Acceleration of the rate of nitrite oxidation by freezing in aqueous solution

WHEN a dilute ionic solution freezes, differences in the partitioning of ions in the aqueous and ice phases can generate electric potentials which may influence electrochemical reactions1–3. Fitter and co-workers4,5 have studied the influence of freezing on the endothermic oxidation of sulphide to sulphate in growing ice crystals inside a cloud chamber. Here we report that the oxidation of nitrite by dissolved oxygen to form nitrate, which is a very slow process in solution, is accelerated markedly when it takes place in a solution undergoing freezing. At pH 4.5 and a temperature of 25 °C, the rate is increased by a factor of about 105 for a freezing rate of 0.2 g solution per minute; the reaction rate increases as the freezing rate increases. Although the mechanism of this acceleration is not yet clear, we are able to eliminate the possibilities of thennochemical, photochemical and simple electrochemical reactions, and of catalysis on the ice surface. Processes of this sort may be important for chemical reactions taking place in freezing cloud and fog droplets in the atmosphere.