Sodium fluoride is an inorganic chemical compound with the formula NaF. A colorless solid, it
is a source of the fluoride ion in diverse applications. Sodium fluoride is less expensive
and less hygroscopic than the related salt potassium fluoride.
Structure, general properties, occurrence Sodium fluoride is an ionic compound, dissolving
to give separated Na+ and F− ions. Like sodium chloride, it crystallizes in a cubic
motif where both Na+ and F− occupy octahedral coordination sites; its lattice spacing, approximately
462 pm, is somewhat smaller than that of sodium chloride.
The mineral form of NaF, villiaumite, is moderately rare. It is known from plutonic nepheline
syenite rocks. Production
NaF is prepared by neutralizing hydrofluoric acid or hexafluorosilicic acid, byproducts
of the reaction of fluorapatite3F) from the production of superphosphate fertilizer. Neutralizing
agents include sodium hydroxide and sodium carbonate. Alcohols are sometimes used to
precipitate the NaF: HF + NaOH → NaF + H2O
From solutions containing HF, sodium fluoride precipitates as the bifluoride salt NaHF2.
Heating the latter releases HF and gives NaF. HF + NaF ⇌ NaHF2
In a 1986 report, the annual worldwide consumption of NaF was estimated to be several million
tonnes. Applications
Fluoride salts is often added to drinking water and some food products for dental health.
Other fluoride sources are used as well, e.g., salts of hexafluorosilicate. The fluoride
enhance the strength of teeth by the formation of fluorapatite, a naturally occurring component
of tooth enamel. Although sodium fluoride is also used to fluoridate water and, indeed,
is the standard by which other water-fluoridation compounds are gauged, hexafluorosilicic acid
and its salt sodium hexafluorosilicate are more commonly used additives in the U.S. Toothpaste
often contains sodium fluoride to prevent cavities.
Sodium fluoride is used as a cleaning agent. A variety of specialty chemical applications
exist in synthesis and extractive metallurgy. It reacts with electrophilic chlorides including
acyl chlorides, sulfur chlorides, and phosphorus chloride. Like other fluorides, sodium fluoride
finds use in desilylation in organic synthesis. The fluoride is the reagent for the synthesis
of fluorocarbons. In medical imaging, fluorine-18-labelled sodium
fluoride is used in positron emission tomography. Relative to conventional bone scintigraphy
carried out with gamma cameras or SPECT systems, PET offers more sensitivity and spatial resolution.
A disadvantage of PET is that fluorine-18 labelled sodium fluoride is less widely available
than conventional technetium-99m-labelled radiopharmaceuticals.
Sodium fluoride is used as a stomach poison for plant-feeding insects. Inorganic fluorides
such as fluorosilicates and sodium fluoride complex magnesium ions as magnesium fluorophosphate.
They inhibit enzymes such as enolase that require Mg2+ as a prosthetic group. Thus,
fluoride poisoning prevents phosphate transfer in oxidative metabolism.
Safety
Fluorides, particularly aqueous solutions of sodium fluoride, are rapidly and quite
extensively absorbed. Fluorides interfere with electron transport
and calcium metabolism. Calcium is essential for maintaining cardiac membrane potentials
and in regulating coagulation. Large ingestion of fluoride salts or hydrofluoric acid may
result in fatal arrhythmias due to profound hypocalcemia. Recreational inhalation of fluoridated
hydrocarbon refrigerants like Freon has been associated with "sudden sniffing death", which
is thought to be a fatal arrhythmia caused by myocardial sensitization to catecholamines.
Chronic over-absorption can cause hardening of bones, calcification of ligaments, and
buildup on teeth. Fluoride can cause irritation or corrosion to eyes, skin, and nasal membranes.
The lethal dose for a 70 kg human is estimated at 5–10 g. Sodium fluoride is classed as
toxic by both inhalation and ingestion. In high enough doses, it has been shown to affect
the heart and circulatory system. For occupational exposures, the Occupational Safety and Health
Administration and the National Institute for Occupational Safety and Health have established
occupational exposure limits at 2.5 mg/m3 over an eight-hour time-weighted average.
In the higher doses used to treat osteoporosis, plain sodium fluoride can cause pain in the
legs and incomplete stress fractures when the doses are too high; it also irritates
the stomach, sometimes so severely as to cause ulcers. Slow-release and enteric-coated versions
of sodium fluoride do not have gastric side effects in any significant way, and have milder
and less frequent complications in the bones. In the lower doses used for water fluoridation,
the only clear adverse effect is dental fluorosis, which can alter the appearance of children's
teeth during tooth development; this is mostly mild and is unlikely to represent any real
effect on aesthetic appearance or on public health. A chronic fluoride ingestion of 1
ppm of fluoride in drinking water can cause mottling of the teeth and an exposure of 1.7
ppm will produce mottling in 30–50 % of patients.
See also Cryolite
References