Which chemical is used to de-chlorinate a water sample that is analyzed for bacteria?

• A. Sodium bicarbonate
• B. Sodium chlorate
• C. Sodium thiosulfate
• D. Sodium fluoride

Answer: (C)

Residual chlorine in a water sample can keep killing or inhibiting bacteria, which would give an inaccurately low count during analysis. To prevent this, a dechlorinating agent is added before testing. Sodium thiosulfate works best here because it quickly reduces chlorine species (free chlorine and chloramines) to chloride ions, effectively neutralizing the oxidizing power of chlorine without harming the bacteria you’re trying to measure. The other chemicals won’t remove chlorine: sodium bicarbonate is just a buffering agent, sodium chlorate is an oxidizer, and sodium fluoride serves as a fluoride additive. Using sodium thiosulfate ensures an accurate bacterial analysis by eliminating chlorine interference.