Pure has not been described. The same holds true for the superacids that are known by the simplified formulas Hexafluorophosphoric acid| and Hexafluoroantimonic acid|. However, a solution of in HF is highly acidic, having an approximate speciation of (fluoronium tetrafluoroborate) and a
Hammett acidity function of −16.6 at , easily qualifying as a superacid. Although the solvent-free has not been isolated, its solvates are well characterized. These salts consist of protonated solvent as a cation, e.g., and , and the tetrahedral anion. The anion and cations are strongly hydrogen-bonded. Aqueous solutions of 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: : An anhydrous fluoroboric acid solution can be prepared by adding aqueous fluoroboric acid to an excess of
acetic anhydride at , which produces a solution of fluoroboric acid,
acetic acid, and residual acetic anhydride.
Acidity Determination of the acidity of fluoroboric acid is complicated by the fact that its name refers to a range of different compounds, e.g. (dimethyloxonium tetrafluoroborate), (oxonium tetrafluoroborate), and (
hydrogen fluoride-
boron trifluoride 1:1 adduct) – each with a different acidity. The aqueous pKa| is quoted as −0.44.
Titration of (tetrabutylammonium tetrafluoroborate) in
acetonitrile solution indicates that , i.e., , has a of 1.6 in that solvent. Its acidity is thus comparable to that of
fluorosulfonic acid. == Applications ==