Michèle Freche

Calcium phosphate precipitation in the 60–80°C range

Abstract In the 60–80°C temperature range and in acidic medium, the precipitation of either octacalcium phosphate (OCP) or dicalcium phosphate dihydrate (DCPD) or dicalcium phosphate anhydrous (DCPA) onto DCPA seeds occurs. High temperature and low pH favor the precipitation of DCPA. A plot of temperature versus pH is given to summarize the results. The kinetics of DCPA growth onto DCPA seeds were studied by constant composition method. At 70°C the rate of growth is small and the order of reaction is 4, contrasting to 80°C where the rate of growth is higher and the order of reaction is 2. At 70°C the edge free energy is 1.7×10 -11 and 2.0×10 -11 J m -1 at pH = 4.3, respectively. The study of the nucleation of OCP onto DCPA seeds allows us to calculate the surface energy of OCP (51–59 mJ m -2 at 60–80°C).