1911 Encyclopædia Britannica/Phosphorus

PHOSPHORUS (Gr. φῶς, light, φέρειν, to bear), the name originally given to any substance which possessed the property of phosphorescence (q.v.), i.e. the power of shining in the dark, but now generally restricted to a non-metallic element, which was first known as Phosphorus mirabilis or igneus. This element is very widely distributed in nature in combination, but is never found free. In the mineral kingdom it is exceptionally abundant, forming large deposits of phosphates (q.v.). It is also necessary to animal and vegetable life (see Manure). It occurs in the urine, blood, tissues, and bones of animals, calcium phosphate forming about 58% of bones, which owe their rigidity to its presence.

The element appears to have been first obtained in 1669 by Brand of Hamburg; Krafft bought his secret and in 1677 exhibited specimens in England, where it created an immense sensation. Its preparation was assiduously sought for, and Kunckel in 1678 and Boyle in 1680 succeeded in obtaining it by the same process as was discovered by Brand, i.e. by evaporating urine to dryness and distilling the residue with sand. This method was generally adopted until 1775, when Scheele prepared it from bones, which had been shown by Gahn in 1769 to contain calcium phosphate. Scheele treated bone ash with nitric acid, precipitated the calcium as sulphate, filtered, evaporated and distilled the residue with charcoal. Nicolas and Pelletier improved the process by decomposing the bone-ash directly with sulphuric acid, whilst Fourcroy and Vauquelin introduced further economies. In modern practice degreased bones (see Gelatin), or bone-ash which has lost its virtue as a filtering medium, &c., or a mineral phosphate is treated with sufficient sulphuric acid to precipitate all the calcium, the calcium sulphate filtered off, and the filtrate concentrated, mixed with charcoal, coke or sawdust and dried in a muffle furnace. The product is then distilled from Stourbridge clay retorts, arranged in a galley furnace, previously heated to a red heat. The temperature is now raised to a white heat, and the product led by malleable iron pipes into condensing troughs containing water, when it condenses. The chemical reactions are as follows: the treatment of the calcium phosphate with the acid gives phosphoric acid, H₃PO₄, which at a red heat loses water to give metaphosphoric acid, HPO₃; this at a white heat reacts with carbon to give hydrogen, carbon monoxide and phosphorus, thus: 2HPO₃+6C＝H₂+6CO+P₂.

Electrothermal processes are also employed. Calcium phosphate, mixed with sand and carbon, is fed into an electric furnace, provided with a closely fitting cover with an outlet leading to a condenser. At the temperature of the furnace the silica (sand) attacks the calcium phosphate, forming silicate, and setting free phosphorus pentoxide, which is attacked by the carbon, forming phosphorus and carbon monoxide. As phosphorus boils at 290° C. (554° F.), it is produced in the form of vapour, which, mingled with carbon monoxide, passes to the condenser, where it is condensed. It is then cast under water. The calcium silicate remains in the furnace in the form of a liquid slag, which may be run off, so that the action is practically continuous. Kaolin may with advantage be used in addition to or in part substitution for sand, because the double silicate thus formed is more fusible than the single silicate of lime. The alternating current is generally used, the action not being electrolytic. One of the special advantages of the electrical over the older process is that the distilling vessels have a longer life, owing to the fact that they are not externally heated, and so subjected to a relatively high temperature when in contact with the corrosive slag formed in the process. The Readman-Parker process (see Jour. Soc. Chem. Ind., 1891, x. 445) appears to be very generally adopted. Readman, experimenting with a Cowles furnace in Staffordshire in 1888, patented his process, and in the same year Parker and Robinson, working independently, patented a similar one. The two inventors then cooperated, an experimental plant was run successfully, and the patents were taken over by the leading manufacturers. With the object of obtaining a valuable by-product in place of the slag produced in this furnace, several patentees (e.g. Hilbert and Frank, Billaudot, Bradley and Jacobs, and others) have sought to combine the manufacture of calcium carbide and phosphorus by using only calcium phosphate and carbon, effecting direct reduction by carbon at a high temperature.

