Issue 16, 1997

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.

Article information

Article type
Paper

J. Chem. Soc., Faraday Trans., 1997,93, 2893-2897

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

G. V. Buxton, T. N. Malone and G. Arthur Salmon, J. Chem. Soc., Faraday Trans., 1997, 93, 2893 DOI: 10.1039/A701472D

To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page.

If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given.

If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page.

Read more about how to correctly acknowledge RSC content.

Spotlight

Advertisements