These concentrations are used in the calculation of the supersaturation degree using the computer program PHREEQC (18). The supersaturation degree calculated for the control and the system with acetate present was the same, within error, at 10.55 and 10.56 respectively. This is to be expected, from the concentrations listed in table 2. In the presence of citrate however, much higher concentrations of calcium and phosphate were necessary for precipitation to occur. The higher concentration is partly due to complex formation with citrate in the solution, which decreases free ion concentration of calcium and phosphate. However the supersaturation degree is calculated based on free ion concentration, and therefore excludes the effect of complexation. In other words, variations in the degree of supersaturation imply an effect at the mineral surface. The supersaturation degree in the presence of citrate was found to be 11.33, which is significantly higher than the control or the acetate experiment. As this is not an aqueous effect, it has to be the result of a surface inhibition. It is therefore likely that it is due to citrate binding onto active sites of the newly formed nuclei and inhibiting growth, until there is enough critical mass of phosphate and calcium in solution for precipitation to occur regardless.
The precipitate was studied by X-ray diffraction. In figure 3 the pattern of one of the precipitates is given in comparison with the pattern of natural hydroxylapatite; all precipitates showed a similar pattern.
Figure 3.XRD-pattern of the precipitate formed in comparison with natural hydroxylapatite.