Iron-Catalyzed Oxidation of Sulfite * : From Established Results to a New Understanding

Abstract

A survey is made of the iron-catalyzed oxidation of sulfite describing a conceptual framework to explain the key processes involved, with a focus on kinetics. Perhaps most importantly, the incorporation of the HSO₅^- + Fe(II) step into the regeneration of catalytically active ferric ions which does not deplete its role over the iron redox cycle. The radical-radical recombination SO₅^-• + SO₅^-•, which terminates the cycling between ferric and ferrous ions, represents a gross but not a net loss of the chain-carriers, because nearly all of them are reformed through the branching step HSO₅^- + Fe(II) → Fe²⁺ + H₂O + SO₄^-•, ${\text{HSO}}_5^{-}+\text{Fe}\left(\text{II}\right)\to {\text{Fe}}^{2+}+{\text{H}}_{\text{2}}\text{O}+{\text{SO}}_4^{-·},{\text{SO}}_4^{-·}\stackrel{{\text{HSO}}_{\text{3}}^{\text{-}}{\text{,O}}_{\text{2}}}{\to }{\text{SO}}_5^{-·}$ in just a few seconds or somewhat longer. A branching mechanism is thus the only possible means of allowing the catalytic process to reach a stationary state. Observations that may be considered as evidence (fingerprints) of rate variations in sulfite depletion due to the branching mechanism are explored in detail, and the related dynamics of the chain-carriers and metal ions cycles are discussed. In particular, the most important is found to be the aspect related to the intrinsic limitation of the cycle of metal ions. This limitation governs the extent of the oxidative/reducing potential of sulfite solutions with respect to the Fe(III/II) couple, thereby governing the quasi-state partioning between ferric and ferrous ions. Such a view enables examination of those conditions under which the limitation to the rate of the catalytic reaction is controlled by the reduction or re-oxidation of ferric ions. Readily applicable kinetic criteria and kinetic diagrams to delimit the conditions are given. In such a framework, the majority of known anomalies of the catalytic reaction receive an explanation.