the difference in electric potential between an electrode and the electrolyte with which it is in contact (usually, between a metal and an electrolyte solution).

An electrode potential is due to the transfer of charged particles across a phase boundary, to the specific adsorption of ions, or, when polar molecules are present (including those of a solvent), to the polar adsorption of the molecules. The magnitude of the electrode potential in a nonequilibrium state depends both on the nature and composition of the phases in contact and on the kinetic behavior of the electrode reactions at the phase boundary. The equilibrium value of the jump in potential at an electrode-solution boundary is determined exclusively by the specifics of the electrode reaction and is independent of the nature of the electrode and its adsorption of surfactants. This absolute potential difference between points located in two different phases cannot be measured experimentally or calculated theoretically. Relative electrode potentials have practical significance. Usually called simply electrode potentials, they represent the difference between the electrode potential of the electrode in question and that of a reference electrode—most often a normal hydrogen electrode, whose electrode potential is assumed by convention to be equal to zero.

When there is electrochemical equilibrium at the electrode, the value of the electrode potential E can be expressed in terms of the change in the Gibbs free energy ΔG of the reaction: E = –ΔG/zF, where z is the number of electrons involved in the electrochemical process and F is the Faraday constant. In this case the electrode potential depends on the activity a of the substances involved in the reaction (potential-determining substances). For Me/Meⁿ⁺ electrodes E = E₀ + (RT/zF) In a_Meⁿ⁺, where R is the gas constant, T is the temperature, and E₀ is the standard potential. For oxidation-reduction with inert electrodes, in which all components of the electrochemical reaction are in solution, the electrode potential (oxidation-reduction potential) is determined by the activity both of the oxidized form a_ox and the reduced form a_red of the substance:

where v is the stoichiometric coefficient.

If the simultaneous occurrence of more than one electrode reaction is possible at an electrode, the concept of a steady-state potential is used. When an electric current is passed, the measured electrode potential will differ from the equilibrium potential by the amount of polarization (seePOLARIZATION, ELECTROCHEMICAL).