What are the Chemical Properties of Fluoride?
A chemical compound, like Sodium Fluoride (NaF) is simply a combination of Sodium and Fluorine, two common elements. Fluorine belongs to the group known as Halogens, which in the Greek means “salt-formers.” (The best-known example of a halogen compound is Sodium Chloride – common table salt.) Fluorine is never found in a free state in nature, but is always in compound form. When dissolved in water, salt compounds dissociate into ions. It is the fluoride ions in the drinking water that cause the reduction in tooth decay. Fluoride ions are free to bind with the enamel to form a more insoluble structure. There is no difference between artificially [chemically] fluoridated water and naturally fluoridated water in that the ions that are released are identical in their chemical structure regardless of the origin of the fluoride ion.
Fluoride compounds used for water fluoridation:
- Sodium Fluoride (NaF) was the first compound used. It is a white, odorless powder or crystal; the crystalline form is preferred if manual handling is used, as it minimizes dust. It is more expensive than the other compounds, but is easily handled and is usually used by smaller utility companies. In water, this compound separates into sodium (Na) and fluoride (F) ions.
- Fluorosilicic acid (H2SiF6) is the most commonly used additive for water fluoridation in the United States. It is an inexpensive liquid by-product removed when apatite rock (a type of limestone) is ground into the phosphorus fertilizer used for plant nutrition. It comes in varying strengths, typically 23–25%; because it contains so much water, shipping can be expensive. In water, this compound separates into hydrogen (H), sand (Si), and fluoride (F) ions.
- Sodium fluorosilicate (Na2SiF6) is the sodium salt of fluorosilicic acid. It is a powder or very fine crystal that is easier to ship than fluorosilicic acid. It is also known as sodium silicofluoride. In water, this compound separates into sodium (Na), sand (Si), and fluoride (F) ions.
These compounds were chosen for their solubility, safety, availability, and low cost. A 1992 census found that, for U.S. public water supply systems reporting the type of compound used, 63% of the population received water fluoridated with fluorosilicic acid, 28% with sodium fluorosilicate, and 9% with sodium fluoride.