Chemical elements
  Phosphorus
    Isotopes
    Energy
    Preparation
    Applications
    Physical Properties
    Chemical Properties
      Alkali Phosphides
      Alkaline Earth Phosphides
      Copper Silver and Gold Phosphides
      Zinc Group Phosphides
      Aluminium Phosphide
      Titanium Group Phosphides
      Tin Phosphides
      Lead Phosphides
      Arsenic Phosphides
      Antimony Phosphides
      Bismuth Phosphides
      Chromium Phosphides
      Molybdenum and Tungsten Phosphides
      Manganese Phosphides
      Iron Phosphides
      Cobalt Phosphides
      Phosphonium Chloride
      Phosphonium Bromide
      Phosphonium Iodide
      Hydrogen Phosphides
      Alkylphosphines
      Phosphorus Trifluoride
      Phosphorus Pentafluoride
      Phosphorus Trifluorodichloride
      Phosphorus Trifluorodibromide
      Fluophosphoric Acid
      Phosphorus Dichloride
      Phosphorus Trichloride
      Phosphorus Pentachloride
      Phosphorus Chlorobromides
      Phosphorus Chloroiodides
      Phosphorus Tribromide
      Phosphorus Pentabromide
      Phosphorus Diiodide
      Phosphorus Triiodide
      Phosphorus Oxytrifluoride
      Phosphorus Oxychloride
      Pyrophosphoryl Chloride
      Metaphosphoryl Chloride
      Phosphoryl Monochloride
      Phosphoryl Dichlorobromide
      Phosphoryl Chlorodibromide
      Phosphoryl Tribromide
      Metaphosphoryl Bromide
      Phosphoryl Oxyiodides
      Phosphorus Thiotrifluoride
      Phosphorus Thiotrichloride
      Phosphorus Thiotribromide
      Mixed Phosphorus Thiotrihalides
      Phosphorus Suboxides
      Phosphorus Trioxide
      Phosphorus Dioxide
      Phosphorus Pentoxide
      Hypophosphorous Acid
      Phosphorous Acid
      Meta- and Pyro-phosphorous Acids
      Hypophosphoric Acid
      Tetraphosphorus Trisulphide
      Diphosphorus Trisulphide
      Tetraphosphorus Heptasulphide
      Phosphorus Pentasulphide
      Phosphorus Oxysulphides
      Phosphorus Thiophosphites
      Phosphorus Thiophosphates
      Phosphorus Selenophosphates
      Phosphorus Sulphoselenides
      Diamidophosphorous Acid
      Phosphorus Triamide
      Monamidophosphoric Acid
      Diamidophosphoric Acid
      Triamidophosphoric Acid
      Dimetaphosphimic Acid ≡P=
      Trimetaphosphimic Acid
      Tetrametaphosphimic Acid
      Penta- and Hexametaphosphimic Acid
      Monamidodiphosphoric Acid
      Diamidodiphosphoric Acid
      Triamidodiphosphoric Acid
      Nitrilotrimetaphosphoric acid
      Monothioamidophosphoric Acids
      Thiophosphoryl Nitride
      Di- Tri-imido- and -amido-thiophosphates
      Imidotrithiophosphoric Acid =
      Phosphorus Chloronitrides
      Triphosphonitrilic Chloride
      Tetraphosphonitrilic Chloride
      Pentaphosphonitrilic Chloride
      Hexaphosphonitrilic Chloride
      Heptaphosphonitrilic Chloride
      Triphosphonitrilic Bromide
      Phosphorus Halonitrides
      Phosphorus Nitride
      Phosphine
      Pyrophosphoric Acid
      Phosphoric acids
    Slow Oxidation
    Phosphatic Fertilisers

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


Physical Properties of Phosphonium Iodide

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.

Chemical Reactions of Phosphonium Iodide

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.
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