| IUPAC name
| Other names
Tetrafluoroboric acid, oxonium
|Molar mass||87.81 g/mol|
|Melting point||−90 °C (−130 °F; 183 K)|
|Boiling point||130 °C (266 °F; 403 K)|
|Solubility||Soluble in glacial acetic acid|
|Acidity (pKa)|| −0.44 (water)|
|Safety data sheet||Sigma-Aldrich (48%)|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Fluoroboric acid or tetrafluoroboric acid is an inorganic compound with the chemical formula HBF4.
Pure HBF4 has been described as a "nonexistent compound", as a sufficiently 'naked' proton is expected to abstract a fluoride from the tetrafluoroborate ion to give hydrogen fluoride and boron trifluoride:
- [H+][BF4–] → HF + BF3
However, a solution of BF3 in HF is highly acidic, having an approximate speciation of [H2F+][BF4–] and a Hammett acidity function of –16.6 at 7 mol % BF3, easily qualifying as a superacid. Although the solvent-free HBF4 has not been isolated, its solvates are well characterized. These salts consist of protonated solvent as a cation, e.g., H3O+ and H
2, and the tetrahedral BF−
4 anion. The anion and cations are strongly hydrogen-bonded.
Its acidity is thus comparable to that of fluorosulfuric acid.
Like hydrofluoric acid, fluoroboric acid attacks glass and metals.
Since HBF4 has not been isolated as pure compound, its properties largely derive from its solution.
Fluoroboric acid is sold as solution by chemical suppliers.
Aqueous solutions of HBF4 are produced by dissolving boric acid in aqueous hydrofluoric acid. Three equivalents of HF react to give the intermediate boron trifluoride and the fourth gives fluoroboric acid:
- B(OH)3 + 4 HF → H3O+ + BF−
4 + 2 H2O
Anhydrous solutions can be prepared by treatment of aqueous fluoroboric acid with acetic anhydride.
- Aluminum etching and acid pickling
- Catalyst for alkylations and polymerizations
- Glass etching
Fluoroboric is corrosive and toxic.
Best to prepare it in situ. Can be kept in plastic bottles.
Can be neutralized with a solution of Ca(OH)2.