was taken to minimise the chance of any transformations (e.g. the filtration of the precipitate was carried out as soon as the precipitate was visible), it is still possible that the first phase formed was another calcium phosphate, which rapidly transformed into HAP. Appropriate experiments are currently being designed to address this possibility (precipitation of calcium phosphate from supersaturated solution in a specially designed chamber within an X-ray diffractometer with a position sensitive detector). The possible formation of a mixture of HAP with ACP cannot be disregarded as a non-crystalline phase cannot be identified using X-ray diffraction. Chemical analyses of the precipitate can only provide information on the calcium to phosphate molar ratio of the precipitate, but will not necessarily prove which calcium phosphate phase has formed, as it is very likely a non-stoichiometric hydroxylapatite may precipitate (23, 24). Finally, it should be noted that the supersaturated solutions used had the stoichiometric HAP molar ratio, although the precipitated apatites had a molar ratio ranging from 1.49 to 1.65.
The results of this study show that reproducible homogeneous precipitation experiments can be performed under thoroughly controlled conditions. It was found that precipitation in a control experiment at pH 7 took place at a supersaturation degree of 10.55. The influence of acetate was found to be negligible as supersaturation was similar, at 10.56. Citrate, however, had a pronounced effect, as it required a higher degree of supersaturation (11.33) for precipitation to take place. This is interpreted as the result of citrate binding to the active sites of the nuclei inhibiting precipitation. All experiments at pH 7 showed the precipitation of poorly crystalline hydroxylapatite with no apparent precursor phase.
The authors wish to thank CEEP (Centre Européen d’ Etudes des Polyphosphates, a sector group of CEFIC, the European Chemical Industry Council) for funding of this project. Gordon Cressey assisted with XRD identification. Vic Din and Gary Jones are thanked for support and advice with chemical analyses and experimental set-up. Alan Pethybridge’s academic supervision of Jacqueline van der Houwen is acknowledged.