The oxidation of sulfur(iv) by reaction with iron(iii): a critical review and data analysis

Abstract

The dependences on ionic strength of the hydrolysis constants of Fe³⁺ and of the first dissociation constant of sulfurous acid are briefly reviewed. The data are needed to derive from apparent stability constants reported in the literature the stability constants for the three iron–sulfito complexes defined by the equilibria (c1) FeOH²⁺ + HSO₃⁻ = FeSO₃⁺ + H₂O, (c2) FeSO₃⁺ + HSO₃⁻ = Fe(SO₃)₂⁻ + H⁺, (c3a) Fe(SO₃)₂⁻ + HSO₃⁻ = Fe(SO₃)₃H²⁻, where K_c1 = 1982 ± 518 dm³ mol⁻¹, K_c2 = 0.72 ± 0.08, K_c3a = 189 ± 9 dm³ mol⁻¹ (ionic strength μ = 0.1 mol dm⁻³). The rapid formation of these complexes is followed by a slower decomposition leading to the formation of SO₃⁻ radicals; the associated rate coefficients are k₁ = 0.19 s⁻¹, k_1a ≈ 0.04 s⁻¹, and k_1b ≈ 0.08 s⁻¹, respectively. The subsequent reaction leads to dithionate and sulfate as products. Overall rates and product yields from a variety of studies of the slow reaction are found to be consistent with a mechanism, in which the production of dithionate occurs mainly by the reaction of SO₃⁻ with FeSO₃⁺ and that of sulfate by the reaction of SO₃⁻ with FeOH²⁺ and/or Fe³⁺. The role of copper as a catalyst is also analyzed. Rate coefficients for individual reactions are estimated from the data at low pH (μ = 1.0 mol dm⁻³) under conditions where the 1 : 1-complex is prevalent. They are extrapolated to lower ionic strengths for an analysis of the data obtained at higher pH to explore conditions when reactions of the higher complexes become important. The overall rate and the product yields of the reaction depend critically on the pH, the initial ratio of S(IV) to Fe(III) and the ionic strength of the solution.