Abstract
The nature of the calcium phosphate precipitate formed when equal volumes of 20mM CaCl 2 and 15mM or 12mM KPO 4 with pH 7.4 are rapidly mixed at 15, 30 and 42°C has been investigated by chemical analysis, TEM, IR and by following the pH of the suspension as a function of time. At 30 and 42°C the final product is a deficient form of calcium hydroxyapatite, called d-HA. The formation of d-HA was preceded by an initial fast precipitation of a spherular amorphous form of calcium phosphate, called ACP1, which transformed into a floccular amorphous form, called ACP2, after which octacalcium phosphate, OCP, and d-HA crystallized. These transformations, which are considered to be solution-mediated, the one phase disappearing with the development of the next, occurred faster, the higher the temperature. The presence of MgCl 2 slowed the ACP1-ACP2 transformation, but apparently not the crystallization of OCP and d-HA. At 15°C ACP1 was formed initially, but the product formed was calcium monohydrogen phosphate dihydrate, DCPD, which also nucleated soon after mixing. Filtering off ACP1 from a reaction mixture at 30°C also resulted in the formation of DCPD and not OCP and d-HA. The amorphous phase, ACP2, which has not previously been clearly identified as a separate phase, is thus considered necessary for the development of OCP and d-HA under the conditions studied.
Published Version
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