CHLORIDES OF PHOSPHORUS.

237

itself through hydrogen, carbonic oxide, or any other combustible gas, it confers upon it the property of taking fire spontaneously when mixed with atmospheric air or with oxygen.

(456) Solid Phosphide of Hydrogen (HP).—The liquid phosphide is immediately decomposed by hydrochloric acid, and the solid yellow phosphide of hydrogen is formed. This substance is readily prepared by treating phosphide of calcium (649) with hot hydrochloric acid. It is insoluble in water and in alcohol. When heated with a solution of hydrate of potash the compound is dissolved, and phosphuretted hydrogen gas is liberated. There appear to be two varieties of the solid phosphide, one of a yellow, the other of a green colour; they do not differ from each other in composition. The solid yellow hydride of phosphorus takes fire at about 300°.

(457) CHLORIDES OF PHOSPHORUS.-With chlorine phosphorus forms two compounds, a terchloride, PC,, corresponding to phosphorous anhydride, and a pentachloride, PCI,, which corresponds to phosphoric anhydride. So strong is the chemical attraction between these elements, that in an atmosphere of chlorine, phosphorus immediately takes fire. The following table shows the composition of these chlorides, and of two of their derivatives: :

(458) Terchloride of Phosphorus (PCl=1375): Sp. Gr. of Vapour, Theoretic, 4750; Observed, 4875; of Liquid, 1616 at 32°; Boiling-pt. 173°4; Mol. Vol.-This liquid is sometimes prepared by causing the vapour of phosphorus to pass over corrosive sublimate placed in a long tube, and gently heated; but it may be obtained more easily and abundantly by transmitting a gentle stream of perfectly dry chlorine gas through dry and melted phosphorus contained in a retort; the operation may be conducted in the same manner as in the preparation of the chloride of sulphur (fig. 309); the chloride distils as a very volatile, transparent, colourless, fuming liquid. It dissolves phosphorus freely, and is itself soluble in benzol and in bisulphide of carbon; alcohol and ether decompose it with evolution of great heat, giving rise to various new compounds. It is also immediately decomposed by a large excess of water, and forms phosphorous and hydrochloric

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CHLORIDES AND OXYCHLORIDE OF PHOSPHORUS.

acids; PC1, +3 H2O, yielding H2PHO2+3 HCl. Terchloride of phosphorus absorbs chlorine greedily, and is converted into the pentachloride; at a boiling temperature it also absorbs oxygen and furnishes the oxychloride.

(459) Pentachloride or Perchloride of Phosphorus (PCI.= 2085); Theoretic Sp. Gr. of Vapour, 3'601; Observed, at 572°, 3654; Mol. Vol. -This compound is obtained by placing

-

dry phosphorus in a flask provided with a stopcock, exhausting the air, and allowing chlorine to enter so long as it is absorbed ; or it may be formed by treating terchloride of phosphorus in a tall glass with an excess of chlorine. Pentachloride of phosphorus is also now prepared on a considerable scale by dissolving phosphorus in bisulphide of carbon, and transmitting dried chlorine in excess through the solution which is cooled artificially during the operation; the pentachloride is deposited in crystals from the solution on evaporation. It forms a white crystalline solid, which volatilizes below 212° whilst still solid, but it may be fused under pressure. In the flame of a lamp it burns, producing chlorine and phosphoric acid: with ammonia it combines readily. It is very deliquescent, and by a large excess of water is immediately decomposed into phosphoric and hydrochloric acids; PCl¿+4 H ̧Ð forming H,PO4+5 HCl.

(460) Oxychloride of Phosphorus (PCl ̧0=153'5); Sp. Gr. of Liquid, 17; of Vapour, 5'29; Boiling-pt. 230°; Mol. Vol. -This compound is formed when the vapour of water is allowed to mingle slowly with that of the pentachloride, hydrochloric acid and oxychloride of phosphorus being the result. The reaction is as follows: PC1, +H20=PC10+2 HCl. The oxychloride is a limpid, volatile, fuming liquid, which is decomposed by the further addition of water into phosphoric and hydrochloric acids.

