Hydrofluoric acid is a
solution of
hydrogen fluoride
Hydrogen fluoride (fluorane) is an Inorganic chemistry, inorganic compound with chemical formula . It is a very poisonous, colorless gas or liquid that dissolves in water to yield hydrofluoric acid. It is the principal industrial source of fluori ...
(HF) in
water
Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
. Solutions of HF are colorless,
acid
An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...
ic and highly
corrosive
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
. A common concentration is 49% (48–52%) but there are also stronger solutions (e.g. 70%) and pure HF has a boiling point near room temperature. It is used to make most
organofluorine compounds; examples include the commonly used pharmaceutical antidepressant medication
fluoxetine (Prozac) and the material
PTFE
Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene, and has numerous applications because it is chemically inert. The commonly known brand name of PTFE-based composition is Teflon by Chemours, a spin-off fro ...
(Teflon). Elemental
fluorine
Fluorine is a chemical element; it has Chemical symbol, symbol F and atomic number 9. It is the lightest halogen and exists at Standard temperature and pressure, standard conditions as pale yellow Diatomic molecule, diatomic gas. Fluorine is extre ...
is produced from it. It is commonly used to
etch glass and silicon wafers.
Uses
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,
fluorocarbon
Fluorocarbons are chemical compounds with carbon-fluorine bonds. Compounds that contain many C-F bonds often have distinctive properties, e.g., enhanced stability, volatility, and hydrophobicity. Several fluorocarbons and their derivatives are ...
s, and
refrigerants
A refrigerant is a working fluid used in the cooling, heating, or reverse cooling/heating cycles of air conditioning systems and heat pumps, where they undergo a repeated phase transition from a liquid to a gas and back again. Refrigerants are ...
such as
freon
Freon ( ) is a registered trademark of the Chemours Company and generic descriptor for a number of halocarbon products. They are stable, nonflammable, low toxicity gases or liquids which have generally been used as refrigerants and as aerosol p ...
. Many pharmaceuticals contain fluorine.
[
]
Production of inorganic fluorides
Most high-volume inorganic fluoride compounds are prepared from hydrofluoric acid. Foremost are Na3AlF6, cryolite
Cryolite ( Na3 Al F6, sodium hexafluoroaluminate) is a rare mineral identified with the once-large deposit at Ivittuut on the west coast of Greenland, mined commercially until 1987.
It is used in the reduction ("smelting") of aluminium, in pest ...
, and AlF3, aluminium trifluoride. A molten mixture of these solids serves as a high-temperature solvent for the production of metallic aluminium
Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...
. Other inorganic fluorides prepared from hydrofluoric acid include sodium fluoride and uranium hexafluoride.[
]
Etchant, cleaner
It is used in the semiconductor
A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...
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 treatment with silicon dioxide
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundan ...
to form gaseous or water-soluble silicon fluorides. It can also be used to polish and frost glass.
: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. 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. 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. Hydrofluoric acid is also 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.
Oil refining
In a standard oil refinery
An oil refinery or petroleum refinery is an industrial processes, industrial process Factory, plant where petroleum (crude oil) is transformed and refining, refined into products such as gasoline (petrol), diesel fuel, Bitumen, asphalt base, ...
process known as alkylation Alkylation is a chemical reaction that entails transfer of an alkyl group. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion, or a carbene (or their equivalents). Alkylating agents are reagents for effecting al ...
, isobutane is alkylated with low-molecular-weight alkene
In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins.
The Internationa ...
s (primarily a mixture of propylene
Propylene, also known as propene, is an unsaturated organic compound with the chemical formula . It has one double bond, and is the second simplest member of the alkene class of hydrocarbons. It is a colorless gas with a faint petroleum-like o ...
and butylene) in the presence of an acid catalyst
Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...
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
Hydrofluoric acid was first prepared in 1771, by Carl Wilhelm Scheele
Carl Wilhelm Scheele (, ; 9 December 1742 – 21 May 1786) was a Swedish Pomerania, German-Swedish pharmaceutical chemist.
Scheele discovered oxygen (although Joseph Priestley published his findings first), and identified the elements molybd ...
