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Rh acids. It does not dissociate on heating as do the pentachloride and pentabromide, thus indicating the existence of pentavalent phosphorus in a gaseous compound, dissociation, however, into the trifluoride and free fluorine may be brought about by induction sparks of 150 to 200 mm. in length. It combines directly with ammonia in the proportion 2PF$5$:5NH$3$, and with nitrogen peroxide at –10° in the proportion PF$5$:NO$2$. Phosphorus trifluorodichloride, PF$3$Cl$2$, prepared from chlorine and the trifluoride, is a pungent-smelling gas, which at 250° gives the pentachloride and fluoride. The trifluorodibromide (see above) is an amber-coloured mobile liquid. Phosphoryl trifluoride, POF3, may be obtained by exploding 2 volumes of phosphorus trifluoride with 1 volume of oxygen (Moissan, 1886); by heating 2 parts of finely-divided cryolite and 3 parts of phosphorus pentoxide (Thorpe and Hambly, Jour. Chem. Soc., 1889, p. 759); or from phosphoryl chloride and zinc fluoride at 40° to 50°. It is a colourless fuming gas, which liquefies under ordinary pressure at –50°, and under a pressure of 15 atmospheres at 16°; it may be solidified to a snow-like mass. Water gives hydrofluoric and phosphoric acids The corresponding sulphur compound, thiophosphoryl fluoride, PSF$3$, obtained by heating lead fluoride and phosphorus pentasulphide to 200°, is a colourless gas, which may be condensed to a clear transparent liquid. It spontaneously inflames in air or oxygen, and when the gas is issuing from a jet into air the flame is greyish green, with a faintly luminous and yellow tip; the flame is probably one of the coldest known. The combustion probably follows the equation PSF$3$+O$2$=PF$3$+SO$2$, the trifluoride at a higher temperature decomposing according to the equations: 10PF$3$+5O$2$=6PF$5$+2P$2$O$5$, 2PF$3$+O$2$=2POF$3$, the complete reaction tending to the equation: 10PSF$3$+15O$2$=6PF$5$+2P$2$O$5$+10SO$2$. The gas dissolves in water on shaking; PSF$3$+4H$2$O=H$2$S+H$3$PO$4$+3HF, but is more readily taken up by alkaline solutions with the formation of fluoride and thiophosphate: PSF$3$+6NaOH=Na$3$PSO$3$+3NaF. Heated in a glass tube it gives silicon fluoride, phosphorus and sulphur, PSF$3$=PF$3$+S; 4PF$3$+3SiO$2$= 3SiF$4$+P$4$+3O$2$. Electric sparks give at first free sulphur and the trifluoride, the latter at a higher temperature splitting into the pentafluoride and phosphorus. With dry ammonia it gives ammonium fluoride and a compound P(NH$2$)$2$SF.

Phosphorus trichloride or phosphorous chloride, PCl$3$, discovered by Gay-Lussac and Thénard in 1808, is obtained by passing a slow current of chlorine over heated red phosphorus or through a solution of ordinary phosphorus in carbon disulphide (purifying in the latter case by fractional distillation). It is a colourless, mobile liquid of specific gravity 1.6128 at 0° and boiling-point 76°. With chlorine it gives the pentachloride, PCl$5$, and with oxygen when heated phosphoryl chloride, POCl$3$. Water gives hydrochloric and phosphorous acids, with separation of red phosphorus if the water be hot. When led with hydrogen into liquid ammonia it gives NH:PNH$2$, which on elevation of temperature gives P$2$(NH)$3$ (Joannis, Comptes rendus, 1904, 139, p. 364) By submitting a mixture of phosphorous chloride and hydrogen to an electric discharge A. Besson and A. Fournier (Comptes rendus, 1901, 150, p. 102) obtained phosphorus dichloride, P$2$Cl$4$, as a colourless, oily, strongly fuming liquid, freezing at –28° and boiling at 180° with decomposition. With water it gave phosphorous acid and a yellow indefinite solid. It decomposes slowly at ordinary temperatures. Phosphorus pentachloride, PCl$5$, discovered by Davy in 1810 and analysed by Dulong in 1816, is formed from chlorine and the trichloride. It is a straw-coloured solid, which by fusion under pressure gives prismatic crystals. It sublimes when heated, but under pressure it melts at 148°, giving a normal vapour density, but on further heating it dissociates into the trichloride and chlorine; this dissociation may be retarded by vaporizing in an atmosphere of chlorine. It fumes strongly in moist air, giving hydrochloric acid and phosphoryl chloride, POCl$3$; with water it gives phosphoric and hydrochloric acids.

