hydrogen fluoride


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hydrogen fluoride,

chemical compound, HF, a colorless, fuming liquid or colorless gas that boils at 19.54°C;. It is miscible with water and is soluble in benzene, toluene, and concentrated sulfuric acid. Hydrofluoric acid is a water solution of hydrogen fluoride; hydrofluoric acid containing 35.35% hydrogen fluoride by weight is an azeotrope with a constant boiling point of 120°C;. Whether gaseous, liquid, or in solution, hydrogen fluoride is a dangerous chemical and must be handled with caution, since it attacks the skin and other tissue. Hydrogen fluoride has a number of properties that distinguish it from the other hydrogen halides. It polymerizes, forming molecules such as H2F2 and H6F6; this explains in part its relatively high boiling point. It is a relatively weak acid. It attacks glass, reacting with the silica, SiO2, to form the gas silicon tetrafluoride, SiF4, and water; this leaves the surface of the glass etched. Major industrial uses of hydrogen fluoride include the synthesis of fluorocarbons (e.g., FreonFreon
[trade name], any one of a special class of chemical compounds that are used as refrigerants, aerosol propellants, and solvents. These compounds are haloalkanes, i.e., halogen derivatives of saturated hydrocarbons (see alkane).
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 and teflon) and the production of aluminum fluoride and synthetic cryolite for use in aluminum refining. It is also employed in refining uranium for use as a nuclear fuel, in manufacturing various organic chemicals, in producing stainless steel, and for various other applications. Hydrogen fluoride is produced commercially by heating purified fluorspar (calcium fluoride) with concentrated sulfuric acid to produce the gas, which may be condensed by cooling or dissolved in water. Hydrogen fluoride is available commercially either in an anhydrous (water-free) state or in water solutions of various concentrations. Because it attacks glass, it is usually stored in steel tanks, cylinders, or drums, or, in small amounts, in plastic bottles.

Hydrogen Fluoride

 

HF, a compound of fluorine with hydrogen. Hydrogen fluoride has a density of 0.98 g/cm3 at 12°C, a melting point of –83.37°C, and a boiling point of 19.43°C. Above 19.43°C, it is a colorless gas, with a pungent odor that irritates the respiratory tract; below this temperature, it is a colorless mobile liquid. The critical temperature is 230.2°C, the critical pressure, 9.45 meganewtons/m2 (94.5 kilograms-force/cm2), and the enthalpy of formation, –271 kilojoule/mole (–64.8 kcal/mole). The hydrogen fluoride molecules are associated, and the degree of association depends on the state of aggregation, temperature, and pressure. In gaseous hydrogen fluoride, the associated molecules include three or four HF molecules. Hydrogen fluoride is soluble in water in any proportions. The aqueous solution of hydrogen fluoride is hydrofluoric acid.

Anhydrous hydrogen fluoride reacts with metals in the electromotive series up to hydrogen, with the exception of Al, Mg, Pb, Fe, and Ni. Hydrogen fluoride interacts with many oxides; for example, SiO2 + 4HF = SiF4 + 2H2O (glass-etching reaction). It combines with organic compounds across the multiple bond and in a number of cases induces the polymerization of the compounds. The action of hydrogen fluoride, in the presence of a catalyst, on polyhaloalkanes causes the halogen in polyhaloalkanes to be replaced by fluoride. Hydrogen fluoride is a good solvent of inorganic and organic compounds: moreover, unlike aqueous solutions, it is hydrogen fluoride itself and not the dissolved substance that is subjected to electrolytic dissociation.

The commercial preparation of hydrogen fluoride is based on the reaction of fluorspar (fluorite) with 98-percent sulfuric acid: CaF2 + H2SO4 = 2HF + CaSO4.

Hydrogen fluoride is widely used in the preparation of synthetic cryolite, in the production of uranium, and in the synthesis of organic fluorine compounds. It is also used in glass etching and as a catalyst in alkylation during the extraction of gasoline from petroleum.

Hydrogen fluoride is toxic. Upon contact with skin, it induces burns. HF vapors irritate the upper respiratory tract; its maximum permissible concentration in the air is 0.0005 mg/liter. Inhalation of oxygen is recommended treatment for inhalation of HF vapors. Burned areas of the skin should be immersed in an iced saturated solution of magnesium sulfate or 70-percent ethanol.

REFERENCES

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A. V. PANKRATOV

hydrogen fluoride

[′hī·drə·jən ′flu̇r‚īd]
(inorganic chemistry)
HF The hydride of fluoride; anhydrous HF is a mobile, colorless, liquid that fumes in air, melts at -83°C, boils at 19.8°C; used to make fluorine-containing refrigerants (such as Freon) and organic fluorocarbon compounds, as a catalyst in alkylate gasoline manufacture, as a fluorinating agent, and in preparation of hydrofluoric acid.
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