On the impedance of galvanic cells XXV. The double-layer capacitance of the dropping mercury electrode in 1 M HCl, 7.5 M and 5.2 M HClO4 and the kinetic parameters of the hydrogen electrode reaction as a function of temperature in these solutions

Abstract

The impedance of the dropping mercury electrode in aqueous solutions in 1 M HCl, 7.5 M HCl and 5.2 M HClO 4 , saturated with hydrogen at one atmosphere was measured at temperatures between −39° and +72° both in and outside the potential region where the electrode reaction, e +H + H 2 , proceeds. Analysis of the impedance data gave information about both the kinetics of the electrode reaction and the double-layer capacity. Exchange current densities and transfer coefficients are reported. The apparent heat of activation is 21.1 kcal mole −1 for 1 M HCl, 19.5 kcal mole −1 for 5.2 M HClO 4 , and 11.1 kcal mole −1 for 7.5 M HCl. In the latter solution the transfer coefficient is found to be a linear function of temperature. In 1 M HCl and 5.2 M HClO 4 the transfer coefficient is virtually independent of temperature. In 1 M HCl the hump in the capacity curve is situated anodically to the potential of zero charge but in 7.5 M HCl and 5.2 M HClO 4 this is reversed. In both 1 M HCl and 7.5 M HCl the capacity-potential curves show one potential where the double-layer capacitance is independent of temperature. In the case of 5.2 M HClO 4 , this occurs at two potentials, situated at either side of the potential of zero charge.