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.