Capacitance of a metal/liquid interface during anion adsorption: phase-selective measurement in the presence of d.c. voltage sweep and finite solution resistance

Anion adsorption plays a critical role in pitting corrosion of metals in aqueous media. Frequently, this adsorption occurs as a non-faradaic process, without any electron transfer across the interface. Such processes cannot be studied efficiently by using standard electrochemical methods such as linear sweep voltammetry (LSV). Phase-selective measurement of the interfacial differential capacitance is necessary in such cases. However, when accompanied by LSV, even these latter measurements can be affected severely by the uncompensated solution resistance. In this paper, we describe a relatively simple phase-selective impedance method where both the solution resistance and the differential capacitance can be measured in situ during the d.c. voltage scan of LSV. We apply this technique to study the adsorption properties of ClO4−, Cl− and SCN− on a polycrystalline Au electrode. The results indicate that this method can be standardized easily for the analysis of adsorption isotherms involving similar systems.