Kinetics of dissolution of calcium hydroxyapatite

Abstract

The rate of dissolution of calcium hydroxyapatite microcrystals [Ca₁₀(PO₄)₆(OH)₂, HAP] in aqueous suspension is controlled by a surface reaction, the diffusion of dissolved substance away from the crystal/liquid interface not influencing the rate. The rate can be described by a polynuclear mechanism. The rate cannot be expressed purely in terms of thermodynamic expressions; kinetic effects are important. The rate constant for the polynuclear mechanism depends on pH. This can be explained by hydrogen-ion catalysis of the linear rate of growth of the dissolution nuclei. Application of the model leads to a value of the Gibbs surface energy of 47 ± 3 mJ m^–2 for HAP in the range 5.0 ⩽ pH ⩽ 7.2. The acidity constant for the HPO₄^2– surface complex is found to be 10^–6–10^–7 mol dm^–3, depending on the electrical potential difference between the crystal surface and the solution. Inhibition of the rate of dissolution of calcium hydroxyapatite owing to adsorption of foreign ions or molecules can often be described by a simple Langmuir adsorption isotherm.