Fluoroboric acid

Fluoroboric acid is the chemical compound with the formula HBF4. It is the conjugate acid of tetrafluoroborate. 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.

Production
Pure fluoroboric acid is unknown. Aqueous solutions of 45-50% HBF4 are produced by combining technical grade boric acid and hydrofluoric acid in aqueous solution at room temperature. 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.

Salts
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, 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 in 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.

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 sys

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.