Fluoroboric acid

Fluoroboric acid

Names
IUPAC name Tetrafluoroboric acid
Other names Borofluoric acid Hydrogen tetrafluoroborate Tetrafluoroboric acid, oxonium
Properties
Chemical formula	HBF4
Molar mass	87.81 g/mol
Appearance	Colorless liquid
Odor	Odorless
Density	1.84 g/cm3
Melting point	−90 °C (−130 °F; 183 K)
Boiling point	130 °C (266 °F; 403 K)
Solubility in water	Soluble
Solubility	Soluble in glacial acetic acid
Acidity (pKa)	−0.44 (water) 1.6 (acetonitrile)
Hazards
Safety data sheet	Sigma-Aldrich (48%)
Flash point	Non-flammable
Related compounds
Related compounds	Hydrogen fluoride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Fluoroboric acid or tetrafluoroboric acid is an inorganic compound with the chemical formula HBF₄.

It is classified as strong acid, but unlike sulfuric acid or fluorosulfuric acid it cannot be isolated as pure compound.

Properties

Chemical

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
5O+
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.

Physical

Since HBF₄ has not been isolated as pure compound, its properties largely derive from its solution.

Availability

Fluoroboric acid is sold as solution by chemical suppliers.

Preparation

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.

Projects

• Aluminum etching and acid pickling
• Catalyst for alkylations and polymerizations
• Glass etching

Handling

Safety

Fluoroboric is corrosive and toxic.

Storage

Best to prepare it in situ. Can be kept in plastic bottles.

Disposal

Can be neutralized with a solution of Ca(OH)₂.

References

Relevant Sciencemadness threads

• Preparation of HBF4...