Chemical Properties of Phosphorus

Phosphorus unites directly with many of the more electronegative elements, forming oxides, halides, sulphides and selenides. The conditions of formation are described under the respective sections in this volume. The energies of combination with oxygen and the halogens are great. The oxidation probably always proceeds in stages, as described under "Oxides." The combustion proceeds not only in air and in oxygen, but also in many compounds containing oxygen, such as oxides of nitrogen and sulphur. Halogenation also proceeds in stages in those cases where both a higher and a lower halide are formed. Phosphorus combines with nitrogen only under special conditions, and under the influence of the electric discharge nitrogen is absorbed by phosphorus. A mixture of phosphorus vapour with nitrogen under the influence of the electric discharge forms a solid nitride.

Phosphorus also unites directly with many metals giving phosphides, which are described under the respective metals in the volumes of this Series.

Phosphorus does not, like nitrogen, combine directly with hydrogen. In this respect it resembles the succeeding members of the Group.

Phosphorus is readily oxidised by strong oxidising agents such as nitric acid to phosphorous and phosphoric acids, eventually the latter. Hydrogen peroxide of concentration greater than 6 per cent, reacts violently when warmed with red phosphorus, less violently with white phosphorus, giving phosphine, phosphorous and phosphoric acids.

The reaction of phosphorus with the alkalies is described under "Phosphine." It is really a case of hydrolysis. This can also be effected by boiling water in the presence of certain metallic salts, which probably act by the intermediate formation of phosphides. Superheated steam at 238° to 260° C. and under a pressure of 57 to 360 atmospheres gives phosphine and orthophosphoric acid, thus:—

4P₂ + 12H₂O = 5PH₃ + 3H₃PO₄

The simplest reaction will be represented by the equation

2P + 3H₂O = PH₃ + H₃PO₃

The phosphorous acid then decomposes, as it is known to do, giving more phosphine.

The Action of Phosphorus on Solutions of Metallic Salts.—When white phosphorus is placed in solutions of the salts of the more " noble " metals the metals are deposited and oxy-acids of phosphorus are found in the solution. The ratios of metal deposited to phosphorus oxidised are not constant except under certain carefully regulated conditions. It is stated that in the case of copper sulphate the ratio was 5Cu:2P when only that phosphorus was included which was present as phosphoric acid. The ratio 2Cu:P (total phosphorus) has also been found when air was excluded. The reaction proceeds in stages. When the copper has been completely precipitated from decinormal copper sulphate about 13 per cent, is present as phosphide, but this phosphorus is oxidised later to oxy-acids. During the later part of the reaction the ratio is 2Cu:P, or 4 equivalents of copper are deposited to 1 atom of phosphorus oxidised. The metals which are deposited from neutral or slightly acid solution are those which react with phosphine, i.e. Cu, Hg, Ag, Pd, Pt, Au; those above copper in the electrochemical series are not deposited. In ammoniacal solution deposits are also obtained from salts of Pb, Ni, Tl, Sn, Co, Cd, Zn.

The reaction with silver nitrate has been thoroughly investigated at different stages. The deposit is dark at first, of a bright crystalline appearance later, and finally grey and spongy. The ratio Ag:P may be more than 5:1 at the beginning and 3.6:1 or less at the end of the reaction. During the middle part the ratio is 4:1. Both the phosphorous and phosphoric acids were determined, and it was suggested that these were produced in equimolecular amounts after the first deposition of phosphide had ceased.

The metal may be deposited on a piece of platinum, gold or carbon at some distance from the stick of phosphorus with which this is in contact. The silver phosphide which was first produced had the formula Ag₃P and phosphorous acid was formed simultaneously. The following equations were suggested as representing the various stages:—

2P + 3H₂O = PH₃ + H₃PO₃
PH₃ + 3AgNO₃ = Ag₃P + 3HNO₃
Ag₃P + 5AgNO₃ + 4H₂O = 8Ag + 5HNO₃ + H₃PO₄

These equations perhaps give a general representation of the reactions which occur between phosphorus and solutions containing salts of the "noble" metals.

Red Phosphorus.—The chief chemical properties of red or amorphous phosphorus were determined by the discoverer and other early investigators. As compared with white phosphorus, both red and scarlet phosphorus are relatively inert, except in respect to certain reactions which depend largely on the extent of surface exposed to aqueous reagents.

Red phosphorus does not glow in the air, but shows a faint luminescence in ozone. When heated in the air or moist oxygen it does not ignite below about 260° C., at which temperature the vapour pressure has become appreciable. In warm moist air it is gradually oxidised to phosphorous and phosphoric acids.

It is even more readily oxidised by concentrated nitric acid than is white phosphorus, the product in both cases being a phosphoric acid (q.v.). It is not affected by concentrated sulphuric acid in the cold, but on heating SO₂ is evolved and oxidation of the phosphorus takes place. It is not affected by aqueous alkalies but dissolves in alcoholic potash, giving a deep red solution from which acids reprecipitate the red element containing a suboxide.

