Abstract
Hydroxyapatite (HAP) in water exhibited a buffer function in the region from weakly acidic through weakly alkaline pH. Equilibrium pH depends on the amount of HAP added, and on the species and concentration of added electrolyte. This function seems to originate from the ability of HAP to adsorb OH-and/or from the protonation-deprotonation equilibrium of surface phosphate ion. Theoretical calculation of the suspension pH was attempted. The isotherm of OH- adsorption from aqueous solutions of NaOH mixed with NaCl was of Langmuir type. The saturated amount of OH- adsorption was almost constant irrespective of the amount of NaCl added, but the binding constant increased with the amount of added NaCl. The adsorption isotherms, however, formed just one curve irrespective of the amount of NaCl added when they were expressed as a function of the thermodynamic activity of NaOH. On the other hand, the amount of adsorbed OH- increased with the amount of Ca2+ added, because Ca2+ is adsorbed on the HAP surface and forms a positively charged adsorption site for OH-. An electrophoresis experiment confirmed that HAP particles have positive charges due to adsorbed Ca2+ even in solutio at high pH.
