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The water−phosphoric acid−cyclohexane ternary system was studied at 25 and 35 °C under atmospheric pressure, and the binodal curve was determined by the cloud-point method. In addition, the isoplethic section between 2H 3 PO 4 •H 2 O−cyclohexane was investigated using a Tammann diagram based on the thermal analysis of different samples with the determination of hemihydrate melting temperature. The concentration of phosphoric acid and cyclohexane was assayed by volumetric titration and gas chromatography, respectively; the amount of water was calculated by the material balance equation. The miscibility between phosphoric acid and cyclohexane was found to be very low with the appearance of a three-phase stability domain liquid−liquid−solid by the presence of phosphoric acid crystals: hemihydrate (2H 3 PO 4 •H 2 O) and anhydrous phosphoric acid (H 3 PO 4 ).

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The freezing depression points of aqueous phosphoric acid have been determined using a method based on measurement of solution conductivity. The osmotic coefficient for freezing point lowering was calculated by the condition of solid-liquid equilibrium and by the equation proposed by Bjerrum (1918). A calculation method is presented in this study to evaluate the ion interaction parameters for the Pitzer model from freezing point and isopiestic molalities of aqueous phosphoric acid solution. The model of Chen, Bromely and the Three-Characteristic Parameter Correlation (TCPC) are also used to model this system. The isopiestic molality values from literature data are used to optimize these models. Using these parameters, we were able to estimate thermodynamic properties: osmotic coefficient, mean activity coefficient and water activity for the H 3 PO 4-H 2 O system, and contribution to a better understanding of the thermodynamic behaviour of the aqueous system containing phosphoric acid.