Fluoroboric acid

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Fluoroboric acid
Names
IUPAC name
Tetrafluoroboric acid
Other names
Borofluoric acid
Hydrogen tetrafluoroborate
Tetrafluoroboric acid, oxonium
Properties
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)
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
5
O+
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