Fluorine

Typical wastewaters

• phosphate fertiliser manufacture fluoride ion in treated calcium sulfate storage pile runoff and contaminated non-process wastewater; 75 daily maximum and 25 mg/L monthly average, otherwise no discharge of process wastewater
• hydrofluoric acid production total fluoride in process wastewater with TSS, nickel and zinc; limits in kg per 1000 kg of product
• primary beryllium extraction (bertrandite raffinate) fluoride in acid solvent extraction raffinate, 78,610 mg/kg daily maximum per kg of beryllium carbonate produced
• semiconductor and aluminium plant effluent (etching, smelting, aluminium fluoride) fluoride ion precipitated with lime to the CaF₂ solubility floor of 10 to 20 mg/L, then polished where the permit is lower

1 · Identity

Symbol, number

F, 9

Oxidation states in water

-1 only, as the fluoride ion F⁻, its weak acid HF below about pH 3, and its complexes with aluminium, iron, magnesium, calcium, silicon (fluorosilicate) and boron. The fluoride in finished water is always the fluoride ion, whether natural or added (WHO). No redox chemistry in water.

Note

The element entry carries fluorspar, fluorapatite, the phosphoric acid BAT on fluoride to water and the fluoropolymer story. This chapter is fluoride in water: occurrence, fluorosis, fluoridation and defluoridation.

2 · Occurrence in water

Natural sources

Fluorides in fluorspar, cryolite and fluorapatite; traces in many waters with higher concentrations in groundwater, where they vary with the rock the water flows through; well water in fluoride mineral areas up to about 10 mg/L and much higher locally, the highest natural level reported 2800 mg/L; high fluoride in parts of India, China, Central Africa and South America and locally in most parts of the world (WHO fact sheet). Volcanic and geothermal waters and sodium bicarbonate groundwaters low in calcium carry the most because calcium fluoride solubility no longer caps it.

Anthropogenic sources

Fluoridation of supplies with sodium fluoride, sodium fluorosilicate or fluorosilicic acid at 0.5 to 1 mg/L final concentration (WHO); phosphate fertiliser manufacture (fluoride escapes phosphoric acid plants in gas and water; US limits 75 and 25 mg/L on storage pile runoff), hydrofluoric acid and aluminium fluoride plants, aluminium smelting, semiconductor and glass etching, primary beryllium raffinate (78,610 mg/kg daily maximum); brick tea and high fluoride coal in some regions add to dietary intake (WHO).

3 · Speciation

Fluoride is the anion of a weak acid (HF pKa about 3.2), so it is F⁻ at every pH met in treatment, and it is a hard ligand: it complexes aluminium strongly (AlF²⁺ through AlF₆³⁻), iron(III), magnesium and boron, and it is held in fluorosilicate SiF₆²⁻ in the fluoridation chemicals until that hydrolyses. Calcium fluoride is the sparingly soluble salt that caps fluoride in calcium rich water and lets it rise in soft, sodium bicarbonate water. The ion selective electrode measures free fluoride only, so complexed fluoride must be released with a buffer before analysis (WHO: sample preparation is critical).

Solubility

Sodium fluoride and fluorosilicate are soluble; calcium fluoride (fluorite) is sparingly soluble and controls fluoride in hard water; aluminium fluoride and magnesium fluoride are of low solubility. No solubility product is quoted because none was read this session.

Hydrolysis

Fluoride is a weak base; hydrolysis is negligible above pH 5. Fluorosilicate hydrolyses to fluoride and silicic acid on dilution, releasing acid.

Complexation

Strong with Al³⁺, Fe³⁺, Be²⁺ and B(OH)₃ (giving BF₄⁻ only in acid); weaker with Mg²⁺ and Ca²⁺; oral absorption is reduced by complex formation with aluminium, phosphorus, magnesium or calcium (WHO). Constants not quoted.

