Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a precursor to almost
all fluorine compounds, including pharmaceuticals such as fluoxetine, diverse materials such
as PTFE, 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. Because of its high reactivity toward glass and moderate reactivity toward
many metals, hydrofluoric acid is usually stored in plastic containers.
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 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:
HF + H2O H3O+ + F− It 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 anion is stabilized by the very strong hydrogen–fluorine hydrogen bond.
Production Hydrofluoric acid is produced by treatment
of the mineral fluorite 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, 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 is generated. Uses
Oil refining In a standard oil refinery process known as
alkylation, isobutane is alkylated with low-molecular-weight alkenes in the presence of the strong acid
catalyst derived from hydrofluoric acid. The catalyst protonates the alkenes to produce
reactive carbocations, which alkylate isobutane. The reaction is carried out at mild temperatures
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. 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.
Etchant and cleaning agent 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. For similar reasons, dilute
hydrofluoric acid is a component of household rust stain remover and in car washes in "wheel
cleaner" compounds. 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 oxides and silicates, hydrofluoric acid is useful
for dissolving rock samples 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, which adheres
to the organic component and allows the rock to be dissolved around it.
Diluted hydrofluoric acid is used in the petroleum industry in a mixture with other 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
attacking the soda content of the glass. Health and safety
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.
Once absorbed into blood through the skin, it reacts with blood calcium and may cause
cardiac arrest. Burns with areas larger than 160 cm2 have the potential to cause serious
systemic toxicity from interference with blood and tissue calcium levels. 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. 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. or special rinsing solutions.
However, because it is absorbed, medical treatment is necessary; rinsing off is not enough. Intra-arterial
infusions of calcium chloride have also shown great effectiveness in treating burns.
Hydrogen fluoride is generated upon combustion of many fluorine-containing compounds such
as products containing Viton and polytetrafluoroethylene parts. 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.
See also Vapour phase decomposition
References
External links International Chemical Safety Card 0283
NIOSH Pocket Guide to Chemical Hazards CID 14917 from PubChem
CID 144681 from PubChem CID 141165 from PubChem
CID 144682 from PubChem Hydrofluoric Acid Burn, The New England Journal
of Medicine Acid burn case study