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.