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

Abstract

The oxidation of Fe ²⁺ , Mn ²⁺ and Cu ²⁺ to the corresponding trivalent ions by SO ₄ ^- has been studied in aqueous solution at pH 3–5 using pulse radiolysis to generate SO ₄ ^- . For Fe ²⁺ 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 ^II SO ₄ ^- ) ⁺ , 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 [Fe ²⁺ ] 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)×10 ⁹ dm3 mol ^-1 s ^-1 . By applying the steady-state approximation to (Fe ^II SO ₄ ^- ) ⁺ , a value of 1.1×10 ⁹ s ^-1 is obtained for the rate constant for the electron-transfer step. Reaction of SO ₄ ^- with Mn ²⁺ is also first order in [Mn ²⁺ ] with a bimolecular rate constant of 1.4×10 ⁷ 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 2.6×10 ⁶ dm3 mol ^-1 s ^-1 . For Mn ²⁺ , like Fe ²⁺ , 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 SO ₄ ^- with Fe ²⁺ and Mn ²⁺ , respectively. For Cu ²⁺ , the rate of decay of SO ₄ ^- was independent of [Cu ^II ] and was largely accounted for by its reaction with Bu ^t OH 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.