# Hydrogen fluoride

Structural formula
General
Surname Hydrogen fluoride
other names
• Hydrogen fluoride
• Hydrogen fluoride gas
• Fluoran
Molecular formula HF
Brief description

colorless gas with a pungent odor

External identifiers / databases
 CAS number 7664-39-3 EC number 231-634-8 ECHA InfoCard 100,028,759 PubChem 14917 DrugBank DB11072 Wikidata Q2468
properties
Molar mass 20.00 g mol −1
Physical state

gaseous

density
• 0.968 g ml −1 (liquid at boiling point)
• 0.92 g l −1 (gas density)
Melting point

−83.37 ° C

boiling point

19.5 ° C

Vapor pressure

1031 h Pa (20 ° C)

pK s value

3.2

solubility

miscible with water

safety instructions
GHS hazard labeling from  Regulation (EC) No. 1272/2008 (CLP) , expanded if necessary

danger

H and P phrases H: 300-310-330-314
EUH: 071
P: 260-262-280-303 + 361 + 353 + 315-304 + 340 + 315-305 + 351 + 338 + 315-403-405
MAK

DFG / Switzerland: 1 ml m −3 or 0.83 mg m −3

As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Hydrogen fluoride is an inorganic chemical substance that belongs to the group of hydrogen halides . At room temperature it is a colorless, pungent odor, very poisonous gas , which has a lower density than air . It is hygroscopic and dehydrates many substances. In the anhydrous form it is present as a smoking liquid below the boiling point of 19.51 ° C.

Hydrogen fluoride is miscible with water in any ratio and reacts acidic . The aqueous solution is known as hydrofluoric acid or hydrofluoric acid .

## Extraction and presentation

To produce hydrogen fluoride, its salts, the fluorides (example: calcium fluoride , fluorspar) , are first concentrated to over 98% by flotation and then reacted with concentrated sulfuric acid in rotary kilns heated to around 200 ° C. Hydrogen fluoride gasses out, which condenses and forms the raw acid:

${\ displaystyle {\ ce {CaF2 + H2SO4 -> CaSO4 + 2HF}}}$

This reaction is also used when detecting fluorides using the creep test .

The pure acid is then obtained by repeated rectification . Rectification is necessary because the crude acid still contains high-boiling components such as water and fluorosulfonic acid (HSO 3 F), but also low-boiling sulfur dioxide .

Hydrogen fluoride is also produced in the production of aluminum and in the firing of bricks and fine ceramics. It is also released during the incineration of waste containing fluorine or fuels such as hard coal.

## properties

### Chemical properties

Partial charges: positive , negative

The chemical behavior of hydrogen fluoride is characterized by the extremely high electronegativity of fluorine , which has the highest of all elements at 4 ( Pauling scale ). The hydrogen fluoride molecule is characterized by a pronounced dipole character with a strong negative partial charge for fluorine. Oligomers such as (HF) 6 are formed through hydrogen bonds . Compared to its higher analogue HCl, HF has a relatively high boiling point of 19.5 ° C; In contrast, HCl boils at −85.0 ° C. As with water, this phenomenon is due to the hydrogen bonds.

#### Reactions as acid

Aqueous HF solution, hydrofluoric acid, is a weak acid; however, anhydrous hydrogen fluoride is a very strong protonic acid and autoprotolyses as follows:

${\ displaystyle {\ ce {3HF <=> H2F ^ + + HF2 ^ -}}}$

Hydrogen fluoride etches silicate glass and reacts with other silicates to form gaseous silicon tetrafluoride :

${\ displaystyle {\ ce {SiO2 + 4HF <=> SiF4 + 2H2O}}}$

In an aqueous solution of hydrogen fluoride, sulfuric acid is not stable and forms fluorosulfonic acid after splitting off water:

${\ displaystyle {\ ce {H2SO4 + HF -> HSO3F + H2O}}}$

Within the range of hydrogen halides, hydrogen fluoride shows the weakest tendency to dissociate in water. Compared to the other hydrogen halides, which completely dissociate to form oxonium ions (H 3 O + ) and their corresponding counterions, hydrofluoric acid contains a mixture of different fluoride-containing ions.