5

Oxychloride of phosphorus may be obtained with facility by Gerhardt's plan of distilling 1 part of crystallized boracic acid with 4 parts of pentachloride of phosphorus, when the following reaction occurs: 3 PC1, +2 (HBO,,H,→)=3 PC12+6 HCl +В2Ð ̧.

*The vapour volume of this compound is anomalous. Perchloride of phosphorus may be supposed to be formed by the union of equal volumes of chlorine, and the vapour of the terchloride (Cahours). In a large number of cases where two bodies combine in the proportion of equal volumes of their components, no condensation occurs, as was long since indicated by GayLussac.

SULPHOCHLORIDE, BROMIDES, AND IODIDES OF PHOSPHORUS. 239

The oxychloride is readily condensed, whilst hydrochloric acid passes off in the form of gas, leaving boracic anhydride in the retort. Crystallized oxalic acid may be substituted for boracic acid in this operation, but it does not answer quite so well. The oxychloride may also be prepared by heating the pentachloride with phosphoric anhydride; 3 PCI, + P20,= 5 PC1,0.

Both the chlorides and the oxychloride of phosphorus have been extensively used in the preparation of various organic substitution-products, particularly the oxychlorides and anhydrides of the organic acids.

(461) Sulphochloride of Phosphorus (PCl,S=1695; Sp. Gr. of Liquid, 1'631; of Vapour, 5·878; Mol. Vol.I]; Boiling-pt. 257°) is a compound corresponding to the oxychloride, but containing sulphur instead of oxygen. It is obtained by decomposing pentachloride of phosphorus with sulphuretted hydrogen: PC1, + H2S, yielding PCI,S+2 HCl. It may be procured still more easily by the gradual addition of powdered sulphide of antimony to the pentachloride of phosphorus; 3 PC1, + Sb2S ̧=3 PC1 ̧S +2 SbCl ̧. Sulphochloride of phosphorus is a fuming, colourless liquid, which, if heated with a solution of caustic soda in excess, exchanges its chlorine for oxygen; chloride of sodium is formed, and a sulphoxyphosphate of sodium may be obtained in six-sided tabular crystals which contain 12 H2O. The composition of this salt is analogous to that of the tribasic phosphate of sodium, but the two are not isomorphous. The following equation explains the changes which accompany its production (Wurtz) :—

PC13S+6 NaHO=Na,PO,S+3 NaCl +3 H2O.

One half of the soda is decomposed, imparting its oxygen to the sulphochloride, from which it receives a corresponding amount of chlorine. Corresponding compounds with barium, calcium, and strontium may be formed by double decomposition with the sodium salt; they are white and insoluble.

(462) BROMIDES OF PHOSPHORUS.-A Terbromide (Sp. Gr. at 32°, 2925; Boiling-pt. 347°5). Pentabromide and Oxybromide of phosphorus, analogous to the corresponding compounds with chlorine, may be formed by similar methods.

(463) IODIDES OF PHOSPHORUS.-Two iodides of phosphorus may be formed, viz., a biniodide and a teriodide (Corenwinder, Ann. de Chimie, III. xxx. 242). The Biniodide (PI,=285) may be obtained by dissolving 1 part (or 1 atom) of phosphorus in bisulphide of carbon and adding 84 parts (or 2 atoms) of iodine: by

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TERIODIDE OF PHOSPHORUS-PHOSPHAM.

cooling the mixture artificially, thin flexible prismatic crystals of the iodide are deposited, of a bright orange colour. This iodide melts at 230°,and is decomposed by water, hydriodic acid being one of the products.