The crude phosphorus is purified by melting under water and then filtering through animal black and afterwards through chamois leather, or by treating it, when molten, with chromic acid or a mixture of potassium bichromate and sulphuric acid; this causes the impurities to rise to the surface as a scum which can be skimmed off. It is usually sent on the market in the form of sticks, which were at one time prepared by sucking the molten material up glass tubes; but the dangers to the workmen and other disadvantages of this method have led to its replacement by a continuous process, in which the phosphorus leaves the melting-pot for a pipe surrounded by water, in which it solidifies and can be removed as a continuous rod.

Properties.—When perfectly pure phosphorus is a white, transparent, waxy solid, but as usually prepared it is yellowish owing to the presence of the allotropic “red phosphorus,” J. Böeseken (Abs. Jour. Chem. Soc., 1907, ii. 343, 760) prepares perfectly pure phosphorus by heating the crude product with chromic acid solution, washing and drying in a vacuum, first at 40°, then at 80°. It remains colourless in vacuum tubes in the dark, but on exposure it rapidly turns yellow. At 25° to 30° C. it is soft and flexible, but it hardens when strongly cooled, and can then only be cut with difficulty. The fracture is distinctly crystalline; large crystals, either regular dodecahedra or octahedra, may be obtained by crystallization from carbon bisulphide, sulphur chloride, &c., or by sublimation. It is a non-conductor of electricity. Its density at 0° is 1·836; this regularly diminishes up to the melting-point, 44·3°, when a sudden drop occurs. Molten phosphorus is a viscid, oily, highly refractive liquid, which may be supercooled to 32° before solidification. It boils at 200°, forming a colourless vapour which just about the boiling-point corresponds in density to tetratomic molecules, P₄; at 1500° to 1700°, however, Biltz and Meyer detected dissociation into P₂ molecules. Beckmann obtained P₄ molecules from the boiling-point of carbon bisulphide solutions, and Hertz arrived at the same conclusion from the lowering of the freezing-point in benzene solution; E. Paternò and Nasini, however, detected dissociation. Phosphorus is nearly insoluble in water, but dissolves in carbon bisulphide, sulphur chloride, benzene and oil of turpentine.

The element is highly inflammable, taking fire in air at 34° and burning with a bright white flame and forming dense white clouds of the pentoxide, in perfectly dry air or oxygen, however, it may be distilled unchanged, H. B. Baker showing that a trace of water vapour was necessary for combination to occur. When exposed to the air a stick of phosphorus undergoes slow combustion, which is revealed by a greenish-white phosphorescence when the stick is viewed in the dark. This phenomenon was minutely studied by Boyle, who found that solutions in some essential oils (oil of cloves) showed the same character, whilst in others (oils of mace and aniseed) there was no phosphorescence. He also noticed a strong garlic-like odour, which we now know to be due to ozone. Frederick Slare noticed that the luminosity increased when the air was rarefied, an observation confirmed by Hawksbee and Homberg, and which was possibly the basis of Berzelius’s theory that the luminosity depended on the volatility of the element and not on the presence of oxygen. Lampadius, however, showed that there was no phosphorescence in a Torricellian vacuum; and other experimenters proved that oxygen was essential to the process. It depends on the partial pressure of the oxygen and also on temperature. In compressed air at ordinary temperature there is no glowing, but it may be brought about by heating. Again, in oxygen under ordinary conditions there is no phosphorescence, but if the gas be heated to 25° glowing occurs, as is also the case if the pressure be diminished or the gas diluted. It is also remarkable that many gases and vapours, e.g. Cl, Br, I, NH₃, N₂O, NO₂, H₂S, SO₂, CS₂, CH₄, C₂H₄, inhibit the phosphorescence.

The theory of this action is not settled. It is certain that the formation of hydrogen peroxide and ozone accompany the glowing, and in 1848 Schonbein tried to demonstrate that it depended on the ozone. E. Jungfleisch (Comptes rendus, 1905, 140, p. 444) suggested that it is due to the combustion of an oxide more volatile than phosphorus, a view which appears to be supported by the observations of Scharff (Zeit. physik. Chem., 1908, 62, p. 178) and of L. and E. Bloch (Comptes rendus, 1908, 147, p. 842).

The element combines directly with the halogens, sulphur and selenium, and most of the metals burn in its vapour forming phosphides. When finely divided it decomposes water giving hydrogen phosphide; it also reduces sulphurous and sulphuric acids, and when boiled with water gives phosphine and hypophosphorous acid; when slowly oxidized under water it yields hypophosphoric acid.

