Thermodynamics, Kinetics and Mechanisms of the Reactions of S(IV) with Cu(II) and Fe(III)

Citation

Conklin, Martha Harriet (1987) Thermodynamics, Kinetics and Mechanisms of the Reactions of S(IV) with Cu(II) and Fe(III). Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/83k6-ex73. https://resolver.caltech.edu/CaltechETD:etd-03242008-134119

Abstract

Spectrosopic methods are used to determine the stability constant for the formation of CuSO₃, K = 1.8 ± 0.6 x 10⁴ M^-1 for µ = 0.4 M. Infrared and Raman measurements indicate that sulfite binds to the metal through both sulfur and oxygen. These results are compared to those of other first-row transition metal-sulfite complexes.

The reduction of Cu(II) is shown to proceed via (Cu(II))₂SO₃²⁺ and CuSO₃CuOH⁺ intermediates. Copper(I), SO₄^2- and a mixed valence compound Cu^||SO₃Cu₂^|SO₃•2H₂O are determined to be the principal products. The rate law is consistent with consecutive first-order reactions. Results are interpreted in terms of the initial formation of an inner-sphere complex which is followed by a rate-limiting electron transfer step. Previously accepted mechanisms for the trace metal catalysis of the autoxidation of SO₃^2- are discussed in light of these results.

A conditional stability constant for the formation of a Fe(III)-S(IV) complex at µ = 0.4 M and pH 2.1 was determined spectroscopically. Raman measurements indicate that sulfite binds to the metal through oxygen. EPR experiments show that the reduction of Fe(III) to Fe(II) by S(IV) is a slow reaction at pH 2 (τ_1/2 ≃ 8 min). Various pathways for the formation of the Fe(III)-S(IV) species are examined to determine the most probable equilibrium species. Results are interpreted by comparing the stability and bonding of Fe(III)-S(IV) species with other Fe(III) complexes.

The rates of these internal redox reactions are too slow for this reaction to be important in the atmospheric autoxidation of S(IV), instead ternary metal-oxygen-sulfito complexes are proposed as the active catalytic species in aqueous atmospheric systems. Calculations based on the equilibrium constants obtained in this study indicate that metal-S(IV) complexes may be important equilibrium species in the absence of α-hydroxyalkylsulfonates. The catalytic autoxidation of SO₂ in aqueous systems appears to proceed via the formation of metal-sulfite complexes as a prelude to electron-transfer.

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