Determination and Correlation of the Ca2+ and Mg2+ Solubility in Ammonium Polyphosphate Solution from 278.2 to 323.2 K

The solubility data of Ca2+ and Mg2+ in ammonium polyphosphate (APP) solution are the key to the formulation of fluid fertilizers, especially in areas where hard water with excessive Ca2+ and Mg2+ is conventionally used for irrigation. The solubility of Ca2+ and Mg2+ in APP solutions of four different Mg2+/Ca2+ molar ratios is determined by the dynamic method at the temperature ranging from 278.2 to 323.2 K and at solution pH values ranging from 5.5 to 8.0 under an atmospheric pressure. The X-ray diffraction patterns of the solid phase formed in the four abovementioned types of mixed solutions showed that their crystal structures are different and that the crystal structure conversion occurs in the process of dissolution and crystallization. The solubility of Ca2+ and Mg2+ increased with the ascending temperature and the solution pH values. The solubility of Ca2+ in APP solutions decreased with the increase of the Mg2+ molar ratio [x(Mg2+)] relative to the total Ca2+ and Mg2+ amount, while the solubility of Mg2+ increased with the increase of x(Mg2+). The experimental solubility data, expressed in molar ratios of components, were correlated with the modified Apelblat equation. The correlation showed good agreement with the experimental results, and the average values of the root-mean-square deviation were 1.46 × 10–2 for Ca2+ and 4.58 × 10–2 for Mg2+, respectively.