Sodium fluoride

Did You Know?

• Sodium fluoride crystallizes in a cubic motif with a lattice spacing of about 462 pm, smaller than sodium chloride's 564 pm.
• The annihilation photons from fluorine-18 decay have a high energy of 511 keV, compared to 140 keV for technetium-99m.
• A chronic fluoride ingestion of 1 ppm in drinking water can cause mottling of teeth, and 1.7 ppm produces mottling in 30%–50% of patients.

Uses

Fluoride salts are added to municipal drinking water and some food products to maintain dental health by forming fluorapatite, a natural component of tooth enamel. Although sodium fluoride is the standard for gauging other water-fluoridation compounds, hexafluorosilicic acid and sodium hexafluorosilicate are more commonly used in the United States. Fluoride supplementation for postmenopausal osteoporosis has been extensively studied; it increases bone density but does not reduce fracture risk.

In medical imaging, fluorine-18-labelled sodium fluoride has been used as a PET tracer since the 1960s. Compared to conventional bone scintigraphy, PET offers greater sensitivity and spatial resolution. Fluorine-18 has a 110-minute half-life, requiring prompt use, which limited adoption against technetium-99m-labelled radiopharmaceuticals, though it is considered superior for skeletal imaging due to high and rapid bone uptake and fast blood clearance.

Sodium fluoride also has specialty chemical applications, reacting with electrophilic chlorides and used in desilylation and the Finkelstein reaction to produce fluorocarbons, though industrial production favors other methods.

Safety

The estimated lethal dose for a 70 kg human is 5–10 g. Aqueous sodium fluoride is quickly absorbed and interferes with electron transport and calcium metabolism, affecting cardiac membrane potentials and coagulation. It irritates eyes, skin, and nasal membranes; chronic over-absorption can cause tooth buildup, ligament calcification, and bone hardening. High ingestion may cause fatal arrhythmias from hypocalcemia. Sodium fluoride is toxic by inhalation and ingestion, and at high doses can affect the heart and circulatory system.

Occupational exposure limits are set at 2.5 mg/m³ over an eight-hour time-weighted average. In osteoporosis treatment doses, plain sodium fluoride can cause leg pain, stress fractures, and gastric irritation leading to peptic ulcers; slow-release and enteric-coated versions have fewer side effects. At water fluoridation levels, the main adverse effect is dental fluorosis, which alters children's tooth appearance and can negatively affect adolescent self-esteem.

Production

Sodium fluoride is prepared by neutralizing hydrofluoric acid or hexafluorosilicic acid, both byproducts from processing fluorapatite in phosphate rock during superphosphate fertilizer production. Sodium hydroxide or sodium carbonate are used as neutralizing agents, and alcohols may precipitate the NaF. From solutions containing HF, sodium bifluoride forms and heating it releases HF and yields NaF. Annual worldwide consumption was estimated at several million tonnes in a 1986 report.