Thermodynamic Properties of the Mixture of Potassium Phosphate Fertilizer with KCl: Water Activity, Osmotic and Activity Coefficients, Excess Energy, and Solubility of KH2PO4–KCl–H2O at 298.15 K

The aim of this research is to investigate the impact of salts on fertilizer phosphate KH₂PO₄ using a hygrometric method at 298.15 K by measuring the thermodynamic properties of the KH₂PO₄–KCl–H₂O system. These measurements of relative humidity are conducted from 0.2 mol·kg^–1 to the saturation of the solutes and for different ionic strength fractions y of KCl (y = I_KCl/(I_KH2PO4 + I_KCl)). The obtained data allowed us to determine the water activities and osmotic coefficients. Four thermodynamic models are used to validate our results, including the Dinane ECA rule, Leitzke–Stoughton (LS II), Robinson and Stokes (RS), and Lin et al. equations. Additionally, we have employed the Pitzer–Simonson–Clegg (PSC) model to correlate the obtained data and to predict solute activity coefficients and excess Gibbs energy for various compositions. The hygrometric method is also used to determine the solubility of the components in the ternary system. To identify the nature of the solid phase, the Powder X-ray Diffraction (PXRD) is used and diagram solubility of KH₂PO₄–KCl–H₂O is plotted. The results obtained from this work can serve as crucial information on the behavior of the system KH₂PO₄–KCl–H₂O for the development of new fertilizers aimed at improving soil fertility, crop yields, agricultural productivity, and sustainable agriculture.