How many drops of sodium thiosulfate are needed to neutralize residual halogen in 8 ounces of sampled potable water?

• A. 2-3 drops
• B. 4-6 drops
• C. 6-8 drops
• D. 1-2 drops

Answer: (B)

Neutralizing residual halogen with sodium thiosulfate is about stopping the oxidizing chlorine species so downstream tests aren’t affected. In an 8-ounce (about 237 mL) potable water sample, the standard field protocol uses four to six drops of sodium thiosulfate. This amount reliably neutralizes typical residual chlorine found in sampled drinking water without introducing excess thiosulfate that could skew subsequent measurements. The chemistry is that chlorine species (like Cl2 or HOCl) react with thiosulfate to form chloride and other reduced sulfur species, effectively removing the oxidant. Too few drops may leave some chlorine active and alter test results, while too many could over-zer the sample. Four to six drops hits the balance, so four to six drops is the best answer.