Hydrogen fluoride

Hydrogen fluoride is a chemical compound with the formula HF. It is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers (e.g. Teflon). HF is widely used in the petrochemical industry and a component of many superacids. HF boils just below room temperature whereas the other hydrogen halides condense at much lower temperatures. Unlike the other halogen hydrides, HF is lighter than air and its odour is particularly penetrating. Aqueous solutions of HF, called hydrofluoric acid, are strongly corrosive.

tructure

HF forms orthorhombic crystals, consisting of zig-zag chains of HF molecules. The HF molecules, with a short H-F bond of 0.95 Å, are linked to neighboring molecules by intermolecular H--F distances of 1.55 Å. [cite journal | author = Johnson, M. W.; Sándor, E.; Arzi, E. | title = The Crystal Structure of Deuterium Fluoride | journal = Acta Crystallographica | year = 1975 | volume = B31 | pages = pages 1998–2003 | doi = 10.1107/S0567740875006711]

Liquid HF also consists of chains of HF molecules, but the chains are shorter, consisting on average of only five or six molecules. [cite journal | title = On the Structure of Liquid Hydrogen Fluoride | journal = Angewandte Chemie, International Edition | year = 2004 | volume = 43 | pages = 1952–55 | doi = 10.1002/anie.200353289 | author = Mclain, Sylvia E.] The higher boiling point of HF relative to analogous species, such as HCl, is attributed to hydrogen bonding between HF molecules, as indicated by the existence of chains even in the liquid state.

Acidity

Dilute aqueous HF solutions are weakly acidic in contrast to corresponding solutions of the other hydrogen halides. A qualitative explanation for this behavior is related to the tendency of HF to hydrogen-bond and form ion-pair clusters such as F^-·H₃O⁺. [cite journal
doi = 10.1021/ja00537a008
title = The nature of hydrofluoric acid. A spectroscopic study of the proton-transfer complex H₃O⁺...F⁻
year = 1980
author = Giguere, Paul A.
journal = J. Am. Chem. Soc.
volume = 102
pages = 5473] [cite journal | author = Radu Iftimie, Vibin Thomas, Sylvain Plessis, Patrick Marchand, and Patrick Ayotte| title = Spectral Signatures and Molecular Origin of Acid Dissociation Intermediates | journal = J. Am. Chem. Soc. | doi = 10.1021/ja077846o| year = 2008| volume = 130| pages = 5901]

In concentrated hydrogen fluoride solution, F⁻ ions forms a [hydrogen difluoride| [HF₂] ⁻] (aq) complex with HF molecules. HF molecules remaining ionize to compensate the loss of F⁻ ions. More H⁺ ions are thus formed, making concentrated HF an effectively strong acid.

Anhydrous hydrogen fluoride is an extremely strong acid ("H"₀ ~ -11), comparable in strength to anhydrous sulfuric acid ("H"₀ ~ -12).

Uses

HF is used for fluorinating polymers giving fluorocarbons, petroleum refining, glassmaking, aluminium manufacturing, titanium pickling, quartz purification, and metal finishing. It is also used to synthesize UF₆, which is key to separating uranium isotopes.

Hydrogen fluoride can be found in consumer products for removing rust, cleaning brass, and glass etching, although use in consumer products is discouraged Fact|date=February 2007 due to HF's corrosiveness and toxicity. Hydrogen fluoride is typically marketed in three common forms: anhydrous HF, aqueous 70% HF, aqueous 49% HF. HF is manufactured by the reaction of calcium fluoride (fluorspar) and sulfuric acid::CaF₂ + H₂SO₄ → CaSO₄ + 2 HFThe vapors from this reaction are a mixture of hydrogen fluoride, sulfuric acid, and a few minor byproducts.

Health effects

Upon contact with moisture, including tissue, hydrogen fluoride immediately converts to hydrofluoric acid, which is highly corrosive and toxic, and requires immediate medical attention.

References

* [http://www.atsdr.cdc.gov/MHMI/mmg11.html "ATSDR - MMG: Hydrogen Fluoride"] . Retrieved May 14, 2006
*Barbalace, Kenneth. [http://environmentalchemistry.com/yogi/chemicals/cn/Antimony%A0Pentafluoride.html "Chemical Database - Hydrogen Fluoride. EnvironmentalChemistry.com"] . 1995 - 2006. Retrieved May 14, 2006
* [http://www.honeywell.com/sites/sm/chemicals/hfacid/Anhydrous_HF.htm Honeywell, Industrial Fluorines G525-521, "Recommended Medical Treatment for Hydrofluoric Acid Exposure"]
*Cotton, F. A. and Wilkinson, G., "Advanced Inorganic Chemistry", John Wiley and Sons: New York, 1988. ISBN 0471849979