Invariance of Double Layer Capacitance to Polarized Potential in Halide Solutions

Abstract

This paper aims at evaluating the dependence of double-layer capacitance at a platinum wire electrode both on concentrations of four kinds of halide ions and the dc potential in the double-layer region (from 0.0 to 0.4 V vs. Ag|AgCl) from a viewpoint of frequency-dispersion of the capacitance. Attention was paid to shape of the double layer capacitance vs. dc-potential curves. The equivalent circuit used for analysing the double-layer impedance was a parallel combination of a double layer capacitance and an apparent resistance. The latter is caused by the time-derivative of the capacitance, which belongs to an in-phase component. It was inversely proportional to the frequency and the electrode area, but unaffected by the potential and the variation of concentrations of halide ions. On the other hand, the parallel capacitance decreased linearly with logarithm of the frequency, and was proportional to the electrode area. It showed almost a value common to concentrations from 1 mM to 2 M (= mol∙dm^-3) and four halide ions. Variations of the capacitance with the dc-potential in 0.1 M KF, KCl and KBr solutions are ca. 10 %. Therefore, the capacitance has negligible influence on the localization of ions by the Gouy-Chapman theory.