Mohsen Nosrati - Biotechnology Group, Department of Chemical Engineering, Faculty of Engineering, Tarbiat Modarres University
Ali Pirouzi - Biotechnology Group, Department of Chemical Engineering, Faculty of Engineering, Tarbiat Modarres University
Samira Vasheghani Farahani - Biotechnology Group, Department of Chemical Engineering, Faculty of Engineering, Tarbiat Modarres University

The electrolyte Nonrandom Two Liquid–Nonrandom Factor (NRTL–NRF) of Haghtalab and Nosrati was modified to determine the excess Gibbs free energy of non–completely dissociable electrolyte systems. In such electrolyte systems, dislike strong salts and acids, undissociated part of the weak acid would remain in the aqueous solution and causes ambiguity and complexity in molecule–ion and molecule–molecule interactions. When excess Gibbs free energy of an aqueous solution which contains non–completely dissociable electrolyte is determined, mean ionic activity coefficient of ions (cation and anion), activity coefficient of undissociated part, and osmotic coefficient of the solvent (water) will be calculated precisely. Our model was applied to some weak electrolyte aqueous solutions i.e. acetic acid, boric acid, arsenic acid, and phosphoric acid solutions as applied and wide– spread industrial examples. Weak electrolytes' thermodynamic behaviors (such as activity coefficients of ionic and molecular species, as well as water osmotic coefficient and pH) were investigated in a wide range of concentration by choosing an appropriate equilibrium reaction between molecular and ionic species for each weak electrolyte solutions. The model contains four adjustable parameters which were determined using water osmotic experimental data. The results of dissociation and of activity coefficients are presented separately.

Non–Completely Dissociable Electrolytes, Excess Gibbs Free Energy, Activity Coefficient