hydrofluoric acid

Hydrofluoric acid
Other names	fluoric acid; fluohydric acid
Identifiers
CAS number	7664-39-3
RTECS number	MW7875000
Properties
Molecular formula	HF(H2O)x
Molar mass	not applicable (see hydrogen fluoride)
Appearance	Colorless solution
Density	1.15 g/mL (for 48% soln.)
Melting point	not applicable (see hydrogen fluoride)
Boiling point	not applicable (see hydrogen fluoride)
Solubility in water	Miscible.
Acidity (pKa)	3.15 (in water)
Hazards
MSDS	External MSDS
Main hazards	Toxic, corrosive.
NFPA 704	0 4 1
R-phrases	R26/27/28, R35
S-phrases	(S1/2), S7/9, S26, S36/37, S45
Flash point	nonflammable
Related Compounds
Other anions	Hydrochloric acid Hydrobromic acid Hydroiodic acid
Related compounds	Hydrogen fluoride fluorosilicic acid
Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
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 solution of hydrogen fluoride in water. Together with hydrogen fluoride, hydrofluoric acid is a valued source of fluorine, being the precursor to numerous pharmaceuticals, diverse polymers (e.g. Teflon), and most other synthetic materials that contain fluorine. Hydrofluoric acid is best known to the public for its ability to dissolve glass by reacting with SiO₂, the major component of most glasses. This dissolution process can be described as follows:

SiO_2(s) + 4HF_(aq) → SiF_4(g) + 2H₂O_(l)

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

Because of its high reactivity toward glass, hydrofluoric acid is typically stored in polyethylene or Teflon containers. It is also unique in its ability to dissolve many metal and semimetal oxides. It is extremely corrosive, as explained below:-

Acidity

Hydrogen fluoride dissociates in aqueous solution in a similar fashion to other common acids:

HF + H₂O → H₃O⁺ + F^-

When the concentration of HF approaches 100%, the acidity increases dramatically due to the following equilibrium:

2HF → H⁺ + FHF⁻

The FHF⁻ anion is stabilized by the very strong hydrogen - fluorine hydrogen bond. In acetic acid and similar solvents, hydrofluoric acid is the strongest of the hydrohalic acids.

Production

Main article: hydrogen fluoride

Industrially, hydrofluoric acid is produced by treatment of the mineral fluorite (CaF₂) with concentrated sulfuric acid. When combined at 250 °C, these two substances react to produce hydrogen fluoride according to the following chemical equation:

CaF₂ + H₂SO₄ → 2HF + CaSO₄

Uses

Because of its ability to dissolve metal oxides, hydrofluoric acid is used 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. Dilute hydrofluoric acid is sold as a household rust stain remover. Recently it has even been used in car washes in "wheel cleaner" compounds.^[1] Due to its ability to dissolve silicate compounds, hydrofluoric acid is often used 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,fluoropolymers, perfluorocarbons and refrigerants such as freon.

Safety

Symptoms of skin exposure to dilute HF are not felt immediately, but can be fatal. Highly concentrated solutions may lead to acute hypocalcemia, followed by cardiac arrest and death. This will usually be fatal in as little as 2% body exposure (about the size of the sole of the foot). This substance should be handled with extreme care, beyond that accorded to hydrochloric, sulfuric, or other mineral acids.

Due to low dissociation constant, HF can penetrate tissues quickly. 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.^{[citation needed]}

Calcium gluconate is used to treat hydrofluoric acid exposure

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 thorough rinsing (for up to 15 minutes) of to the exposed areas followed by the application of calcium gluconate gel. If exposure is high, or too much time has passed, a calcium gluconate 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, potentially necessitating amputation of the affected limbs.

The highest concentration of HF in air that can be tolerated by a human for 1 minute is 100 mg/m³.^{[citation needed]} This causes a definite sensation of pain on the skin, a definite sour taste, and some degree of eye and respiratory irritation^{[citation needed]}. If the air contains 50 mg/m³, the sour taste is apparent and there is irritation of the eyes and nose, but no pain is sensed on the skin^{[citation needed]}. The concentration of 26 mg/m³ can be tolerated for several minutes, but the sour taste becomes evident after a short time, and there is mild pain in the nose and eyes^{[citation needed]}. The American Conference of Governmental Industrial Hygienists has adopted 2 mg/m³ 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.^{[citation needed]}

A concern for emergency services is the theft of drums of hydrofluoric acid, possibly after being mistaken for hydrochloric acid which has uses in the preparation of all kinds of substances, including in the clandestine production of drugs such as methamphetamines.

Hydrofluoric acid is a known hazard in car engine fires, forming when Viton o-rings and hoses are exposed to temperatures in excess of 400 °C.

References

1. ^ Strachan, John (January, 1999). "A deadly rinse: The dangers of hydrofluoric acid". Professional Carwashing & Detailing. Retrieved on 2006-08-30. 

External links

• International Chemical Safety Card 0283
• National Pollutant Inventory - Fluoride and compounds fact sheet
• NIOSH Pocket Guide to Chemical Hazards
• CID 14917 from PubChem (HF)
• CID 144681 from PubChem (5HF)
• CID 141165 from PubChem (6HF)
• CID 144682 from PubChem (7HF)
• Computational Chemistry Wiki
• Hydrofluoric Acid Burn, The New England Journal of Medicine Acid burn case study

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