Phosphorus Trifluoride, PF₃

Some early indications were obtained that phosphorus reacted with fluorides of zinc and lead, and that the fluoride resulting was a gas. This gas was prepared in 1884 by heating lead fluoride with copper phosphide in a brass tube. It is also produced by the action of fluorides of zinc, silver or lead on phosphorus trichloride or tribromide. One of the most convenient methods of preparation is by the reaction between arsenic trifluoride, which is easily prepared and purified, and phosphorus trichloride, thus:—

PCl₃ + AsF₃ = PF₃ + AsCl₃

The apparatus consists of a distillation flask fitted with a tap-funnel. The side-tube of the flask is sealed to a condensing wash-bottle, which can be cooled in a mixture of solid carbon dioxide and alcohol, and the wash-bottle is joined to a condenser which is immersed in liquid air. After the whole apparatus has been thoroughly dried, the arsenic trifluoride is allowed to drop into the trichloride. The vapours of these two compounds are removed in the first condenser, and the phosphorus trifluoride is solidified in the second.

Phosphorus trifluoride can also be made by the action of lead fluoride on phosphorus trichloride and by the decomposition of the pentafluoride by means of electric sparks.

The composition has been established by the vapour density and by analysis. The fluoride is a colourless gas which does not fume in the air, and is highly poisonous. It condenses to a colourless liquid at -95° C. and freezes to a white solid at -160° C. The heat of formation is 106.2 to 109.7 Cals. per mol, therefore much greater than that of the trichloride, and the trifluoride also proves to be the more stable of the two compounds. It can be decomposed by electric sparks with deposition of phosphorus and formation of the pentafluoride, thus:—

5PF₃ = 3PF₅ + 2P

It can be reduced by heating with hydrogen:—

PF₃ + 3H₂ = PH₃ + 3HF

and also by heating with silicon or boron:—

4PF₃ + 3Si = 3SiF₄ + 4P

but is not reduced by heating with sulphur, phosphorus or arsenic. It is not affected by the metals copper, mercury, iron, cobalt and nickel at ordinary temperatures, but the same metals give phosphides at a red heat. It is completely absorbed by sodium at the melting- point of this metal.

Phosphorus trifluoride is easily oxidised. Although it does not burn in air it can be exploded with oxygen by electric sparks, one volume of the gas mixed with half a volume of oxygen giving one volume of phosphorus oxyfluoride:—

PF₃ + ½O₂ = POF₃

It combines vigorously with all the halogens giving mixed pentahalides (q.v.). It is hydrolysed very slowly by cold water, rather more quickly by boiling water, and quickly by aqueous alkalies, giving in the last case alkali fluoride and phosphite. It was considered by Moissan and by Berthelot that the first product of hydrolysis was a fluophosphoric acid (q.v.), the potassium salt of which was fairly stable.