Octacalcium Phosphate Solubility Product from 4 to 37 °C

Octacalcium phosphate (OCP) is proving to be an important intermediary in the formation of tooth and bone mineral and various pathological calcifications. Before this mineral can form, its solubility product must be exceeded. Thus, a knowledge of its precise values under various conditions is required for a basic understanding of calcification processes. The methodology suitable for measuring the solubility of metastable phases was developed and used to determine the negative logarithms of the solubility products of OCP, pK(sp)(OCP), at 4, 4.8, 6, 18, 23.5, and 37 °C. This methodology includes (1) the use of high solid-to-liquid ratio, 10 mg/mL, to minimize the effects of hydrolysis, (2) frequent sampling during equilibration to detect possible effects of hydrolysis, (3) equilibration from supersaturation and from undersaturation, and (4) equilibration in the absence and presence of a CO(2)-containing atmosphere. The resulting pK(sp)(OCP) values are 48.3±0.2, 48.3±0.2, 48.2, 48.3, 48.4±0.1 and 48.7±0.2 at 4, 4.8, 6, 18, 23.5, and 37 °C. A 5.5% CO(2) atmosphere did not change the apparent pK(sp)(OCP) value significantly. The value of pK(sp)(OCP) obtained by approaching equilibrium from supersaturation was essentially the same as that from undersaturation. The effects of (1) the use of different ionic models, (2) OCP hydrolysis, and (3) differences in equilibrium constants on the apparent pK(sp)(OCP) values are described; the latter two contribute significantly to the differences in pK(sp)(OCP).