Fluorosulfuric acid

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Fluorosulfuric acid
Skeletal formula of fluorosulfuric acid
Spacefill model of fluorosulfuric acid
IUPAC name
Sulfurofluoridic acid
Systematic IUPAC name
Fluorosulfuric acid[citation needed]
Other names
Fluorosulfonic acid,
Fluorosulphonic acid,
Fluorinesulfonic acid,
Fluorinesulphonic acid,
Fluoridosulfonic acid,
Fluoridosulphonic acid,
Sulfuric fluorohydrin,
Epoxysulfonyl fluoride
3D model (JSmol)
ECHA InfoCard 100.029.227 Edit this at Wikidata
EC Number
  • 232-149-4
MeSH Fluorosulfonic+acid
RTECS number
  • LP0715000
UN number 1777
  • InChI=1S/FHO3S/c1-5(2,3)4/h(H,2,3,4) checkY
  • InChI=1/FHO3S/c1-5(2,3)4/h(H,2,3,4)
  • OS(F)(=O)=O
  • FS(=O)(=O)O
Molar mass 100.06 g·mol−1
Appearance Colorless liquid
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
Acidity (pKa) -10
Conjugate base Fluorosulfate
Tetragonal at S
Tetrahedral at S
GHS labelling:
GHS07: Exclamation markGHS05: Corrosive
H314, H332[1]
P280, P305+P351+P338, P310[1]
Safety data sheet (SDS) ICSC 0996
Related compounds
Related compounds
Antimony pentafluoride
Trifluoromethanesulfonic acid
Hydrofluoric acid
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 (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.[2]

Chemical properties[edit]

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.[3] 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)


Fluorosulfuric acid is prepared by the reaction of HF and sulfur trioxide:[2]

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.[4]


HSO3F is one of the strongest known simple Brønsted acids, although carborane-based acids are still stronger.[5] 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,[6] although it is unclear if such applications are of commercial importance. It can also be used as a laboratory fluorinating agent.[4]


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.

See also[edit]


  1. ^ a b Sigma-Aldrich Co., Phosphoric acid. Retrieved on 2014-05-09.
  2. ^ a b Erhardt Tabel, Eberhard Zirngiebl, Joachim Maas "Fluorosulfuric Acid" in "Ullmann's Encyclopedia of Industrial Chemistry" 2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a11_431
  3. ^ Olah, G. A.; Prakash, G. K.; Wang, Q.; Li, X.-Y. (2001). "Fluorosulfuric Acid". Encyclopedia of Reagents for Organic Synthesis. Encyclopedia of Reagents for Synthesis. John Wiley & Sons. doi:10.1002/047084289X.rf014. ISBN 0471936235.
  4. ^ a b Cotton, F. A.; Wilkinson, G. (1980). Advanced Inorganic Chemistry (4th ed.). New York: Wiley. p. 246. ISBN 0-471-02775-8.
  5. ^ Christopher A. Reed "Myths about the Proton. The Nature of H+ in Condensed Media" Acc. Chem. Res., 2013, 46 (11), pp 2567–2575. doi:10.1021/ar400064q
  6. ^ Olah, G.; Farooq, O.; Husain, A.; Ding, N.; Trivedi, N.; Olah, J. (1991). "Superacid HSO3F/HF-Catalyzed Butane Isomerisation". Catalysis Letters. 10 (3–4): 239–247. doi:10.1007/BF00772077. S2CID 94408218.