What substance will sodium arsenite in the SPADNS reagent effectively remove?

Sodium arsenite in the SPADNS reagent is specifically effective at removing chlorine. This is particularly relevant in the context of water quality testing, where the presence of chlorine can interfere with the detection of other substances, such as fluoride. The SPADNS method relies on the reaction of fluoride ions in the sample without the interference of chlorine, so it's crucial to effectively remove chlorine to ensure accurate measurements.

In contrast, the other substances listed do not experience similar removal through sodium arsenite. For example, fluoride detection can be compromised by chlorine, but the purpose of sodium arsenite here is to address that interference by removing chlorine. Heavy metals and organic compounds require different treatments for removal or analysis and are not targeted by the SPADNS reagent in the same way.