How many drops of sodium thiosulfate are needed to neutralize residual halogen in 4 ounces of sampled potable water?

• A. 1-2 drops
• B. 2-3 drops
• C. 3-4 drops
• D. 4-5 drops

Answer: (B)

Neutralizing residual halogen with sodium thiosulfate is done to prevent chlorine from interfering with the tests you’ll perform on the water sample. For a 4-ounce (about 120 mL) potable water sample, about two to three drops of sodium thiosulfate are typically enough to react with and neutralize the residual chlorine present.

This amount is chosen because it’s enough to quench the oxidizing halogen without overdoing it. Too few drops may leave some residual chlorine that can skew downstream measurements, while too many drops can dechlorinate more than intended and distort results that rely on the presence of halogen. In standard field practice, two to three drops strike the right balance for a small sample like this. If chlorine levels are unusually high, you might need more, but for normal residuals, two to three drops is the guideline.