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 octacalc ium phosphate carboxylates. The kinetics of formation of OCP-SUCC by c onversion of alpha-tricalcium phosphate (alpha-TCP), alpha-Ca3(PO4)2, in ammonium succinate solutions was followed at varied initial pH, I a pproximately 0.5 mol dm-3, 37-degrees-C. The changes in the liquid pha se were monitored by pH measurements. Solid phases were characterized by means of chemical analyses, X-ray diffraction and infrared spectros copy. The rate of conversion decreased with increasing initial pH. In the solubility experiments the concentrations of calcium, phosphate, s uccinate, ammonium and pH were determined in solutions equilibrated wi th OCP-SUCC (6.1 < pH < 7.4). Utilizing these data and association con stants 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.