It combines with halogens, although not so violently as white phosphorus; with chlorine, either gaseous or in aqueous solution; with bromine in the cold; and with iodine on warming.

Red phosphorus is less soluble than white in all solvents. In water and alcohol it is almost insoluble. It is somewhat soluble in ether and in hot acetic acid, from which it is reprecipitated by water. It is slightly soluble in phosphorus trichloride. These solubilities refer to the ordinary preparation, which, usually contains residual quantities of the white form. Red phosphorus is able to reduce salts, especially those of the " noble " metals, in aqueous solution on boiling. Salts of mercury are reduced to the metal; those of gold and silver give insoluble phosphides; while ferric and stannic salts are reduced to ferrous and stannous respectively.

Scarlet Phosphorus, sometimes called "Schenck's phosphorus," can be prepared by boiling a 10 per cent, solution of phosphorus in phosphorus tribromide. It appears to be an intermediate form between the white and the red. The conditions of its formation and its physical properties, so far as these are known, are more fully described under " Scarlet Phosphorus,".

The chemical properties partly resemble those of white, partly those of red, phosphorus. It does not glow in the air, but does so in ozone. It is rapidly attacked by alkalies, giving hypophosphite and phosphine which is not spontaneously inflammable. It is coloured intensely black by ammonia. It dissolves in aqueous alcoholic potash giving red solutions from which acids precipitate a mixture of phosphorus and solid hydride. It dissolves in phosphorus tribromide to the extent of about 0.5 gram in 100 grams of the solvent at about 200° C. It is said to be non-poisonous; its physiological properties probably resemble those of red phosphorus (q.v.).

An aqueous collosol has been prepared by boiling commercial red phosphorus with water to which has been added stabilising substances such as gelatin, dextrin or sucrose, etc. Contrary to the usual order, the last-named substances seem to have the strongest effect in protecting the phosphorus against coagulation by salts. When an alcoholic solution of white phosphorus is poured into water a colloidal solution is obtained. A colloidal solution in isobutyl alcohol was made by passing arcs between red phosphorus suspended in this medium.

White phosphorus is easily detected by its well-marked property of glowing in the dark as well as by its peculiar smell and reducing properties (q.v.). Smaller quantities may be detected by the well-known Mitscherlich test. The material is boiled with water in a flask furnished with a long glass reflux condenser cooled by air. A luminous band is seen (in the dark) at the point where the steam is condensed. The phosphorus may be distilled with steam and collected under water in small globules. The distillate will reduce ammoniacal silver nitrate and mercuric salts. The presence of phosphorus in the steam may also be demonstrated by allowing the latter to impinge upon a piece of paper wetted with silver nitrate, which is at once blackened. Other vapours and gases (such as AsH₃) which have the same effect are not likely to be produced under the conditions. Traces of white phosphorus in matches may be found by extraction with benzene. Strips of filter paper soaked in this extract, suspended in a glass tube and exposed to a current of air at 40-50° C. become luminescent if 0.01 milligram or more of phosphorus is present.

Phosphorus combined in organic compounds, or as phosphide in metals, is also estimated after oxidation by precipitation as ammonium phosphomolybdate or magnesium ammonium phosphate, with the subsequent treatment. The methods by which the phosphorus is brought into solution vary with the nature of the material which is being analysed. Organic compounds are oxidised in a sealed tube with fuming nitric acid or in a flask by a mixture of concentrated sulphuric and nitric acids.

Alloys of copper and tin such as the phosphor bronzes are dissolved in nitric acid of density 1.5, and the metastannic acid, which contains all the phosphoric oxide, after ignition and weighing is fused with KCN. The aqueous solution of the melt is freed from tin globules by filtration and from traces of soluble copper and tin by H₂S, then containing all the phosphorus as potassium phosphate, which is determined as described below.

Iron and steel are dissolved in 1:1 nitric acid, the solution evaporated to dryness and the residue taken up with concentrated hydrochloric acid until all the silica is rendered insoluble. The solution containing the phosphoric and hydrochloric acids is evaporated to dryness once more to get rid of the latter acid, and the residue then taken up with nitric acid and ammonium nitrate solution and precipitated with ammonium molybdate.

Small quantities of phosphorus may be estimated quickly by the molybdate method, the amount of phosphomolybdate being estimated colorimetrically by comparison in Nessler glasses or test-tubes with a standard prepared under conditions which are made identical as far as possible.

