Hydrofluoric acid

Hydrofluoric acid is a solution of hydrogen fluoride in water. Together with hydrogen fluoride, hydrofluoric acid is a valued source of fluorine, being the precursor to numerous pharmaceuticals, diverse polymers (e.g. Teflon), and most other synthetic materials that contain fluorine. Hydrofluoric acid is best known to the public for its ability to dissolve glass by reacting with SiO₂, the major component of most glasses. This dissolution process can be described as follows:

SiO_2(s) + 4HF_(aq) → SiF_4(g) + 2H₂O_(l)

SiO_2(s) + 6HF_(aq) → H₂[SiF₆]_(aq) + 2H₂O_(l)

Because of its high reactivity toward glass, hydrofluoric acid is typically stored in polyethylene or Teflon containers. It is also unique in its ability to dissolve many metal and semimetal oxides. It is corrosive, as explained below.

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Acidity

Hydrogen fluoride dissociates in aqueous solution in a similar fashion to other common acids:

HF + H₂O → H₃O⁺ + F⁻

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

2HF → H⁺ + FHF⁻

The FHF⁻ anion is stabilized by the very strong hydrogen - fluorine hydrogen bond. In acetic acid and similar solvents, hydrofluoric acid is the strongest of the hydrohalic acids.

Production

Main article: hydrogen fluoride

Industrially, hydrofluoric acid is produced by treatment of the mineral fluorite (CaF₂) with concentrated sulfuric acid. When combined at 250 °C, these two substances react to produce hydrogen fluoride according to the following chemical equation:

CaF₂ + H₂SO₄ → 2HF + CaSO₄

Uses

Because of its ability to dissolve metal oxides, hydrofluoric acid is used in the purification of both aluminium and uranium. It is also used to etch glass, to remove surface oxides from silicon in the semiconductor industry, as a catalyst for the alkylation of isobutane and butene in oil refineries, and to remove oxide impurities from stainless steel in a process called pickling. Dilute hydrofluoric acid is sold as a household rust stain remover. Recently it has even been used in car washes in "wheel cleaner" compounds.^[1] Due to its ability to dissolve silicate compounds, hydrofluoric acid is often used to dissolve rock samples (usually powdered) prior to analysis.

Hydrofluoric acid is also used in the synthesis of many fluorine-containing organic compounds, including Teflon,fluoropolymers, perfluorocarbons, and refrigerants such as freon.

Affinity for magnesium and calcium

Hydrofluoric acid attacks many metal oxides and forms stable fluorides or fluoro complexes. In the body, hydrofluoric acid reacts with the ubiquitous biologically important ions Ca²⁺ and Mg²⁺. In some cases, exposures can lead to hypocalcemia. Treatment of hydrofluoric acid exposure, aside from thorough rinsing of the exposed areas, often entails treatment with calcium gluconate, applied topically as a gel or by injection.

Safety

    Hydrofluoric acid is corrosive and a contact poison. Symptoms of exposure to hydrofluoric acid may not be immediately evident but can be fatal. It should be handled with extreme care, beyond that accorded to other mineral acids. Due to its low dissociation constant, HF penetrates tissues quickly.^{[citation needed]} Hydrogen fluoride is released upon combustion of fluorine-containing compounds such as products made from Viton and Teflon. Hydrogen fluoride converts immediately to hydrofluoric acid upon contact with moisture.

References

1. ^ Strachan, John (January, 1999). "A deadly rinse: The dangers of hydrofluoric acid". Professional Carwashing & Detailing. Retrieved on 2006-08-30.