Fluoboric acid

When boron trifluoride in sufficiently large amount is passed into cold water, a colourless, syrupy liquid is produced. A similar liquid is obtained when as much boric acid as possible is dissolved in a cold, concentrated solution of hydrofluoric acid and the solution concentrated on the water-bath, boiled until it evolves white fumes, and cooled over concentrated sulphuric acid.

The liquid thus obtained, of density 1.584, has a composition in agreement with the formula H₂B₂O₄.6HF. It is decomposed by excess of water with the precipitation of boric acid, an aqueous solution of hydrofluoboric acid being produced. Berzelius regarded the liquid as a definite acid, of which he could form the alkali salts M₂B₂O₄.6MF.H₂O by neutralisation with alkali. This view, however, is in all probability erroneous, although the evidence against the individuality of the liquid is not as conclusive as could be desired. Distillation of the liquid is accompanied by rise of boiling-point, and the successive liquid fractions differ in density and composition, whilst initially boron trifluoride is evolved. The " salts," when recrystallised, yield first MF and subsequently a mixture of MF and M2B₂O₄, while the "acid" in aqueous solution gives with silver nitrate a precipitate of silver metaborate mixed with silver oxide. According to Basarow, the liquid is merely a mixture of metaboric, hydrofluoboric, and hydrofluoric acids.

Two other fluoboric acids have been stated to exist, namely, H₄B₂O₇.3HF and H₄B₂O₉.2HF, but little is known concerning them. A physico-chemical study of mixtures of boric acid, hydrofluoric acid, and potassium fluoride has been made by Abegg; Fox, and Herz, but no definite conclusions could be deduced.

Although no fluoboric acids are definitely known, two compounds have been prepared which may be looked upon as salts of such acids. These are B₂O₃.2KF and KBO₂.KF, to which the constitutions



have been assigned. The former is made by fusing 7 parts of boron sesqui- oxide with 12 parts of potassium fluoride, allowing the melt to cool slowly, and extracting soluble impurities with alcohol; the latter, by fusing the former with the requisite amount of potassium carbonate. The compounds dissolve without decomposition in a little water, but much water decomposes them.