Hydrofluoboric acid, HBF₄

Hydrofluoboric acid, HBF₄, was discovered by Berzelius. It is obtained in aqueous solution when boron trifluoride is passed into water until the liquid reacts strongly acid, and the solution is then cooled. The boric acid simultaneously formed is said to separate out as metaboric acid. An aqueous solution may also be prepared by dissolving boric acid in a cold, dilute, aqueous solution of hydrogen fluoride.

In aqueous solution, hydrofluoboric acid reacts as a monobasic complex acid, the ions of which are H^• and BF₄'. Apparently there is also a slight secondary dissociation of the ion BF₄' into BF₃ and F', the BF₃ partly hydrolysing to hydrofluoric and boric acids. The diluted acid does not attack glass; when the solution is concentrated, however, it does so, since hydrogen fluoride is produced. The acid is poisonous.

The salts of hydrofluoboric acid are called borofluorides. They may be prepared (i.) by neutralising the acid with metallic hydroxides, oxides, or carbonates, (ii.) by the action of the so-called fluoboric acid on metallic fluorides, and (iii.) by the action of a metallic fluoride and hydrofluoric acid, or an acid fluoride, on boric acid. In the last case the curious phenomenon may be observed of a mixture of two acid liquids producing an alkaline solution, e.g.: -

H₃BO₃ + 2NaHF₂ = NaBF₄ + NaOH + 2H₂O.

Most borofluorides are soluble in water. The insolubility of the potassium salt in 50 per cent, alcohol may be utilised for separating potassium from sodium and magnesium. When heated to redness, borofluorides yield boron trifluoride and a metallic fluoride; with concentrated sulphuric acid, boron trifluoride and hydrogen fluoride are evolved. A mixture of fluoride and borate is produced by fusing a borofluoride with an alkali carbonate.