Behaviors salts of diammonium hydrogenphosphate or sulfate containing in fertilizers, solubility and thermodynamic study in aqueous solutions at various temperatures

Abstract

In this study, both experimental and calculated solubilities of binary aqueous solutions of diammonium (hydrogenphosphate or sulfate) were determined at various temperatures. The experimental solubilities of ${\left({\mathrm{NH}}_4\right)}_2\mathrm{HP}{O}_4\left(\mathrm{aq}\right)$ and ${\left({\mathrm{NH}}_4\right)}_{2}S{O}_4\left(\mathrm{aq}\right)$ ranged from $m_{s\left(\exp \right)}$ = 5.260 to 8.641 $\mathrm{mol}·{\mathrm{kg}}^{-1}$ and $m_{s\left(\exp \right)}$ = 5.799 to 7.122 $\mathrm{mol}·{\mathrm{kg}}^{-1}$, respectively, between the temperatures of 298.15 K and 353.15 K. The thermodynamic properties of the studied systems were derived from water activity measurements. Thus, the water activities of these aqueous solutions were measured from dilute to saturated solutions using the hygrometric method at different temperatures. From the new experimental water activities, the osmotic coefficients of water for the electrolyte solutions were obtained at the temperature range. The treatment of the obtained coefficients was made by the expanded ion interaction models. The related thermodynamic properties of the sub-binary systems were assessed using the developed models of Pitzer and Clegg–Pitzer–Brimblecombe. Indeed, the binary parameters of diammonium hydrogenphosphate or sulfate aqueous solutions were calculated and used for the computation of solute activity and osmotic coefficients; a good description of the calculated activity coefficients is acquired by the adequate models. The dissolution Gibbs energies and the solubility product of ${\left({\mathrm{NH}}_4\right)}_2\mathrm{HP}{O}_4\left(s\right)$ and ${\left({\mathrm{NH}}_4\right)}_{2}S{O}_4\left(s\right)$ were also evaluated at different temperatures. In addition, excess thermodynamic functions such as $∆G^E,∆H^E$, and $∆S^E$, along with the partial molar entropies of solvent and solute, were obtained. Based on these results, the sulfate and phosphate systems exhibit opposite hydration behavior and thermodynamic stability.