The left-hand side of one of the various cross-differential relationships that can be obtained from the Gibbs adsorption equation when only one chemical potential (\(\mu \)) is considered as variable, viz. \[(\frac{\partial E}{\partial \mu })_{T,p,\sigma }=- (\frac{\partial \mathit{\Gamma }}{\partial \sigma })_{T,p,\mu }\] where \(E\) is the potential difference, \(T\) is the temperature, \(p\) is the pressure, \(\mathit{\Gamma}\) is the surface excess and \(\sigma \) is the charge density.