Metal Ion Catalyzed Autoxidation of Sulfur(IV)‐Oxides: Redox Cycling of Metal Ions Induced by Sulfite

The important role of metal ion catalyzed autoxidation reactions of sulfur(IV)‐oxides in aqueous solution has been confirmed by numerous investigations. In general, transition metals such as Co, Fe and Mn in the 3+ oxidation state can rapidly oxidize HSO3−/SO32‐ to produce the reduced metal ions in the 2 + oxidation state and the SO3− radical, which can subsequently initiate a series of free radical propagation reactions. Experimental results are reported to demonstrate how low concentration levels of sulfite can effectively induce the autoxidation of the reduced metal ions in order to complete the catalytic cycle. In this way the mentioned metal ions exhibit redox cycling in which the nature of the redox process is determined by the sulfite and oxygen concentration levels in solution. A general mechanism to account for these observations, as well as for the synergistic effect usually observed for such reactions, is presented.