Influence of ionic strength on aqueous oxidation of SO2 catalyzed by manganese

We studied the influence of ionic strength as well as type of supporting electrolyte on the rate of oxidation of dissolved SO2 catalyzed by Mn(II) under dark conditions at pH 3. Two electrolytes were chosen; NaCl and NaNO3. NaCl is an important component of atmospheric particles of marine origin and NO3− represents a considerable part of aerosols of continental origin. Laboratory experiments were performed in a system that enables continuous introduction of gas mixture (SO2/air) into synthetic aqueous solution. The solubility of SO2 in aqueous solution increases to some extent with an increase in ionic strength from 0.001 to 0.1 mol l−1 of both supporting electrolytes. The results suggest that the effect of concentration and nature of electrolyte on the catalytic S(IV) autoxidation are not negligible parameters. In NaNO3 medium, the rate constant decreases with increasing ionic strength, and the inhibition appears to be a consequence of the primary salt effect. In NaCl medium, the dependence of the rate constant on the ionic strength is more complex. The reaction rate of the catalyzed reaction is almost constant or increases slightly with the ionic strength in the concentration range of 0.001–0.02 mol l−1, and subsequently a considerable decrease was noticed. It seems that two opposite effects of Cl− could be important, the inhibiting and catalytic one.