Sodium monofluorophosphate

Sodium monofluorophosphate (also disodium monofluorophosphate or MFP) is a chemical with the formula Na₂FPO₃. Its molecular weight is 143.95 g/mol. Typical for a salt, MFP is odorless, colorless, and water-soluble. It melts at 625 ºC.

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Uses of MFP

MFP is best known as an ingredient in toothpastes for both humans and animals. It is claimed to protect tooth enamel from attack by bacteria that cause dental caries (cavities). Though developed by a chemist at Procter and Gamble, its use in toothpaste (Colgate toothpaste) was patented by Colgate-Palmolive, as Procter and Gamble was engaged in the marketing of Crest toothpaste (containing stannous fluoride, marketed as "Fluoristan"). In the early 1980s, Crest was reformulated to use MFP, under the trademark "Fluoristat."

MFP is also used in some medications for the treatment of osteoporosis.

In 1991, sodium monofluorophosphate was found by Calgon to inhibit the solubility of lead in drinking water when used in concentrations between 0.1 mg/L and 500 mg/L.^[1]

Tooth decay

Tooth decay is caused by bacteria normally present in one's mouth. These bacteria form a sticky, colorless soft film on the teeth called plaque. When foods containing carbohydrates, (starches, and sugars) are eaten, the bacteria that form plaque use the sugar as a form of energy. They also turn it into a glue-like substance that helps them stick to the surface of the tooth. The plaque produces acid , which attacks the enamel.^[2]

Chemistry of decay

Tooth enamel consists mostly of calcium hydroxyphosphate, Ca₅(PO₄)₃OH, also known as the mineral apatite. Apatite is a hard, insoluble compound. Acid (H⁺), produced especially after a high-sugar meal, attack the apatite:

Ca₅(PO₄)₃OH_(s) + H⁺_(aq) → Ca₅(PO₄)₃⁺_(aq) + H₂O_(l)

Chemistry of enamel fluoridation

The degradation of apatite by loss of OH^- causes the enamel to dissolve. The process is reversible as saliva supplies back OH^- to reform apatite. If fluoride, F^-, ions are present in saliva, fluorapatite, Ca₅(PO₄)₃F, also forms.

Ca₅(PO₄)₃⁺_(aq) + F^-_(aq) → Ca₅(PO₄)₃F_(s)

Fluorapatite resists attacks by acids better than apatite itself, so the tooth enamel resists decay better than enamel containing no fluoride.^[3]

Preparation

MFP is prepared by heating a difluorophosphate solution in dilute sodium hydroxide.

PO₂F₂^2-_(aq) + 2 NaOH_(aq) → Na₂PO₃F_(aq) + 2 HF_(aq)

Discovery and development

Sodium monofluorophosphate was first described in 1929 by the German chemist Dr. Willy Lange, who was then with the University of Berlin. His fruitless attempts to prepare the free monofluorophosphoric acid led him to check the stability of its esters. Together with Gerda von Krueger, one of his students, Lange thus synthesized diethyl fluorophosphate and some analogs, which proved to be quite toxic; the first nerve agents were discovered. In the 1930's, Gerhard Schrader, working for the German company IG Farben, tried to develop synthetic insecticide. His work focused on esters of phosphoric acid and resulted in an accidental discovery of some other nerve agents such as DFP (= di-isopropyl fluorophosphate), Tabun, Soman, and Sarin. In the meantime, Lange, who was married to a Jewish woman, emigrated from Germany to the United States and started work for Procter and Gamble Company. In 1947, he and Ralph Livingston of Monsanto Company published reports on the preparation of the free fluorophosphoric acids and mentioned the use of some toxic esters of monofluorophosphoric acid (like DFP) in the treatment of glaucoma and myasthenia gravis. The well known toxicity of these esters led to fears that the simple salts might also be toxic, and such fears precluded any large scale commercial use of the salts. In 1950, under sponsorship of the manufacturer of the compounds, Ozark Chemical Company, the toxicity of sodium monofluorophosphate was studied by Harold C. Hodge at the University of Rochester who included anti-cavity testing. In 1967 Colgate-Palmolive filed several patents on the use of sodium monofluorophosphate in toothpaste.^[4]

Content and toxicity

The usual content of MFP in toothpaste is 0.76%. It is not used for water fluoridation. Currently accepted research indicates that by using such toothpaste, cavities may be reduced 17-38%.

The compound is not very toxic. (LD₅₀ in rats = 0.9 g/kg).

Structure of fluorophosphate

The structure of the fluorophosphate anion consists of phosphorus at the center of a tetrahedron defined by three oxygen atoms and one fluorine. Formal representations depict a double bond between one oxygen atom and phosphorus, with single bonds for the other two oxygen atoms and the fluorine. In this very formal depiction, negative charge is localized on the O atoms of the single P-O bonds. MFP is similar to and isoelectronic with Na₂SO₄.

References

1. ^ Peter Meiers Monofluorophosphate History
2. ^ Healthy Teeth [1]
3. ^ Davis, R. E., Ph.D., Metcalfe, H. C., Williams, J. E., Castka, J. F. (1999). Modern Chemistry. Austin, TX: Harcourt Brace & Company.
4. ^ Peter Meiers Monofluorophosphate History