Kinetics and mechanism of the oxidation of sulfur(iv) by iron(iii) at metal ion excess

Abstract

The kinetics and mechanism of the reaction between iron(III) and sulfur(IV) was studied at high iron(III) excess at 10.0 and 25.0 °C in 1.0 M NaClO₄ by the stopped-flow method. The number of absorbing species in this system was determined by matrix rank analysis of time-resolved spectra. The reaction exhibits composite kinetic features which could be explained by considering the reactions of Fe(H₂O)₆³⁺, Fe(H₂O)₅OH²⁺, Fe₂(OH)₂(H₂O)₈⁴⁺ and two sulfito complexes, FeSO₃(H₂O)₅⁺ and Fe₂(μ-SO₃)(μ-OH)(H₂O)₈³⁺. In contrast to previous observations at sulfur(IV) excess, it was found that the kinetic traces are not influenced by the presence or absence of molecular oxygen. A detailed, ten-step kinetic model was proposed for the interpretation of the experimental observations at 340 and 430 nm. The model was validated by simultaneously fitting kinetic traces recorded at various initial concentrations to the corresponding differential equation system. It was confirmed that the dinuclear sulfito complex is not involved directly in the redox process, and the rate-determining step of the overall redox reaction is the decomposition of the mono complex in an intramolecular electron transfer step, FeSO₃(H₂O)₅⁺ (+H₂O) → Fe(H₂O)₆²⁺ + SO₃⁻˙. The first-order rate constant for the decomposition was determined at 10.0 and 25.0 °C to be 0.052 ± 0.012 and 0.19 ± 0.03 s⁻¹, respectively. The results imply that the sulfite ion radical is quickly oxidised to sulfate ion by iron(III) in a subsequent reaction step.