Phosphoryl trichloride or phosphorus oxychloride, POCl$3$, corresponding to phosphoric acid, (HO)$3$PO, discovered in 1847 by Wurtz, may be produced by the action of many substances containing hydroxy groups on the pentachloride; from the trichloride and potassium chlorate, by leaving phosphorus pent oxide in contact with hydrochloric acid: 2P$2$O$5$+3HCl=POCl$3$+3HPO$3$; or by heating the pentachloride and pentoxide under pressure. 3PCl$5$+P$2$O$5$=5POCl$3$ it is a colourless liquid, boiling at 107.2°, and when solidified it melts at 0.8°. Water gives hydrochloric and phosphoric acids; dilute alcohol gives monoethyl phosphoric acid, C$2$H$5$·H$2$PO$4$, whilst absolute alcohol gives triethyl phosphate, (C$2$H$5$)$3$PO$4$. Pyrophosphoryl chloride, P$2$O$3$Cl$4$, corresponding to pyrophosphoric acid, was obtained by Geuther and Michaelis (Ber., 1871, 4, p. 766) in the oxidation of phosphorus trichloride with nitrogen peroxide at low temperature; it is a colourless fuming liquid which boils at about 212° with some decomposition. With water it gives phosphoric and hydrochloric acids. Thiophosphoryl chloride, PSCl$3$, may be obtained by the direct combination of sulphur with the trichloride; from sulphuretted hydrogen and the pentachloride, from antimony trisulphide and the pentachloride; by heating the pentasulphide with the entachloride, and by dissolving phosphorus in sulphur chloride and distilling the solution 2P+3S$2$Cl$2$= 4S+2PSCl$3$. It is a colourless mobile liquid, boiling at 125.1° and having a pungent, slightlv aromatic odour it is slowly decomposed by water giving phosphoric and hydrochloric acids, with sulphuretted hydrogen; alkalis form a thiophosphate, e.g. PS(OK)$3$, and a chloride.

Phosphorus tribromide, PBr$3$, prepared by mixing solutions of its elements in carbon disulphide and distilling, is a transparent, mobile liquid, boiling at 173° and resembling the trichloride chemically. The pentabromide, PBr$5$, which results from phosphorus and an excess of bromine, is a yellow solid, and closely resembles the pentachloride. The bromochloride, PCl$3$Br$2$, is an orange-coloured solid formed from bromine and the trichloride, into which components it decomposes at 35°. Phosphoryl tribromide, POBr$3$, is a solid, melting at 45° and boiling at 195°. Thiophosphoryl bromide, PSBr$3$, obtained after the manner of the corresponding chloride, forms yellow octahedral which melt at 38°, and have a penetrating, aromatic odour. With water it gives sulphur, sulphuretted hydrogen, hydrobromic, phosphorous and phosphoric acids, the sulphur and phosphorous acid being produced by the interaction of the previously formed sulphuretted hydrogen and phosphoric acid. Pyrophosphoryl thiobromide, (PBr$2$S)$2$S, and metaphosphoryl thiobromide, PS$2$Br, are also known.

Phosphorus forms three iodides. The subiodide, P$4$I, was obtained by R. Boulough (Comptes rendus, 1905, 141, p. 256), who acted with dry iodine on phosphorus dissolved in carbon disulphide; with alkalis it gives P$4$(OH). The di-iodide and tri-iodide are formed similarly; the first is deposited as orange-coloured prisms which melt at 110° to a red liquid (see Doughty, Jour. Amer. Chem Soc., 1905, 27, p. 1444), whilst the second forms dark-red hexagonal plates which melt at 55°.

