactions that may alter the water sample. Thus, sodium thiosulphate is specifically chosen for its effectiveness in this role.