The sorption and desorption of calcium were studied on hydroxyapatite (Ca-10(PO4)(6)(OH)(2)) in 0.01 M Ca(ClO4)(2) aqueous solutions at 25 degreesC in the pH range 4.5-11.5, under conditions of solubility equilibrium. Thus, the isotopic exchange of Ca-45 can be utilized to obtain the partition coefficients of calcium on Ca-10(PO4)(6)(OH)(2), whose solubility product was previously estimated from a literature review: log *K-s(0) = -87.2 +/- 1.4. Under these conditions, calcium was partially desorbed from hydroxyapatite particles. The experimental data were analyzed assuming a reversible component of the partition coefficient and a nonreversible sorption component that might result from the polycrystalline nature of the hydroxyapatite particle. (C) 2003 Elsevier Science (USA). All rights reserved.