A calculation method is presented in this work to evaluate the ion interaction parameters for the Pitzer model from freezing points of aqueous solutions of pure electrolytes and solubility. To show the reliability of this method, one system that was not covered by the open literature was chosen. Using a small number of experimental data we were able to determine Pitzer parameters for this system reaching a concentration up to 1.8 mol(-1) kg(-1) Using this set of parameters thermodynamic properties such as osmotic coefficient, water activity, and mean activity coefficient were estimated at low temperatures for ferric nitrate-water system.