|Jmol-3D images||Image 1|
|Molar mass||87.81 g mol−1|
-90 °C, 183 K, -130 °F
130 °C, 403 K, 266 °F
|S-phrases||(S1/2), S26, S27, S45|
|Related compounds||Hydrogen fluoride
| (what is: / ?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Fluoroboric acid or fluoboric acid (systematically named oxonium tetrafluoroboranuide or oxonium tetrafluoridoborate(1-)) is an inorganic compound with the chemical formula H
4. As fluoroboric acid has not been isolated, its descriptive properties remain largely unknown. It is mainly produced as a precursor to other fluoroborate salts. It is classified as 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. With a strength comparable to nitric acid, fluoroboric acid is a strong acid with a weakly coordinating, non-oxidizing conjugate base.
Pure fluoroboric acid has never been produced but aqueous solutions of HBF4 can be produced by dissolving boric acid in aqueous hydrofluoric acid solution at 20-25 °C. Three equivalents of HF react to give the intermediate boron trifluoride and the fourth gives fluoroboric acid.
- B(OH)3 + 4 HF → H3O+ + BF4− + 2 H2O
Aqueous solutions of fluoroboric acid can also be prepared by treating impure hexafluorosilicic acid with solid boric acid followed by removal of precipitated silicon dioxide. Anhydrous solutions can be prepared by treatment with acetic anhydride.
Fluoroboric acid is the principal precursor to fluoroborate salts, which are typically prepared by acid-base reactions. The inorganic salts are intermediates in the manufacture of flame-retardant materials and glazing frits, and in electrolytic generation of boron. HBF4 is also used in aluminum etching and acid pickling.
HBF4 is used as a catalyst for alkylations and polymerizations. In carbohydrate protection reactions, ethereal fluoroboric acid is an efficient and cost-effective catalyst for transacetalation and isopropylidenation reactions. Acetonitrile solutions cleave acetals and some ethers, while neat fluoroboric acid removes tert-butoxycarbonyl groups.
Aqueous HBF4 is used as an electrolyte in galvanic cell oxygen sensor systems, which consist of an anode, cathode, and oxygen-permeable membrane. The solution of HBF4 is able to dissolve lead(II) oxide from the anode in the form of lead tetrafluoroborate while leaving the rest of the system unchanged.
A mixture of CrO3, HBF4, and sulfonic acids in conjunction with a cathode treatment give tin-plated steel. Tin(I) fluoroborate/fluoroboric acid mixtures and organic reagents are used as the electrolyte in the cathode treatment of the tin plating process. Similar processes of electrodeposition and electrolytic stripping are used to obtain specific metal alloys.
Other fluoroboric acids
A series of fluoroboric acids is known in aqueous solutions. The series can be presented as follows:
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