Effect of dimethylsulfoxide as a masking agent for aqueous chlorine in the determination of oxychlorines

Abstract

The aim of this work was to demonstrate that dimethylsulfoxide (DMSO) is an excellent masking agent for aqueous chlorine in the determination of other oxychlorines. By the addition of excess DMSO, specific absorbance of free chlorine disappeared, and the oxidation of iodide to iodine by chlorine was completely prevented. Chlorine dioxide, chlorite and chlorate were not affected by the co-existence of excess DMSO, but chlorosulfamic acid, showed results comparable to free chlorine. By using ion chromatographic analysis of the mixed solution of free chlorine and DMSO, chloride was recovered as the only anionic species and its molar concentration was approximately twice the initial chlorine concentration. DMSO seems to reduce and mask chlorine completely, without affecting other oxychlorines. Chlorine and DMSO reacted in the molar ratio of 1:1 The reaction seemed to be second-order. The rate constant was larger at a lower pH, and it was dependent not on a concentration of total chlorine, but on that of hypochlorous acid. The redox potential of DMSO was higher at a lower pH, and only hypochlorous acid would have a redox potential high enough to oxidize DMSO in acidic conditions. These results suggest that DMSO reacts with hypochlorous acid stoichiometrically. In practical use, DMSO may be successfully used as a masking agent for aqueous chlorine.