Hydrofluoric acid is a solution of hydrogen fluoride (HF) in water. Solutions of HF are colorless, acidic and highly corrosive. A common concentration is 49% (48–52%) but there are also stronger solutions (e.g. 70%) and pure HF has a boiling point near room temperature. It is used to make most organofluorine compounds; examples include the commonly used pharmaceutical antidepressant medication fluoxetine (Prozac) and the material polytetrafluoroethylene (Teflon). Elemental fluorine is produced from it. It is commonly used to etch glass and silicon wafers.

Uses

Production of organofluorine compounds

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. Many pharmaceuticals contain fluorine. For similar reasons, dilute hydrofluoric acid is a component of household rust stain remover, in car washes in "wheel cleaner" compounds, in ceramic and fabric rust inhibitors, and in water spot removers. 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. 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.

Oil refining

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 an 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

Hydrofluoric acid was first prepared in 1771, by Carl Wilhelm Scheele. It is now mainly produced by treatment of the mineral fluorite, CaF<sub>2</sub>, with concentrated sulfuric acid at approximately 265&nbsp;°C.

:CaF<sub>2</sub> + H<sub>2</sub>SO<sub>4</sub> → 2 HF + CaSO<sub>4</sub>

The acid is also a by-product of the production of phosphoric acid from apatite and fluoroapatite. Digestion of the mineral with sulfuric acid at elevated temperatures releases a mixture of gases, including hydrogen fluoride, which may be recovered.

Because of its high reactivity toward glass, hydrofluoric acid is stored in fluorinated plastic (often PTFE) containers.

Properties

In dilute aqueous solution, hydrogen fluoride behaves as a weak acid.

Infrared spectroscopy has been used to show that, in solution, dissociation is accompanied by formation of the ion pair .

: + HF ⋅F<sup>−</sup>pK<sub>a</sub> = 3.17

This ion pair has been characterized in the crystalline state at very low temperature.

Further association has been characterized both in solution and in the solid state.

:HF + F<sup>−</sup> log K = 0.6

It is assumed that polymerization occurs as the concentration increases. This assumption is supported by the isolation of a salt of a tetrameric anion and by low-temperature X-ray crystallography. while later compilations show −11, comparable to values near −12 for pure sulfuric acid.

Acidity

Unlike other hydrohalic acids, such as hydrochloric acid, hydrogen fluoride is only a weak acid in dilute aqueous solution. This is in part a result of the strength of the hydrogen–fluorine bond, but also of other factors such as the tendency of HF, , and anions to form clusters. At high concentrations, HF molecules undergo homoassociation to form polyatomic ions (such as bifluoride, ) and protons, thus greatly increasing the acidity. This leads to protonation of very strong acids like hydrochloric, sulfuric, or nitric acids when using concentrated hydrofluoric acid solutions. Although hydrofluoric acid is regarded as a weak acid, it is very corrosive, even attacking glass when hydrated. Paul Giguère and Sylvia Turrell

: + HF

With increasing concentration of HF the concentration of the hydrogen difluoride ion also increases. Despite its irritating vapor, HF may reach dangerous levels without an obvious odor. due to HF strongly interacting with calcium in bones. In a concentrated form, HF can cause severe tissue destruction through lesions and mucous membrane damage, but dilute HF is still dangerous because of its high lipid affinity, leading to cellular death of nerves, blood vessels, tendons, bones, and other tissues. Cases of intestinal damage from contact with hydrofluoric acid requiring surgical intervention are rare but known in medical literature.

Hydrofluoric acid burns can be treated with "hexafluorine", a name used for calcium gluconate gel.

  • In the episodes "Cat's in the Bag..." and "Box Cutter" of the crime drama television series Breaking Bad, Walter White and Jesse Pinkman use hydrofluoric acid to chemically degrade the corpses of gangsters.
  • In "Of Past Regret and Future Fear", a Season 4 episode of the television medical drama ER, a patient is brought in with severe chemical burns from hydrofluoric acid that prove to be fatal.
  • Hydrofluoric acid is mentioned in the novel Virtual Destruction by Kevin J. Anderson and Doug Beason. The character Jose Aragon comes in contact with the acid on his hand, prompting him to seek treatment at a medical center.

See also

  • Vapour phase decomposition
  • 2019 Philadelphia Energy Solutions refinery explosion

References

  • International Chemical Safety Card 0283
  • NIOSH Pocket Guide to Chemical Hazards
  • (HF)
  • (5HF)
  • (6HF)
  • (7HF)
  • "Hydrofluoric Acid Burn", The New England Journal of Medicine—Acid burn case study