Lippman's equation

An equation which gives the electric charge per unit area of an interface (electrode):
( ∂ γ ∂ E A ) T , p , μ i ≠ μ = − Q A
where γ is the interfacial tension, E A is the potential of a cell in which the reference electrode has an interfacial equilibrium with one of the ionic components of A, Q A is the charge on unit area of the interface, μ i is the chemical potential of the combination of species i whose net charge is zero, T is the thermodynamic temperature and p is the external pressure. Since more than one type of reference electrode may be chosen, more than one quantity Q may be obtained. Consequently Q cannot be considered as equivalent to the physical charge on a particular region of the interphase. It is in fact an alternative way of expressing a surface excess or combination of surface excess of charged species.
PAC, 1974, 37, 499 (Electrochemical nomenclature) on page 508
PAC, 1986, 58, 437 (Interphases in systems of conducting phases (Recommendations 1985)) on page 445