Ionic interaction with bone mineral IV. Varying affinity of synthetic calcium phosphates for Ca²⁺

The concentration of Ca²⁺ and PO₄^3- in the liquid film surrounding the crystals of several synthetic calcium phosphates suspended in an aqueous medium was calculated from ⁴⁵Ca²⁺ and ³²PO₄^3- exchange studies performed under physiological conditions. This permitted an estimation of the uptake of Ca²⁺ and PO₄^3- from solution by the solid phase alone, exclusive of the uptake by the adherent liquid film. The compounds with low calcium-to-phosphate molar ratio (Ca/P) showed a greater affinity for Ca²⁺ than those with higher Ca/P. Octacalcium phosphate (Ca/P = 1.33) incorporated considerably more Ca²⁺ than PO₄^3-. Tricalcium phosphate (Ca/P = 1.5), Bio-Rad apatite (Ca/P = 1.51), G-D apatite (Ca/P = 1.63), and L-apatite (Ca/P = 1.67), all of which gave apatitic X-ray diffraction patterns, demonstrated an uptake of Ca²⁺ in decreasing order, and an uptake of PO₄^3- in increasing order.