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
