Use of Sodium Thiosulfate to Quench Hypochlorite Solutions Prior to Chlorate Analysis

Abstract

Sodium thiosulfate is a commonly used quenching agent for sodium hypochlorite and has previously been deemed acceptable when quenching hypochlorite solutions before conducting chlorate analysis. However, using thiosulfate to quench hypochlorite solutions with compositions similar to those produced by electrolytic onsite generation process (OSG; free available chlorine content of 0.8% with a pH range of 8–10) can result in artificially low chlorate measurements. Reactions between thiosulfate and hypochlorite can result in a substantial decrease in pH that in turn causes the production of bisulfite ions from thiosulfate. In turn, these ions react with chlorate ions, resulting in artificially decreased measurements of chlorate in the initial hypochlorite solutions. Malonic acid does not cause a similar effect when used as a hypochlorite quenching agent and should therefore be used in place of thiosulfate when conducting chlorate analysis of hypochlorite solutions produced through electrolytic OSG processes.