Effect of Fluoride Pretreatment on the Solubility of Synthetic Carbonated Apatite

Abstract

The purpose of this research was to address the following question: How is the solubility of fluoride-pretreated carbonated apatite (CAP) in aqueous acidic media related to the equilibrium solution fluoride and/or the CAP adsorbed fluoride levels? A CAP sample prepared by a precipitation method at 70 degrees C containing approximately 6% carbonate was fluoride-treated (F adsorption from neutral aqueous solutions) to yield a approximately 1000 ppm F CAP and a approximately 3300 ppm F CAP. Metastable equilibrium solubility distributions were determined in acetate buffers at pH 5.0. Solution fluoride, calcium, phosphate, and pH were determined from the equilibrated solutions. The equilibrium solution fluoride levels were extremely low, e.g., as low as approximately 0.10 ppb to approximately 0.30 ppb at 50% dissolved for the two CAP preparations. The approximately 3300 ppm F CAP yielded a lower solubility than the approximately 1000 ppm F CAP (shift in the mean pKHAP value of 1.5-2 units). This can be attributed to the lower solution F(-) for the sample containing approximately 1000 ppm fluoride compared with the approximately 3300 ppm fluoride-containing CAP. These important findings suggest that a fluoride treatment simply may provide an adsorption fluoride depot for subsequent release, providing a solution fluoride effect upon the CAP solubility and not necessarily any intrinsic alteration of the mineral solubility.

Apparent solubility distributions of hydroxyapatite and enamel apatite

“…This conclusion is supported by the large deviations with pH when distributions were constructed on the basis of pI OCP instead of pI HA . Approximate calculations also indicated that fluorapatite is not the solubility-controlling phase, but further work on the role of F is certainly required, in the light of recent work [37].…”

Influence of crystallite microstrain on surface complexes governing the metastable equilibrium solubility behavior of carbonated apatites

“…Work in our laboratory [11][12][13][14][15][16][19][20][21][22][23][24] has been focused on developing and applying methods for investigating the solubility behavior of CAPs in acidic aqueous media. Within the time scale of laboratory experiments, CAPs do not follow conventional solubility theory (i.e., the thermodynamic solubility product principle) but exhibit a behavior termed "metastable equilibrium solubility (MES)" [12,13].…”

“…Within the time scale of laboratory experiments, CAPs do not follow conventional solubility theory (i.e., the thermodynamic solubility product principle) but exhibit a behavior termed "metastable equilibrium solubility (MES)" [12,13]. All apatite preparations investigated to date appear to demonstrate the MES and the MES distribution phenomenon [8,[12][13][14][15][16][19][20][21][22][23][24][25] including CAPs synthesized at various temperatures, possessing different carbonate levels and crystallinities, bone mineral from rats of different ages, and human dental enamel. Recent work has established that, for a given CAP preparation, a relationship may exist between the MES (and the MES distribution) and a MES driving force, which may be expressed in terms of a relevant ion activity product, K MES .…”

Quantitative Determination of Lattice Fluoride Effects on the Solubility and Crystallinity of Carbonated Apatites with Incorporated Fluoride

“…Calcium content was analyzed by the method of Sarkar and Chauhan [1967], phosphate by the method of Gee et al[1954], and carbonate by the micro diffusion method of Conway [1962]. Fluoride content was determined by dissolving a small amount of the F-CAPs with 1 ml of 1M HClO 4 solution, then adjusting to pH 5.0 with NaOH; the final solution being measured with a fluoride ion selective electrode (Radiometer) [Barry et al 2003]. The specific surface area of each F-CAP sample was determined by the BET method with nitrogen as adsorbate (Monosorb, Quantachrome, NY).…”