Precipitates

CaF₂ (fluorite) in lime treatment and hard water; fluorapatite Ca₁₀(PO₄)₆F₂ in contact precipitation and bone char; AlF₃ and fluoride adsorbed on Al(OH)₃ in alum treatment and activated alumina.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Plastic bottles only; fluoride etches glass and adsorbs on it. No preservation is needed. Aluminium coagulated water reads low on the electrode unless buffered; fluorosilicate stock must be fully hydrolysed before analysis. Do not confuse fluoride with fluorine or with total fluorine (organofluorine, PFAS), which needs combustion methods.

7 · Regulatory limits

Limits change, and many are set locally. Treat these as the published values to start from, not as your compliance target: check the standard in force at your site and the numbers written into your own permit.

8 · Health and environmental effects

Toxicity

Water soluble fluorides are rapidly and almost completely absorbed, reduced by complexes with aluminium, phosphorus, magnesium or calcium; distributed to teeth and bone with virtually no soft tissue storage; excreted in urine. Acute intoxication needs about 1 mg/kg body weight. Caries protection from about 0.5 mg/L up to about 2 mg/L; mild dental fluorosis at 0.9 to 1.2 mg/L; skeletal fluorosis at 3 to 6 mg/L, crippling above 10 mg/L; clear excess skeletal risk at 14 mg/day total intake and suggestive evidence above 6 mg/day; no support for a cancer or birth defect link (WHO fact sheet). Fluoride may be essential but essentiality is not demonstrated unequivocally (WHO).

Bioaccumulation

Accumulates in bone and teeth, which can release it after exposure ends; tea leaves average 100 mg/kg dry (WHO). Not bioaccumulative through food chains in the usual sense.

Ecotoxicity

No US EPA aquatic life criterion for fluoride in the national recommended criteria table read; irrigation guidance treats fluoride as a trace element (FAO 29 Table 21, not read).

Flags

• US fluoridation policy is time-sensitive: the 0.7 mg/L level is the 2015 Public Health Service recommendation as shown on the CDC page read in September 2026; in April 2025 HHS announced a review of the recommendation and an expert panel, and the outcome was not verified this session.
• The HF pKa, fluorosilicate hydrolysis, the CaF₂ residual of 10 to 20 mg/L and the alumina exchange schematic are cited to Stumm and Morgan and MWH chapter 16 from memory of the texts, not re-read.
• The Nalgonda efficiency and the aluminium residual remark are general practice, not from WHO.
• The US phosphate and hydrofluoric acid limits are read from the CFR sections; the phosphate limits apply to a narrow stream (storage pile runoff) and the HF limits are per unit production.
• Abu Dhabi values cover two media (marine outfall 10 mg/L, sewer 15 mg/L); other GCC states not read.

Gaps

• No surface water or effluent fluoride survey was read.
• The WHO 2004 fluoride background document and the Fawell et al. 2006 monograph were not read; the expanded fact sheet stands in.
• US industrial categories beyond parts 415, 418 and 421 (aluminium smelting part 421 subpart B, semiconductors part 469 which limits only TTO and pH) were not transcribed for fluoride.
• Other GCC discharge standards were not read.
• No solubility product for CaF₂ or fluorapatite, no aluminium fluoride stability constants and no activated alumina or bone char capacity figures are quoted; the aluminium fluoride, fluorapatite and alum with lime equations are from Stumm and Morgan chapters 3 and 6 and MWH chapter 16, from the chapter, not re-read.

