An equation which gives the electric charge per unit area of an interface (electrode): $$(\frac{\partial \gamma }{\partial E_{\text{A}}}{)}_{T,p,{\mu }_i\ne \mu }=-Q_{\text{A}}$$ where $\gamma$ is the interfacial tension, $E_{\text{A}}$ is the potential of a cell in which the reference electrode has an interfacial equilibrium with one of the ionic components of A, $Q_{\text{A}}$ is the charge on unit area of the interface, ${\mu }_i$ 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.