A new method has been developed for the determination of the solubility pro
duct of complexonate of known composition; because of the initial compositi
on of the heterogeneous mixture (lot a), it uses only the equilibrium value
s of the pH. For the determination of the stability constant in solution, t
hese data have been completed with information on the solubility of the pre
cipitate. We have shown that, on the basis of lot a and in the absence of r
eactions of formation of complexes in solution, it is possible to evaluate
the equilibrium constants, since they are residuals of the compositions of
the precipitate. We conclude on the possibility of applying this method to
the test and standardization of ion-selective electrodes. The qualities and
defect of the method are discussed. Otherwise, the values for the equilibr
ium constants and the equilibrium as well as the residual concentrations th
at we have calculated for a series of real systems from known experimental
data are in good accord with those reported in tables under the same condit
ions.