Binary compounds of the metals and the less electronegative non- metals with phosphorus are made by methods which recall those employed in the preparation of nitrides. The most important of these methods may be classified as follows:-

1. Direct union of the element with phosphorus under various conditions. The metal may be heated with red phosphorus in an indifferent gas, or the vapour of phosphorus may be passed over the heated metal. By this means phosphides of the alkali metals, of the alkaline earth metals, of many ferrous and non-ferrous base metals, as well as of the " noble " metals such as gold and platinum, may be prepared.
2. Reduction of phosphates with carbon at a high temperature. This method is chiefly applicable to the phosphates of the alkaline earth metals. The action of phosphine or phosphorus in liquid ammonia upon a solution of an alkali metal in liquid ammonia.
3. The action of phosphine on aqueous solutions of metallic salts or passage over the dry salts.
4. Heating the metals in the vapour of phosphorus trifluoride or trichloride.

Although the metallic phosphides are described under the respective metals in the appropriate Volumes of this Series, a selection will also be brought under review here, since they illustrate the reactivity of phosphorus and of phosphine towards elements of the different groups.

Phosphine, like ammonia, has a much lower affinity for water than it has for the halogen hydracids, no doubt on account of the fact that water gives a very low concentration of hydrogen ion and the halogen hydracids in water high concentrations of hydrogen ion (Werner), the hydrates in both cases being less stable than the hydrohalides. The formation of hydroxides and hydrohalides consists partly in the addition of hydrogen ion to the anhydro-base. This process takes place to a much greater extent with ammonia than with phosphine. Indeed phosphine does not appear to form a hydroxide at all with water, but simply dissolves as an indifferent gas. The case is different with the halogen hydracids; these do form phosphonium salts of varying stability, the iodide being the most stable.

Phosphorus reacts energetically with iodine when heated in contact with it, or in dry organic solvents, giving orange to red crystalline products. Two of these, the diiodide and the triiodide, have been prepared by various reactions, and their properties well ascertained.

Oxyfluorides are the most stable of the oxyhalides, and the stability decreases with increasing atomic weight of the halogen. Special methods are used in the preparation of oxyfluorides, while the other oxyhalides can be prepared by general reactions, such as the partial hydrolysis of the pentahalides, or the oxidation of the trihalides. The compounds fume in the air and readily undergo further hydrolysis giving hydrogen halides and oxyacids of phosphorus.

Phosphorus forms two well-defined oxides, namely, diphosphorus trioxide and diphosphorus pentoxide, having the empirical formulae P₂O₃ and P₂O₅ respectively; an intermediate oxide, diphosphorus tetroxide, P₂O₄, and possibly the suboxides P₄O and P₂O, are known.

The numerous oxyacids of phosphorus may be regarded as derived from three prototypes, namely, hypophosphorous acid, H₃PO₂, phosphorous acid, H₃PO₃, and orthophosphoric acid, H₃PO₄, in all of which phosphorus probably has the co-ordination number 4.

In the hypophosphorous and phosphorous series the phosphorus undoubtedly is in a lower state of oxidation, and may be wholly in the trivalent state, which corresponds to a symmetrical structure of the molecules. More probably, however, these acids consist of a mixture containing the more symmetrical molecules in tautomeric equilibrium with less symmetrical molecules which contain hydrogen directly united to phosphorus. In either form the unsaturated acids and their salts are powerful reducing agents and are easily oxidised to the stable phosphate series.

Phosphorus combines directly with sulphur in various proportions to give sulphides, the formulae of some of which resemble those of the oxides. It also gives oxysulphides, thiophosphites, thiophosphates and the corresponding acids. The latter salts may be made by the action of alkalies or alkali sulphides on phosphorus sulphides.

Historical

It was recognised early that phosphorus combines violently with sulphur when the two are heated together to a sufficiently high temperature, and various products were examined, some of which afterwards proved to be compounds and others mixtures. Among the earliest products to be prepared and analysed were P₄S and P₄S₃. The substance P₄S was shown later to be merely a solid solution of the two elements, while the latter, P₄S₃, is one of the best-known compounds and is prepared in large quantities for use in the match industry (q.v.).

Physical Mixtures

When sulphur and phosphorus are melted together at temperatures below 100° C. each lowers the melting-point of the other but there is no sign of combination. The eutectic mixture solidified at +9.8° C. and contained 22.8 per cent, of sulphur. The mixed crystals deposited on the sulphur side of the eutectic were isomorphous with the octahedral form of sulphur up to a maximum of about 20 per cent, of phosphorus, while the crystals on the phosphorus side were isomorphous with phosphorus up to a maximum of about 5 per cent, of sulphur. On distillation at low temperatures (under reduced pressures) the products behaved as mixtures; all the phosphorus distilled away and the sulphur was left.

The System Phosphorus-Sulphur and Compounds

The two elements mixed in various proportions were fused in sealed tubes at about 200° C. The solids so formed were heated and the temperatures determined at which complete liquefaction took place. These temperatures are the initial freezing-points of liquid at that temperature in equilibrium with the solid phases.

