Phase Equilibria and Thermodynamic Properties in the Oxalic Acid–Citric Acid–Water System

The liquid–solid and liquid–vapor phase equilibria in the oxalic acid (H2Ox)–citric acid (H3Cit)–water system at a temperature of 298.15 K (25°C) and 298.15–308.15 K, respectively, are studied. The stability regions of the H3Cit⋅H2O and H2Ox⋅2H2O crystalline hydrates are determined. Experimental data on water activity for solutions of the oxalic acid–citric acid–water ternary system are obtained; the saturated vapor pressure above solutions of the H3Cit–H2Ox–H2O system is measured by the static method; water activity is determined by the dew point method. It is shown that water activity depends on temperature only slightly in a temperature range of 298.15–308.15 K. The solubility products of H3Cit⋅H2O and H2Ox⋅2H2O are calculated using the Pitzer model. A model that describes the thermodynamic properties of solution and phase equilibria in the H3Cit–H2Ox–H2O ternary system with an accuracy that is not inferior to the experimental accuracy is developed.