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 nit rite and sulfite can promote the rate of autoxidation, instead of retarding it. Experiments show that nitrite and sulfite undergo rapid oxidation to n itrate and sulfate, respectively, in high yield (> 90% under certain condit ions) when their dilute (100 muM) aqueous solutions are frozen for 10-60 mi n. at -10 to -40 degreesC. For example, the pseudo-second-order rate consta nt for nitrite autoxidation, k('), defined in d[NO3-]/dt = 2k(') [HNO2](2), reaches a maximum value of 117 +/- 14 M(-1)s(-1) at -15.5 degrees. This co unterintuitive result is hypothesized to be the result of a freeze-concentr ation effect that occurs when reactants are concentrated into liquid microp ockets ahead of the advancing ice front. Oxidation by hydrogen peroxide is also accelerated upon freezing. Since the yields and rates may be high comp ared to other competing pathways, this process may be significant where fre eze/thaw cycles occur naturally, e.g., in glaciating clouds, snow packs, gl aciers, and melt ponds on polar sea ice.