22. Electrode Reactions and Galvani Potential Differences

Initially, the terms Galvani potential for the electric potential in the bulk of a phase, electrochemical potential \( \tilde{\mu} \), and electron potential μ _e are introduced to characterize processes in which charge-carrying species are involved. The electrochemical potentials can be used to determine the Galvani potential difference between two phases in equilibrium, as an especially simple example between two different metals. The formation of an electric double layer at the interface of both metals as well as the corresponding Galvani potential difference, the so-called contact voltage, will be presented. More important for practical use like that in galvanic cells is, however, the Galvani potential difference between a metal electrode and an electrolyte solution. The electrochemical potentials and their possible composition dependence are used to describe the underlying charge transfer reaction and to derive Nernst’s equation. This type of charge transfer reaction can be regarded from a formal point of view as a special kind of a so-called redox reaction. Redox reactions in which electrons are transferred from one species to another are together with the proton transfer typical of acid–base reactions central to chemistry and its applications. Subsequently, different types of half-cells such as redox electrodes, gas electrodes, as well as film electrodes and the corresponding Galvani potential differences are discussed. The Galvani potential differences across liquid–liquid interfaces and membranes will be the topic of the last section. Such membrane voltages described by Donnan’s equation play an important role in biological membranes, for example, for information transfer in nerve cells.