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

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.