Phosphonium Iodide, PH₄I

Phosphonium Iodide, PH₄I, is the most stable of these compounds, being formed by direct union of the gases at ordinary temperatures and atmospheric pressure with the production of a crystalline compound. It may also be made by a considerable number of reactions between phosphorus, iodine and water. These substances may be heated together in a retort:—

2P + I₂ + 4H₂O = PH₄I + H₃PO₄ + HI

Or the water may be allowed to drop on to phosphorus triiodide or on to a mixture of phosphorus and iodine. The preparation of a small quantity is most conveniently effected as follows:—

10 grams of phosphorus are placed in a retort with a wide tubulure through which is passed a tap-funnel and a delivery tube connected with a source of dry carbon dioxide. An equal weight of carbon disulphide is added, and then 17 grams of iodine. The carbon disulphide is then distilled off in a current of carbon dioxide by placing the retort in a basin of warm water. The mouth of the retort is then connected with a wide tube which fits into the mouth of a wide-mouthed bottle, which is also connected with a draught exit to draw off the uncombined hydrogen iodide. 8.5 grams of water are then placed in the tap- funnel and allowed to drop slowly on to the phosphorus and iodine. The phosphonium iodide which sublimes into the wide tube is afterwards pushed into the bottle—

5I + 9P + 16H₂O = 5PH₄I + 4H₃PO₄

The crystals appear to belong to the cubic system and were so described by the earlier investigators. They are really, however, tetragonal, the ratio of the longer axis to the shorter axis being 1:0.729. X-ray photographs showed that the dimensions of the unit cell were 6.34, 6.34, 4.62 Å, and that the space-lattice was very similar to that of the form of ammonium chloride which is stable at low temperatures. The density of the solid is 2.860. The heat of formation of the solid from gaseous phosphine and hydrogen iodide is +24.17 Cals., and the heat of decomposition by water giving gaseous phosphine and a solution of HI is +4.77 Cals.

Phosphonium iodide is hydrolysed by water, and is still more rapidly decomposed by alkalies, giving phosphine and an iodide. The phosphine is displaced by ammonia giving ammonium iodide, and even by ethyl alcohol giving ethyl iodide, with phosphine in both cases. It is hardly affected by aqueous acids, except those which are also oxidising agents.

As might be expected, phosphonium iodide acts as a reducing agent in most cases, although it is not oxidised by oxygen or air at ordinary temperatures. Chlorine water gives phosphorus and P₂H, and chlorine has gives similar products at first, while an excess gives PCl₅ even at low temperatures. Chloric, bromic and iodic acids and their salts, silver nitrate and nitric acid oxidise it with inflammation. Carbon disulphide at temperatures above 140° C. is said to give P(CH₃)₃HI.