A Thermodynamic Model for the NH4+, K+ // H2PO4-, H2P2O72-, NO3-, Cl- - H2O System

We have developed a quantitative thermodynamic description of the NH4H2PO4-(NH4)2H2P2O7-KH2PO4-K2H2P2O7-NH4NO3-KNO3-NH4Cl-KCl-H2O system, which is of great importance for fertilizers applications. The liquid phase and all solid solutions were modeled using the Modified Quasichemical Model for short-range ordering. The liquid model includes the reaction of dimerization of AH2PO4 (where A = NH4+ or K+) according to: 2 AH2PO4(liquid) ↔ A2H2P2O7(liquid) + H2O(liquid or gas). The model parameters are mainly obtained by the critical evaluation and optimization of available thermodynamic and phase equilibrium data for the lower-order subsystems. The model is then used to estimate the properties of multicomponent salts from these assessed parameters for the lower-order subsystems, using interpolation methods along with Gibbs energy minimization software. When used in conjunction with thermodynamic databases for solids, solid solutions, and gases, these databases permit the calculation of complex multiphase equilibria in multicomponent systems, using the FactSage thermochemical software. In particular, calculations of interest for the production and storage of fertilizers may be performed.