Hydrofluoric acid
Other names fluoric acid; fluorhydric acid
Identifiers
CAS number	7664-39-3 Y
ChemSpider	14214 Y
UNII	RGL5YE86CZ Y
EC number	231-634-8
ChEBI	CHEBI:29228 Y
RTECS number	MW7875000
Jmol-3D images	Image 1
SMILES F
InChI InChI=1S/FH/h1H Y Key: KRHYYFGTRYWZRS-UHFFFAOYSA-N Y InChI=1/FH/h1H Key: KRHYYFGTRYWZRS-UHFFFAOYAC
Properties
Molecular formula	HF (aq)
Molar mass	not applicable (see hydrogen fluoride)
Appearance	Colorless solution
Density	1.15 g/mL (for 48% soln.)
Melting point	not applicable (see hydrogen fluoride)
Boiling point	not applicable (see hydrogen fluoride)
Solubility in water	Miscible.
Acidity (pKa)	3.17 [1]
Hazards
MSDS	duPont MSDS
EU Index	009-003-00-1
EU classification	Very toxic (T+) Corrosive (C)
R-phrases	R26/27/28, R35
S-phrases	(S1/2), S7/9, S26, S36/37, S45
NFPA 704	0 4 2 COR
Flash point	Non-flammable
Related compounds
Other anions	Hydrochloric acid Hydrobromic acid Hydroiodic acid
Related compounds	Hydrogen fluoride
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Hydrofluoric acid (HF) is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine (Prozac) and diverse materials such as PTFE (Teflon).

Hydrofluoric acid is a highly corrosive acid, capable of dissolving many materials, especially oxides. Its ability to dissolve glass has been known since the 17th century, even before hydrofluoric acid had been prepared in large quantities by Carl Wilhelm Scheele in 1771.^[2] Because of its high reactivity toward glass and moderate reactivity toward many metals, hydrofluoric acid is usually stored in plastic containers (although PTFE is slightly permeable to it).^[3]

Hydrogen fluoride gas is a severe poison that may immediately and permanently damage lungs and the corneas of the eyes. Aqueous hydrofluoric acid is a contact-poison with the potential for deep, initially painless burns and ensuing tissue death. By interfering with body calcium metabolism, the concentrated acid may also cause systemic toxicity and eventual cardiac arrest and fatality, after contact with as little as 160 cm² (25 square inches) of skin.

Acidity[link]

Hydrofluoric acid is classified as a weak acid because of its lower dissociation constant compared to the strong acids. It ionizes in aqueous solution in a similar fashion to other common acids:^[4]

HF + H₂O H₃O⁺ + F⁻

It is the only hydrohalic acid that is not considered a strong acid, i.e. it does not fully ionize in dilute aqueous solutions.

When the concentration of HF approaches 100%, the acidity increases dramatically because of the following equilibrium:

2 HF H⁺ + FHF⁻

The bifluoride (FHF⁻) anion is stabilized by the very strong hydrogen–fluorine hydrogen bond.

Production[link]

Hydrofluoric acid is produced by treatment of the mineral fluorite (CaF₂) with concentrated sulfuric acid. When combined at 265 °C, these two substances react to produce hydrogen fluoride and calcium sulfate according to the following chemical equation:

CaF₂ + H₂SO₄ → 2 HF + CaSO₄

Although bulk fluorite is a suitable precursor and a major source of world HF production, HF is also produced as a by-product of the production of phosphoric acid, which is derived from the mineral apatite. Apatite sources typically contain a few percent of fluoroapatite, acid digestion of which releases gaseous stream consisting of sulfur dioxide (from the H₂SO₄), water, and HF, as well as particulates. After separation from the solids, the gases are treated with sulfuric acid and oleum to afford anhydrous HF. Owing to the corrosive nature of HF, its production is accompanied by the dissolution of silicate minerals, and, in this way, significant amounts of fluorosilicic acid is generated.^[3]

Uses[link]

Oil refining[link]

In a standard oil refinery process known as alkylation, isobutane is alkylated with low-molecular-weight alkenes (primarily a mixture of propylene and butylene) in the presence of the strong acid catalyst derived from hydrofluoric acid. The catalyst protonates the alkenes (propylene, butylene) to produce reactive carbocations, which alkylate isobutane. The reaction is carried out at mild temperatures (0 and 30 °C) in a two-phase reaction.