Allatropio Phosphorus.—Several allotropic forms of phosphorus have been described, and in recent years much work has been done towards settling their identities. When the ordinary form immersed in water is exposed to light, it gradually loses its transparency and becomes coated with a thin film. This substance was regarded as an allotrope, but since it is not produced in non-aerated water it is probably an oxide. More important is the so-called “ red phosphorus,” which is produced by heating yellow phosphorus to about 230° for 24 hours in an inert atmosphere, or in closed vessels to 300°, when the change is effected in a few minutes. E. Kopp in 1844 and B. C. Brodie in 1853 showed that a trace of iodine also expedited the change. The same form is also produced by submitting ordinary phosphorus to the silent electric discharge, to sunlight or the ultraviolet light. Since this form does not inflame until heated to above 350°, it is manufactured in large quantities for consumption in the match industry. The process consists in heating yellow phosphorus in iron pots provided with air-tight lids, which, however, bear a long pipe open to the air. A small quantity of the phosphorus combines with the oxygen in the vessel, and after this the operation is practically conducted in an atmosphere of nitrogen with the additional safety from any risk of explosion. The product is ground under water, and any unchanged yellow form is eliminated by boiling with caustic soda, the product being then washed and dried and finally packed in tin boxes. The red variety is remarkably different from the yellow. It is a dark red microcrystalline powder, insoluble in carbon bisulphide, oil of turpentine, &c, and having a density of 2·2, It is stable to air and light, and does not combine with oxygen until heated to above 350° in air or 260° in oxygen, forming the pentoxide. It is also non-poisonous. When heated in a vacuum to 530° it sublimes, and on condensation forms microscopic needles.

Hittorf’s phosphorus is another crystalline allotrope formed by heating phosphorus with lead in a sealed tube to redness, and removing the lead by boiling the product with nitric and 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. Elektrochem, 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., 1835 [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 heating 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.  . 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 dialkyl 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:—

	Primary.	Secondary.	Tertiary.
Methyl Ethyl Isopropyl Isobutyl Isoamyl	−14° C. +25° C.  41° C.  62° C. 107° C.	25° C. 85° C. 118° C. 153° C. 210–215° C.	40–42° C.   128° C.   — 215° C.   300° C.(?)

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 other oxides, when the element is burnt in a limited supply of air or in pure oxygen under reduced pressure (E. Jungfleisch, Abs. Jour. Chem. Soc., 1907, ii. 761), and also when a solution of phosphorus in the trichloride or tribromide is exposed to light. It is a yellow or red powder which becomes dark red on heating; it is stable in air and can be heated to 300° without decomposition. Its existence, however, has been denied by A. Stock (Abs. Jour. Chem. Soc., 1910, ii. 121). The oxide P₂O was obtained by Besson (Comptes rendus, 1897, 124, p. 763; 1901, pp. 132, 1556) by heating a mixture of phosphonium bromide and phosphorus oxychloride in sealed tubes to 50°.

