The potential differences observed at metal liquid interfaces arise as a result of two distinct phenomena: 1) at reversible electrodes, phenomena associated with Faraday’s Law; 2) at polarized electrodes, phenomena associated with the charging of the double layer. At reversible (or nearly reversible) electrodes, electricity can flow through the interface by the mechanism of an electron exchange reaction between the metal and the chemicals present at the electrode surface. When electrons are donated by the metal (reduction), the electrode is a cathode. When electrons are removed by the metal (oxidation), the electrode is an anode. When electron exchange is at equilibrium, the electrode potential is governed by the Nernst equation. A kinetic derivation of this equation is given. Its two terms, the standard potential term, and the Mass Action term are discussed. The standard hydrogen electrode (SHE) provides a convenient reference point from which to measure electrode potentials. The conventions regarding the signs and nomenclature of electrode potentials are reviewed.
With a net current flowing, the electron exchange is not at equilibrium and the potential of the electrode is displaced from the reversible Nernst value (polarization potential). Polarization as an anode shifts the potential in a more noble direction; as a cathode, in a less noble direction. The application of cathodic polarization to the cathodic protection of a metal is discussed. The properties of the electrical double layer at an ideal polarized electrode are surveyed both from the point of view of the potential of zero charge (the isoelectric point of the electrode) and from the point of view of the differential capacity of the double layer.
