Note 9 — Chapter XIX · PHOSPHORUS AND THE OTHER ELEMENTS OF THE FIFTH GROUP

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.