[9] The periodic law and direct experiment (the molecular weight) show that PH3 is the normal compound of P and H and that it is more simple than PH2 or P2H4, just as methane, CH4, is more simple than ethane, C2H6, whose empirical composition is CH3. The formation of liquid phosphuretted hydrogen may be understood from the law of substitution. The univalent radicle of PH3 is PH2, and if it is combined with H in PH3 it replaces H in liquid phosphuretted hydrogen, which thus gives P2H4. This substance corresponds with free amidogen (hydrazine), N2H4 (Chapter [VI.]) Probably P2H4 is able to combine with HI, and perhaps also with 2HI, or other molecules—that is, to give a substance corresponding to phosphonium iodide.
Phosphonium iodide, PH4I, may be prepared, according to Baeyer, in large quantities in the following manner:—100 parts of phosphorus are dissolved in dry carbon bisulphide in a tubulated retort: when the mixture has cooled, 175 parts of iodide are added little by little, and the carbon bisulphide is then distilled off, this being done towards the end of the operation in a current of dry carbonic anhydride at a moderate temperature. The neck of the retort is then connected with a wide glass tube, and the tubulure with a funnel furnished with a stopcock, and containing 50 parts of water. This water is added drop by drop to the phosphorous iodide, and a violent reaction takes place, with the evolution of hydriodic acid and phosphonium iodide. The latter collects as crystals in the glass tube and the retort itself. It is purified by further distillations; more than 100 parts may be obtained. Baeyer expresses the reaction by the equation P2I + 2H2O = PH4I + PO2; and the compound PO2 may be represented as phosphorous phosphoric anhydride: P2O5 + P2O3 = 4PO2. As a better proportion we may take 400 grams of phosphorus, 680 grams of iodine, and 240 grams of water, and express the formation thus: 13P + 9I + 21H2O = 3H4P2O7 + 7PH4I + 2HI (Chapter XI., Note [77]).
Phosphonium iodide and even phosphine act as reducing agents in solutions of many metallic salts. Cavazzi showed that with a solution of sulphurous anhydride phosphine gives sulphur and phosphoric acid.
[10] The air must first be expelled from the flask by hydrogen, or some other gas which will not support combustion, as otherwise an explosion might take place owing to the spontaneous inflammability of the phosphuretted hydrogen.
The combustion of phosphuretted hydrogen in oxygen also takes place under water when the bubbles of both gases meet, and it is very brilliant. The phosphuretted hydrogen obtained by the action of phosphorus on caustic potash always contains free hydrogen, and often even the greater part of the gas evolved consists of hydrogen.
Pure phosphuretted hydrogen (not containing hydrogen or liquid or solid phosphides) is obtained by the action of a solution of potash on phosphonium iodide: PH4I + KHO = PH3 + KI + H2O (in just the same way as ammonia is liberated from ammonium chloride). The reaction proceeds easily, and the purity of the gas is seen from the fact that it is entirely absorbed by bleaching powder and is not spontaneously inflammable. Its mixture with oxygen explodes when the pressure is diminished (Chapter XVIII., Note [8]). The vapours of bromine, nitric acid, &c., cause it to again acquire the property of inflaming in the air; that is, they partially decompose it, forming the liquid hydride, P2H4. Oppenheim showed that when red phosphorus is heated at 200° with hydrochloric acid in a closed tube it forms the compound PCl3(H3PO3), together with phosphine.
[10 bis] If there be a deficiency of oxygen, phosphorous anhydride P2O3 is formed. It was obtained by Thorpe and Tutton (1890) and is easily volatilised, melts at 22°·5, boils without change (in an atmosphere of N2 or CO2) at 173°, and is therefore easily separated from P2O3, which volatilises with difficulty. The vapour density shows that the molecular weight is double, i.e. P4O6 (like As2O3). Although colourless, phosphorous anhydride (its density in a state of fusion at 24° = 1·936) turns yellow and reddens in sun-light (possibly red phosphorus separates out ?), and decomposes at 400° forming hypophosphorous anhydride P2O4 (Note [11]) and phosphorus. It passes into P2O5 in air and oxygen, and when slightly heated in oxygen becomes luminous, and ultimately takes fire. Cold water slowly transforms P2O3 into phosphoric acid, but hot water gives an explosion and leads to the formation of PH3, (P4O6 + 6H2O = PH3 + 3PH3O4). Alkalis act in the same manner. It takes fire in chlorine and forms POCl3 and PO2Cl, and combines with sulphur at 160°, forming P2S2O3 (the molecular formula is double this) a substance which volatilises in vacuo and is decomposed by water into H2S and phosphoric acid, i.e. it may be regarded as P2O5, in which O2 has been replaced by two atoms of sulphur. Judging from the above, the mixture of P2O3 and P2O5 formed in the combustion of phosphorus in air is transformed into P2O5 in an excess of oxygen.
[11] Salzer proved the existence of hypophosphoric acid (it is also called subphosphoric acid), in which many chemists did not believe. Drawe (1888) and Rammelsberg (1892) investigated its salts. It may be obtained in a free state by the following method. The solution of acid produced by the slow oxidation of moist phosphorus is mixed with a solution (25 p.c.) of sodium acetate. A salt, Na2H2P2O6,6H2O, crystallises out on cooling; it is soluble in 45 parts of water, and gives a precipitate of Pb2P2O6 with lead salts (Ag4P2O6 with salts of silver). The lead salt is decomposed by a current of hydrogen sulphide, when lead sulphide is precipitated, while the solution, evaporated under the receiver of an air-pump, gives crystals of H4P2O6,2H2O, which easily lose water and give H4P2O6. The salts in which the H4 is replaced by Ni2, or NiNa2, or CdNa2, &c., are insoluble in water.
In order to see the relation between phosphoric acid and hypophosphoric acid which does not contain the elements of phosphorous acid (because it does not reduce either gold or mercury from their solutions), but which nevertheless is capable of being oxidised (for example, by potassium permanganate) into phosphoric acid, it is simplest to apply the law of substitution. This clearly indicates the relation between oxalic acid, (COOH)2, and carbonic acid, OH(COOH). The relation between the above acids is exactly the same if we express phosphoric acid as OH(POO2H2), because in this case P2H4O6, or (POO2H2)3, will correspond with it just as oxalic does with carbonic acid. A similar relationship exists between hyposulphuric or dithionic acid, (SO2OH)2, and sulphuric acid, OH(SO2OH), as we shall find in the following chapter. Dithionic acid corresponds with the anhydride S2O5, intermediate between SO2 and SO3; oxalic acid with C2O3, intermediate between CO and CO2; hypophosphoric acid corresponds with the anhydride P2O4, intermediate between P2O3 and P2O5, and the analogue of N2O4.
[12] Besides the hydrates enumerated, a compound, PH3O, should correspond with PH3. This hydrate, which is analogous to hydroxylamine, is not known in a free state, but it is known as triethylphosphine oxide, P(C2H5)3O, which is obtained by the oxidation of triethylphosphine, P(C2H5)3. It must be observed that there may also be lower oxides of phosphorus corresponding with PH3, like N2O and NO, and there are even indications of the formation of such compounds, but the data concerning them cannot be considered as firmly established.