Chemical elements
    Physical properties
    Chemical properties
      Boron Hydrides
      Boron trihydride
      Boron halogen
      Boron trifluoride
      Hydrofluoboric acid
      Potassium borofluoride
      Fluoboric acid
      Perfluoboric acid
      Boron subchloride
      Boron trichloride
      Boron tribromide
      Boron tri-iodide
      Oxides of Boron
      Tetraboron trioxide
      Boron dioxide
      Tetraboron pentoxide
      Boron sesqui-oxide
      Boron trioxide
      Boric anhydride
      Boric Acids
      Orthoboric acid
      Boric acid
      Boracic acid
      Complex Boric Acids
      Perboric Acid and Perborates
      Sodium perborate
      Sodium hyperborate
      Potassium perborate
      Rubidium perborate
      Ammonium perborate
      Barium perborate
      Boron sesquisulphide
      Boron trisulphide
      Boron pentasulphide
      Boron selenide
      Boron nitride
      Boron amide
      Boron imide
      Boron phosphide
      Boron phospho-iodides
      Boron carbide
      Boron thiocyanate
      Boron Alkyls
      Boron trimethyl
      Boron Silicides and

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.

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