Thermochemical Properties and Decomposition Kinetics of Ammonium Magnesium Phosphate Monohydrate

Ammonium magnesium phosphate monohydrate NH₄MgPO₄·H₂O was prepared via solid state reaction at room temperature and characterized by XRD, FT-IR and SEM. Thermochemical study was performed by an isoperibol solution calorimeter, non-isothermal measurement was used in a multivariate non-linear regression analysis to determine the kinetic reaction parameters. The results show that the molar enthalpy of reaction above is (28.795±0.182) kJ/mol (298.15 K), and the standard molar enthalpy of formation of the title complex is (−2185.43±13.80) kJ/mol (298.15 K). Kinetics analysis shows that the second decomposition of NH₄MgPO₄·H₂O acts as a double-step reaction:an nth-order reaction (F_n) with n=4.28, E₁=147.35 kJ/mol, A₁=3.63×10¹³ s⁻¹ is followed by a second-order reaction (F₂) with E₂=212.71 kJ/mol, A₂=1.82×10¹⁸ s⁻¹.