Hydrofluoric acid

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Hydrofluoric acid

General
Systematic name	hydrogen fluoride
Other names	fluoric acid fluohydric acid hydrofluoride hydrofluoric acid fluorine monhydride AHF
Molecular formula	HF
Acidic Ion Concentration	(1 N) (pH = 1.57)
Molar mass	20.0063 g/mol
Appearance	Colorless gas.
CAS number	[7664-39-3]
Properties
Density and phase	0.818 g/L (gas), 1.015 g/mL (liquid O °C)
Solubility in water	Miscible.
Melting point	−83.38 C (189.77 K)
Boiling point	19.54°C (293.15 K)
Acidity (pK_a)	3.15 (in water) ~ -11 (neat)
Structure
Molecular shape	Linear.
Dipole moment	? D
Hazards
MSDS	External MSDS
Main hazards	Highly toxic, corrosive.
NFPA 704	0 4 1
Flash point	−37.8°C
R/S statement	R: R26/27/28, R35 S: S1/2, S7/9, S26, S36/37, 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 toxic and corrosive solution of hydrogen fluoride in water. Neat hydrogen fluoride is sometimes called anhydrous hydrofluoric acid. Hydrofluoric acid is notoriously known to dissolve glass by reacting with SiO₂, the major component of most glasses:

SiO_2(s) + 4HF_(aq) → SiF₄_(g) + 2H₂O_(l)

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

Consequently, it must be stored in polyethylene or Teflon containers. It is also unique in its ability to dissolve almost all inorganic metal and semimetal oxides. Because of its low tendency to dissociate into H⁺ and F^- ions in water, it is properly considered a weak acid, but it is nonetheless extremely corrosive and poisonous, as explained below.

1 Production
2 Uses
3 Safety
4 Acidity
5 References
6 External links

[edit] Production

Industrially, hydrofluoric acid is produced from the mineral fluorite, also known as calcium fluoride (CaF₂) and 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

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.

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.

[edit] 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. 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 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.

[edit] 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 cardiac arrest 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 that 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.

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 applying calcium gluconate gel to the exposed areas. 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 limb/s.

The highest concentration of HF in air that can be tolerated by a human for 1 minute is 100 mg/m³. This causes a definite sensation of pain on the skin, a definite sour taste, and some degree of eye and respiratory irritation. 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. 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. 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.

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 methamphetamines. The consequences of using HF instead of HCl in a process that involves baking under pressure are likely to be very hazardous to those involved, but records indicating the occurrence of such incidents are not well documented.

[edit] Acidity

While HF is a weak acid in water, as the mole percent concentration of HF is increased to near 100%, the acidity increases dramatically. This is because of the disproportionation reaction:

2HF → H⁺ + FHF⁻

The FHF⁻ anion is stabilized by the very strong hydrogen - fluorine hydrogen bond, but this is disrupted by water in less concentrated situations.

Although the pH of a solution of hydrofluoric acid does not suggest that hydrofluoric acid is a strong acid, this is because of the extensive hydrogen bonding that occurs. Hydrofluoric acid does get completely deprotonated in water, as do other strong acids, but there is very little free hydronium ion in solution because the fluoride ion hydrogen bonds with the hydronium ion.