Oxidative Reaction of Thiosulfate with Hydrogen Peroxide by Freezing

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.