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.

Source:

PAC, 1974, 37, 499 (Electrochemical nomenclature) on page 508

PAC, 1986, 58, 437 (Interphases in systems of conducting phases (Recommendations 1985)) on page 445