Hydrofluoric acid

Hydrofluoric acid
White plastic bottle with safety cap is labeled "QP Panreac" above smaller text "Hydrofluoric Acid 40% QP" with 6 translations. In a bright orange region along the side, warning symbols are clearly visible.
Names
Other names
fluoric acid; fluorhydric acid
Identifiers
7664-39-3 YesY
ChEBI CHEBI:29228 YesY
ChemSpider 14214 YesY
EC Number 231-634-8
Jmol interactive 3D Image
RTECS number MW7875000
UNII RGL5YE86CZ YesY
Properties
HF (aq)
Molar mass not applicable
(see hydrogen fluoride)
Appearance Colorless solution
Density 1.15 g/mL (for 48% soln.)
Melting point Not applicable
(see hydrogen fluoride)
Boiling point Not applicable
(see hydrogen fluoride)
Miscible.
Acidity (pKa) 3.17[1]
Hazards
Safety data sheet duPont MSDS
T+ C
R-phrases R26/27/28, R35
S-phrases (S1/2), S7/9, S26, S36/37, S45
NFPA 704
Flash point Non-flammable
Related compounds
Other anions
 Hydrochloric acid
Hydrobromic acid
Hydroiodic acid
Related compounds
 Hydrogen fluoride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Hydrofluoric acid is a solution of hydrogen fluoride (HF) in water. It is a precursor to almost all fluorine compounds, including pharmaceuticals such as fluoxetine (Prozac), diverse materials such as PTFE (Teflon), and elemental fluorine itself. It is a colourless solution that is highly corrosive, capable of dissolving many materials, especially oxides. Its ability to dissolve glass has been known since the 17th century, even before Carl Wilhelm Scheele prepared it in large quantities in 1771.[2] Because of its high reactivity toward glass and moderate reactivity toward many metals, hydrofluoric acid is usually stored in plastic containers (although PTFE is slightly permeable to it).[3]

Hydrogen fluoride gas is an acute poison that may immediately and permanently damage lungs and the corneas of the eyes. Aqueous hydrofluoric acid is a contact-poison with the potential for deep, initially painless burns and ensuing tissue death. By interfering with body calcium metabolism, the concentrated acid may also cause systemic toxicity and eventual cardiac arrest and fatality, after contact with as little as 160 cm2 (25 square inches) of skin.

Acidity

Hydrofluoric acid is classified as a weak acid because of its lower dissociation constant compared to the strong acids. It ionizes in aqueous solution in a similar fashion to other common acids:[4]

HF + H2O is in equilibrium with H3O+ + F−

HF is the only hydrohalic acid that is not considered a strong acid, i.e. it does not fully ionize in dilute aqueous solutions.

When the concentration of HF approaches 100%, the acidity increases dramatically because of homoassociation:

3 HF is in equilibrium with H2F+ + FHF−

The bifluoride (FHF−) anion is stabilized by the very strong hydrogen–fluorine hydrogen bond.

Production

Hydrofluoric acid is produced by treatment of the mineral fluorite (CaF2) with concentrated sulfuric acid. When combined at 265 °C, these two substances react to produce hydrogen fluoride and calcium sulfate according to the following chemical equation:

CaF2 + H2SO4 → 2 HF + CaSO4

Although bulk fluorite is a suitable precursor and a major source of world HF production, HF is also produced as a by-product of the production of phosphoric acid, which is derived from the mineral apatite. Apatite sources typically contain a few percent of fluoroapatite, acid digestion of which releases gaseous stream consisting of sulfur dioxide (from the H2SO4), water, and HF, as well as particulates. After separation from the solids, the gases are treated with sulfuric acid and oleum to afford anhydrous HF. Owing to the corrosive nature of HF, its production is accompanied by the dissolution of silicate minerals, and, in this way, significant amounts of fluorosilicic acid are generated.[3]

Uses

Hydrofluoric acid has a variety of uses in industry and research. It is used as a starting material or intermediate in industrial chemistry, mining, refining, glass finishing, silicon chip manufacturing, and in cleaning.[5]

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 the strong 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 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.[3]

Production of fluorides

Most high-volume inorganic fluoride compounds are prepared from hydrofluoric acid. Foremost are Na3AlF6, cryolite, and AlF3, aluminium trifluoride. A molten mixture of these solids serves as a high-temperature solvent for the production of metallic aluminium. Given concerns about fluorides in the environment, alternative technologies are being sought. Other inorganic fluorides prepared from hydrofluoric acid include sodium fluoride and uranium hexafluoride.[3]

