The solubility product of octacalcium phosphate [Ca4H(PO4)3 · 2.5H 2O] has been determined in the system Ca(OH)2-H3PO 4KNO3-H2O at 37°C in experiments involving a range of hydrodynamics, ionic strength, and equilibration time. A value of pK so= 49.3 ± 0.2 was obtained for three different solid preparations by considering activity coefficients and ion-pair corrections.
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