Dissolution of Synthetic Hydroxyapatite in the Presence of Lanthanum Ions.

Numerous cations exhibit strong interactions with dental or synthetic hydroxyapatite (HAP) and of these, there are indications that lanthanum improves the acid resistance of these compounds. In the present study, the reaction of La3+ ions with synthetic HAP in a water suspension has been investigated. It has been demonstrated that these cations react with phosphate ions in equilibrium with HAP to form insoluble LaPO4. This reaction releases protons which induce HAP dissolution up to its solubility equilibrium and to complete precipitation of La3+ ions. The kinetics of HAP dissolution at pH 5 with samples equilibrated for 2 h in the presence of 10 or 50 ppm LaCI3 show no differences compared with pure HAP. This strongly suggests that La3+ ions added to HAP in water suspension have no effect on the surface condition of this compound.