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.
|Systematic IUPAC name
Fluorosulfuric acid
3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||100.06 g·mol−1|
|Density||1.726 g cm−3|
|Melting point||−87.5 °C; −125.4 °F; 185.7 K|
|Boiling point||165.4 °C; 329.6 °F; 438.5 K|
|Tetragonal at S|
|Tetrahedral at S|
|P280, P305+P351+P338, P310|
|Safety data sheet (SDS)||ICSC 0996|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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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. Reflecting its strong acidity, it 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:
HSO3F is one of the strongest known simple Brønsted acids, although carborane-based acids are still stronger. It has an H0 value of −15.1 compared to −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 all fall into the category of "superacids", acids stronger than 100% sulfuric acid.
HSO3F is useful for regenerating mixtures of HF and H2SO4 for etching lead glass.
HSO3F isomerizes alkanes and 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.
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.