Effect of acidified fluoride solutions on dissolution rate of enamel

It is well known that aqueous solutions of SnF₂ and TiF₄ reduce the dissolution rate of enamel in erosive acid, and it has recently been shown that highly diluted solutions of hydrofluoric (HF) acid shares this property. All these solutions have “natural” acidity, and induce formation of calcium fluoride on enamel. Objective: To compare the dissolution-reducing capacity of HF with NaF acidified with HCl or H₃PO₄. The latter are also known to cause calcium fluoride formation. Material and methods: The diluted HF and acidified NaF-solutions all had a pH of 3 and contained 0.09% fluorine. Enamel samples (n=18) were individually pre-treated with 10ml of 0.01M citric acid for 10 min (Etch 1) and the amount of calcium released measured by atomic absorption. These enamel samples were subsequently treated with one of the fluoride containing solutions for 10 min, and again etched and analyzed as before (Etch 2). The difference between Etch 1 and Etch 2 was assumed to be due to the respective fluoride treatments. Data were analysed using one way ANOVA. Results: It was found that the HF-treatment was more effective than treatment with acidified NaF-solutions. Mean reductions were HF - 50%, NaF with H₃PO₄ - 30% (p=0.26) and NaF with HCl - 20% (p=0.03). Conclusion: As HF contains mainly un-dissociated molecules at pH 3, it is suggested that these very small, uncharged molecules of covalently bound H and F, can diffuse rapidly together into enamel, opposed to ionic F^-Na⁺ in acidified solutions. The HF molecules can form calcium fluoride as follows: CaHPO₄ + 2HF > CaF₂ + H₂PO₄ + H⁺. The acids used with NaF are fully dissociated and these protons are free to individually penetrate into enamel and cause mineral loss. The movement of fluoride is however restricted due to its high affinity for Na.