EFFECTS OF SODIUM AND AMMONIUM-IONS ON OCCURRENCE, EVOLUTION AND CRYSTALLINITY OF CALCIUM PHOSPHATES

Calcium phosphates are precipitated at 37-degrees-C from aqueous solutions at same ionic strength (I = 0.15), the initial pH ranging from 5 to 10. I and pH are adjusted either by sodium or by ammonium chlorides and hydroxides, so that sodium and ammonium are never together in the same solution. Three concentration ratios of total calcium over total phosphate are used: 3/17, 5/15 and 10/10 in millimol/liter. When pH > 6.5, amorphous calcium phosphate (ACP) first precipitates in either solution. In the solutions containing sodium, it transforms into hydroxyapatite (HAP) or octocalcium phosphate (OCP) which, in turn, slowly transforms into HAP. In the solutions containing ammonium, ACP directly transforms into HAP. This is due to the buffer effect of ammonium which holds the pH at a high level when ACP occurs. Accordingly, concentration in PO43- ions and supersaturation with respect to HAP remain high, which favours nucleation and growth of HAP. As concerns the crystallinity of the latter phase, it depends on the solution composition: it is the best in the solutions with sodium ions, when the calcium-over-phosphate ratio is 10/10. The interpretation was supported by calculating pH and supersaturation with respect to all possible crystalline phases during the crystallization course.