Equilibrium Solubility Behavior and Thermodynamic Properties of Potassium and Ammonium Di-Hydrogenophosphate Aqueous Solutions at T = 298.15 K

The thermodynamic properties of potassium and ammonium dihydrogenophosphate aqueous solutions and equilibrium solubility were investigated at a temperature of 298.15 K. The calculated solubility required the thermodynamic properties of the ternary system. Then, the water activities were measured at molar concentrations ranging from dilute to saturated solutions of { y KH 2 PO 4 + (1 – y )NH 4 H 2 PO 4 }(aq) and at various ionic strength fractions ( y ) using the hygrometric method. From these newly measured data, relevant properties of the mixed system, such as the osmotic coefficients, were evaluated by the Pitzer and Clegg–Pitzer–Brimblecombe ion interaction models. The parametrization of binary subsystems KH 2 PO 4 (aq) and (NH 4 )H 2 PO 4 (aq) was also developed by equations based on the mole fraction composition scales. The activity coefficients were calculated for the electrolytes KH 2 PO 4 and NH 4 H 2 PO 4 in the ternary system using the mixing parameters from the developed models. The experimental and calculated solubilities of the mixed compounds of KH 2 PO 4 and NH 4 H 2 PO 4 in aqueous solutions were determined at various ionic strength fractions from y = 0.00 to 1.00. The solubility behaviors are also presented with the iso-activity of the ternary system. The calculated results agree well with those obtained experimentally, and both models are able to better correlate with the experimental data.