Etchant and cleaning agent

Wet etching tanks

In metalworking, hydrofluoric acid is used as a pickling agent to remove oxides and other impurities from stainless and carbon steels because of its limited ability to dissolve steel. It is used in the semiconductor industry as a major component of Wright Etch and buffered oxide etch, which are used to clean silicon wafers. In a similar manner it is also used to etch glass by reacting with silicon dioxide to form gaseous or water-soluble silicon fluorides. It can also be used to polish and frost glass.[5]

SiO2 + 4 HF → SiF4(g) + 2 H2O
SiO2 + 6 HF → H2SiF6 + 2 H2O

A 5% to 9% hydrofluoric acid gel is also commonly used to etch all ceramic dental restorations to improve bonding.[6] 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.[5][7] 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.

Niche applications

Because of its ability to dissolve (most) oxides and silicates, 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.[8]

Diluted hydrofluoric acid (1 to 3 %wt.) is used in the petroleum industry in a mixture with other acids (HCl or organic acids) in order to stimulate the production of water, oil, and gas wells specifically where sandstone is involved.

Hydrofluoric acid is also used by some collectors of antique glass bottles to remove so-called 'sickness' from the glass, caused by acids (usually in the soil the bottle was buried in) attacking the soda content of the glass.

Offset printing companies use hydrofluoric acid to remove unwanted images from printing plates. Felt-tip markers called "deletion pens" are available to make the process safer for the worker.

Health and safety

A hydrofluoric acid burn of the hand
left and right hands, two views, burned index fingers
HF burns, not evident until a day after

In addition to being a highly corrosive liquid, hydrofluoric acid is also a contact poison. It should therefore be handled with extreme care, using protective equipment and safety precautions beyond those used with other mineral acids. Owing to its low acid dissociation constant, HF as a neutral lipid-soluble molecule penetrates tissue more rapidly than typical mineral acids. Because of the ability of hydrofluoric acid to penetrate tissue, poisoning can occur readily through exposure of skin or eyes, or when inhaled or swallowed. Symptoms of exposure to hydrofluoric acid may not be immediately evident, and this can provide false reassurance to victims, causing them to delay medical treatment.[9] Despite having an irritating odor, HF may reach dangerous levels without an obvious smell.[5] HF interferes with nerve function, meaning that burns may not initially be painful. Accidental exposures can go unnoticed, delaying treatment and increasing the extent and seriousness of the injury.[9] Symptoms of HF exposure include irritation of the eyes, skin, nose, and throat, eye and skin burns, rhinitis, bronchitis, pulmonary edema (fluid buildup in the lungs), and bone damage.[10]

Once absorbed into blood through the skin, it reacts with blood calcium and may cause cardiac arrest. Burns with areas larger than 160 cm2 (25 square inches) have the potential to cause serious systemic toxicity from interference with blood and tissue calcium levels.[11] In the body, hydrofluoric acid reacts with the ubiquitous biologically important ions Ca2+ and Mg2+. Formation of insoluble calcium fluoride is proposed as the etiology for both precipitous fall in serum calcium and the severe pain associated with tissue toxicity.[12] In some cases, exposures can lead to hypocalcemia. Thus, hydrofluoric acid exposure is often treated with calcium gluconate, a source of Ca2+ that sequesters the fluoride ions. HF chemical burns can be treated with a water wash and 2.5% calcium gluconate gel[13][14][15] or special rinsing solutions.[16][17] However, because it is absorbed, medical treatment is necessary;[11] rinsing off is not enough. Intra-arterial infusions of calcium chloride have also shown great effectiveness in treating burns.[18]

Hydrogen fluoride is generated upon combustion of many fluorine-containing compounds such as products containing Viton and polytetrafluoroethylene (Teflon) parts.[19] Hydrofluorocarbons in automatic fire suppression systems can release hydrogen fluoride at high temperatures, and this has led to deaths from acute respiratory failure in military personnel when a rocket-propelled grenade hit the fire suppression system in their vehicle.[20] Hydrofluoric acid can be released from volcanoes, sea salt aerosol, and from welding or manufacturing processes.[5]

Hydrofluoric acid is also a highly reactive compound and must be stored carefully to prevent dangerous reactions, though it is not flammable. It reacts with bases, acids, and oxidants and attacks glass, ceramics, concrete, some forms of plastic, rubber, and coatings. When combined with methanesulfonic acid or polymerizing cyanogens, it produces explosive gases.[5]

