Potassium borofluoride, KBF₄

Potassium borofluoride, KBF₄, may be prepared as follows: - Boric acid (1 part) and calcium fluoride (2.5 parts) are heated with boiling sulphuric acid (5.25 parts). The liquid is cooled and filtered, and a solution of a potassium salt added. Potassium borofluoride is precipitated; it is re- crystallised from hot water. Thus obtained, it forms anhydrous, shining, six-sided, prismatic crystals of density 2.5, and is orthorhombic (a:b:c = 0.7898:1:1.2830), being isomorphous with the rubidium salt, RbBF₄ (a:b:c = 0.8067:1:1.2948; density = 2.820). When prepared from hydrofluoric acid, boric acid, and potassium carbonate, however, it forms a gelatinous mass which is converted at 100° into crystals belonging to the cubic system and showing the faces of the octahedron and dodecahedron.

The following borofluorides are also known (Berzelius): -

LiBF₄; CsBF₄; Ca(BF₄)₂.xH₂O; Pb(BF₄)₂; NaBF₄; (NH₄)BF₄; Ba(BF₄)₂.H₂O; Cu(BF₄)₂; KBF₄; Mg(BF₄)₂.xH₂O; Al(BF₄)₃.xH₂O; RbBF₄; Zn(BF₄)₂; Y(BF₄)₃.xH₂O