Reaction behaviour of hydroxyapatite toward the Zn2+ ion.

The reaction of synthetic hydroxyapatite, Ca5(PO4)3OH (HAP), with Zn2+ ions is investigated by means of pH, pZn, pCa and pNa measurements as a function of the time, temp. (40 and 60) and salts compn. and at different Zn2+/Ca2+ molar ratios in the range 1 - 0.1. The solid phases sepd. after different times of reaction were investigated with XRD and IR techniques. The Zn2+ ions added to HAP give rise to a soln. with pH >6 which inhibits any reaction involving HAP. No effect is obsd. by addn. of NaCl. At pH <5 Zn2+ forms Zn3(PO4)2.2H2O and does not replace Ca2+ in HAP structure. The addn. of an excess of NaHCO3 or a mixt. of NaHCO3 and Na2HPO4, with a molar ratio NaHCO3/Na2HPO4 = 30 gives rise to the formation ZnCO3 (smithsonite).