Apparent solubility of hydroxyapatite in aqueous medium and its influence on the morphology of nanocrystallites with precipitation temperature

Two differing wet-chemical synthesis routes, Ca(OH)₂ + H ₃PO₄ and CaCl₂ + Na₃PO ₄/NaOH, were used to prepare hydroxyapatite (HA) at various temperatures ranging from 30 to 95°C. The electrical conductivity of the solution was measured at regular intervals of time during H₃PO ₄ and Na₃PO₄ addition to the suspension/solution containing Ca²⁺ ions. The rate of change of conductivity is used to note the end point of the reaction. X-ray diffraction of the dried, precipitated particles revealed HA as the predominant phase, and the FTIR spectroscopy studies indicated the presence of CO₃^2- groups which substituted PO₄^3- groups in the HA lattice (B-type). FESEM observations revealed that the aspect ratio of the particles decreased with increasing precipitation reaction temperature in one system [Ca(OH)₂ + H₃PO₄] and in the other system it increased with increasing temperature. The changes in the morphology with temperature were analyzed through conductivity measurements and the thermochemical properties of the reaction systems. Conductivity measurements showed that the concentration of dissolved ions at the end point of the reaction between Ca(OH)₂ and H₃PO₄ increased, indicating an increased apparent solubility of HA with increasing temperature, whereas the end-point conductivity did not increase noticeably in the other reaction system.