Defined for species B in phase β as $${\alpha }_{\text{B}}^{\beta }={\mu }_{\text{B}}^{\beta }-z_{\text{B}}F{\psi }^{\beta }$$ where ${\mu }_{\text{B}}^{\beta }$ is the electrochemical potential of species B in phase β, $z_{\text{B}}$ is the charge number of species B, $F$ is the Faraday constant, and ${\psi }^{\beta }$ is the outer electric potential of phase β. Since ${\psi }^{\beta }$ 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.