Observations on the apparent solubility of carbonate-apatites

Previous reports indicating that the apparent solubilities of carbonate-apa tites are low appear to conflict with findings that carbonate incorporation into the apatite structure tends to reduce stability. Carbonate-apatites w ere prepared by a precipitation method and by hydrolysis of monetite and br ushite, Apparent solubility profiles were determined by measuring dissoluti on after exposure for 24 h to acetate buffers with known saturations with r espect to hydroxyapatite, All preparations showed a range of apparent solub ilities, in agreement with previous work. Precipitated samples had higher a pparent solubilities than samples prepared by hydrolysis and this was corre lated with differences in crystallinity, Further experiments showed that py rophosphate ion reduced the apparent solubility, but it was concluded that pyrophosphate occurring naturally in synthetic apatites would be insufficie nt to cause low apparent solubilities. Microscopical observations showed th at precipitated carbonate-apatites were composed of small crystals of unifo rm size, whereas carbonate-apatites prepared by hydrolysis consisted of bot h small crystals and very large crystals. The low apparent solubilities obs erved in the latter preparations are attributed to the large crystals. The surface phenomena possibly involved in apparent solubility behavior are dis cussed. (C) 1999 Academic Press.