On the impedance of galvanic cells XVIII. The potential dependence of the faradaic impedance in the case of an irreversible electrode reaction, and its application to A.C. polarography

Abstract

A theoretical treatment for the potential dependence of the faradaic impedance, in the case that the electrode reaction behaves irreversibly with respect to the direct current, using the steady-state concept, is presented. An analysis of the expressions obtained in the complex impedance plane is given and a method is described to measure even a very small rate constant, ksh. The consequences for a.c. polarography are discussed. It has been found that even very irreversible electrode reactions exhibit substantial peaks in an a.c. polarogram. If only the oxidized form of the reactant is present, one peak appears even for very small ksh-values. For decreasing ksh, the peak height decreases to a limiting value, independent of ksh andsufficiently large to be detectable in an a.c. polarogram. If both the oxidized and reduced form are present, two peaks occur for irreversible systems, the peak heights again being independent of the ksh-value.