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Hydrofluoric acid

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Title: Hydrofluoric acid  
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Subject: Hydrogen fluoride, Etching (microfabrication), Carl Wilhelm Scheele, Calcium fluoride, Tantalum pentoxide
Collection: Fluorides, Mineral Acids, Nonmetal Halides
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Hydrofluoric acid

Hydrofluoric acid
White plastic bottle with safety cap is labeled
Other names
fluoric acid; fluorhydric acid
ChemSpider  Y
EC number 231-634-8
Jmol-3D images Image
RTECS number MW7875000
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)
Acidity (pKa) 3.17[1]
Safety data sheet duPont MSDS
Very Toxic T+ Corrosive 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).
 Y  (: Y/N?)

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 1
  • Production 2
  • Uses 3
    • Oil refining 3.1
    • Production of organofluorine compounds 3.2
    • Production of fluorides 3.3
    • Etchant and cleaning agent 3.4
    • Niche applications 3.5
  • Health and safety 4
  • See also 5
  • References 6
  • External links 7


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 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 H2F+ + FHF

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


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]


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 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, 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.

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.[5] For similar reasons, dilute hydrofluoric acid is a component of household rust stain remover and in car washes in "wheel cleaner" compounds.[6] 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

  • International Chemical Safety Card 0283
  • NIOSH Pocket Guide to Chemical Hazards
  • CID 14917 from PubChem (HF)
  • CID 144681 from PubChem (5HF)
  • CID 141165 from PubChem (6HF)
  • CID 144682 from PubChem (7HF)
  • Hydrofluoric Acid Burn, The New England Journal of Medicine Acid burn case study

External links

  1. ^ Harris, Daniel C. (2010). Quantitative Chemical Analysis (8th international ed.). New York: W. H. Freeman. pp. AP14.  
  2. ^  
  3. ^ a b c d 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?".  
  5. ^ Powers, John M. and Sakaguchi, Ronald L. (2006) Craig's Restorative Dental Materials, 12th ed., Mosby, ISBN 0323036066
  6. ^ Strachan, John (January 1999). "A deadly rinse: The dangers of hydrofluoric acid". Professional Carwashing & Detailing 23 (1). 
  7. ^ Edwards, D. (1982). "Fragmentary non-vascular plant microfossils from the late Silurian of Wales". Botanical Journal of the Linnean Society 84 (3): 223–256.  
  8. ^ 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.  
  9. ^ a b "Recommended Medical Treatment for Hydrofluoric Acid Exposure" (PDF). Honeywell Specialty Materials. Retrieved 2009-05-06. 
  10. ^ Hoffman, Robert S. et al. (2007) Goldfrank's Manual of Toxicologic Emergencies. New York: McGraw-Hill Professional, p. 1333, ISBN 0071509577.
  11. ^ 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.  
  12. ^ 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.  
  13. ^ "Calcium Gluconate Gel as an Antidote to HF Acid Burns". Northwestern University. Retrieved 2012-10-01. 
  14. ^ 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.  
  15. ^ "News & Views". Chemical Health and Safety 12 (5): 35–37. September–October 2005.  
  16. ^ Siegel DC, Heard JM; Heard (March 1992). "Intra-arterial calcium infusion for hydrofluoric acid burns". Aviat Space Environ Med (NCBI) 63 (3): 206–11.  
  17. ^ Koch, Ernst-Christian (2002). "Metal-Fluorocarbon-Pyrolants IV: Thermochemical and Combustion Behaviour of Magnesium/Teflon/Viton (MTV)".  
  18. ^ Chauviere, Matt and Zierold, Dustin (2011-09-17). "Hydrogen Fluoride Inhalation Injury from a Fire Suppression System". NATO. Retrieved 2013-08-22. 


See also

Hydrogen fluoride is generated upon combustion of many fluorine-containing compounds such as products containing Viton and polytetrafluoroethylene (Teflon) parts.[17] 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.[18]

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.[9] 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.[10] 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[11][12][13] or special rinsing solutions.[14][15] However, because it is absorbed, medical treatment is necessary;[9] rinsing off is not enough. Intra-arterial infusions of calcium chloride have also shown great effectiveness in treating burns.[16]

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. 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.[8]

left and right hands, two views, burned index fingers
HF burns, not evident until a day after

A hydrofluoric acid burn of the hand

Health and safety

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.

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.

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.


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