Reaction of SO₄^- with Fe²⁺, Mn²⁺ and Cu²⁺ in aqueous solution

Abstract

The oxidation of Fe²⁺, Mn²⁺ and Cu²⁺ to the corresponding trivalent ions by SO₄^- has been studied in aqueous solution at pH 3–5 using pulse radiolysis to generate SO₄^-. For Fe²⁺ the reaction has a negative energy of activation of -(18±2) kJ mol^-1 at low ionic strength, and k_obs shows a very small dependence on ionic strength, indicating that a precursor complex, (Fe^IISO₄^-)⁺, is kinetically significant. The stability constant, K_a, of the complex is estimated to be 5.3 dm3 mol^-1 at 298 K. The observed rate is first order in [Fe²⁺] and the overall bimolecular rate constant, at ca. 20°C and an ionic strength of 0.06 mol dm^-3, is (4.6±0.2)×10⁹ dm3 mol^-1 s^-1. By applying the steady-state approximation to (Fe^IISO₄^-)⁺, a value of 1.1×10⁹ s^-1 is obtained for the rate constant for the electron-transfer step. Reaction of SO₄^- with Mn²⁺ is also first order in [Mn²⁺] with a bimolecular rate constant of 1.4×10⁷ dm3 mol^-1 s^-1, at an ionic strength of 0.065 mol dm^-3 at 20°C, and an activation energy of (34±2) kJ mol^-1. The rate constant for electron transfer is obtained as 2.6×10⁶ dm3 mol^-1 s^-1. For Mn²⁺, like Fe²⁺, k_obs shows a small ionic strength dependence consistent with that expected for the formation of an outer-sphere ion-pair complex. Treatment of the data according to the classical Marcus theory for electron transfer yields ΔH°=-277 kJ mol^-1 and -54 kJ mol^-1 for the reaction of SO₄^- with Fe²⁺ and Mn²⁺, respectively. For Cu²⁺, the rate of decay of SO₄^- was independent of [Cu^II] and was largely accounted for by its reaction with Bu^tOH which was present to scavenge OH. No rate constant for the oxidation step could be determined; that some oxidation did occur is deduced from spectral changes assigned to the formation of a Cu^III species.

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