A diffusion (or transport) and a kinetically (or surface) controlled [9-26] dissolution models, as well as the two-site model [15, 27-29], are devoted to comparison of transport rates of chemical reagents (H+ and anions of acid) from bulk solution to the surface of apatite crystals and those of products (Ca2+, F− and orthophosphate anions) obtained backwards with the kinetics of chemical interactions on the crystal surface. According to the diffusion-controlled and two-site models, a transport of either the chemical reagents and/or the products of chemical reactions mainly determine the overall dissolution rate, whereas, the kinetically controlled model emphasizes chemical transformations on the surface as the limiting factor (Fig. 7.1) [30]. An important consequence of the two types of rate-controlling mechanism is that the solution immediately next to the crystal surface is undersaturated with respect to apatite in the surface controlled process and this solution is saturated in the diffusion-controlled process [31]. Both models usually operate with a so-called driving force which means either a concentration gradient within the Nernst diffusion layer (the diffusion controlled model) or a gradient of ionic chemical potentials between the apatite crystal surface and bulk solution (the kinetically controlled model). To investigate whether a dissolution process follows either of these models, one should study the influence of hydrodynamics, concentration of reagents, ionic strength, solution undersaturation,

pH, temperature, crystal dimensions, and the presence of chemical modifiers on the dissolution kinetics. A rotary disk [10, 13, 14], constant composition [9, 16, 18, 19, 20, 32-34] or dual constant composition [22, 34, 35] techniques are used for experimental investigations. The results obtained are usually plotted as an uptake of H+ ions (e.g., as a titrant volume added) and/or a release of calcium, orthophosphate, and fluoride ions versus the dissolution time. Afterwards, calculations of the numeric values for activation energy, rate constants, effective reaction order, diffusion layer thickness, characteristic adsorption impedance, diffusive jump distance, etc., are performed [9-29, 32-36]. The conclusion on whether a dissolution process is kinetically or diffusion controlled is made based on the numeric values calculated. For example, when analysis of the rate data gave a numeric value of the effective reaction order n = 6, a surface controlled dissolution mechanism was suggested [22].