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Rh hydrochloric acid. It is also obtained by heating red phosphorus under pressure to 580°. It forms a lustrous, nearly black crystalline mass, composed of minute rhombohedra. G. E. Linck and P. Möller (Ber., 1908, 41, p. 1404) have affirmed that the product of the first process always contains lead. E. Cohen and J. Olie, Jun. (Abs. Jour. Chem. Soc., 1909, ii. 998) regard red phosphorus as a solid solution of the white in Hittorf's, but this is contradicted by A. Stock (Ber., 1909, 42, p. 4510), who points out that ordinary red phosphorus melts at 605°–610°, whilst Hittorf's melts at 620°; moreover, the latter is less reactive than the former at high temperatures.

Another form was obtained by R. Schenck (Zeit. Elektrochemie, 1905, ii. 117) as a scarlet amorphous powder by deposition of solutions of phosphorus in the tri-iodide, tribromide or sulphide (P4S3). It phosphoresces in ozone, but not in air, and is nonpoisonous; from its solution in alcoholic potash acids precipitate the hydride P12H6, and when heated it is transformed into the red modification. It has been used in combination with potassium chlorate as a composition for matches to strike on any surface. Finally a black phosphorus was described by Thénard as formed by rapidly-cooling melted phosphorus.

Phosphine (phosphoretted hydrogen), PH3, a gas formed in the putrefaction of organic matter containing phosphorus, was obtained by Gengembre (Crell's Ann., 1789, i. 450) by the action of potash upon phosphorus, the gas so prepared being spontaneously inflammable. Some time later Davy, by heating phosphorous acid, obtained a phosphoretted hydrogen which was not spontaneously inflammable. These gases were considered to be distinct until Le Verrier (Ann. chim. phys., 1815 [2], 60. p. 174) showed that the inflammability of Gengembre's phosphine was due to small quantities of liquid phosphoretted hydrogen, P2H4. Phosphine may be prepared by the decomposition of calcium phosphide with water (P2H4 being formed simultaneously); by the decomposition of phosphorous and hypophosphorous acids when strongly heated; and by the action of solutions of the caustic alkalis on phosphorus: P4+3NaOH+3H2O = PH3+3NaH2PO2; hydrogen and P2H4, are produced at the same time, and the gas may be freed from the latter substance by passing into a hydrochloric acid solution of cuprous chloride and eating the solution, when pure phosphine is liberated (Riban, Comptes rendus, 58, p. 581) The pure gas may also be obtained by heating phosphonium iodide with caustic potash (A. W. Hofmann, Ber., 1871, 4, p. 200); by the decomposition of crystalline calcium phosphide or of aluminium phosphide with water (H. Moissan, Bull. soc. chim., 1899 (3), 21, p 926; Matignon, Comptes rendus 1900, 130, p. 1391); and by the reduction of phosphorous acid with nascent hydrogen.

It is a colourless, extremely poisonous gas, possessing a characteristic offensive smell, resembling that of rotting fish. It becomes liquid at –90° C., and solid at –133° C. (K. Olszewski, Monats., 1866, 7, p. 371). It is only slightly soluble in water, but is readily soluble in solutions of copper sulphate, hypochlorous acid, and acid solutions of cuprous chloride. It burns with a brightly luminous flame, and is spontaneously inflammable at about 100° C. When mixed with oxygen it combines explosively if the mixture be under diminished pressure, and is violently decomposed by the halogens. It is also decomposed when heated with sulphur or with most metals, in the latter case with the liberation of hydrogen and formation of phosphide of the metal. It combines with the halide derivatives of boron and silicon to form, e.g. PH3·2BF3, 2PH3·SiCl, (Besson, Comptes rendus, 1890, 110, 80, pp. 240, 516; 1891, 113, p. 78), with the halogen acids to form phosphonium salts, PH4X (X =Cl, Br, I), and with sodammonium and potassammonium to form PH2Na, PH2K (Joannis, Comptes rendus, 1894, 119, p. 557). It oxidizes slowly in air, and is a reducing agent. It decomposes when heated, hydrogen and red phosphorus being formed.

Liquid Phosphoretted Hydrogen, P2H4, first obtained by P. Thénard (Comptes rendus, 1844, 18, p. 652) by decomposing calcium phosphide with warm water, the products of reaction being then passed through a U tube surrounded by a freezing mixture (see also L. Gattermann, Ber., 1890, 23, p. 1174). It is a colourless liquid which boils at 57°–58° C. It is insoluble in water, but soluble in alcohol and ether. It is very unstable, being readily decomposed by heat or light. By passing the products of the decomposition of calcium phosphide with water over granular calcium chloride, the P2H4 gives a new hydride, P12H6 and phosphine, the former being an odourless, canary-yellow, amorphous powder. When heated in a vacuum it evolves phosphine, and leaves an orange-red residue of a second new hydride, P9H2 (A. Stock, W. Böttcher, and W. Lenser, Ber., 1909, 42, pp. 2839, 2847, 2853).