. It is now mainly produced by treatment of the mineral fluorite, CaF2, with concentrated sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...
at approximately 265 °C.
:CaF2 + H2SO4 → 2 HF + CaSO4
The acid is also a by-product of the production of phosphoric acid
Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, ...
from apatite
Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of Hydroxide, OH−, Fluoride, F− and Chloride, Cl− ion, respectively, in the crystal. The formula of the admixture of ...
and fluoroapatite. Digestion of the mineral with sulfuric acid at elevated temperatures releases a mixture of gases, including hydrogen fluoride, which may be recovered.
Because of its high reactivity toward glass, hydrofluoric acid is stored in fluorinated plastic
Plastics are a wide range of synthetic polymers, synthetic or Semisynthesis, semisynthetic materials composed primarily of Polymer, polymers. Their defining characteristic, Plasticity (physics), plasticity, allows them to be Injection moulding ...
(often PTFE
Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene, and has numerous applications because it is chemically inert. The commonly known brand name of PTFE-based composition is Teflon by Chemours, a spin-off fro ...
) containers.
Properties
In dilute aqueous solution hydrogen fluoride behaves as a weak acid,
Infrared spectroscopy
Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or functio ...
has been used to show that, in solution, dissociation is accompanied by formation of the ion pair ·F−.
: + HF ⋅F− p''K''a = 3.17
This ion pair has been characterized in the crystalline state at very low temperature.[
]
Further association has been characterized both in solution and in the solid state.
:HF + F− log ''K'' = 0.6
It is assumed that polymerization occurs as the concentration increases. This assumption is supported by the isolation of a salt of a tetrameric anion and by low-temperature X-ray crystallography.[ The species that are present in concentrated aqueous solutions of hydrogen fluoride have not all been characterized; in addition to which is known] the formation of other polymeric species, , is highly likely.
The Hammett acidity function, ''H''0, for 100% HF was first reported as −10.2, while later compilations show −11, comparable to values near −12 for pure sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...
.
Acidity
Unlike other hydrohalic acids, such as hydrochloric acid
Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...
, hydrogen fluoride is only a weak acid in dilute aqueous solution. This is in part a result of the strength of the hydrogen–fluorine bond, but also of other factors such as the tendency of HF, , and anions to form clusters. At high concentrations, HF molecules undergo homoassociation to form polyatomic ions (such as bifluoride, ) and proton
A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...
s, thus greatly increasing the acidity. This leads to protonation
In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), usually denoted by H+, to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brø ...
of very strong acids like hydrochloric, sulfuric, or nitric acids when using concentrated hydrofluoric acid solutions. Although hydrofluoric acid is regarded as a weak acid, it is very corrosive, even attacking glass when hydrated.
Dilute solutions are weakly acidic with an acid ionization constant (or ), in contrast to corresponding solutions of the other hydrogen halides, which are strong acids (). However concentrated solutions of hydrogen fluoride are much more strongly acidic than implied by this value, as shown by measurements of the Hammett acidity function ''H''0(or "effective pH"). During self ionization of 100% liquid HF the ''H''0 was first measured as −10.2 and later compiled as −11, comparable to values near −12 for sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...
.
In thermodynamic terms, HF solutions are highly non-ideal, with the activity of HF increasing much more rapidly than its concentration.
The weak acidity in dilute solution is sometimes attributed to the high H—F bond strength, which combines with the high dissolution enthalpy
Enthalpy () is the sum of a thermodynamic system's internal energy and the product of its pressure and volume. It is a state function in thermodynamics used in many measurements in chemical, biological, and physical systems at a constant extern ...
of HF to outweigh the more negative enthalpy
Enthalpy () is the sum of a thermodynamic system's internal energy and the product of its pressure and volume. It is a state function in thermodynamics used in many measurements in chemical, biological, and physical systems at a constant extern ...
of hydration of the fluoride ion. Paul Giguère and Sylvia Turrell have shown by infrared spectroscopy
Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or functio ...
that the predominant solute species in dilute solution is the hydrogen-bonded ion pair ·F−.
