EFFECTS OF SODIUM-FLUORIDE, POTASSIUM FLUORIDE AND AMMONIUM FLUORIDE SOLUTIONS ON THE HYDROLYSIS OF CAHPO4 AT 37.4-DEGREES-C

The variations in pH during CaHPO4 hydrolysis in water and in 18.75-60 0 mM NaF, KF and NH4F solutions were determined at a 20.0 liquid-to-so lids weight ratio and 37.4 degrees C and the equilibrium solution chem istry and morphology were determined after 3 months. Hydrolysis kineti cs were rapid in all solutions containing fluoride. The pH variations demonstrate complex kinetic behavior. The cations with fluoride alter the reaction mechanisms over the range of concentrations studied. CaHP O4 dissolution changes from incongruent to congruent as the concentrat ions of cation-phosphate complexes increased regardless of whether the cation is Na, K or NH4. When hydrolysis is carried out in fluoride co ncentrations below 75 mM, CaHPO4 and fluoroapatite coexist at equilibr ium. The fluoroapatite formed discrete needles on the surface of the C aHPO4. The density of fluoroapatite needles on the CaHPO4, surfaces an d edges was not uniform. Fluoroapatite was the only phase present when hydrolysis was carried out in 75 mM fluoride. At fluoride concentrati ons above 75 mM, fluoroapatite and CaF2 coexist at equilibrium. Thus, the evolution of phases when CaHPO4 is hydrolyzed in increasing fluori de concentrations is CaHPO4 + fluoroapatite, fluoroapatite and fluoroa patite + CaF2. The fluoroapatite and CaF2 coexist as components of pse udomorphs of the original CaHPO4 crystallites. Pseudomorphs of the ori ginal CaHPO4 crystallites were the only morphology observed when CaHPO 4 was hydrolyzed in 600 mM fluoride. (C) 1998 Elsevier Science B.V. Al l rights reserved.