The chemical potential of the positive ions in the solution is given by: \begin{equation} \mu_{+}=\left(\frac{\partial G}{\partial n_+}\right)_{T,P, n_{j\neq +}} \end{equation} We cannot measure $\mu_{+}$ or $\mu_{-}$ since we cannot add only negative or positive ions to the solution.
Therefore, the chemical potential of the electrolyte as a whole is defined. \begin{equation} \mu_{i}=\left(\frac{\partial G}{\partial n_i}\right)_{T,P,n_A} \end{equation} where $n_i$ represents the moles of dissolved electrolyte.