Kinetics of hydroxyapatite precipitation at pH 7.4 to 8.4

Abstract

The rate of hydroxyapatite (HAP) precipitation was studied using a reproducible seeded growth technique under pH stat conditions. Thirty different experiments were performed at initial Ca 2+ and PO 4 3− concentrations ranging from 0.37–0.86 and 0.29–1.0 mmol L −1, respectively, ionic strengths from 0.015–0.043 mol L −1, HAP seed concentrations from 7.1–28.4 m 2 L −1, temperatures from 10–40°C, and pH from 7.4 to 8.4. Initial rates expressed as mole HAP L −1 s −1 were used to test several empirical rate equations and derive a rate equation based on experimentally determined reaction orders. The rate equation: R = k f sγ 2 γ 3[ Ca 2+][ PO 4 3−], where R = rate of HAP precipitation ( mol HAP L −1 s −1), k f = rate constant ( L 2 mol −1 m −2 s −1), γ 2 and γ 3 = divalent and trivalent ion activity coefficients, s = surface area ( m 2 L −1), and brackets = concentrations of Ca 2+ and PO 4 3− (mol L −1), was derived based on the reaction orders with respect to S, [Ca 2+] and [PO 4 3−]. The equation was also verified using the integral method, and the average value for the precipitation rate constant was 173 ± 11 L 2 mol −1 m −2 s −1. The Arrhenius activation energy was 186 ± 15 kJ mol −1, indicative of a surface controlled precipitation mechanism. We speculate that the rate limiting steps include migration of surface Ca 2+ and HPO 4 2− into lattice vacancies, with subsequent dehydration and incorporation into the crystal lattice.