Reactions of Tin (II) and Fluoride Ions with Etched Enamel

¹ Procter & Gamble Company, Miami Valley Laboratories, Cincinnati, Ohio

Acid-etched but otherwise sound and uniform samples of human incisor enamel were treated by brushing with sodium fluoride or tin (II) fluoride solutions for various times at different concentrations and pH's. After treatment, the samples were exposed to strong acid to dissolve specific thicknesses of enamel. The acid solutions were analyzed chemically for tin (II) or fluoride ions. The experiments support the following conclusions.

Both tin (II) and fluoride reacted with enamel much more rapidly during the first few minutes of brushing than they did afterward. Fluoride taken up by enamel from either sodium fluoride or tin (II) fluoride solution was found at appreciable depths in the enamel. Most of the tin (II) from tin (II) fluoride solution was laid down as a uniform coat on the surface or slightly within the enamel.

Tin (II) and fluoride solutions in the concentration range 0.00001-0.001 M deposited measurable amounts of tin (II) and fluoride onto enamel, and the reactivity increased with increasing concentration. Tin (II) or fluoride solutions from 0.001 to 0.2 M showed little change in reactivity with enamel as concentration was varied. As sodium fluoride concentration was raised from 0.2 M to saturation, the reactivity of the fluoride with enamel increased with concentration, reaching a maximum value of 40-50 µg. of fluoride laid down per square centimeter of enamel in 4 hours under the conditions used in this work.

In the reaction of tin (II) with enamel, high tin (II) concentration, high pH, long treatment times, or the presence of rapidly hydrolyzing tin (II) salts favored an adhesion process whereby precipitated tin (II) oxide accumulated on the surface. If fluoride was present, some of it was occluded in the precipitate.