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Hydrofluoric acid
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Hydrofluoric acid is a solution of hydrogen fluoride in water. While it is extremely corrosive and dangerous to handle, it is technically a weak acid. Hydrogen fluoride, often in the aqueous form as hydrofluoric acid, is a valued source of fluorine, being the precursor to numerous pharmaceuticals (e.g., Prozac), 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 (silicon dioxide), the major component of most glass.
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Encyclopedia
Hydrofluoric acid is a solution of hydrogen fluoride in water. While it is extremely corrosive and dangerous to handle, it is technically a weak acid. Hydrogen fluoride, often in the aqueous form as hydrofluoric acid, is a valued source of fluorine, being the precursor to numerous pharmaceuticals (e.g., Prozac), 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 (silicon dioxide), the major component of most glass. This property has been known since the 17th century, even before hydrofluoric acid had been prepared in large quantities by Scheele in 1771. This dissolution process can be described as follows:
- SiO2(s) + 4 HF(aq) ? SiF4(g) + 2 H2O(l)
- SiO2(s) + 6HF(aq) ? H2[SiF6](aq) + 2H2O(l)
Because of its high reactivity toward glass, hydrofluoric acid must be stored (for small quantities) in polyethylene or Teflon containers. It is also unique in its ability to dissolve many metal and semimetal oxides. It is corrosive, as explained below.
Acidity
Hydrogen fluoride ionizes 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. Hydrofluoric acid is the only hydrohalic acid that is not considered a strong acid due to its low degree of ionization in aqueous solution.
Production
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
Although bulk fluorite is a suitable precursor, most HF is 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 fluorite. The acid hydrolysis of fluorite-containing minerals generates an impure gas stream consisting of sulfur dioxide (from the H2SO4), water, and HF, as well as particulates. After separation from the solids, the gases are treated with sulfuric 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 fluorosilic acid is generated.
Uses
Oil refining
In a standard oil refinery process, isobutane is alkylated with low-molecular-weight alkenes (primarily a mixture of propylene and butylene) in the presence of a strong acid catalyst, hydrofluoric acid. The catalyst is able to protonate 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. It is important to keep a high ratio of isobutane to alkene at the point of reaction to prevent side reactions that lead to a lower octane product, so the plants have a high recycle of isobutane back to feed. The phases separate spontaneously, so the acid phase is vigoriously mixed with the hydrocarbon phase to create sufficient contact surface.
Production of organofluorine compounds
The principal uses of hydrofluoric acid and HF 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 serve as high temperature solvent for the production of metallic aluminium. Given concerns about flourides 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
The ability of hydrofluoric acid to dissolve metal oxides is the basis of several applications. It removes oxide impurities from stainless steel in a process called pickling. Surface oxides are removed from silicon with hydrofluoric acid in the semiconductor industry. Similarly, it is also used to etch glass. For these reasons, dilute hydrofluoric acid is sold as a household rust stain remover. Recently it has even been used in car washes in "wheel cleaner" compounds.
Niche applications
Due to its ability to dissolve oxides, hydrofluoric acid is useful for dissolving rock samples (usually powdered) prior to analysis. Similarly, 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.
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.
Safety
Hydrofluoric acid is extremely corrosive and a contact poison. It should be handled with extreme care, beyond that accorded to other mineral acids, in part because of its low dissociation constant, which allows HF to penetrate tissue more quickly. Symptoms of exposure to hydrofluoric acid may not be immediately evident. HF interferes with nerve function and burns may not initially be painful. Accidental exposures can go unnoticed, delaying treatment and increasing the extent and seriousness of the injury. HF is known to etch bone, and since it penetrates the skin it can weaken bones without destroying the skin. More seriously, it can be absorbed into blood through skin and react with blood calcium, causing cardiac arrest.
In the body, hydrofluoric acid reacts with the ubiquitous biologically important ions Ca2+ and Mg2+. 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. In some cases, amputation may be required.
Hydrogen fluoride is generated upon combustion of many fluorine-containing compounds such as products containing Viton and Teflon parts. Hydrogen fluoride converts immediately to hydrofluoric acid upon contact with liquid water.
External links
(HF)
(5HF)
(6HF)
(7HF)
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