Hydrofluoric acid

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Hydrofluoric acid
Image:Hydrofluoric acid.png
General
Systematic name	Hydrogen fluoride (aqueous solution)
Other names	Hydrofluoric acid fluohydric acid
Molecular formula	HF
Molar mass	20.0063 g/mol
Appearance	Colorless gas.
CAS number	Template:CASREF
Properties
Density and phase	0.818 g/L, gas.
Solubility in water	Miscible.
Melting point	-83.38°C (189.77 K)
Boiling point	20°C (293.15 K)
Acidity (pK_a)	3.15
Structure
Molecular shape	Linear.
Dipole moment	? D
Hazards
MSDS	External MSDS
Main hazards	Highly toxic, corrosive.
NFPA 704	Template:Nfpa
Flash point	-37.8°C
R/S statement	R: Template:R26/27/28, Template:R35 S: Template:S1/2, Template:S7/9, Template:S26, Template:S36/37, Template:S45
RTECS number	MW7875000
Supplementary data page
Structure and properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Other anions	Hydrogen chloride hydrogen bromide hydrogen iodide
Other cations	?
Related compounds	Hydrogen fluoride fluorosilicic acid
Except where noted otherwise, data are given for materials in their standard state (at 25°C, 100 kPa) Infobox disclaimer and references

Hydrofluoric acid is a highly corrosive solution of hydrogen fluoride in water. Gaseous hydrogen fluoride is sometimes called anhydrous hydrofluoric acid. Hydrofluoric acid is a weak acid. This is because it does not dissociate completely in water because of the relatively high strength of hydrogen-fluorine bonds. Hydrofluoric acid is notoriously known to dissolve glass (SiO₂):

SiO_2(s) + 6 HF_(aq) → H₂[SiF₆]_(aq) + 2 H₂O_(l)

Consequently, it must be stored in plastic containers, though ideally it should be stored in Teflon bottles. It is also unique in its ability to dissolve almost all inorganic oxides.

1 Production
2 Uses
3 Safety
4 External links

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Production

Industrially, hydrofluoric acid is produced from the mineral fluorspar, also known as calcium fluoride (chemical formula CaF₂) and concentrated sulfuric acid. When combined at 250°C, these two substances react to produce hydrogen fluoride according to the chemical equation

CaF₂ + H₂SO₄ → 2 HF + CaSO₄

The vapors from this reaction are a mixture of hydrogen fluoride, sulfuric acid, and a few minor byproducts, from which hydrogen fluoride can be isolated by distillation.

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Uses

Hydrofluoric acid's ability to dissolve oxides makes it important in the purification of both aluminium and uranium. It is also used to etch glass, to remove surface oxides from silicon in the semiconductor industry, as a catalyst for the alkylation of iso-butane and butene in oil refineries and to remove oxide impurities from stainless steel in a process called pickling. Hydrofluoric acid is also used to clean brickwork. Due to its ability to dissolve silicate compounds, hydrofluoric acid is often used during the rock and mineral analysis process to dissolve rock samples (usually powdered) prior to analysis.

Hydrofluoric acid is also used in the synthesis of many fluorine-containing organic compounds, including teflon and refrigerants such as freon.

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Safety

Symptoms of skin exposure to dilute HF are not felt immediately, but exposure of less than 10% of the body to it can be fatal, even with immediate medical treatment. Highly concentrated solutions may lead to acute hypocalcemia, followed by heart attack and death, and will usually be fatal in as little as 2% body exposure (about the size of the sole of the foot). This substance is extremely toxic and has the capacity to kill upon exposure rather than simply damage skin and eyes. It should be handled with extreme care, beyond what is given to hydrochloric, sulfuric, or other mineral acids.

Due to low dissociation constant, HF can penetrate tissues quickly like a small non-polar particle. Hydrofluoric acid which comes into direct contact with the fingers can severely damage or destroy the tissue underneath the nail without causing any damage to the nail itself. It is this ability to cause little harm to outer tissues but considerable harm to inner tissues which can produce dangerous delays in treatment of hydrofluoric acid exposure. Once the pain starts, it is out of proportion to the burns produced. Patients often describe the feeling as if they have struck their fingers with a hammer. HF that penetrates under the skin causes later development of painful ulcers, which heal slowly.

Solutions of less than 20% HF can produce pain and redness with delay up to 24 hours after skin exposure. 20 to 50% HF produces pain and redness within 8 hours, and solutions of more than 50% produce immediate burning, redness and blister formation. Contact of the skin with the anhydrous liquid produces severe burns.

In the body, hydrofluoric acid reacts with the ubiquitous ions of calcium and magnesium and so can disable tissues and organs whose proper function depends on these metal ions. Exposure to hydrofluoric acid may not be initially painful, and symptoms may not occur until several hours later, when the acid begins to react with calcium in the bones. Under most circumstances, hydrofluoric acid exposure results in severe or even lethal damage to the heart, liver, kidneys, and nerves. Initial treatment to hydrofluoric acid exposure usually involves applying calcium gluconate gel to the exposed areas. If exposure is high, or too much time has passed, a calcium solution may be injected directly into a local artery or surrounding tissues. In all cases, hydrofluoric acid exposure requires immediate professional medical attention. If coming in contact with human skin or bone, the acid can severely burn and then decompose the bone.

The highest concentration of HF in air that can be tolerated by man for 1 minute is 100 mg/cu. meter. This causes a definite smarting of skin, a definite sour taste, and some degree of eye and respiratory irritation. If the air contains 50 mg/cu. meter, the sour taste is apparent and there is irritation of the eyes and nose, but no smarting of the skin. The concentration of 26 mg/cu. meter can be tolerated for several minutes, but the sour taste becomes evident after a short time, and there is mild smarting of the nose and eyes. The American Conference of Governmental Industrial Hygienists has adopted 2 mg/cu. meter as the threshold limit for hydrogen fluoride. This comes to about 3 ppm (parts per million). Inhalational exposure to concentrated HF for as little as 5 minutes is usually fatal, producing death within 2-10 hours.

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