The notion of the electrode charge and the Lippmann equation

Abstract

Summary The notion of the charge has been considered for the case of an ideally polarized electrode. It has been proved necessary to distinguish between the total charge, which figures in thermodynamic relations, and the free charge, which can be determined only in terms of a certain electric double-layer model. A definition of the total charge is given, equally applicable to ideally polarized and reversible electrodes, as the amount of electricity to be supplied to the electrode to keep the electrode potential constant when its surface is increased by unity and the composition of the bulk phases of the system is maintained constant. The total charge thus determined satisfies in all cases the Lippmann equation. Expressions are given for the Lippmann equation for reversible redox systems, as exemplified by platinum-hydrogen and amalgam-thallium electrodes. It has been shown that in such systems two kinds of electrocapillary curves can be obtained, depending on which chemical potential is held constant: that of the oxidised or that of the reduced component. The number of characteristic electrocapillary curves in the general case of a reversible redox system has been shown to be equal to that of the independent variables in the Nernst equation expressing the conditions of electrochemical equilibrium of the system. The results obtained have been used for the interpretation of the electrocapillary dependences observed under polarographic conditions.