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 (µ) 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, Γ is the surface excess and σ is the charge density.