Sulphides and Thio-acids.—Phosphorus and sulphur combine energetically with considerable rise of temperature to form sulphides. The researches of A. Stock (Ber., 1908, 41, pp. 558, 657; 1909, 42, p. 2062; 1910, 43, pp. 150, 414) show that three exist, P$4$S$3$, P$4$S$7$, P$2$S$5$. The first is prepared by heating red phosphorus with finely powdered sulphur in a tube sealed at one end and filled with carbon dioxide. The product is extracted with carbon disulphide and the residue distilled in carbon dioxide. It forms light yellow crystals from benzene, which melt at 1725° and boil at 407°–408° with slight decomposition. Alkalis give hydrogen and phosphine. The second, P$4$S$7$, is obtained by heating a mixture of red phoshorus and sulphur in the proportions given by P$4$S$7$+5% P$4$S$3$, and crystallizing from carbon disulphide in which P$4$S$3$ is readily soluble. It forms small, slightly yellow prisms, which melt at 310° and boil at 523°. The third, or pentasulphide, P$2$S$5$, was obtained as a substance resembling flowers of sulphur by A. Stock and K. Thiel (Ber., 1905, 38, p. 2719; 1910, 43, p. 1223), who heated sulphur with phosphorus in carbon disulphide solution with a trace of iodine to 120°–130°. It exists in two forms, one having the formula P$4$S$10$, and the other a lower molecular weight. With liquid ammonia it gives P$2$S$5$·7NH$3$, which is a mixture of ammonium iminotrithiophosphate, P(SNH$4$)$3$:NH, and ammonium nitrilodithiophosphate, P(SNH$4$)$2$$$\dot :$$N. Water converts the former into ammonium thiophosphate, PO(SNH$4$)$3$·H$2$O, whilst the latter heated to 300° in a vacuum gives thiophosphoric nitrile, N$$\dot :$$P:S (Stock, ibid., 1906, 39, p. 1967).

Thiophosphates result on dissolving the pentasulphide in alkalis. Sodium monothiophosphate, Na$3$PSO$3$·12H$2$O, is obtained by adding one P$2$S$5$ to six NaOH, adding alcohol, dissolving the precipitate in water and heating to 90°. n cooling the salt separates as white six-sided tablets. Sodium dithiophosphate, Na$3$PS$2$O$2$·11H$2$O, is obtained by heating the above solution only to 50°–55°, cooling and adding alcohol, which precipitates the dithio salt. On heating it gives the monothio salt. Sodium trithiophosphate appears to be formed when the pentasulphide acts with sodium hydrosulphide at 20°. All thiophosphates are decomposed by acids giving sulphuretted hydrogen and sometimes free sulphur. They also act in many cases as reducing agents.

Nitrogen Compounds.—Phosphorus pentachloride combines directly with ammonia, and the compound when heated to redness loses ammonium chloride and hydrochloric acid and gives phospham, PN$2$H$4$, a substance first described by Davy in 1811. It is a white, infusible, very stable solid, which decomposes water on heating, giving ammonia and metaphosphoric acid, whilst alkalis give an analogous reaction. With methyl and ethyl alcohols it forms secondary amines (Vidal, Comptes rendus, 1891, 112, p. 950; 1892, 115, p. 123). The diamide, PN$2$H$4$, was obtained by Hugot (ibid., 1905, 141, p. 1235) by acting with ammonia gas on phosphorus tribromide or tri-iodide at –70°; it is very unstable, and decomposes at –25°. Phosphorus combines with nitrogen and chlorine to form several polymeric substances of the general formula (PNCI2)x, where x may be 1, 3, 4, 5, 6, 7, or 11; they may be obtained by heating the pentachloride with ammonium chloride in a sealed tube and separating the mixture by fractional distillation (H. N. Stokes, Amer. Chem. Jour., 1898, 20, p. 740; also see Besson and Rosset, Comptes rendus, 1906, 37, p. 143) the commonest form is P$3$N$3$Cl$6$, a crystalline solid, insoluble in water, but soluble in alcohol and ether. Several phosphoamides have been described. The diamide, PO(NH2)(NH), results when the pentachloride is saturated with ammonia gas and the first formed chlorophosphamide, PCl3(NH2)2, is decomposed by water. The triamide, PO(NH2)3, results from ammonia and phosphorus oxychloride. Both these compounds on heating give phosphomonamide, PON, of which a polymer (PON)2 had been described by Oddo (Gazz. chim. Ital., 1899, 29 (ii), p. 330). Stokes (Amer. Chem. Jour.,