Thermodynamic representation of the solubility for potassium dihydrogen phosphate (KDP) + water + alcohols systems

The solubility of potassium dihydrogen phosphate (KDP) in water and in aqueous solutions of ethanol, 1-propanol and 2-propanol was experimentally determined by means of a visual polythermal method. Solubility measurements were performed in the temperature range of 293 to 333 K, for an alcohol content varying between 0 and 0.30 kg/kg water for ethanol and 2-propanol, and from 0 to 0.15 kg/kg water for 1-propanol. The phase equilibria condition and the mean ionic activity coefficients calculated by the Chen model allowed to describe the ternary solubilities. The mean ionic activity coefficients were calculated with nine binary parameters: six parameters for the binaries KDP/water and water/alcohol (taken from literature), and three parameters for the binary KDP/alcohol, regressed in this work. The binary interaction energy parameters were defined as τij = ΔgijRT, where Δgij = f(T). The optimized parameters were two Δgij energy parameters and the nonrandomness factor αij for the KDP/alcohol binaries. Ternary solubility was described with a standard deviation of 0.0020 kg salt/kg water and less than 0.9°C, respectively in terms of concentration and temperature, for a 95% confidence interval. The sensitivity analysis of the model allows to conclude that the uncertainty on the activity coefficients values obtained by the Chen model is mainly due to the nonrandomness factor αij and to the temperature.