Fluorosulfuric acid

Fluorosulfuric acid (IUPAC name: sulfurofluoridic acid) is the inorganic compound with the chemical formula HSO3F. It is one of the strongest acids commercially available. It is a tetrahedral molecule and is closely related to sulfuric acid, H2SO4, substituting a fluorine atom for one of the hydroxyl groups. It is a colourless liquid, although commercial samples are often yellow.

Properties
Fluorosulfuric acid is a free-flowing colorless liquid. It is soluble in polar organic solvents (e.g. nitrobenzene, acetic acid, and ethyl acetate), but poorly soluble in nonpolar solvents such as alkanes.

HSO3F is one of the strongest known simple Brønsted acids. It has an H0 value of −15.1 compared to &minus;12 for sulfuric acid. The combination of HSO3F and the Lewis acid antimony pentafluoride produces "Magic acid", which is a far stronger protonating agent. These acids are categorized as "superacids", acids stronger than 100% sulfuric acid.

Reflecting its strong acidity, HSO3F dissolves almost all organic compounds that are even weak proton acceptors. HSO3F hydrolyzes slowly to hydrogen fluoride (HF) and sulfuric acid. The related triflic acid (CF3SO3H) retains the high acidity of HSO3F but is more hydrolytically stable. The self-ionization of fluorosulfonic acid also occurs:
 * 2 HSO3F <-> [H2SO3F]+ + [SO3F]-     K = 4.0 × 10−8 (at 298 K)

HSO3F isomerizes alkanes and catalyzes the alkylation of hydrocarbons with alkenes, although it is unclear if such applications are of commercial importance. It can also be used as a laboratory fluorinating agent.

Production
Fluorosulfuric acid is prepared by the reaction of HF and sulfur trioxide:
 * SO3 + HF → HSO3F

Alternatively, KHF2 or CaF2 can be treated with oleum at 250 °C. Once freed from HF by sweeping with an inert gas, HSO3F can be distilled in a glass apparatus.

Safety
Fluorosulfuric acid is considered to be highly toxic and extremely corrosive. It hydrolyzes to release HF. Addition of water to HSO3F can be violent, similar to the addition of water to sulfuric acid but much more violent.