Experimental Determination of Osmotic Coefficient and Salt Solubility of System NH₄NO₃–NH₄H₂PO₄–H₂O and Their Correlation and Prediction with the Pitzer–Simonson–Clegg Model

The mixed aqueous electrolyte system of ammonium nitrate and ammonium dihydrogen phosphate was investigated at a temperature of 298.15 K. The relative humidities of this system were measured against a total molality ranging from 0.4 mol kg–1 to saturation, for various ionic-strength fractions (y) of NH4H2PO4 (y = INH4H2PO4/(INH4NO3 + INH4H2PO4)) including 1/4, 1/3, 1/2, 2/3, and 3/4. Water activities, osmotic coefficients, and solubilities were calculated from the measurements. The solid state was characterized by using powder X-ray diffraction (XRD) and attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy. The Pitzer–Simonson–Clegg model was used to correlate the results and predict the solute activity coefficients in the mixture for various system compositions. The excess Gibbs energy and solubility in the investigated molality range were also calculated based on the obtained results