Thermodynamic Properties\nof the Mixture of Potassium\nPhosphate Fertilizer with KCl: Water Activity, Osmotic and Activity\nCoefficients, Excess Energy, and Solubility of KH<sub>2</sub>PO<sub>4</sub>–KCl–H<sub>2</sub&

The aim of this research is to investigate the impact\nof salts\non fertilizer phosphate KH<sub>2</sub>PO<sub>4</sub> using a hygrometric\nmethod at 298.15 K by measuring the thermodynamic properties of the\nKH<sub>2</sub>PO<sub>4</sub>–KCl–H<sub>2</sub>O system.\nThese measurements of relative humidity are conducted from 0.2 mol·kg<sup>–1</sup> to the saturation of the solutes and for different\nionic strength fractions <i>y</i> of KCl (<i>y</i> = <i>I</i><sub>KCl</sub>/(<i>I</i><sub>KH<sub>2</sub>PO<sub>4</sub></sub> + <i>I</i><sub>KCl</sub>)).\nThe obtained data allowed us to determine the water activities and\nosmotic coefficients. Four thermodynamic models are used to validate\nour results, including the Dinane ECA rule, Leitzke–Stoughton\n(LS II), Robinson and Stokes (RS), and Lin et al. equations. Additionally,\nwe have employed the Pitzer–Simonson–Clegg (PSC) model\nto correlate the obtained data and to predict solute activity coefficients\nand excess Gibbs energy for various compositions. The hygrometric\nmethod is also used to determine the solubility of the components\nin the ternary system. To identify the nature of the solid phase,\nthe Powder X-ray Diffraction (PXRD) is used and diagram solubility\nof KH<sub>2</sub>PO<sub>4</sub>–KCl–H<sub>2</sub>O is\nplotted. The results obtained from this work can serve as crucial\ninformation on the behavior of the system KH<sub>2</sub>PO<sub>4</sub>–KCl–H<sub>2</sub>O for the development of new fertilizers\naimed at improving soil fertility, crop yields, agricultural productivity,\nand sustainable agriculture.