Supriya Ghosh (Editor)

Fluoroboric acid

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Formula
  
HBF4

Appearance
  
Colourless liquid

Fluoroboric acid httpsuploadwikimediaorgwikipediacommonsthu

Related compounds
  
Hydrogen fluorideTriflic acid

Fluoroboric acid or tetrafluoroboric acid is an inorganic compound with the chemical formula H
3
OBF
4
.

Contents

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.

Tetrafluoroboric acid meaning


Structure and production

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.

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(OEt2)+BF
4
, H3O+BF
4
, and HF.BF3 - each with a different acidity. The aqueous pKa is quoted as -0.44. Titration of NBu+
4
BF
4
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 glazing frits, and in electrolytic generation of boron. HBF4 is also used in aluminum etching and acid pickling.

Organic chemistry

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. Many reactive cations have been obtained using fluoroboric acid, e.g. tropylium tetrafluoroborate (C
7
H+
7
BF
4
), triphenylmethyl tetrafluoroborate (Ph
3
C+
BF
4
), triethyloxonium tetrafluoroborate (Et
3
O+
BF
4
), and benzenediazonium tetrafluoroborate (PHN+
2
BF
4
).

Galvanic cells

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.

Metal plating

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.

A 10% solution of fluoroboric acid, tin fluoroborate and thiourea is used to electroless tin plate the copper traces of printed circuit boards for protection and easier soldering.

Other fluoroboric acids

A series of fluoroboric acids is known in aqueous solutions. The series can be presented as follows:

  • H[B(OH)4]
  • H[BF(OH)3]
  • H[BF2(OH)2]
  • H[BF3(OH)]
  • H[BF4]
  • References

    Fluoroboric acid Wikipedia