By this method points on the temperature-composition curves were obtained corresponding to the compounds P₄S₃, P₂S₃, P₂S₅ and perhaps P₃S₆, P₄S₇, PS₆

Freezing-points and compositions of the system phosphorus-sulphur [The melting-points of compounds and eutectics are printed in heavy- type, the corresponding compositions of the liquid phases in italics. Solid phases of uncertain composition are enclosed in brackets.]

Per cent. Sulphur.	Solid Phase.	Freezing- point, °C.
0.0	P	+44
6.0	P	27
10.0	P	20
12.0	P	13
16.0	P	+3
20.0	P+(P2S)	-7
24.0	(P2S)	+5
26.0	(P2S)	11
30.0	(P2S)	24
34.0	(P2S)	38
36.0	(P2S)+P4S3	44
38.0	P4S3	86
40.0	P4S3	122
41.0	P4S3	146
43.6	P4S3	167
45.0	P4S3	154
50.0	P4S3+P2S3	46
55.0	P2S3	230
60.8	P2S3	296
67.5	P2S3+P2S5	230
72.1	P2S5	272
75.0	P2S5+(PS6)	243
80.0	(PS6)	300
86.1	(PS6)	314
90.0	(PS6)	308
95.0	(PS6)	260
100.0	S	115.2

Other compounds not included in this set of experiments have been described, e.g. P₄S₇ (m.pt. 303° C.), P₃S₆ (m.pt. 298° C.),3 and P₄S₈ (m.pt. 311° C.).

A metastable series between phosphorus and P₄S₃ has also been reported, with a eutectic at -40° C. and 33.5 per cent, sulphur, but the latter point may be due to supercooling.

Uses of the Sulphides of Phosphorus

The pentasulphide of phosphorus is used to replace the oxygen of organic compounds by sulphur; thus ethyl alcohol gives ethyl mercaptan, and acetic acid thioacetic acid. The reactions, however, are somewhat complex; thus with ethyl alcohol the first product has been shown to be diethyl-dithiophosphate, SP(SH)(OEt)₂, the mercaptan being produced by a secondary reaction. Phosphorus pentasulphide, boiling under atmospheric or other definite pressure, has been recommended for use in constant temperature baths in place of sulphur. The compound P₄S₃, which is one of the most stable sulphides in dry air, but resembles phosphorus in some respects, is used as a substitute for this element in the manufacture of matches.

When red phosphorus is melted with selenium in a current of carbon dioxide a reaction is said to occur without notable loss of weight. The products are sensitive to moist air, phosphine and seleniuretted hydrogen being evolved. The action of concentrated alkalies or alkali selenides gives selenophosphates (q.v.).

The solution of selenium in yellow phosphorus is also extremely sensitive to moisture, and quantities of phosphine and hydrogen selenide are evolved during the preparation, unless the selenium is dried at a temperature which is high enough to convert the red partly into the black modification. The melting-point of the phosphorus is greatly lowered; the solution containing phosphorus 4.4 parts and selenium 3.0 parts melts at -7° C. On distillation of such solutions, approximating to P₄Se and P₂Se, phosphorus passes over with only traces of selenium. The residue, or other mixtures containing more selenium, when distilled in a current of carbon dioxide at a higher temperature gave a distillate of oily drops which solidified to a red mass which had a composition closely approximating to P₄Se₃. The second residue, a black vitreous mass, distilled at a red heat and had approximately the composition P₂Se₅.

A compound P₄Se₃ has also been prepared by warming 5.6 grams of powdered selenium with 2.8 grams of yellow phosphorus in 30 c.c. of tetralin (tetrahydronaphthalene). After the reaction has begun the mixture is boiled for a long time in an atmosphere of carbon dioxide. The liquid is decanted and immediately deposits an orange substance, sometimes in crystalline form (needles). Further quantities extracted with boiling tetralin and washed with alcohol increase the yield to 3.5 grams. The substance may be purified by extraction with a 1:1 mixture of carbon disulphide and petroleum ether. A crystalline deposit forms in the extraction flask. The substance melts at 242° C. to a dark red liquid with the formation of a slight sublimate. It is inflammable, and slightly decomposed by boiling water with evolution of the hydrides of selenium and phosphorus. It is oxidised powerfully by cold nitric acid. The resulting solution was used for analysis, which gave results agreeing fairly well with the formula P₄Se₃ and also with the analysis of Meyer, being about 1 per cent, high in phosphorus and 1 per cent, low in selenium.

Amido-phosphorous and -phosphoric acids are prepared by the action of dry ammonia on the anhydrous oxides or oxyhalides such as phosphoryl chlorides. The imido-derivatives, in which =NH replaces =O, can often be obtained from the amido-derivatives by heating. Other methods are the hydrolysis of amido-esters and of phosphorus chloronitrides (q.v.).