Effects of sodium fluoride, potassium fluoride and ammonium fluoride solutions on the hydrolysis of CaHPO 4 at 37.4°C

Abstract

The variations in pH during CaHPO 4 hydrolysis in water and in 18.75–600 mM NaF, KF and NH 4F solutions were determined at a 20.0 liquid-to-solids weight ratio and 37.4 C and the equilibrium solution chemistry and morphology were determined after 3 months. Hydrolysis kinetics 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. CaHPO 4 dissolution changes from incongruent to congruent as the concentrations of cation-phosphate complexes increased regardless of whether the cation is Na, K or NH 4. When hydrolysis is carried out in fluoride concentrations below 75 mM, CaHPO 4 and fluoroapatite coexist at equilibrium. The fluoroapatite formed discrete needles on the surface of the CaHPO 4. The density of fluoroapatite needles on the CaHPO 4 surfaces and edges was not uniform. Fluoroapatite was the only phase present when hydrolysis was carried out in 75 mM fluoride. At fluoride concentrations above 75 mM, fluoroapatite and CaF 2 coexist at equilibrium. Thus, the evolution of phases when CaHPO 4 is hydrolyzed in increasing fluoride concentrations is CaHPO 4 + fluoroapatite, fluoroapatite and fluoroapatite + CaF 2. The fluoroapatite and CaF 2 coexist as components of pseudomorphs of the original CaHPO 4 crystallites. Pseudomorphs of the original CaHPO 4 crystallites were the only morphology observed when CaHPO 4 was hydrolyzed in 600 mM fluoride.