An analysis of impedance in the kolbe reaction on platinum and gold anodes in aqueous acetate solutions.

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The Kolbe reaction occurring on platinum or gold anodes in aqueous acetate was studied by analyzing the equivalent circuit of the impedance obtained with a phase-sensitive detector. In the case of platinum, the equivalent circuit at the potential for the Kolbe reaction is represented by a parallel combination of the double-layer capacity, Cdl, and the charge-transfer resistance, θ, in series with the solution resistance, Rsol. The Cdl value of 2.00 V obtained by a complex impedance plane plot is nearly identical to that calculated by assuming that the Helmholtz layer is occupied by CH3COO−. In the potential range more negative than 2.00 V, the Kolbe reaction occurs. The steep decrease in the differential capacity is considered to be a result of the decrease in the adsorption pseudocapacity. The θ value of the oxygen evolution increases with an increase in the acetate concentrations. It is shown that the surface coverage of CH3COO− may be obtained by "Frumkin's treatment" in conjunction with the high-frequency capacitance, CHF, and the low-frequency capacitance, CLF, from the complex capacitance plane plot (Y/ω plane plot). The Kolbe reaction does not occur at all with gold anodes, and the specific circular arc was not seen in the Y/ω plane plot. Impedance measurements, therefore, confirm that an adsorption pseudocapacity of CH3COO− is not present with gold anodes.
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