Kinetics of hydroxyapatite precipitation at pH 7.4 to 8.4

William P. Inskeep, Jeffrey Silvertooth

Research output: Contribution to journalArticle

45 Citations (Scopus)

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 Ca2+ and PO4 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 m2 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 = kf2γ3[Ca2+][PO4 3-], where R = rate of HAP precipitation (mol HAP L-1 s-1), kf = rate constant (L2 mol-1 m-2 s-1), γ2 and γ3 = divalent and trivalent ion activity coefficients, s = surface area (m2 L-1), and brackets = concentrations of Ca2+ and PO4 3- (mol L-1), was derived based on the reaction orders with respect to S, [Ca2+] and [PO4 3-]. The equation was also verified using the integral method, and the average value for the precipitation rate constant was 173 ± 11 L2 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 Ca2+ and HPO4 2- into lattice vacancies, with subsequent dehydration and incorporation into the crystal lattice.

Original languageEnglish (US)
Pages (from-to)1883-1893
Number of pages11
JournalGeochimica et Cosmochimica Acta
Volume52
Issue number7
DOIs
StatePublished - 1988
Externally publishedYes

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Durapatite
kinetics
Kinetics
Crystal lattices
Rate constants
Activity coefficients
Ionic strength
Dehydration
Vacancies
Seed
Activation energy
rate
Ions
activity coefficient
dehydration
activation energy
surface area
crystal
Experiments
seed

ASJC Scopus subject areas

  • Geochemistry and Petrology

Cite this

Kinetics of hydroxyapatite precipitation at pH 7.4 to 8.4. / Inskeep, William P.; Silvertooth, Jeffrey.

In: Geochimica et Cosmochimica Acta, Vol. 52, No. 7, 1988, p. 1883-1893.

Research output: Contribution to journalArticle

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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 Ca2+ and PO4 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 m2 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 = kfsγ2γ3[Ca2+][PO4 3-], where R = rate of HAP precipitation (mol HAP L-1 s-1), kf = rate constant (L2 mol-1 m-2 s-1), γ2 and γ3 = divalent and trivalent ion activity coefficients, s = surface area (m2 L-1), and brackets = concentrations of Ca2+ and PO4 3- (mol L-1), was derived based on the reaction orders with respect to S, [Ca2+] and [PO4 3-]. The equation was also verified using the integral method, and the average value for the precipitation rate constant was 173 ± 11 L2 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 Ca2+ and HPO4 2- into lattice vacancies, with subsequent dehydration and incorporation into the crystal lattice.",
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