Fundamental equation in electrochemistry that describes the dependence of the equilibrium electrode potential on the composition of the contacting phases, written as a reduction: $$E_{\mathrm{e}\mathrm{q}}=E^{\circ }-(RT/zF)\sum _i{\nu }_i\ln (a_i)$$ where $E_{\mathrm{e}\mathrm{q}}$ is the equilibrium electrode potential, $E^{\circ }$ the standard electrode potential of the reaction, $R$ the gas constant, $T$ the thermodynamic temperature, $F$ the Faraday constant, $z$ the electron number of an electrochemical reaction, and ${\nu }_{\mathrm{i}}$ are the stoichiometric coefficients (numbers of species) in the equation of the electrode reaction, positive for products and negative for reactants, while $a_{\mathrm{i}}$ represents the activities of the species involved (most usually ions).