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

Phosphorus Triiodide, PI3






When solutions of iodine and phosphorus in CS2, in the proportions of 3I to P, are mixed, a red colour appears and on cooling dark red needles separate.

Other methods can be used. The action of hydrogen iodide on phosphorus trichloride either alone or in CCl4 solution, or HI on POCl3, also yields the triiodide. Or, PI3 can be separated from the products formed when PCl5 is heated with PH4I:—

3PCl5 + 3PH4I = PI3 + PCl3 + 12HCl + 4P


Properties

Phosphorus triiodide forms deep red tabular crystals belonging to the hexagonal system, which melt at 55° to 61° C. The formula has been established by analysis and by the vapour density, namely, 14.3 to 14.6 (air = l) at 250° C. and reduced pressure. The heat of formation is slightly greater than that of the diiodide, thus:—

P + 1½I2 (solid) = PI3 (solid) + 10.5 Cals.

The surface tension of the liquid is 56.5 dynes/cm. at 75.3° C. The dielectric constant is for the solid 3.66, for the liquid 4.12.

Chemical Reactions

The iodine can be replaced by chlorine, using either the gas itself or chlorides, such as those of mercuric mercury, arsenic, antimony and tin. A sulphoiodide is formed when the triiodide is heated with the trisulphide:—

2P2S3 + 2PI3 = 3P2S2I2

and the same compound may be produced by the action of hydrogen sulphide on the liquid triiodide:—

2PI3 + 2H2S = 4HI + P2S2I2

Hydrolysis proceeded in the usual manner giving hydrogen iodide and phosphorous acid, but a solid product was formed at the same time. The reaction with ethyl alcohol gave ethyl phosphorous acid as well as ethyl iodide:—

PI3 + 3C2H5OH = 2C2H5I + OP(C2H5)(OH)2 + HI

Like the other halides it reacted with anhydrous ammonia. If this was in the liquid form and below -65° C. an amide was formed:—

15NH3 + PI3 = P(NH2)3 + 3NH4(NH3)3I
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