Sources

Identity

Name and symbol

Fluorine, F

Atomic number

9 protons

Position

group 17 · period 2 · p-block · diatomic nonmetal

CAS number

7782-41-4

Atomic structure

Atomic mass

18.998 u

Electron configuration

1s² 2s² 2p⁵
[He] 2s²²p⁵

Electrons per shell

2, 7

Valence electrons

7 outer shell

Physical properties

State at room temperature

Gas

Melting point

53.53 K (-219.62 °C)

Boiling point

85.03 K (-188.12 °C)

Density

0.0017 g/cm3 (gas at STP, so 1.696 g/L)

Appearance

gas: very pale yellowliquid: bright yellowsolid: alpha is opaque, beta is transparent

Thermal conductivity

0.02591 W/(m·K)

Electrical resistivity

not in sources

Electrical conductivity

not in sources

Crystal structure

cubic

Molar heat capacity

not in sources

Chemical properties

Oxidation states

-1

Electronegativity

3.98 (Pauling Scale)

Ionisation energy

17.423 eV
1st 1,681, 2nd 3,374.2, 3rd 6,050.4 kJ/mol

Electron affinity

3.339 eV

Atomic radius

empirical 57, covalent 57 pm

Ionic radius

F⁻ 133; F⁷⁺ 8 pm

Reactivity

The most electronegative and most reactive element; the pale yellow gas reacts, often violently, with every element except helium, neon and argon (argon only as the fluorohydride under extreme conditions) and with most organic and inorganic substances.

with water

Water burns in a jet of fluorine; water vapour is oxidised to hydrogen fluoride and oxygen, with some ozone: $\ce{2 F2 + 2 H2O -> 4 HF + O2}$

with oxygen, air

Does not combine under ambient conditions; oxygen fluorides form only in an electric discharge at low temperature and pressure and fall apart when heated.

with acids

Sulfuric acid reacts with fluorine only at elevated temperature; hydrogen sulfide and sulfur dioxide combine readily, the latter sometimes explosively.

with halogens

Reacts readily with chlorine, bromine and iodine to interhalogens (mono-, tri- and pentafluorides, and iodine heptafluoride): $\ce{Cl2 + F2 -> 2 ClF}$

Typical compounds

• HF hydrogen fluoride hydrofluoric acid; etches glass, route to most fluorochemicals
• CaF₂ calcium fluoride fluorite or fluorspar, the main ore; infrared lenses
• UF₆ uranium hexafluoride volatile; separates uranium isotopes
• NaF sodium fluoride toothpaste and water fluoridation
• SF₆ sulfur hexafluoride very inert insulating gas; strongest greenhouse gas
• Na₃AlF₆ cryolite most fluorine-rich mineral; flux in aluminium smelting

Occurrence, production and use

Crustal abundance

5.85×102 milligrams per kilogram

Oceanic abundance

1.3 milligrams per liter

Occurrence and sources

• dissolved in seawater about 1.3 mg/L; crustal estimate 585 mg/kg (Jefferson Lab figures via PubChem)
• fluorspar, fluorite (CaF2) vein and replacement deposits in China, Mexico, Mongolia, South Africa, Spain and Germany; mines being developed or reopened in Australia, Canada, Germany, Kenya and the United States (2024)
• fluorapatite Ca10(PO4)6(F,OH)2 in phosphate rock sedimentary and igneous phosphate deposits worldwide; about 3.5 percent fluorine, recovered as fluorosilicic acid at phosphoric acid plants
• cryolite Greenland, no longer widely used; replaced in aluminium smelting by a synthetic mixture of sodium, aluminium and calcium fluorides

Extraction, production

Anhydrous hydrogen fluoride from acid-grade fluorspar and sulfuric acid

$\ce{CaF2 + H2SO4 -> 2 HF + CaSO4}$

Dried fluorspar and concentrated sulfuric acid at elevated temperature, delta HR = 59 kJ/mol; impurities in the spar give side reactions to silicon tetrafluoride and sulfur dioxide (AAF BREF, PDF p286). HF production is by far the leading use of acid-grade fluorspar and aqueous HF is the feedstock for virtually all fluorine chemicals (USGS).

Fluorosilicic acid (FSA) recovered from phosphoric acid plants, converted to HF or AlF3

An estimated 40,000 tonnes of FSA, equivalent to about 65,000 tonnes of 100 percent fluorspar, was recovered from three US phosphoric acid plants in 2024, and a plant in Aurora, North Carolina, began making anhydrous HF from FSA (USGS, printed pp. 72 to 73). No equation printed.

