A theoretical analysis of the diffuse layer in solutions of a 1:1 electrolyte near the zero charge point was carried out without using the ideal solution concept Accordance with the Gibbs adsorption equation demands fulfilment of the equality of the cross concentration derivatives of the chemical potentials of cations and anions. An expression was found for the differential capacity of the diffuse layer at the zero charge point as a function of the concentration derivatives of the chemical potentials of cations and anions. On this basis, and on the basis of Trasatti and coworkers’ idea of using the ideally polarizable electrode as a constant reference electrode, it was shown that there exists the possibility, in principle, of finding experimentally the derivatives of the chemical potentials of ions with respect to both the cation concentration and the anion concentration. A plausible explanation for conserving the Parsons-Zobel dependence in the non-ideal solutions is given.