Oxidation reaction of [Ni(1,4,7-triazacyclononane)2]2+ by [Fe(o-phenanthroline)3]3+ in aqueous acidic solution at elevated pressures: determination of the activation and reaction volumes for the Ni(iii)/(ii) couple

Abstract

The reaction volume corresponding to the self-exchange process of the [Ni(tacn)₂]^3+/2+ couple was determined in aqueous acidic solution. Theoretical equations on the basis of the Mean Spherical Approximation were proposed for the estimation of reaction volumes for M^{n+/(n − 1)+} couples in solution, and the calculated reaction volumes were compared with the experimentally estimated values. The activation volume for the [Ni(tacn)₂]^3+/2+ couple was determined in the acidic condition from the cross reaction of [Ni(tacn)₂]²⁺ and [Fe(o-phen)₃]³⁺ at elevated pressures. The agreement of the experimentally estimated activation volume for the [Ni(tacn)₂]^3+/2+ couple, −8.2 ± 2.4 cm³ mol⁻¹, with the theoretically calculated value, −7.5 cm³ mol⁻¹, within the allowed uncertainty (±1 cm³ mol⁻¹) indicates that the electron self-exchange reaction of this redox couple obeys the Marcusian behavior in aqueous acidic solution.