OPTIMALIZATION OF THE PREPARATION OF NA+-CONTAINING AND CO-3(2)-CONTAINING HYDROXYAPATITE BY THE HYDROLYSIS OF MONETITE

In this study, we investigated the preparation of Na+- and CO32- containing apatites by the hydrolysis of monetite (CaHPO4)at 95-degrees-C in solutions with Na2CO3 concentrations ranging from 0.001M to 0.250M. When the pH of the solution is relatively low ( < 9), unreacted CaHPO4 might be found in the precipitates, indicating that a high pH is necessary for the complete conversion of monetite to apatite. Moreover, low Na2CO3 concentrations ( < 0.005M) favour, even at high pH approximately 10), the incorporation of HPO42- in the apatite lattice and suppress the CO32- incorporation corroborating the competition between CO32- and HPO42- for an incorporation in the lattice. It was also found that a coprecipitation of CaCO3 (calcite) occurs after short reaction times. However, after hydrolysis for at least 5 h, no calcite could be detected in the precipitates. It is concluded that uncontaminated, well crystalline and single phase apatites with varying Na- and CO3 content can be obtained in a highly reproducible way by the hydrolysis of CaHPO4 for at least 5 h in Na2CO3 Solutions with a concentration between 0.010M and 0.250M. A suitable choice of the solid/solution ratio permits to restrict the changes of CO32- and Na+ concentration in the solution to acceptable values.