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), SO42- 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 SO32- 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 (τ1/2 ≃ 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.
