Reaction characteristics of dental and synthetic apatites with Fe2 + and Fe3 + ions

Abstract

Various effects of Fe2+ and Fe3+ ions on the reaction characteristics of dental and synthetic apatites have been investigated. It has been found that both Fe2+ and Fe3+ ions inhibit crystallization of calcium phosphates during the calcification process, and that the colour of the reaction product turns brown owing to the incorporation of these ions. The dental and the synthetic hydroxyapatites, the latter being a pure model substance of dental apatite, were also coloured when they were treated with Fe2+ solutions. The results show that Ca2+ ions in the apatites are exchanged for Fe2+ ions in solution and the ion-exchanged Fe2+ ions are partially oxidized to Fe3+ ions by dissolved oxygen in solution. Furthermore, the dissolution rate of the dental apatite treated with Fe2+ is inhibited in organic acid solution. For the reaction of the dental and the synthetic hydroxyapatites with Fe3+ ions, both the reaction rate and the removal of Fe3+ are far larger than those of Fe2+ and the product is a stable amorphous solid even in acidic solution of pH 3.0–5.0. These interesting phenomena are explained in terms of formation of an amorphous ferric phosphate hydrate by an attack reaction of Fe3+ ions in solution for Ca2+ ions of the apatite, assisted by the loosening effect of H+ ions on the skeletal structure of the apatite.