The combined reference electrode concept, the Gibbs adsorption equation, and the Onsager mutual relationship are used to construct a generalized theory of electrochemical faradaic impedance for mixed electrolyte solutions. This theory of impedance takes into account the migration of discharging ions and changes in chemical potential of the base electrolyte near the electrode surface due to passage of alternating current. It is shown that there is a relationship between the Onsager relations and the Gibbs adsorption equation.