#### Hydrogen fluoride storage

Pure gaseous hydrogen fluoride can be transported in tank wagons made of iron, as the surface of the iron is covered with a fluoride layer that protects the material against further attack. Monel metal (a nickel-copper alloy) is also used for valves, pipes and equipment . The aqueous solutions, on the other hand, would deeply oxidize the iron due to the dissolving of the fluoride layer. That is why hydrofluoric acid is transported in plastic tanks.

#### Hydrogen fluoride as a solid

The chain structure of crystalline HF

Solid hydrogen fluoride consists of orthorhombic crystals with chains arranged in a zigzag pattern. The HF molecules have an H – F bond length of 95  pm , the neighboring molecules have an intermolecular H – F distance of 155 pm.

## use

Hydrogen fluoride is used in the production of fluorocarbons , aluminum fluoride , synthetic cryolite (Eisstein, Na 3 [AlF 6 ]), uranium hexafluoride , other fluorine compounds, hydrofluoric acid , in the production process of surfactants and in dye chemistry . Under certain conditions, the electrolysis of hydrogen fluoride produces elemental fluorine (F 2 ).

Hydrogen fluoride is used as a catalyst in gasoline production , as a solvent, in organic chemistry for substitution with fluorine and in low-temperature technology.

Hydrogen fluoride was used in the 50s and 60s for wood protection treatment e.g. B. used by roof trusses, trade name z. B. "Osmol WB4".

## Biological importance

The main route of exposure for hydrogen fluoride is through the airways and skin . Due to the high hygroscopicity, inhalation of gaseous hydrogen fluoride should only occur in extreme cases, but in which case almost complete absorption in the upper respiratory tract is to be expected. Much more common is the absorption of aqueous solutions of hydrogen fluoride (hydrofluoric acid or hydrofluoric acid) through the skin. The skin is not an obstacle to absorption. Inadvertent oral absorption is not to be expected for physical-chemical reasons and because of the extremely pungent odor. Aqueous solutions are completely absorbed. The process can be slowed down by possible stomach contents.

Contact with hydrogen fluoride has a highly caustic effect on the skin and mucous membranes , especially the eyes . It must be noted that the symptoms can occur with a delay of up to 24 hours. The first aid in case of skin contact is immediate treatment with calcium gluconate gel. When it is absorbed into the organism, biochemical metabolic processes are significantly influenced by enzyme inhibition . Chronic effects include damage to the skeleton and skin, and impaired lung function .

## Individual evidence

1. Entry on hydrogen fluoride, anhydrous in the GESTIS substance database of the IFA , accessed on January 8, 2020(JavaScript required) .
2. ^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 .
3. Entry on Hydrogen fluoride in the Classification and Labeling Inventory of the European Chemicals Agency (ECHA), accessed on February 1, 2016. Manufacturers or distributors can expand the harmonized classification and labeling .
4. Swiss Accident Insurance Fund (Suva): Limit values ​​- current MAK and BAT values (search for 7664-39-3 or hydrogen fluoride ), accessed on November 2, 2015.
5. a b c V. Kaiser: Boiling equilibrium of the hydrogen fluoride mixtures HF / SO2 and HF / H2O / HSO3F . In: Chemical Engineer Technology . tape 38 , no. 2 , February 1, 1966, ISSN  1522-2640 , p. 151–154 , doi : 10.1002 / cite.330380210 ( wiley.com [accessed October 6, 2017]).
6. Ralf Alsfasser, Erwin Riedel, HJ Meyer: Moderne Anorganische Chemie . Walter de Gruyter, 2007, ISBN 3-11-019060-5 , p. 104 ( limited preview in Google Book search).
7. ^ Johnson, MW; Sándor, E .; Arzi, E. The Crystal Structure of Deuterium Fluoride , Acta Crystallographica , 1975, B31, pp. 1998-2003. doi : 10.1107 / S0567740875006711 .
8. Hydrogen fluoride data sheet from Sigma-Aldrich , accessed on May 18, 2015 ( PDF ).
9. Entry on hydrofluoric acid. In: Römpp Online . Georg Thieme Verlag, accessed on November 13, 2013.