A mathematical model based on ionic equilibrium and balance equations was developed. Water solutions simulating natural water and containing up to 10 ionic and molecular species including strong and weak electrolytes were considered. The model and software allow the calculation of ionic composition, pH and electrical conductivity variation after adding or removing some known quantity of several species. The solution of an inverse problem where the ionic concentration changes are calculated as a result of pH and electrical conductivity variation of natural water is presented. The comparison of experimental and predicted data in a potentiometric titration is considered.