Consider an arbitrary, possibly multi-electron step, redox reaction involving a number of species Si defined as

where νi is the stoichiometric coefficient of the reacting species of index i and n is the number of participating electrons.

The equilibrium potential of the electrode reaction, Eeq (V), is the electrode potential (the difference between the electrode phase and electrolyte phase potentials, ϕs − ϕl) for which the net reaction rate (and the local current density, iloc) is zero.

The equilibrium potential is directly related to the change of Gibbs free energy of the reacting species, ΔG, as

Since the free energy of the reacting species is concentration dependent, also the equilibrium potential is concentration (activity) dependent. Eeq is usually defined according to the Nernst equation as

where Eeq, ref (V) is the equilibrium potential for a reference state for which all species activities ai (unitless) are equal to a chosen set of reference activities ai, ref (unitless).