An equation which gives the @E01923@ per unit area of an @I03082@ (electrode): \[(\frac{\partial \gamma }{\partial E_{\text{A}}})_{T,p,\mu _{i}\neq \mu }=- Q_{\text{A}}\] where \(\gamma \) is the @I03088@, \(E_{\text{A}}\) is the potential of a cell in which the @R05229@ has an interfacial equilibrium with one of the ionic components of A, \(Q_{\text{A}}\) is the charge on unit area of the @I03082@, \(\mu _{i}\) is the @C01032@ of the combination of species \(i\) whose net charge is zero, \(T\) is the @T06321@ and \(p\) is the external pressure. Since more than one type of @R05229@ 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 @I03118@. It is in fact an alternative way of expressing a @S06171@ or combination of @S06171@ of charged species.