Differential capacitance (per unit area of interphase or electrode; SI unit \(\pu{F m-2}\)) is given by \[C=(\frac{\partial Q}{\partial E})_{T,p,\mu _{i},...}\] where \(E\) is the potential of the electrode with respect to a reference electrode, \(\mu _{i}\) is a set of chemical potentials which are held constant, \(T\) is the thermodynamic temperature, \(Q\) is the electric charge (per unit area of interphase or electrode), and \(p\) is the pressure.