Boron trifluoride, BF₃

Boron trifluoride, BF₃, was originally prepared by heating a mixture of boron oxide and calcium fluoride to a white heat in an iron tube. It may be prepared by heating a mixture of boron trioxide (1 pt.) and calcium fluoride (2 pts.) with concentrated sulphuric acid, or better, by mixing 100 parts of potassium borofluoride with 15 to 20 parts of powdered boron sesqui-oxide and heating with concentrated sulphuric acid. The gas may be collected over mercury.

Boron trifluoride is a very stable, colourless gas of suffocating odour. Its density corresponds to that required for the simple formula BF₃. When purified from hydrogen fluoride by passage over sodium fluoride, it melts at -127° and boils at -101°. The liquefaction of the fluoride was first effected by Faraday. Boron trifluoride combines with its own volume of ammonia, producing a white, opaque solid, BF₃.NH₃, which can be sublimed without decomposition. Liquids of the composition BF₃.2NH₃ and BF₃.3NH₃ may be obtained if more ammonia is employed (J. Davy), but their individuality is doubtful. With dry phosphine at -50°, boron fluoride unites to form a white, unstable solid of the formula 2BF₃.PH₃. It also combines with the oxides of nitrogen.

Boron trifluoride is rapidly absorbed by water, 1057 volumes being absorbed at 0° C. The products of the reaction are boric acid and hydro-fluoboric acid: -

4(BF₃) + 3H₂O + Aq. = 3HBF₄Aq. + H₃BO₃Aq. + 98.04 Cals.

Owing to the readiness with which boron trifluoride reacts with water, it has been proposed as a dehydrating agent in organic chemistry.

Boron trifluoride appears to combine with hydrogen fluoride in two proportions, giving rise to BF₃.HF or HBF₄ and BF₃.3HF. Little or nothing is known of the latter beyond the fact that it is a colourless liquid of normal vapour density. The former is a well-known acid, hydrofluoboric acid.