Phosphorus oxide, P₂O₆, discovered by Sage in 1777, is a product of the limited combustion of phosphorus in air. It may be conveniently prepared by passing a rapid current of air over burning phosphorus contained in a combustion tube, and condensing the product in a metal condenser, from which it may be removed by heating the condenser to 50°–60° (Thorpe and Tutton, Jour. Chem. Soc., 1890, pp. 545, 632; 1891, p. 1019). Jungfleisch has obtained it by carrying out the combustion with oxygen under reduced pressure, or diluted with an inert gas. It forms crystals, apparently monoclinic, which melt at 22·5° to a clear, colourless, mobile liquid of boiling-point 173·1°. Its specific gravity is 2·135 at 21°. Vapour density and cryoscopic determinations point to the double formula, P₄O₆. It is comparatively stable up to 200°, but when heated in a sealed tube to 430° it gives phosphorus and the tetroxide P₂O₅. It is unaffected by light when pure, but if phosphorus be present, even in minute quantity, it turns yellow and ultimately dark red. It oxidizes on exposure to air to the pentoxide, and with a brilliant inflammation when thrown into oxygen at 50°–60°. It slowly reacts with cold water to form phosphorous acid; but with hot water it is energetically decomposed, giving much red phosphorus or the suboxide being formed with an explosive evolution of spontaneously inflammable phosphoretted hydrogen; phosphoric acid is also formed. With dilute alkalis phosphites are slowly formed, but with concentrated solutions the decomposition follows the same course as with hot water. With chlorine it gives phosphoryl and “metaphosphoryl” chlorides, the action being accompanied with a greenish flame; bromine gives phosphorus pentabromide and pentoxide which interact to give phosphoryl and “metaphosphoryl” bromides; iodine gives phosphorus di-iodide, P₂I₄, and pentoxide, P₂O₅; whilst hydrochloric acid gives phosphorus trichloride and phosphorous acid, which interact to form free phosphorus, phosphoric acid and hydrochloric acid. It combines violently with sulphur at 160° to form phosphorus sulphoxide, P₄O₆S₄, which forms highly lustrous tetragonal plates (after sublimation), melting at 102° and boiling at 295°, it is decomposed by water into sulphuretted hydrogen and metaphosphoric acid, the latter changing on standing into orthophosphoric acid. Sulphur trioxide and sulphuric acid oxidize phosphorus oxide, giving the pentoxide and sulphur dioxide, whilst sulphur chloride, S₂Cl₂, gives phosphoryl and thiophosphoryl chlorides, free sulphur and sulphur dioxide. Ammonia also reacts immediately. giving phosphorus diamide, P(OH)(NH₂)₂, and the corresponding ammonium salt. Phosphorous oxide is very poisonous, and is responsible for the caries set up in the jaws of those employed in the phosphorus industries (see below). It is probable, however, that pure phosphorous oxide vapour is odourless, and the odour of phosphorus as ordinarily perceived is that of a mixture of the oxide with ozone.

Phosphorus tetroxide, P₂O₄, was obtained by Thorpe and Tutton by heating the product of the limited combustion of phosphorus in vacuo as a sublimate of transparent, highly lustrous, orthorhombic crystals. They are highly deliquescent, and form with water a mixture of phosphorous and phosphoric acids: P₂O₄+3H₂O＝H₃PO₃+H₂PO₄. The vapour density at about 1400° is 230, i.e. slightly less than that required by P₈O₁₆ (West, Jour. Chem. Soc., 1902, p. 923).

Phosphorus oxide, or phosphorus pentoxide, P₄O₁₀, formed when phosphorus is burned in an excess of air or oxygen, or from dry phosphorus and oxygen at atmospheric pressure (Jungfleisch, loc. cit.), was examined by Boyle and named “flowers of phosphorus” by Marggraf in 1740. It is a soft, flocculent powder, which on sublimation forms transparent, monoclinic crystals. It is extremely deliquescent, hissing when thrown into water, with which it combines to form phosphoric acid. It is reduced when heated with carbon to phosphorus, carbon monoxide being formed simultaneously. Its vapour density at 1400° points to the double formula (West, Jour. Chem. Soc., 1896, p. 154).

Oxyacids.—Phosphorus forms several oxyacids: hypophosphorous acid, H₃PO₂, and hypophosphoric acid, H₄P₂O₆ or H₂PO₃, of which the anhydrides are unknown; phosphorous acid, H₃PO₃, derived from P₄O₆; monoperphosphoric acid, H₃PO₅; perphosphoric acid, H₂P₄O₈; and meta-, pyro-, and ortho-phosphoric acids, derived from P₄O₁₀, for which see Phosphates.

Hypophosphorous acid, HP(OH)₂, discovered by Dulong in 1816, and obtained crystalline by Thomson in 1874 (Ber., 7, p. 994), is prepared in the form of its barium salt by warming phosphorus with baryta water, removing the excess of baryta by carbon dioxide, and crystallizing the filtrate. The acid may be prepared by evaporating in a vacuum the solution obtained by decomposing the barium salt with the equivalent amount of sulphuric acid. The acid forms a white crystalline mass, melting at 17·4° and having a strong acid reaction. Exposure to air gives phosphorous and phosphoric acids, and on heating it gives phosphine and phosphoric acid. A characteristic reaction is the formation of a red precipitate of cuprous hydride, Cu₂H₂, when heated with copper sulphate solution to 60°. It is a monobasic acid forming salts which are permanent in air, but which are gradually oxidized in aqueous solution. On heating they yield phosphine and leave a residue of pyrophosphate, or a mixture of meta- and pyrophosphates, with a little phosphorus. They react as reducing agents. On boiling with caustic potash they evolve hydrogen, yielding a phosphate.