Production of organofluorine compounds[link]

The principal use of hydrofluoric acid is in organofluorine chemistry. Many organofluorine compounds are prepared using HF as the fluorine source, including Teflon, fluoropolymers, fluorocarbons, and refrigerants such as freon.^[3]

Production of fluorides[link]

Most high-volume inorganic fluoride compounds are prepared from hydrofluoric acid. Foremost are Na₃AlF₆, cryolite, and AlF₃, aluminium trifluoride. A molten mixture of these solids serves as a high-temperature solvent for the production of metallic aluminium. Given concerns about fluorides in the environment, alternative technologies are being sought. Other inorganic fluorides prepared from hydrofluoric acid include sodium fluoride and uranium hexafluoride.^[3]

Etchant and cleaning agent[link]

The ability of hydrofluoric acid to dissolve metal oxides is the basis of several applications. It removes oxide impurities from stainless steel, a process called pickling, and silicon wafers in the semiconductor industry. In this regard it is also referred to as BHF, (when Buffered with Ammonium Fluoride), and BOE (for Buffered Oxide Etch). It is a significant constituent of Wright Etch and the similar HNA (HF+Nitric+Acetic Acid) etch. In similar manner, it is also used to etch glass. A 5% to 9% hydrofluoric acid gel is also commonly used to etch all ceramic dental restorations to improve bonding.^[5] For similar reasons, dilute hydrofluoric acid is a component of household rust stain remover and in car washes in "wheel cleaner" compounds.^[6] Hydrofluoric acid attacks glass by reaction with silicon dioxide to form gaseous or water-soluble silicon fluorides. This dissolution process proceeds as follows:

SiO₂ + 4 HF → SiF₄ (g) + 2 H₂O

SiO₂ + 6 HF → H₂SiF₆ + 2 H₂O

Because of its ability to dissolve iron oxides as well as silica-based contaminants, hydrofluoric acid is used in pre-commissioning boilers that produce high-pressure steam.

Niche applications[link]

Because of its ability to dissolve oxides, hydrofluoric acid is useful for dissolving rock samples (usually powdered) prior to analysis. In similar manner, this acid is used in acid macerations to extract organic fossils from silicate rocks. Fossiliferous rock may be immersed directly into the acid, or a cellulose nitrate film may be applied (dissolved in amyl acetate), which adheres to the organic component and allows the rock to be dissolved around it.^[7]

Diluted hydrofluoric acid (1 to 3 %wt.) is used in the petroleum industry in a mixture with other acids (HCl or organic acids) in order to stimulate the production of water, oil, and gas wells specifically where sandstone is involved.^{[citation needed]}

Hydrofluoric acid is also used by some collectors of antique glass bottles to remove so-called 'sickness' from the glass, caused by acids (usually in the soil the bottle was buried in) attacking the soda content of the glass.^{[citation needed]}

Health & safety[link]

HF burns, not evident until a day after

Hydrofluoric acid is a highly corrosive liquid and is a contact poison. It should be handled with extreme care, beyond that accorded to other mineral acids. Owing to its low dissociation constant, HF as a neutral lipid-soluble molecule penetrates tissue more rapidly than typical mineral acids. Because of the ability of hydrofluoric acid to penetrate tissue, poisoning can occur readily through exposure of skin or eyes, or when inhaled or swallowed. Symptoms of exposure to hydrofluoric acid may not be immediately evident. HF interferes with nerve function, meaning that burns may not initially be painful. Accidental exposures can go unnoticed, delaying treatment and increasing the extent and seriousness of the injury.^[8]

Once absorbed into blood through the skin, it reacts with blood calcium and may cause cardiac arrest. Burns with areas larger than 25 square inches (160 cm²) have the potential to cause serious systemic toxicity from interference with blood and tissue calcium levels.^[9] In the body, hydrofluoric acid reacts with the ubiquitous biologically important ions Ca²⁺ and Mg²⁺. Formation of insoluble calcium fluoride is proposed as the etiology for both precipitous fall in serum calcium and the severe pain associated with tissue toxicity.^[10] In some cases, exposures can lead to hypocalcemia. Thus, hydrofluoric acid exposure is often treated with calcium gluconate, a source of Ca²⁺ that sequesters the fluoride ions. HF chemical burns can be treated with a water wash and 2.5% calcium gluconate gel.^[11][12][13] or special rinsing solutions.^[14][15] However, because it is absorbed, medical treatment is necessary;^[9] rinsing off is not enough. Intra-arterial infusions of calcium chloride have also shown great effectiveness in treating burns.^[16]

Hydrogen fluoride is generated upon combustion of many fluorine-containing compounds such as products containing Viton and polytetrafluoroethylene (Teflon) parts. ^[17]

See also[link]

• Vapour phase decomposition

References[link]

External links[link]

Wikimedia Commons has media related to: Hydrogen fluoride

• International Chemical Safety Card 0283
• National Pollutant Inventory – Fluoride and compounds fact sheet
• NIOSH Pocket Guide to Chemical Hazards
• CID 14917 from PubChem (HF)
• CID 144681 from PubChem (5HF)
• CID 141165 from PubChem (6HF)
• CID 144682 from PubChem (7HF)
• Hydrofluoric Acid Burn, The New England Journal of Medicine Acid burn case study