An equation which gives the electric charge per unit area of an interface (electrode): \[(\frac{\partial \gamma }{\partial E_{\text{A}}})_{T,p,\mu _{i}\neq \mu }=- Q_{\text{A}}\] where γ is the interfacial tension, EA 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.