Phosphorous acid, P(OH)₃, discovered by Davy in 1812, may be obtained by dissolving its anhydride, P₄O₆, in cold water; by immersing sticks of phosphorus in a solution of copper sulphate contained in a well-closed flask, filtering from the copper sulphide and precipitating the sulphuric acid simultaneously formed by baryta water, and concentrating the solution in vacuo; or by passing chlorine into melted phosphorus covered with water, the first formed phosphorus trichloride being decomposed by the water into phosphorus and hydrochloric acids. It may also be prepared by leading a current of dry air into phosphorus trichloride at 60° and passing the vapours into water at 0°, the crystals thus formed being drained, washed with ice-cold water and dried in a vacuum. The crystals melt at 70°. The acid is very deliquescent, and oxidizes on exposure to air to phosphoric acid. It decomposes on heating into phosphine and phosphoric acid. It is an energetic reducing agent; for example, when boiled with copper sulphate metallic copper is precipitated and hydrogen evolved. Although nominally tribasic the commonest metallic salts are dibasic. Organic ethers, however, are known in which one, two and three of the hydrogen atoms are substituted (Michaelis and Becker, Ber., 1897, 30, p. 1003). The metallic phosphites are stable both dry and in solution; when strongly heated they evolve hydrogen and yield a pyrophosphate, or, especially with the heavy metals, they give hydrogen and a mixture of phosphide and pyrophosphate.

Hypophosphoric acid, H₄P₂O₆; or H₂PO₃, discovered by Salzer in 1877 among the oxidation products of phosphorus by moist air, may be prepared by oxidizing phosphorus in an aqueous solution of copper nitrate, or by oxidizing sticks of phosphorus under water, neutralizing with sodium carbonate, forming the lead salt and decomposing this with sulphuretted hydrogen (J. Cavalier and E. Cornee, Abs. Jour. Chem. Soc., 1910, ii. 31). The aqueous solution may be boiled without decomposition, but on concentration it yields phosphorous and phosphoric acids. Deliquescent, rectangular tablets of H₄P₂O₆·2H₂O separate out on concentrating a solution in a vacuum, which on drying further give the acid, which melts at 55°, and decomposes suddenly when heated to 70° into phosphorous and metaphosphoric acids with a certain amount of hydrogen phosphide. The solution is stable to oxidizing agents such as dilute hydrogen peroxide and chlorine, but is oxidized by potassium permanganate to phosphoric acid; it does not reduce salts of the heavy metals. With silver nitrate it gives a white precipitate, Ag₄P₂O₆. The sodium salt, Na₄P₂O₆·10H₂O, forms monoclinic prisms and in solution is strongly alkaline; the acid salt, Na₃HP₂O₆·9H₂O, forms monoclinic tablets. The formula of the acid is not quite definite. Cryoscopic measurements on the sodium salt points to the double formula, but the organic esters appear to be derived from H₂PO₃, (see A. Rosenheim and M. Pritze, Ber., 1908, 41, 2708; E. Cornee, Abs. Jour. Chem. Soc., 1910, ii. 121).

Monoperphosphoric and perphosphoric acids, H₂PO₅ and H₄P₂O₈, were obtained by J. Schmidlin and P. Massini (Ber., 1910, 43, 1162). The first is formed when 30% hydrogen peroxide reacts with phosphorus pentoxide or meta- or pyrophosphoric acids at low temperatures and the mixture diluted with ice-cold water. The solution is strongly oxidizing, even converting manganous salts to permanganates in the cold, a property not possessed by monopersulphuric acid. Perphosphoric acid is formed when pyrophosphoric acid is treated with a large excess of hydrogen peroxide.

