Ks. Tenhuisen et Pw. Brown, VARIATIONS IN SOLUTION CHEMISTRY DURING CALCIUM-DEFICIENT AND STOICHIOMETRIC HYDROXYAPATITE FORMATION FROM CAHPO4-CENTER-DOT-2H(2)O AND CA-4(PO4)(2)O, Journal of biomedical materials research, 36(2), 1997, pp. 233-241
This study explores the mechanistic paths taken when calcium-deficient
hydroxyapatite, CDHAp (Ca/P = 1.50), and stoichiometric hydroxyapatit
e, SHAp (Ca/P = 1.67), form by reaction between particulate calcium ph
osphate salts. The acidic reactant was CaHPO4 . 2H(2)O (DCPD) and the
basic reactant was Ca4(PO4)(2)O (TetCP). Variations in pH, calcium and
phosphate concentrations, and the solids present during apatite forma
tion, were determined as functions of reaction temperature (25.0 degre
es, 37.4 degrees, and 50.0 degrees C) and time. It was found that the
dissolution of TetCP was rate limiting for both hydroxyapatite (HAp) c
ompositions at all three temperatures. However, the retrograde solubil
ity and incongruent dissolution of DCPD became increasingly important
in influencing the kinetics as the reaction temperature was increased.
An amorphous intermediate phase was observed regardless of the HAp st
oichiometry. The solutions from which the SHAp formed approached equil
ibrium at much shorter reaction times (1-2 days) than those from which
the CDHAp formed. The latter continued to display changes in pH and i
n calcium and phosphate concentrations for 6 months. CD-HAp was shown
to be a thermodynamically stable phase. The dissolution of CDHAp is in
congruent, showing a Ca/P molar ratio in solution less than 0.5. (C) 1
997 John Wiley & Sons, Inc.