https://doi.org/10.1351/goldbook.09065
Equilibrium electrode potential under conditions of unit concentration of species involved in the electrode reaction. \[E_{\rm{eq}} = E^{\circ\prime} - (RT/zF)\mathop \sum \limits_i \nu _i \ln(c_i/c^\circ)\] where \(E_{\rm{eq}}\) is the equilibrium electrode potential, \(R\) the gas constant, \(T\) the thermodynamic temperature, \(F\) the Faraday constant, \(z\) the electron number of an electrochemical reaction, \(\nu_{\rm{i}}\) the stoichiometric coefficients (numbers of species) in the equation of the electrode reaction (positive for products and negative for reactants), and \(c_{\rm{i}}\) are the amount concentrations of the species involved (most usually ions).
Note: It is \(E^{\circ\prime}\) which is calculated in electrochemical experiments when the concentrations of the various species are known, but their activities are not. Its value depends on the composition of the electrolyte solution. The argument of ln is dimensionless, while the concentration \(c_{\rm{i}}\) is normalized through division by the standard concentration, usually \(c^{\circ} = \pu{1 mol dm-3}\) for soluble species..