THE STATE OF FLUORIDE IN DRINKING WATER

¹ Department of Radiation, University of Rochester, School of Medicine and Dentistry, Rochester, N. Y.

When sodium silicofluoride is dissolved in water, hydrolysis occurs; the fluoride exists in any or all of the following states: SiF₆⁼, SiF₄, F^–, and HF. Knowing the hydrolysis constants of SiF₆⁼ and SiF₄ and the dissociation constant of hydrogen fluoride, the distribution of fluoride in these states in a given solution may be calculated from the pH and the initial silicofluoride concentration. The hydrolysis constants of SiF₆⁼ and SiF₄ were determined from pH measurements of silicofluoride solutions to be 7 x 10^–32 and 7 x 10^–26, respectively, at 29° C.

It is concluded that in any drinking water supply with a pH of 5 or higher, fluoridated with sodium silicofluoride to the extent of 16 ppm of F or less, all of the silicofluoride is completely hydrolysed to silicic acid, fluoride ion, and hydrogen fluoride. In the water, HF constitutes only a very small fraction of the total fluoride, and when the water is ingested and absorbed into the blood stream all the fluoride is fluoride ion. There can be no question of toxicity of SiF₄ or SiF₆⁼ under such conditions.

The stability constants of the metallo-fluorides reported by Paul have been used to show that of the many metallic ions in drinking water only calcium and magnesium are in sufficient concentration to bind at least 1 per cent of any fluoride present. When the total fluoride content is 1 ppm the percentage of fluoride bound to calcium ranges from about 0.03 per cent for very soft water to about 2.8 per cent for very hard water; the magnesium-bound fluoride ranges from 0.3 per cent for very soft water to 28 per cent for very hard water.