Relationship Between pH, Age, and Concentration of Solutions of Stannous Fluoride and Sodium Fluoride in Decreasing Enamel Solubility and Affecting the Uptake of Fluorine

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Abstract
The marked effectiveness of freshly prepared SnF2 solns. in reducing the solubility of powdered dental enamel decreases over a period of a few days, but the effectiveness of NaF solns. is not decreased. Enamel takes up larger amts. of F from SnF2 regardless of the age of the solns., and the uptake is greater from old SnF2 solns. than from those which are freshly prepared. The uptake of F from NaF is not affected by age. These changes appear to be related to the pH of the solns. The pH of the stannous compound decreases, through hydrolysis of the salt, but the pH of the Na salt remains unchanged. Aged dilute solns. are decidedly less effective than freshly prepared in reducing the solubility of dental enamel.