Phosphorus Pentasulphide, P₂S₅

The methods which have been already described in connection with the other sulphides have been successfully used in the preparation of Phosphorus Pentasulphide or Diphosphorus Pentasulphide or Phosphorus Tetritadecasulphide, P₂S₅ or P₄S₁₀, from the theoretical proportions of the elements:—

1. By fusion. This method is used in the preparation of the commercial product, which is not pure. A slight excess of sulphur should be used and the heating should take place in an atmosphere of CO₂. The compound may be purified by heating in a vacuous sealed tube for several hours at 700° C., and then by crystallisation from carbon disulphide.
2. By heating phosphorus (20 grams) and sulphur (60 grams) in a sealed tube with iodine (0.5 gram) and carbon disulphide (150 c.c.) at 211° C., followed by recrystallisation.
3. By the action of H₂S on POCl₃.
4. By passing the vapour of PSCl₃ through a red-hot tube.

Details of the preparation have been given by Stock. Pure, dry, red phosphorus (100 grams) is intimately mixed with sulphur (260 grams). Portions of 30 to 40 grams are heated until they combine, in the manner described under P₄S₃. The product is ground up and heated in an evacuated and sealed glass tube to about 700° C. The contents are powdered and extracted with CS₂. They are recrystallised twice from the same solvent and dried at 100° C. in a current of hydrogen. The yield should be about 60 per cent, of the theoretical.

The compound has been prepared in two forms:—

1. Pale yellow crystals obtained by repeated recrystallisation from carbon disulphide, in which they are only sparingly soluble (1 in 195). Density 2.03.
2. A nearly white substance obtained by rapid condensation of the vapour, followed by extraction with carbon disulphide. This form was more soluble in this solvent (1 in 30), had a higher density (2.08) and a lower indefinite melting-point (247° to 276° C.).

The melting-point of the commercial sulphide was 255° C. and that of the recrystallised product 275° to 276° C. The melting-point was raised to 284° to 291° C. by repeated recrystallisation from carbon disulphide. Boiling occurs with partial decomposition at 513° to 515° C. Other values which have been found are 520° C., 523.6° C. The sulphide distils in a water-pump vacuum at 332° to 340° C., probably with considerable dissociation.

The vapour density at temperatures slightly above the boiling- point corresponded to simple molecules P₂S₅. Later investigators, however, found that it was slightly below the theoretical, being 208 at 300° C., and decreased at higher temperatures, down to 133 at 1000° C. The molecular weight in boiling carbon disulphide was 482 to 491, corresponding approximately to double molecules (P₄S₁₀ requires 444).

The compound does not glow in air, but is highly inflammable, giving a mixture of the oxides. It was attacked only slowly by cold water, but rapidly by hot water, giving phosphoric acid and hydrogen sulphide. It did not form addition compounds with bromine or iodine. It was converted into PSCl₃ by phosphorus pentachloride and by several other acid chlorides:—

P₂S₅ + 3PCl₅ = 5PSCl₃
P₂S₅ + 5POCl₃ = 5PSCl₃ + P₂O₅
2P₂S₅ + 6SOCl₂ = 4PSCl₃ + 3SO₂ + 9S
P₂S₅ + SbCl₃ = PSCl₃ + SbPS₄

Ammonia, in the gaseous or liquid form, was readily absorbed by the pentasulphide giving a phosphorus hexammonio-pentasulphide, P₂S₅.6NH₃, as well as lower ammoniates. This compound may be ammonium diimidopentathiopyrophosphate, S{P(SNH₄)₂(NH)}₂.

The pentasulphide dissolved slowly in cold alkalies, quickly in hot, and gave salts of thiophosphoric acid. Sodium sulphide also gave a thiophosphate.