An approach has been developed to determine the equilibrium model in heterogeneous aqueous systems containing a mixture of solid phases from the initial composition of the heterogeneous mixture and the equilibrium values of the pH. This approach allows to significantly reduce the quantity of measured data, properly for solubility measurements. By using the new deduced equations it is possible to determine the solubility products of precipitates on the basis of the known composition of initial mixtures and pH values of saturated solutions avoiding some difficult experimental solubility measurements. The derived relations are useful for solving both direct and indirect equilibrium problems.