Which chemical is specifically used to remove residual chlorine from water?

• A. Chlorine dioxide
• B. Sodium thiosulfate (Na₂S₂O₃)
• C. Calcium hypochlorite
• D. Potassium permanganate

Answer: (B)

Sodium thiosulfate (Na₂S₂O₃) is specifically used to remove residual chlorine from water due to its ability to react chemically with chlorine. When sodium thiosulfate is added to water containing residual chlorine, it undergoes a redox reaction where thiosulfate reduces chlorine to chloride ions, effectively neutralizing the chlorine. This makes sodium thiosulfate a commonly employed dechlorination agent in various water treatment processes, including drinking water treatment.

The other choices do not serve the specific purpose of dechlorination effectively. Chlorine dioxide is more commonly used as a disinfectant rather than a dechlorination agent. Calcium hypochlorite, on the other hand, functions as a chlorine source for disinfection, and potassium permanganate is primarily utilized as an oxidizing agent for removing organic matter and controlling taste and odor rather than for dechlorination purposes.