In this equation, µ M is the Fermi energy of the metal and ∆ S M ϕ = ϕ M -ϕ S represents the inner potential difference between the metal and the solvent. The involved electrochemical quantities are all shown in Figure where the outer (Volta) potential ψ α is defined as the work required to bring a unit point charge from far away to just outside the surface of the phase α.