TEMPERATURE-PROGRAMMED DEHYDRATION OF HYDROXYAPATITE

The temperature-programmed dehydration of several near-stoichiometric hydroxyapatites from aqueous medium has been investigated over the range from 20 to 700°C at constant heating rate. In most of this work the sample was subjected to a vacuum of the order of 10-6 to 10-7 Torr and the effluent water vapor was detected by a mass spectrometer. We shall designate this as the MTA procedure. We find two major peaks in the MTA spectra for all these samples from aqueous medium. The first, at 90-100°C, is attributed to desorption of reversibly adsorbed water on the external surfaces; and the second, at 225-260°C, we attribute to the irreversible removal of water from ultrafine pores in the solid. The MTA technique has been employed to demonstrate that hydroxyapatite in the hydrated state undergoes H2O2O exchange when soaked in heavy water. This applies to both the 90-100°C peaks and the 225-260°C peaks. It is suggested that the considerable variation in the peak temperatures observed over the higher range from 221 to 257°C may be attributed to a difference in pore diameters, and that we must attain a higher temperature to empty a narrower pore. © 1979.