Structural Change of Synthetic Hydroxyapatite by Ion-Exchange Reaction

The cation-exchange characteristics between Sn2+ ions in aqueous solutions containing various counter-anions (Cl-, F-, BF4-) and Ca2+ ions in synthetic hydroxyapatite (samples) [Ca10(PO4)6(OH)2] have been investigated in the pH range below 3 at 25°C by a normal batch method.It was found that the hydroxyapatite samples are transformed into amorphous states by the reactions between Ca2+ ions in the samples and Sn2+ ions in the SnCl2 and Sn(BF4)2 acidic aqueous solutions with pH of 3.0 or below with a molar ratio of Sn2+/Ca2+≅1.0. These results are interesting because the Sn2+ uptake phenomena appear to be an ion-exchange reaction between Sn2+ ions in solution and Ca2+ ions in the apatite even in an acidic atmosphere. However, the hydroxyapatite was decomposed into Sn3F3PO4 in a SnF2 acidic aqueous solution.The apatite samples are quickly transformed into the amorphous state in the SnCl2 acidic aqueous solutions, while they are slowly transformed into the amorphism in Sn(BF4)2 acidic aqueous solutions even with high concentrations of Sn2+ ions (ex. 44.0×10-3mol/200cm3).The existence of hydroxyapatite as amorphism in acidic aqueous solutions such as SnCl2 and Sn(BF4)2 is quite interesting because in general the hydroxyapatite has been found to be dissolved in acidic aqueous solutions. Moreover, the obtained amorphism are found to be stable up to at least 500° to 600°C but to be unstable in alkaline solutions. The characteristics of Sn2+ ions are found to be quite different from those of homologous Pb2+ ions which have been found to form crystalline Pb2+ apatite even in such an acidic atmosphere mentioned above.