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+1(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.
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