Solid Phosphoretted Hydrogen, P4H2, first obtained by Le Verrier (loc. cit.), is formed by the action of phosphorus trichloride on gaseous phosphine (Besson, Comptes rendus, 111, p. 972); by the action of water on phosphorus di-iodide and by the decomposition of calcium phosphide with hot concentrated hydrochloric acid. It is a yellow solid, which is insoluble in water. It burns when heated to about 200° C. Oxidizing agents decompose it with great violence. When warmed with alcoholic potash it yields gaseous phosphine, hydrogen and a hypophosphite. It reduces silver salts.

Phosphonium Salts.—The chloride, PH4Cl, was obtained as a crystalline solid by Ogier (Comptes rendus, 1879, 89, p. 705) by combining phosphine and hydrochloric acid gas under a pressure of from 14–20 atmospheres; it can also be obtained at —30° to –35° C. under ordinary atmospheric pressure. It crystallizes in large transparent cubes, but rapidly dissociates into its constituents on exposure. The bromide, PH4Br, was first obtained by H. Rose (Pogg. Ann., 1832, 24, p. 151) from phosphine and hydrobromic acid, it also results when phosphorus is heated with hydrobromic acid to 100–120° C. in sealed tubes (Damoiseau, Bull. soc. chim., 1881, 35, p. 49). It crystallizes in colourless cubes, is deliquescent, and often inflames spontaneously on exposure to air. It is readily decomposed by water and also by carbonyl chloride (Besson, Comptes rendus, 1896, 122, p. 140): 6PH4Br + 5COCl2 = 10HCl + 5CO + 6HBr + 2PH3 + P4H2. The iodide, PH4I, first prepared by J. Gay-Lussac (Ann. chim. phys., 1814, 91, p. 14), is usually obtained by the action of water on a mixture of phosphorus and iodine (A. W. Hofmann, Ber., 1873, 6, p. 286). It is also prepared by the action of iodine on gaseous phosphine, or by heating amorphous phosphorus with concentrated hydriodic acid solution to 160° C. It crystallizes in large cubes and sublimes readily. It is a strong reducing agent. Water and the caustic alkalis readily decompose it with liberation of phosphine and the formation of iodides or hydriodic acid. It is also decomposed by carbonyl chloride (Besson, loc. cit.). 4PH4I+8COCl2=16HCl+8CO+P2I4+2P.

Just as the amines are derived from ammonia, so from phosphine are derived the primary, secondary and tertiary organic phosphines by the exchange of hydrogen for alkyl groups, and corresponding to the phosphonium salts there exists a series of organic phosphonium bases. The primary and secondary phosphines are produced when the alkyl iodides are heated with phosphonium iodide and zinc oxide to 150° C. (A. W. Hofmann, Ber., 1871, 4, p. 430, 605), thus: 2RI + 2PH4I + ZnO = 2R·PH2·HI + ZnI2 + H2O, 2RI + PH4I + ZnO = R2·PH·HI + ZnI2 + H2O. The reaction mixture on treatment with water yields the primary phosphine, the secondary phosphine being then liberated from its hydriodide by caustic soda. The tertiary phosphines, discovered by L. Thénard (Comptes rendus, 1845, 21, p. 144; 1847, 25, p. 892), are formed (together with the quaternary phosphonium salts) by heating alkyl iodides with phosphonium iodide to 150–180° C.: PH4I+3CH3I=P(CH3)3HI+3HI; P(CH3)3HI+CH3I=P(CH3)4I+HI (see also Fireman, Ber., 1897, 30, p. 1088). They are also formed by the interaction of phosphorus trichloride and zinc alkyls (Cahours and Hofmann, Ann., 1857, 104, p. 1): 2PCl3+3Zn(C2H5)2=3ZnCl2+2P(C2H5)3.

The primary and secondary phosphines are colourless compounds, and with the exception of methyl phosphine are liquid at ordinary temperature. They possess an unpleasant odour, fume on exposure to air, show a neutral reaction, but combine with acids to form salts. They oxidize very rapidly on exposure, in many cases being spontaneously inflammable. On oxidation with nitric acid the primary compounds give monoalkyl phosphinic acids, R·PO(OH)2, the secondary yielding dialky phosphinic acids, R2PO(OH). The primary phosphines are very weak bases, their salts with acids being readily decomposed by water. The tertiary phosphines are characterized by their readiness to pass into derivatives containing pentavalent phosphorus, and consequently they form addition compounds with sulphur, carbon bisulphide, chlorine, bromine, the halogen acids and the alkyl halides with great readiness. On oxidation they yield phosphine oxides, R3P·O. The quaternary phosphonium salts resemble the corresponding nitrogen compounds. They are stable towards aqueous alkalis, but on digestion with moist silver oxide yield the phosphonium hydroxides, which are stronger bases than the caustic alkalis. They differ from the organic ammonium hydroxides in their behaviour when heated, yielding phosphine oxides and paraffin hydrocarbons: R4P·OH =R3PO+RH. The boiling-points of some members of the series are shown in the table:—

The alkyl phosphinic acids are colourless crystalline compounds which are easily soluble in water and alcohol. They yield two series of salts, viz. RHM·PO3 and RM2PO3, (M=metal). The dialkyl phosphinic acids are also colourless compounds, the majority of which are insoluble in water. They yield only one series of salts.

Oxides.—Phosphorus forms three well-defined oxides, P4O6, P2O4 and P2O5 two others, P4O and P2O, have been described.

Phosphorus suboxide, P4O, is said to be formed, mixed with the