Aqueous solutions are involved in many processes. Modelling is now becoming a necessary tool for processes simulation or optimisation. In this paper, the model is based on a thermodynamic description of the electrolytic system. The mathematical formulation includes: chemical or ionic equilibriums, physical equilibriums, between the solid phases that might be present, mass balances and an electroneutrality equation. Due to the electrostatic forces, these kinds of systems cannot be successfully described by the concept of an ideal solution. The calculation of activity coefficients is required. However, choosing a group contribution method to calculate the activity coefficient conserves user friendliness. The non-linear system obtained is solved by using Newton Raphson's method. The model is validated on several examples with concentrations of salts being up to saturation.