OCTACALCIUM PHOSPHATE CARBOXYLATES .4. KINETICS OF FORMATION AND SOLUBILITY OF OCTACALCIUM PHOSPHATE SUCCINATE

Kinetic and thermodynamic studies of octacalcium phosphate succinate (OCP-SUCC), Ca8(HPO4)2-x(succ)x(PO4)4.yH2O, where 0.8 < x < 1.0 and 5.5 < y < 6.5, broaden the present knowledge in the chemistry of octacalcium phosphate carboxylates. The kinetics of formation of OCP-SUCC by conversion of alpha-tricalcium phosphate (alpha-TCP), alpha-Ca3(PO4)2, in ammonium succinate solutions was followed at varied initial pH, I approximately 0.5 mol dm-3, 37-degrees-C. The changes in the liquid phase were monitored by pH measurements. Solid phases were characterized by means of chemical analyses, X-ray diffraction and infrared spectroscopy. The rate of conversion decreased with increasing initial pH. In the solubility experiments the concentrations of calcium, phosphate, succinate, ammonium and pH were determined in solutions equilibrated with OCP-SUCC (6.1 < pH < 7.4). Utilizing these data and association constants of phosphoric and succinic acid and stability constants of the soluble complexes, the solubility product of OCP-SUCC, K(s) = c8(Ca2+)c1.07(H+)c0.93(succ2-)C5.07(PO43-), was determined: pK(s)(OCP-SUCC) = 68.4 +/- 0.9 at I = 0.5 mol dm-3, 37-degrees-C.