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The kinetics of the environmentally important oxidation of sulfur(IV) by oxygen in acetate buffered medium in the presence of Fe(III) and the pH range 5.27-5.70 has been studied. The results were in agreement with the rate law:The role of iron(III) appears to be that of production of SO 3 -radicals in Fe(III)SO 3 2-complex by an internal 1-equivalent redox reaction. Subsequently, a radical mechanism involving oxysulfur radicals, viz., SO 3 -, SO 4 -, and SO 5 -operates. Addition of ethanol leads to the introduction of an induction period and decrease in reaction rate, most likely due to scavenging of SO 4 -radicals. The value of apparent energy of activation is 45.4 kJ mol -1 .

For getting an insight into the mechanism of atmospheric autoxidation of sulfur(IV), the kinetics of this autoxidation reaction catalyzed by CoO, Co 2 O 3 and Ni 2 O 3 in buffered alkaline medium has been studied, and found to be defined by Eqs. I and II for catalysis by cobalt oxides and Ni 2 O 3 , respectively.

In the Ni(II)-S(IV)-O 2 system in the region of pH > 8.4, both Ni(II) and S(IV) are simultaneously autoxidized, and when sulfur is consumed fully NiOOH precipitates. At pH > 8.4, ethanol has no effect on the rate, whereas ammonia strongly inhibits the reaction when pH > 7.0. The kinetics of the reaction, in both the presence and the absence of ethanol, is defined by the rate lawwhere k is the rate constant, K O is the equilibrium constant for the adsorption of O 2 on Ni(OH) 2 particle surface. In ammonia buffer, the factor F is defined by