Relation of lattice ion solution composition to octacalcium phosphate dissolution kinetics

The kinetics of dissolution of octacalcium phosphate (OCP) has been investi gated under conditions of constant relative undersaturation with respect to OCP, sigma(OCP) = -0.57, at pH 4.500, and ionic strength (I), 0.15 mol l(- 1). The molar calcium/phosphate ratio (R) in the solutions was varied from 0.1 to 10. The dissolution rate decreased by 160% as R increased from 0.1 t o 10. The zeta-potential of the OCP surfaces was measured in solutions equi librated with respect to OCP at pH values ranging from 5.0 to 11. Under sto ichiometric conditions (R = 1.33), OCP was positively charged at pH values from 5.0 to 10. As the solution calcium concentration was increased, zeta b ecame more positive over the entire pH range studied. At R = 0.1, two isoel ectric points were apparent at pH values of about 6.3 and pH 9.5. This beha vior may be related to the solubility product (K-sp) of OCP. The relationsh ip between surface characteristics and dissolution rate are discussed in te rms of kink density and the kinetic ionic ratio model developed previously (J. Zhang, G.H. Nancollas, J. Colloid Interf. Sci., 200 (1998) 131. (C) 199 9 Elsevier Science B.V. All rights reserved.