Teriodide of Phosphorus (PI,=412).—This compound may be obtained in a manner similar to the last, by dissolving I part of phosphorus and 121⁄2 parts of iodine in bisulphide of carbon; the liquid is concentrated by evaporation, and on cooling it by a freezing mixture, dark red six-sided plates are formed; it melts between 120° and 130°, and on cooling crystallizes in fine prisms. It deliquesces rapidly when exposed to the air.

Brodie (Q. J. Chem. Soc. v. 289) finds that iodine, when heated with phosphorus in the proportion of 1 atom of iodine to 100 atoms of phosphorus, converts nearly the whole of the phosphorus into the red variety described by Schrötter. When phosphorus was placed in a long tube, and heated till it just melted, and iodine was projected gradually into the phosphorus, the iodine was dissolved, colouring the phosphorus slightly red; when heated by an oil-bath to 212°, the colour became deep red; and between 250° and 266°, a scarlet powder was deposited on the sides of the tube; at 284° the mass was quite solid, and on raising the heat to 392° F., a sharp explosion took place: a sudden evolution of heat occurred, and the cork which closed the tube was blown out by the vapour of phosphorus. The red mass may be distilled in closed tubes, and when it is condensed in the cooler portions of the tubes it is still in the red modification. The changes which occur in the process are supposed to be the following: first, the formation of biniodide of phosphorus ; next, the transformation of this iodide by heat into an allotropic iodide; and thirdly, the decomposition of this new iodide into red phosphorus and a volatile iodide, which acts upon a further portion of the phosphorus; and thus the action is indefinitely continued.

(464) PHOSPHAM (HN,P?).—If terchloride of phosphorus be cooled by a freezing mixture, and saturated with ammoniacal gas, a white saline mass (5 H,N,PCI) is obtained; it is to be introduced into a tube of Bohemian glass, and heated to redness in a current of dry carbonic anhydride as long as any sal ammoniac is sublimed: a yellowish-white bulky amorphous powder remains behind this substance is Rose's phosphide of nitrogen; but there can be no doubt that it contains hydrogen, it is the phospham of Gerhardt; probably its composition should be represented by the formula given above. In closed vessels it sustains a red heat without fusion or volatilization, but when heated in air it is slowly

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241

oxidized, with formation of phosphoric anhydride; if projected into fused hydrate of potash, it is decomposed with incandescence, phosphate of potassium being formed, whilst ammonia and nitrogen are disengaged: but it is remarkable that dry chlorine and hydrochloric acid gases, and the vapour of sulphur, are without action upon it, even at a red heat; and it is but very slowly attacked by concentrated nitric acid. Solutions of the alkalies exert scarcely any action upon it. When heated in hydrogen, ammonia is formed. It combines with sulphuretted hydrogen, and if heated in a current of this gas, the new compound as it is formed is slowly sublimed in the form of a white powder.

(465) SULPHIDES OF PHOSPHORUS.-Sulphur and phosphorus may be melted together in all proportions: several definite compounds exist between them, corresponding in composition with the oxides of phosphorus; and in addition to these, a combination, PS, may be formed (Berzelius). All the sulphides of phosphorus are more fusible than either element separately, and are exceedingly inflammable; most of them may be obtained in crystals. They combine with the sulphides of the alkaline metals, and form a series of definite salts. The combination of sulphur with phosphorus should be gradually effected under warm water; great heat is extricated by their union, and the experiment requires to be conducted very carefully, in order to avoid explosion.

CHAPTER IX.

SILICON AND BORON.

§ I. SILICON, OR SILICIUM: Si=28; or Si=14.

(466) Analogies of the Silicon Group.-Silicon presents a certain analogy with boron in its tendency to unite with fluorine and with nitrogen: but its relationship to niobium and tantalum is still more strongly marked, not only in these particulars, but in its tendency to form an acid with 2 atoms of oxygen, and in its production of a volatile liquid perchloride. Silicon likewise exhibits a similar resemblance to titanium and tin. Zirconium forms a solid perchloride, but in its habits it is also closely allied to silicon. These elements might be arranged thus in parallel series: