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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 Mn+/(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.

The reaction volume corresponding to the self-exchange process of the Nitacn232couple 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 Mnn− 1couples in solution, and the calculated reaction volumes were compared with the experimentally estimated values. The activation volume for the Nitacn232couple was determined in the acidic condition from the cross reaction of Nitacn22and Feo-phen33at elevated pressures. The agreement of the experimentally estimated activation volume for the Nitacn232couple, −8.2 ± 2.4 cm3mol−1, with the theoretically calculated value, −7.5 cm3mol−1, within the allowed uncertainty ±1 cm3mol−1 indicates that the electron self-exchange reaction of this redox couple obeys the Marcusian behavior in aqueous acidic solution.