Incongruent dissolution and surface complexes of hydroxyapatite

Abstract

Calcium/phosphorus ratios in solution were determined when samples of synthetic hydroxyapatite (HA) were subject to acid (HCl) dissolution (pH 4.5) in 0.1 M KCl at various solid-to-solution ratios. At a ratio of 500g in 1 liter, the Ca/P approached 2.0, but when the HA solid was washed in electrolyte and resuspended at approximately the same solid: solution ratio, dissolution at pH 4.5 was congruent (Ca/P = 1.67). Incongruent dissolution was associated with the initial contact between solid and solution. At ratios of less than 8g HA/liter the initial dissolution appeared congruent because the absolute amount of excess calcium or deficit of phosphorus was less than the experimental accuracy of estimation of the elements. At the highest solid: solution ratio used, all the calcium and phosphorus involved in the production of Ca/P greater than 1.67 could be attributed to variations in the composition of the surface layer of unit cells. However, the underlying units are of normal composition. The changes in this surface layer are thought to occur at the end of the preparation of the HA, and may be associated with the acetone washing before drying of the crystals.