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Refer: FL-145 March, 2001

THE FLUORIDE ION

Fluoride is not a generic term for fluorine. Fluorine is an element and a highly reactive pale yellow gas, but fluoride is an negative ion [technically called ionic fluoride] and combines with a positive ion called a cation, by sharing a electron, that generally produces a very stable molecule. To compare the two as just generic for each other is chemically incorrect. There are two forms that the fluoride ion can take. One is as an inorganic chemicals [as described above] and others is an organic chemical that combines the fluoride ion with the carbon ion in some manner. The inorganic form is what we use in water fluoridation and is the one that can produce the reduction in dental caries. The organic form is what is commonly used in the production of pesticides, nerve gas, and many other organic compounds. The organic form has no effect on dental caries. The fluoride ion, as one of the four halogens, can form various inorganic compounds [or chemicals] like calcium fluoride, sodium fluoride, aluminum fluoride, potassium fluoride, magnesium fluoride, etc, all of which are not poisonous to humans, animals, plants, etc, depending on the concentration. Almost any compound or chemical can be poisonous, if the concentration is high enough. The statement that is sometime made, "There are hundreds of fluorides: all are poisonous to human, animal, plant and aquatic life" is not correct! The solubility of any of the inorganic fluoride compounds varies widely. The solubility of calcium fluoride is 8 ppm while the solubility of sodium fluoride is 18,000 ppm. [Each fluoride compound will have a different solubility point, of course.] Most [but not all] drinking waters have calcium in them and thus when the fluoride level reaches about 8 ppm, calcium fluoride starts precipitating out. Lead fluoride has a solubility of about one ppb, thus it is very insoluble. The ionic fluoride is like free fluoride ions, but in chemical terms they are really not free, but ionized. This is very confusing to the non-chemists, because it means in a liquid solution the ionic bond has been broken. The ionic bond is a chemical bond characteristic of salts and is formed by the transfer of one or more electrons from one atom to another. For example, in water the sodium fluoride [or calcium fluoride or any other inorganic fluoride compound] is broken into the sodium ion [with a plus one electron] and fluoride ion [with a minus one electron]. This breakup is called a disassociation. There are always equal numbers of each ion. If the water evaporates or is somehow reduced, the ions will start combining again [precipitating out] and fall out of solution. The fluoride in the water is just fluoride [in the ionic form]. The fluoride in the water does not know if it came from sodium fluoride or calcium fluoride or another fluoride compound [in the inorganic form, of course]. Incidentally, tests that are taken to measure the concentration of fluoride in the water only measure the inorganic form of fluoride. Thomas G. Reeves, P.E. National Fluoridation Engineer Program Services Branch Division of Oral Health National Center for Chronic Disease Prevention and Health Promotion Centers for Disease Control and Prevention

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