·
Empirical Formula (Hill Notation) HF
·
Molecular Weight 20.01
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.
Detailed description
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. Aqueoushydrofluoric 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.
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
In metalworking, hydrofluoric acid is used as
a pickling agent to
remove oxides and other impurities from stainless and carbon steelsbecause of its
limited ability to dissolve steel.[citation needed] 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]
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.
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