Energetics of Calcium Phosphate Nanoparticle Formation by the Reaction of Ca(NO3)2 with (NH4)2HPO4

The energetics of the reaction between Ca(NO3)(2)(aq) and (NH4)(2)HPO4(aq) leading to the formation of calcium phosphate nanoparticles was investigated by flow calorimetry. The relationship between the observed enthalpy change, the pH of the (NR4)(2)HPO4(aq) solution, and the elemental composition and morphology of the obtained compounds was studied. Results of elemental analysis, combined thermogravimetry-infrared spectroscopy, X-ray powder diffraction, and scanning electron microscopy showed that the change of the pH during the precipitation reaction through the addition of controlled amounts of NaOH(s) to the (NH4)(2)HPO4(aq) solution leads to significant and reproducible changes of the chemical composition, morphology, and amorphous character of the obtained materials. These changes are reflected by the corresponding enthalpy of reaction, which seems to be predominantly determined by the differences in chemical composition.