Halogen Compounds.—Phosphorus trifluoride, PF₃, discovered by Davy, may be obtained mixed with the pentafluoride; by direct combination of its elements; from the tribromide and arsenic trifluoride (MacIvor); from the tribromide and zinc fluoride, and from dried copper phosphide and lead fluoride (H. Moissan). It is a colourless, non-fuming gas, which gives a colourless, mobile liquid at −10° and 20 atmospheres; the liquid boils at −95° and solidifies at 160° (Moissan, Comptes rendus, 1904, 138, p. 789). It does not burn in air, but explodes, under the action of a flame or the electric spark, when mixed with half its volume of oxygen, giving the oxyfluoride, POF₃. It is slowly decomposed by water giving hydrofluoric and phosphorous acids, or, in addition, fluorphosphorous acid, HPF₄. It has no action on glass in the cold, but when heated it gives phosphorus and silicon tetrafluoride. Phosphorus pentafluoride, PF₅, discovered by Thorpe (Proc. Roy. Soc., 1877, 25, p. 122), may be obtained by burning the trifluoride in fluorine, from the pentachloride and arsenic trifluoride and from the trifluoride and bromine, the first formed fluorobromide, PF₃Br₂, decomposing into the pentabromide and pentafluoride: 5PF₃Br₂＝3PF₅+2PBr₅. It is a colourless gas 41/2 times heavier than air, and liquefies at 15° under 40 atmospheres, solidifying when the pressure is diminished. It is incombustible and extinguishes flame. It fumes in moist air and is quickly decomposed by water giving hydrofluoric and phosphoric 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 2PF₅:5NH₃, and with nitrogen peroxide at −10° in the proportion PF₅:NO₂. Phosphorus trifluorodichloride, PF₃Cl₂, 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, PSF₃, 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 PSF₃+O₂＝PF₃+SO₂, the trifluoride at a higher temperature decomposing according to the equations: 10PF₃+5O₂＝6PF₅+2P₂O₅, 2PF₃+O₂＝2POF₃, the complete reaction tending to the equation: 10PSF₃+15O₂＝6PF₅+2P₂O₅+10SO₂. The gas dissolves in water on shaking; PSF₃+4H₂O＝H₂S+H₃PO₄+3HF, but is more readily taken up by alkaline solutions with the formation of fluoride and thiophosphate: PSF₃+6NaOH＝Na₃PSO₃+3NaF. Heated in a glass tube it gives silicon fluoride, phosphorus and sulphur, PSF₃＝PF₃+S; 4PF₃+3SiO₂＝ 3SiF₄+P₄+3O₂. 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(NH₂)₂SF.

Phosphorus trichloride or phosphorous chloride, PCl₃, 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, PCl₅, and with oxygen when heated phosphoryl chloride, POCl₃. 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:PNH₂, which on elevation of temperature gives P₂(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, P₂Cl₄, 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, PCl₅, 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 vapourizing in an atmosphere of chlorine. It fumes strongly in moist air, giving hydrochloric acid and phosphoryl chloride, POCl₃; with water it gives phosphoric and hydrochloric acids.

Phosphoryl trichloride or phosphorus oxychloride, POCl₃, corresponding to phosphoric acid, (HO)₃PO, 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: 2P₂O₅+3HCl＝POCl₃+3HPO₃; or by heating the pentachloride and pentoxide under pressure. 3PCl₅+P₂O₅＝5POCl₃ 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, C₂H₅·H₂PO₄, whilst absolute alcohol gives triethyl phosphate, (C₂H₅)₃PO₄. Pyrophosphoryl chloride, P₂O₃Cl₄, 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, PSCl₃, 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 pentachloride, and by dissolving phosphorus in sulphur chloride and distilling the solution 2P+3S₂Cl₂＝ 4S+2PSCl₃. It is a colourless mobile liquid, boiling at 125·1° and having a pungent, slightly 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, PBr₃, 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, PBr₅, which results from phosphorus and an excess of bromine, is a yellow solid, and closely resembles the pentachloride. The bromochloride, PCl₃Br₂, is an orange-coloured solid formed from bromine and the trichloride, into which components it decomposes at 35°. Phosphoryl tribromide, POBr₃, is a solid, melting at 45° and boiling at 195°. Thiophosphoryl bromide, PSBr₃, obtained after the manner of the corresponding chloride, forms yellow octahedra 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, (PBr₂S)₂S, and metaphosphoryl thiobromide, PS₂Br, are also known.