: + HF ⋅F−
With increasing concentration of HF the concentration of the hydrogen difluoride ion also increases. The reaction
:3 HF + H2F+
is an example of homoconjugation.
Health and safety
In addition to being a highly corrosive
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
liquid, hydrofluoric acid is also a powerful contact poison
A poison is any chemical substance that is harmful or lethal to living organisms. The term is used in a wide range of scientific fields and industries, where it is often specifically defined. It may also be applied colloquially or figurati ...
. Since it can penetrate tissue, poisoning can occur readily through exposure of skin or eyes, inhalation
Inhalation (or inspiration) happens when air or other gases enter the lungs.
Inhalation of air
Inhalation of air, as part of the cycle of breathing, is a vital process for all human life. The process is autonomic (though there are exceptions ...
, or ingestion. 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. Despite its irritating vapor, HF may reach dangerous levels without an obvious odor. It 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. Symptoms of HF exposure include irritation of the eyes, skin, nose, and throat, eye and skin burns, rhinitis
Rhinitis, also known as coryza, is irritation and inflammation of the mucous membrane inside the nose. Common symptoms are a stuffy nose, runny nose, sneezing, and post-nasal drip.
The inflammation is caused by viruses, bacteria, irritant ...
, bronchitis
Bronchitis is inflammation of the bronchi (large and medium-sized airways) in the lungs that causes coughing. Bronchitis usually begins as an infection in the nose, ears, throat, or sinuses. The infection then makes its way down to the bronchi. ...
, pulmonary edema
Pulmonary edema (British English: oedema), also known as pulmonary congestion, is excessive fluid accumulation in the tissue or air spaces (usually alveoli) of the lungs. This leads to impaired gas exchange, most often leading to shortness ...
(fluid buildup in the lungs), and bone damage due to HF strongly interacting with calcium in bones. In a concentrated form, HF can cause severe tissue destruction through lesions and mucous membrane damage, but dilute HF is still dangerous because of its high lipid affinity, leading to cellular death of nerves, blood vessels, tendons, bones, and other tissues.
Hydrofluoric acid burns should be rapidly treated with calcium gluconate gel.
In popular culture
*In the episodes " Cat's in the Bag..." and "Box Cutter
A utility knife is any type of knife used for general manual work purposes.Peterson, Harold L., ''Daggers and Fighting Knives of the Western World'', London: Herbert Jenkins Ltd., , p. 1 Such knives were originally fixed-blade knives with dura ...
" of the crime drama television series ''Breaking Bad
''Breaking Bad'' is an American crime drama television series created and produced by Vince Gilligan for AMC (TV channel), AMC. Set and filmed in Albuquerque, New Mexico, the series follows Walter White (Breaking Bad), Walter White (Bryan Cran ...
'', Walter White and Jesse Pinkman use hydrofluoric acid to chemically degrade the corpses of gangsters.
* In Season 4 episode 20 "Of Past Regret and Future Fear" of the television medical drama '' ER'', a patient is brought in with severe chemical burns from hydrofluoric acid that prove to be fatal.
* Hydrofluoric acid is mentioned in the novel Virtual Destruction by Kevin J. Anderson and Doug Beason. A character named Jose Aragon comes in contact with the acid on his hand, and he furiously seeks treatment at a medical center.
See also
* Vapour phase decomposition
Vapour phase decomposition (VPD) is a method used in the semiconductor industry to improve the sensitivity of total-reflection x-ray fluorescence spectroscopy by changing the contaminant from a thin layer (which has an angle-dependent fluorescenc ...
* 2019 Philadelphia Energy Solutions refinery explosion
References
External links
International Chemical Safety Card 0283
* (HF)
* (5HF)
* (6HF)
* (7HF)
"Hydrofluoric Acid Burn"
''The New England Journal of Medicine
''The New England Journal of Medicine'' (''NEJM'') is a weekly medical journal published by the Massachusetts Medical Society. Founded in 1812, the journal is among the most prestigious peer-reviewed medical journals. Its 2023 impact factor w ...
''—Acid burn case study
{{Authority control
Fluorides
Nonmetal halides
Mineral acids
Acid catalysts