Autoxidation of N(III), S(IV), and other species in frozen solution - A possible pathway for enhanced chemical transformation in freezing systems

Freezing dilute aqueous solutions of certain oxidizable species such as nitrite and sulfite can promote the rate of autoxidation, instead of retarding it. Experiments show that nitrite and sulfite undergo rapid oxidation to nitrate and sulfate, respectively, in high yield (>90% under certain conditions) when their dilute (100 μM) aqueous solutions are frozen for 10-60 min. at -10 to -40°C. For example, the pseudo-second-order rate constant for nitrite autoxidation, k′, defined in d[NO₃-]/dt = 2k′ [HNO₂]², reaches a maximum value of 117 ± 14 M^-1s^-1 at -15.5°C. This counterintuitive result is hypothesized to be the result of a freeze-concentration effect that occurs when reactants are concentrated into liquid micropockets ahead of the advancing ice front. Oxidation by hydrogen peroxide is also accelerated upon freezing. Since the yields and rates may be high compared to other competing pathways, this process may be significant where freeze/thaw cycles occur naturally, e.g., in glaciating clouds, snow packs, glaciers, and melt ponds on polar sea ice.