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

• Main Author: Conklin, Martha Harriet
• Format: Others
• Language: en
• Published: 1987
• Online Access: https://thesis.library.caltech.edu/1104/1/Conklin_mh_1986.pdf; 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 <https://resolver.caltech.edu/CaltechETD:etd-03242008-134119>

<p>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.</p> <p>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.</p> <p>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.</p> <p>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.</p>