An Analysis of Impedance in the Kolbe Reaction on Platinum and Gold Anodes in Aqueous Acetate Solutions

Abstract

Abstract 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.