The reaction of sulfide with hydrogen peroxide in an aqueous solution at pH 9 produces thiosulfate, sulfite, and an unknown sulfur compound as intermediates, and, finally, all sulfides convert to sulfate (Chen and Gupta, Environ Lett 1973, 4, 187-200; Yokosuka et al., Nihon Kagaku Kaishi 1975, 11, 1901-1909 Hoffmann, Environ Sci Technol 1977, 11, 61-66; Ràbai et al., J Phys Chem 1992, 96, 5414-5419). This reaction was found to be accelerated by freezing. The decomposition rate of sulfide by H 2 O 2 in freezing at −15 • C was about 5 times faster than the maximum decomposition rate in solution at 25 • C. The decomposition of sulfide by freezing obeys zero-order kinetics, and the rate coefficient was 11.9 mol dm −3 min −1 at a freezing rate of 0.83 cm 3 min −1 . Zero-order kinetics is one of the characteristics of freezing rate-controlled reactions. Thiosulfate, sulfite, and an unknown sulfur compound were also observed as intermediates in the reaction by freezing. The decomposition of thiosulfate obeys first-order kinetics, and the rate coefficient was 0.0496 min −1 at −15 • C. The rate coefficient in freezing at −15 • C is about 47 times faster than that in solution at 25 • C. Sulfide, thiosulfate, and sulfite were consumed after 90 min. However, the unknown sulfur compound was not oxidized and was preserved in ice for a long time even in the presence of an excess of hydrogen peroxide. The concentration of an unknown sulfur compound in the frozen sample can be changed by changing the concentration of H 2 O 2 and the pH of the solution. Freezing could be used for rapid preparation and preservation of unstable substances in solution. C

In order to investigate the characteristics of the oxidative reactions of low-valence sulfur compounds with H2O2 in the freezing process, a Na2S2O3 aqueous solution was mixed with a H2O2 aqueous solution, followed by freezing at −15 °C. After the designated time had passed, the concentrations of sulfur compounds were measured. The oxidation of thiosulfates was accelerated due to the freeze-concentration effect. The maximum reaction rate of the freezing reaction (−15 °C) was about 20-times faster than that in an aqueous solution (25 °C). In the aqueous-phase reaction, thiosulfates were transformed to tetrathionates as intermediates, or directly to sulfates, and all sulfur compounds were finally oxidized to sulfates. On the other hand, the tetrathionates and sulfates transformed by the freezing reaction were preserved in ice for a long time. The maximum amounts of the tetrathionates formed by freezing were about twice those in the aqueous-phase reaction. Once tetrathionates were formed by the freezing reaction, most of the formed tetrathionates coexisting with high concentrations of H2O2 were not transformed to sulfate, regardless of whether or not the sample was re-frozen. This observation is probably attributable to the difference in the pH value when tetrathionates are formed.

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