Dissolution of urea phosphate: A kinetic and thermodynamic study by solution calorimetry

A thermodynamic and kinetic study of the dissolution properties of urea phosphate (UP) in water is performed using a Calvet type microcalorimeter. Calorimetric measurements reveal the dissolution is endothermic and the heat is perfectly proportional to the amount of UP dissolved, yielding an enthalpy of (33.755 ± 0.65) kJ/mol. By combining the enthalpy data of our measurements and literature, we derive the enthalpies of urea-phosphoric acid adduct reaction, formation of solid UP and UP dissociation. Dissolution kinetics parameters are estimated using the collected dynamic calorimetric data that are well represented with quasi-second-order kinetic model, giving an activation energy of (16.7 ± 2.7) kJ/mol. Thermodynamic modeling of the pH difference between UP and phosphoric acid solutions shows the degree of UP dissociation decreases from 74% to 42% when UP molality increases from 0.02 to 1 mol/kg. Furthermore, the species molality distributions in UP solutions are calculated based on mass balance and the dissociation equilibrium relations considered.