The apparent faradaic response in the impedance characteristics of the capacitive current at a streaming mercury electrode

The impedance spectra of processes studied at the streaming mercury electrode have unique features, originating from the characteristics of the capacitive current permanently charging the flowing stream of mercury. For the solutions of the supporting electrolyte, free of any detectable electroactive species, such spectra are qualitatively similar to those of the faradaic process. In particular the Nyquist plot of the capacitive current is a semicircle. Such impedance characteristics of the capacitive current were explained quantitatively in terms of the appropriate electric equivalent circuit, including the virtual resistance Rd=(2πrCdv)−1, connected parallel to the double layer capacitance (with r – the mercury stream radius, v – the stream velocity and Cd – the double layer capacitance per unit area). The experimental results for NaN3 and NaF solutions confirm the theoretical model semi-quantitatively. This apparent faradaic response for the capacitive current has to be subtracted from the impedance spectrum for the sample with both the supporting electrolyte and the electroactive species, in order to extract reliable faradaic characteristics of the electrode process studied.