Sodium Fluoride
Properties
| State | Solid at room temperature |
| Color | White powder or colorless crystals |
| Solubility | Sparingly soluble in water (4.13 g/100 mL at 25 °C) |
| Melting Point | 993 °C |
| Boiling Point | 1704 °C |
About Sodium Fluoride
Sodium fluoride (NaF, MW 41.988) is the small, dense ionic salt behind one of the largest public-health interventions of the 20th century: community water fluoridation at roughly 0.7 ppm F-, which the CDC credits with cutting childhood cavities by about 25%. The mechanism at the tooth surface is a topical reaction. Fluoride ions exchange into hydroxyapatite (Ca5(PO4)3OH), the mineral phase of enamel, replacing OH- to form fluorapatite (Ca5(PO4)3F). Fluorapatite has a tighter crystal lattice, a lower critical pH for dissolution (about 4.5 versus 5.5 for hydroxyapatite), and resists the acid produced when Streptococcus mutans ferments sucrose to lactate. Above the dental story, NaF is unusual among sodium halides for being only sparingly soluble (4.13 g/100 mL at 25 °C versus 35.9 for NaCl). The small F- ion forms an anomalously tight lattice with Na+ (lattice energy around -923 kJ/mol), and the hydration enthalpy of F- cannot quite overcome it. Industrially, NaF is a flux in aluminum smelting (alongside cryolite), a wood preservative against fungi and termites, an HF source for cleaning silicon wafers in semiconductor fabs, and the active in dental varnishes like Duraphat (5% NaF, 22,600 ppm F-). Pharmacologically, high-dose NaF was once prescribed for osteoporosis (it stimulates osteoblasts to lay down new bone), but the bone formed was brittle so the indication has largely been replaced by bisphosphonates.
Where you'll encounter it
If you've ever brushed with Crest, Colgate, or any ADA-accepted toothpaste, you've used 0.24% NaF (1,100 ppm fluoride) or its cousin sodium monofluorophosphate. In the dentist's chair, the bitter-tasting trays from childhood are 1.23% APF (acidulated phosphate fluoride) gels delivering about 12,300 ppm F- in four-minute applications. In a semiconductor fab, dilute NaF or buffered HF (BOE: NH4F + HF) etches SiO2 off silicon wafers between photolithography steps, and the etch rate of about 100 nm/min on thermal oxide is the calibration every process engineer memorizes. In a clinical chemistry lab, gray-top blood collection tubes use sodium fluoride (around 10 mg per 10 mL) as a glycolysis inhibitor: F- inactivates enolase, freezing the glucose level so plasma glucose results stay valid even if the tube sits for hours before centrifugation.
Common Uses
- Community water fluoridation at 0.7 ppm F- for cavity prevention
- Active in 0.24% (1,100 ppm F-) toothpastes and 5% NaF dental varnishes (Duraphat)
- Glycolysis inhibitor in gray-top blood tubes for accurate plasma glucose
- Etchant component in buffered oxide etch (BOE) for silicon wafer fabrication
- Wood preservative in older Wolman salts against decay fungi and termites
- Flux additive in aluminum smelting alongside cryolite
- Source of F- for synthesis of organofluorine compounds and fluoride electrodes
- Calibration standard for fluoride ion-selective electrodes in environmental labs
Safety Information
Toxic by ingestion at gram quantities; the probably toxic dose in children is about 5 mg F-/kg, with the acutely lethal dose around 32-64 mg F-/kg. Chronic over-exposure causes dental fluorosis (mottled enamel) at intakes above 0.05 mg F-/kg/day during tooth development, and skeletal fluorosis at long-term intakes above 10 mg/day. OSHA PEL for fluoride dust as F is 2.5 mg/m3 (8-hour TWA); ACGIH TLV is the same. GHS classifications: H301 (toxic if swallowed), H315 (causes skin irritation), H319 (causes serious eye irritation), H335 (may cause respiratory irritation). Pictograms: Skull-and-crossbones, Exclamation mark. The first-aid for ingestion is oral calcium (milk or calcium gluconate) to precipitate F- as insoluble CaF2 in the gut. Store away from acids (releases HF gas) and silica glass (etches it). Standard PPE: nitrile gloves, safety glasses, dust mask when weighing.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.