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Fluoride

physical science Maturity 9-11 Next article →

Fluoride is an inorganic, monatomic anion of the element fluorine. In chemistry, an anion is an atom that carries a negative charge. The chemical formula for the fluoride ion is F−. Most fluoride salts appear as white or colorless solids. These salts often have a distinctive bitter taste but no smell. Fluoride is classified as a weak base. This means it does not fully associate in a solution. However, concentrated fluoride is corrosive and can attack the skin.

Calcite sur fluorine (USA) 1
Calcite sur fluorine (USA) 1

The fluoride ion is the simplest form of a fluorine anion. In terms of its size and charge, it closely resembles the hydroxide ion. This similarity allows fluoride to act as a nucleophile in chemical reactions. In many inorganic solutions, fluoride ions exist alongside bifluoride ions, which are written as H2F+. Most fluoride salts are not very soluble in water without undergoing hydrolysis. This is a process where the salt reacts with water to change its chemical structure. In certain special environments, scientists can create "naked" fluoride. This refers to fluoride that is not surrounded by other atoms or solvents. A recent example from 2016 is imidazolium fluoride, which is a stable source of naked fluoride.

Fluoride is widely dispersed throughout the natural world. It is estimated to be the 13th-most abundant element in the Earth's crust. Most of this element exists entirely in the form of fluorides. These are found mostly in mineral deposits. The most commercially important mineral is fluorite, which has the formula CaF2.

Calcite sur fluorine (USA) 1
Calcite sur fluorine (USA) 1

Fluoride moves through the environment via the fluorine cycle. This cycle occurs through the natural weathering of rocks. Human activities also release fluorides into the biosphere. These activities can include mining or industrial processes.

Water sources contain varying amounts of fluoride depending on their location. Seawater fluoride levels usually range from 0.86 to 1.4 mg/L. The average concentration in the ocean is about 1.1 mg/L. This is much lower than the chloride concentration, which is about 19 g/L. In freshwater, the levels change significantly. Surface water in lakes and rivers typically contains between 0.01 and 0.3 mg/L. Groundwater levels vary based on local minerals. In Canada, natural levels can be under 0.05 mg/L. In China, levels can reach up to 8 mg/L. In some parts of Asia, groundwater can contain dangerously high levels. These high levels can lead to serious health problems for people.

Plants also absorb fluoride from the soil and water. All vegetation contains some amount of fluoride. Some plants are better at concentrating it than others. The tea plant, Camellia sinensis, is a notable accumulator. All tea leaves contain fluoride. Mature leaves can have 10 to 20 times more fluoride than young leaves. A 2013 study found that drinking one liter of tea daily might provide 4 mg of fluoride. This is the daily recommended intake for some adults. If a person is fasting, they may absorb even more. Fasting can increase absorption from 60–80% up to nearly 100%.

Sodium fluoride tablets
Sodium fluoride tablets

Fluoride is a vital micronutrient for human health. It is necessary to promote healthy bone growth and prevent dental cavities. Because of this, it is used in many oral hygiene products. Sodium fluoride and sodium monofluorophosphate are common ingredients in toothpaste. Some countries also add fluoride to table salt. In the United States, many cities use hexafluorosilicic acid to fluoridate public water. The U.S. Centers for Disease Control calls water fluoridation a great public health achievement.

Sodium fluoride tablets
Sodium fluoride tablets

The Institute of Medicine provides recommended amounts. The Adequate Intake for adult men is 4.0 mg/day. For adult women, it is 3.0 mg/day.

Industry relies heavily on fluoride for many different processes. Fluoride salts and hydrofluoric acid are highly valuable. One major use is in the production of cryolite, which is Na3AlF6. This chemical is essential for smelting aluminum. Mined fluorite is also used in steel-making to separate slag. Additionally, uranium hexafluoride is used to purify uranium isotopes. Fluoride is also used to create many drugs, polymers, and refrigerants. These are known as organofluorine compounds. In laboratories, fluoride salts are used to inhibit the activity of enzymes called phosphatases. This makes them very important tools for biological research.

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