Differential capacitance (per unit area of interphase or electrode; SI unit ${\text{F m}}^{-2}$) 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.