It has been shown that it is possible to find the true ζ value of Pyrex glass particles forming a diaphragm, in contact with solutions of different concentrations of KNO3, from Wijga's experimental data by using an equation of the form 1/ζa = 1/ζ + m/r, where ζa is the electrokinetic potential calculated from Smoluchowski equation, m is a constant under the conditions of the experiments and r is found from χ, the specific surface conductivity of the diaphragm, from data on conductance measurements.