Reaction of Calcium Hydrogenphosphate Dihydrate (DCPD) with a Solution Containing a Small Amount of Fluoride

Abstract

Calcium hydrogenphosphate dihydrate (DCPD) efficiently reacts with fluoride ion in a solution to form fluorapatite (FAp). In this paper, the reaction of DCPD with a solution containing a small amount of fluoride was investigated. 2.0 g of DCPD was added to 2.0 dm3 of solution containing 20 mg dm-3 of fluoride, and then reacted for various periods. We observed a “lag time” within which the concentration of fluoride ions in the solution did not change. After this lag time, the fluoride ion concentration in the solution quickly decreased. From the results of powder X-ray diffraction, fied emmision-scanning electron microscopy (FE-SEM) observation and chemical analysis of the solid phase, DCPD did not directly react with the fluoride ions, but formed a nano-scale particle on the surface of the DCPD particles, and then reacted with the fluoride to form calcium deficient FAp. From these results, the lag time could be considered to be the time needed to form nano-scale particles on the surface of the DCPD particles. It was also found that the amount of fluoride reactable with DCPD was larger at lower temperatures.