Kinetics and mechanism of the iron(III)-catalyzed autoxidation of sulfur(IV) oxides in aqueous solution. The influence of pH, medium and aging

The influence of pH on the oxygen-dependent step observed during the iron(III)-catalyzed autoxidation of sulfur(IV)-oxides was studied over the pH range 1.5 less than or equal to pH less than or equal to 4. A maximum decomposition rate of the iron(III)-sulfito complexes is observed around pH 2.3. The pH-time profile for the iron(III)-catalyzed autoxidation of sulfur(IV)-oxides exhibited no dependence on the initial [O-2] when the reaction is initiated at pH 6.1, but shows an oxygen dependence when the reaction is initiated at pH 2.5. pH-shift experiments clearly demonstrate the reduced catalytic activity of dimeric and polymeric iron(III) hydroxo species. A decrease in the catalytic activity of the iron(III) solutions with time was observed, which depended on the initial pH. This decrease was ascribed to the formation of dimeric and polymeric iron(III) hydroxo species as a result of slow hydrolysis processes during aging. The employed iron(III) salt had no effect on the decrease in the catalytic activity due to aging.