DISSOLUTION OF FLUORAPATITE - A CONSTANT-COMPOSITION KINETICS STUDY

The kinetics of dissolution of fluorapatite (FAP) crystals has been investigated at 37-degrees-C and at pH values of 4.5, 5.0, 5.5, and 6.0 by use of the constant-composition (CC) method. The rate of reaction is surface controlled, probably following a spiral dislocation mechanism with an effective reaction order of 2 with respect to the relative undersaturation. This is in contrast to the dissolution of hydroxyapatite, which was found to be controlled by a polynucleation process with a kinetic order of approximately 6. Although the rate of FAP dissolution was retarded in the presence of both zinc and magnesium ions at pH 5.0, the former additive was much more effective in inhibiting the dissolution. The effective adsorption affinity, calculated from the decrease in rates of dissolution, was 2.5 x 10(5) L mol-1.