Phosphorus Thiotrifluoride, PSF₃

Phosphorus Thiotrifluoride or thiophosphoryl fluoride, PSF₃, was first prepared by heating a mixture of phosphorus and sulphur with an excess of lead fluoride in a lead tube to 250° C. and leading over the mixture a current of dry nitrogen. The gas was purified by standing for about a day over dry lime. It was also prepared by heating arsenic trifluoride in a sealed tube with phosphorus thiotrichloride:—

AsF₃ + PSCl₃ = PSF₃ + AsCl₃

It is a transparent and colourless gas, which has no action on glass at ordinary temperatures. The formula has been established by analysis and from the vapour density. The compound is decomposed by heat, giving sulphur and fluorides of phosphorus, thus:—

PSF₃ = PF₃ + S
5PF₃ = 3PF₅ + 2P

It burns in air with a blue or greyish-green low-temperature flame. The white fumes which are produced contain P₂O₅. The sulphur probably burns first, leaving PF₃, which combines with more oxygen, giving PF₅ and P₂O₅, the whole reaction being summarised by the equation

10PSF₃ + 15O₂ = 6PF₅ + 2P₂O₅ + 10SO₂

With moist oxygen in closed vessels the reaction may proceed explosively, but it does not proceed at all when the gas and the oxygen are intensively dried. The gas was slowly absorbed and hydrolysed by water, more rapidly by alkalies, thus:—

PSF₃ + 4H₂O = H₂S + 3HF + H₃PO₄
PSF₃ + 6NaOH = Na₃PSO₃ + 3NaF + 3H₂O