See also

References

  1. ↑ Harris, Daniel C. (2010). Quantitative Chemical Analysis (8th international ed.). New York: W. H. Freeman. pp. AP14. ISBN 1429263091.
  2. ↑ Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. p. 921. ISBN 0-08-022057-6.
  3. 1 2 3 4 Aigueperse, J. et al. (2005) "Fluorine Compounds, Inorganic" in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, doi:10.1002/14356007.a11_307
  4. ↑ Ayotte, P; Hébert, M; Marchand, P (Nov 2005). "Why is hydrofluoric acid a weak acid?". J. Chem. Phys. 123 (18): 184501. doi:10.1063/1.2090259. PMID 16292908.
  5. 1 2 3 4 5 6 "CDC - The Emergency Response Safety and Health Database: Systemic Agent: HYDROGEN FLUORIDE/ HYDROFLUORIC ACID - NIOSH". www.cdc.gov. Retrieved 2015-12-04.
  6. ↑ Powers, John M. and Sakaguchi, Ronald L. (2006) Craig's Restorative Dental Materials, 12th ed., Mosby, ISBN 0323036066
  7. ↑ Strachan, John (January 1999). "A deadly rinse: The dangers of hydrofluoric acid". Professional Carwashing & Detailing 23 (1).
  8. ↑ Edwards, D. (1982). "Fragmentary non-vascular plant microfossils from the late Silurian of Wales". Botanical Journal of the Linnean Society 84 (3): 223–256. doi:10.1111/j.1095-8339.1982.tb00536.x.
  9. 1 2 Yamashita M, Yamashita M, Suzuki M, Hirai H, Kajigaya H; Yamashita; Suzuki; Hirai; Kajigaya (2001). "Ionophoretic delivery of calcium for experimental hydrofluoric acid burns". Crit. Care Med. 29 (8): 1575–8. doi:10.1097/00003246-200108000-00013. PMID 11505130.
  10. ↑ "CDC - NIOSH Pocket Guide to Chemical Hazards - Hydrogen fluoride". www.cdc.gov. Retrieved 2015-11-28. line feed character in |title= at position 47 (help)
  11. 1 2 "Recommended Medical Treatment for Hydrofluoric Acid Exposure" (PDF). Honeywell Specialty Materials. Retrieved 2009-05-06.
  12. ↑ Hoffman, Robert S. et al. (2007) Goldfrank's Manual of Toxicologic Emergencies. New York: McGraw-Hill Professional, p. 1333, ISBN 0071509577.
  13. ↑ el Saadi MS, Hall AH, Hall PK, Riggs BS, Augenstein WL, Rumack BH; Hall; Hall; Riggs; Augenstein; Rumack (1989). "Hydrofluoric acid dermal exposure". Vet Hum Toxicol 31 (3): 243–7. PMID 2741315.
  14. ↑ Roblin I, Urban M, Flicoteau D, Martin C, Pradeau D; Urban; Flicoteau; Martin; Pradeau (2006). "Topical treatment of experimental hydrofluoric acid skin burns by 2.5% calcium gluconate". J Burn Care Res 27 (6): 889–94. doi:10.1097/01.BCR.0000245767.54278.09. PMID 17091088.
  15. ↑ "Calcium Gluconate Gel as an Antidote to HF Acid Burns". Northwestern University. Retrieved 2012-10-01.
  16. ↑ Hultén P, Höjer J, Ludwigs U, Janson A; Höjer; Ludwigs; Janson (2004). "Hexafluorine vs. standard decontamination to reduce systemic toxicity after dermal exposure to hydrofluoric acid". J. Toxicol. Clin. Toxicol. 42 (4): 355–61. doi:10.1081/CLT-120039541. PMID 15461243.
  17. ↑ "News & Views". Chemical Health and Safety 12 (5): 35–37. September–October 2005. doi:10.1016/j.chs.2005.07.007.
  18. ↑ Siegel DC, Heard JM; Heard (March 1992). "Intra-arterial calcium infusion for hydrofluoric acid burns". Aviat Space Environ Med (NCBI) 63 (3): 206–11. PMID 1567323.
  19. ↑ Koch, Ernst-Christian (2002). "Metal-Fluorocarbon-Pyrolants IV: Thermochemical and Combustion Behaviour of Magnesium/Teflon/Viton (MTV)". Propellants, Explosives, Pyrotechnics 27 (6): 340–351. doi:10.1002/prep.200290004.
  20. ↑ Chauviere, Matt and Zierold, Dustin (2011-09-17). "Hydrogen Fluoride Inhalation Injury from a Fire Suppression System". NATO. Retrieved 2013-08-22.

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