Phosphorus forms three iodides. The subiodide, P₄I, 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 P₄(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, P₄S₃, P₄S₇, P₂S₅. 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, P₄S₇, is obtained by heating a mixture of red phosphorus and sulphur in the proportions given by P₄S₇+5% P₄S₃, and crystallizing from carbon disulphide in which P₄S₃ is readily soluble. It forms small, slightly yellow prisms, which melt at 310° and boil at 523°. The third, or pentasulphide, P₂S₅, 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 P₄S₁₀, and the other a lower molecular weight. With liquid ammonia it gives P₂S₅·7NH₃, which is a mixture of ammonium iminotrithiophosphate, P(SNH₄)₃:NH, and ammonium nitrilodithiophosphate, P(SNH₄)₂${\dot {:}}$N. Water converts the former into ammonium thiophosphate, PO(SNH₄)₃·H₂O, whilst the latter heated to 300° in a vacuum gives thiophosphoric nitrile, N⫶P:S (Stock, ibid., 1906, 39, p. 1967).

Thiophosphates result on dissolving the pentasulphide in alkalis. Sodium monothiophosphate, Na₃PSO₃·12H₂O, is obtained by adding one P₂S₅ to six NaOH, adding alcohol, dissolving the precipitate in water and heating to 90°. On cooling the salt separates as white six-sided tablets. Sodium dithiophosphate, Na₃PS₂O₂·11H₂O, 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, PN₂H₄, 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, PN₂H₄, 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 P₃N₃Cl₆, 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., 1893, 15, p. 198; 1894, 16 pp. 123, 154) has described PO(OH)₂NH₂ and PO(OH)(NH₂)₂, whilst the compound PO(OH)NH was obtained by Schiff (Ann., 1857, 103, p. 168) by acting with ammonia on the pentoxide. Numerous other nitrogen compounds have been obtained.

The atomic weight of phosphorus was determined by Berzelius, Pélouze, Jacquelin, Dumas, Schrotter, Brodie and van der Plaats. More recent are the investigations of G. Ter Gazarian (Compt. rend., 1909, 148, p. 1397) on hydrogen phosphide, which gave the value 30.906, and of G. P. Baxter and G. Jones (Journ. Amer. Chem Soc., 1910, 32, p. 298) on silver phosphate, which gave the value 31·04.

Therapeutics.—The phosphorus used in the British pharmacopoeia is obtained from calcium phosphate, and is a waxlike non-metallic substance soluble in oils and luminous in the dark. There are various medicinal preparations. In young animals phosphorus has a remarkable influence on the growth of bone, causing a proliferation of the jelly-like masses and finally a deposit in them of true bony material. Owing to this influence it has been used in rickets and osteomalacia. Its most effective use, however, is as a nerve tonic in paralysis agitans, locomotor ataxia, impotence and nervous exhaustion. In some skin diseases such as psoriasis, chronic eczema and acne indurata, phosphorus is very useful, and cases of diabetes mellitus and lymphadenoma have improved under some of its compounds. The hypophosphites have been recommended in pulmonary affections, being said to act as free phosphorus without being irritant, and the glycero-phosphates are certainly useful to stimulate metabolism. Dilute phosphoric acid is used as a gastric stimulant. It does not resemble phosphorus in its physiological action and cannot be used to replace it.

Toxicology.—Poisonous amounts of phosphorus are frequently taken or administered, criminally or accidentally, it being easily accessible to the public in the form of matches or of vermin pastes. They may have been swallowed several hours before symptoms of acute poisoning show themselves, with nausea and vomiting, and a burning in the oesophagus, stomach and abdomen. The important thing is to prevent the absorption of the poison, so emetics and purgatives should be given at once. Sulphate of copper, in doses of 3 to 5 gr., freely diluted and repeated every few minutes forms the harmless, black phosphide of copper, which is rapidly eliminated by the kidneys. The stomach may be washed out with warm water and then with a 2% solution of permanganate of potash, an enema of the same solution being given. The old French oil of turpentine is the best antidote to use in phosphorus poisoning, delaying the toxic effects, but ordinary oils are not only useless but harmful. When some time has elapsed before treatment and the phosphorus has become absorbed, the organic degenerative changes cannot be easily controlled. For the chronic form of industrial poisoning in the manufacture of lucifer matches—a form of necrosis, known in England as “phossy jaw” and in France as “mal chimique,” a localized inflammatory infection of the periosteum, ending with the death and exfoliation of part of the bone—see Match.