Fluoroboric acid or tetrafluoroboric acid is an inorganic compound with the chemical formula , where H+ represents the solvated proton. The solvent can be any suitably Lewis basic entity. For instance, in water, it can be represented by , although more realistically, several water molecules solvate the proton: . The ethyl ether solvate is also commercially available: , where n is most likely 2. Unlike strong acids like H2SO4 or HClO4, the pure unsolvated substance does not exist. It is mainly produced as a precursor to other fluoroborate salts. It is a strong acid. Fluoroboric acid is corrosive and attacks the skin. It is available commercially as a solution in water and other solvents such as diethyl ether. It is a strong acid with a weakly coordinating, non-oxidizing conjugate base. It is structurally similar to perchloric acid, but lacks the hazards associated with oxidants.
Structure and production
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: → HF + BF3. However, a solution of BF3 in HF is highly acidic, having an approximate speciation of 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, and the tetrahedral anion. The anion and cations are strongly hydrogen-bonded. 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: Anhydrous solutions can be prepared by treatment of aqueous fluoroboric acid with acetic anhydride.
Acidity
The acidity of fluoroboric acid is complicated by the fact that the name refers to several different species H+BF, H3O+BF, and HF.BF3 – each with a different acidity. The aqueous pKa is quoted as −0.44. Titration of NBuBF in acetonitrile solution indicates that HBF4, i.e., HF.BF3, has a pKa of 1.6 in that solvent. Its acidity is thus comparable to that of fluorosulfonic acid.
Applications
Fluoroboric acid is the principal precursor to fluoroborate salts, which are typically prepared by treating the metal oxides with fluoroboric acid. The inorganic salts are intermediates in the manufacture of flame-retardant materials and glazingfrits, and in electrolytic generation of boron. HBF4 is also used in aluminum etching and acid pickling.