Elemental fluorine by electrolysis of potassium hydrogendifluoride in anhydrous HF

$\ce{2 HF (l) -> H2 (g) + F2 (g)}$

Moissan's 1886 method, still used; described in words only.

Uses

Fluorine is added to city water supplies in the proportion of about one part per million to help prevent tooth decay. Sodium fluoride (NaF), stannous(II) fluoride (SnF2) and sodium monofluorophosphate (Na2PO3F) are all fluorine compounds added to toothpaste, also to help prevent tooth decay. Hydrofluoric acid (HF) is used to etch glass, including most of the glass used in light bulbs. Uranium hexafluoride (UF6) is used to separate isotopes of uranium. Crystals of calcium fluoride (CaF2), also known as fluorite and fluorspar, are used to make lenses to focus infrared light. Fluorine joins with carbon to form a class of compounds known as fluorocarbons. Some of these compounds, such as dichlorodifluoromethane (CF2Cl2), were widely used in air conditioning and refrigeration systems and in aerosol spray cans, but have been phased out due to the damage they were causing to the earth's ozone layer.

Fluorine and its compounds are used in producing uranium (from the hexafluoride) and more than 100 commercial fluorochemicals, including many high-temperature plastics. Hydrofluoric acid etches glass of light bulbs. Fluorochlorohydrocarbons are extensively used in air conditioning and refrigeration.

The presence of fluorine as a soluble fluoride in drinking water to the extent of 2 ppm may cause mottled enamel in teeth when used by children acquiring permanent teeth; in smaller amounts, however, fluoride helps prevent dental cavities.

Elemental fluorine has been studied as a rocket propellant as it has an exceptionally high specific impulse value.

• Fluorochemicals and polymers: refrigerants; hydrofluorocarbon quotas were cut to 40 percent below baseline in 2024 under the US AIM Act after CFCs were banned for destroying the ozone layer; fluoropolymers such as PTFE for non-stick coatings, cable insulation and waterproof membranes; lithium-ion battery binders, electrolyte salts and separator coatings; sulfur hexafluoride insulating gas for transformers HF manufacture is by far the leading use of acid-grade fluorspar (USGS 2024)
• Aluminium smelting: aluminium fluoride and synthetic cryolite bath for the electrolysis of alumina; HF is a key ingredient in aluminium processing and smelters recycle HF and fluorides
• Nuclear: uranium hexafluoride for isotope separation; the US Department of Energy sells aqueous HF recovered from converting depleted UF6 to oxide
• Steel, cement and glass: metallurgical-grade fluorspar as a flux in steelmaking and iron casting; cement, enamels, glass manufacture, welding rod coatings; HF etching of glass
• Water treatment: fluoridation of drinking water below 2 ppm to prevent dental cavities

Safety, toxicity

Elemental fluorine and the fluoride ion are highly toxic. The free element has a characteristic pungent odor, detectable in concentrations as low as 20 ppb, which is below the safe working level. The recommended maximum allowable concentration for a daily 8-hour time-weighted exposure is 1 ppm.

Safe handling techniques enable the transport liquid fluorine by the ton.

Discovery and name

Discovered by

André-Marie Ampère

Discovered

1810

First isolated

Henri Moissan

Named by

*Humphry Davy *André-Marie Ampère

Origin of the name

after the mineral fluorite, itself named after Latin fluo (to flow, in smelting)

Fluorine is the most electronegative and reactive of all elements. It is a pale yellow, corrosive gas, which reacts with most organic and inorganic substances. Finely divided metals, glass, ceramics, carbon, and even water burn in fluorine with a bright flame.

Until World War II, there was no commercial production of elemental fluorine. The nuclear bomb project and nuclear energy applications, however, made it necessary to produce large quantities.