Tetraphosphorus Trisulphide, P₄S₃

Tetraphosphorus Trisulphide or Phosphorus Tetritatrisulphide, P₄S₃, may be prepared by heating together the theoretical proportions of red phosphorus and sulphur in a sealed tube or in an atmosphere of carbon dioxide. It is recommended to use an excess of red phosphorus, mix intimately with powdered sulphur and heat to 100° C. in a wide tube in a current of carbon dioxide. The reaction is started by stronger heating in one spot. After the reaction has ceased, the contents of the tube are heated until distillation begins, in order to dissociate the higher sulphides of phosphorus. The product, after cooling in an atmosphere of carbon dioxide, may be extracted with carbon disulphide, which dissolves the P₄S₃, or distilled, when P₄S₃ passes over.

Details of the preparation according to Stock are as follows:—The sulphur, phosphorus, carbon disulphide and benzene should be both pure and dry. The red phosphorus (155 grams) is mixed with the sulphur (95 grams). This mixture, in portions of 40 to 50 grams is carefully warmed to 100° C. in a beaker standing on a sand-bath, while a current of CO₂ is passed on to the surface. The part of the beaker which is on a level with the upper edge of the mixture is then heated with a small flame until the reaction starts and spreads rapidly through the mass. The melted substance is then heated to the point of distillation in a current of CO₂, which is maintained until the product is cold. It is then crushed and extracted with about twice its weight of warm CS₂. On evaporation of this, the sulphide, having a melting-point of 130° to 150° C., remains. It is powdered, boiled with water and steamed. The powder is again extracted with CS₂, which is shaken with P₂O₅, filtered and evaporated until crystals form. These are dried over P₂O₅ in a water-pump vacuum. The product now melts at 160° to 171° C. but still contains CS₂, which may be removed by extraction in a Soxhlet apparatus with benzene. As this proceeds, fine crystals are deposited from the benzene. These are sucked dry on a filter and finally freed from solvent in a current of dry hydrogen. Another crop of crystals may be obtained by evaporation of the mother-liquors. The yellowish needles, of melting-point about 173° C., are soluble in benzene and carbon disulphide. Solutions are turbid on exposure to air, but remain unaltered in an atmosphere of hydrogen. They can be heated to 700° C. without alteration in the absence of oxygen and moisture.

In the commercial process phosphorus and sulphur are heated together in a current of carbon dioxide to 330° or 340° C., and the compound sublimed. The commercial product may contain 83 to 98 per cent, of P₄S₃ with free sulphur, water, volatile matter, phosphoric acid and other impurities.2 If it is to be used in the match industry it should not contain free phosphorus.

The purified compound is a yellow crystalline substance which has a constant composition and which does not alter on fractional distillation or crystallisation. The vapour density was found to agree with the formula P₄S₃. It has been confirmed that the molecular weight of the vapour agrees with the theoretical (220) at about 700° C., but above this temperature dissociation takes place, a molecular weight of 179 being found at 1000° C. Molecular weights slightly above the theoretical (228 to 264) were found for this compound when dissolved in benzene at its boiling-point.

The density of the solid was 2.0. The melting-point was found to be 165° to 167° C. by Isambert, Helff, Mai and Scheffer, also by Rebs and Giran, but Stock and his collaborators give 171° to 172.5° C. The boiling-point is above 400° C.; it is given as 410° to 420° C., 408° to 418°, 407° to 408°. In a water-pump vacuum the sulphide may be distilled at about 230° C.

The heat of formation was low, namely, 16.4 Cals. per mol P₄S₃ and of the same order as the heat of transformation of yellow into red phosphorus. As already mentioned, the vapour is dissociated at higher temperatures, and in an atmosphere of carbon dioxide this dissociation appears to begin at about 380° C.

The compound ignites in air at about 100° C. and shows a greenish glow of slow combustion at about 80° C. The conditions of this glow resemble, but are not identical with, those required in the cases of white phosphorus and phosphorous oxide. It did not appear in pure oxygen until, at 65° C., the pressure was reduced below 300 mm., and was steady at 242 mm. in dry and 250.3 mm. in moist oxygen. At higher temperatures the glow appeared and disappeared at higher pressures. Inflammation occurred between 80° and 90° C. The products of combustion are phosphoric oxide and sulphur dioxide. Chlorine converts the sulphide into phosphoric and sulphuric acids, while aqua regia also dissolves it.

Cold water has no effect, but boiling water slowly decomposes the sulphide into phosphoric acid and hydrogen sulphide. With potassium hydroxide solution the sulphide behaves like a mixture of sulphur and phosphorus, giving phosphine, hydrogen, etc., and potassium sulphide.

Like its constituent elements, this sulphide of phosphorus dissolves very freely in CS₂ (solubility 60 to 100 at ordinary temperatures). It is also moderately soluble in solvents such as benzene and toluene.