Defined for species B in phase β as \[\alpha _{\text{B}}^{\beta } = \mu _{\text{B}}^{\beta }\,-\,z_{\text{B}}\ F\ \psi ^{\beta }\] where µ B β is the electrochemical potential of species B in phase β, z B is the charge number of species B, F is the Faraday constant, and ψ β is the outer electric potential of phase β. Since ψ β is zero when the charge on the phase β is zero, the real potential may be regarded as the value of the electrochemical potential of the uncharged phase.