Phosphorus triiodide

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Phosphorus triiodide
IUPAC names
Phosphorus triiodide
Phosphorus(III) iodide
Other names
Phosphorus iodide
Jmol-3D images Image
Molar mass 411.68717 g/mol
Appearance Dark red solid
Odor Acrid (moist air)
Density 4.18 g/cm3
Melting point 61.2 °C (142.2 °F; 334.3 K)
Boiling point 200 °C (392 °F; 473 K) (decomposes)
Solubility Reacts with alcohols
Soluble in benzene, carbon disulfide, hexane, 1,2-dichloroethane
Poorly soluble in acetonitrile, sulfur dioxide
Insoluble in dichloromethane
Vapor pressure ~0 mmHg
192 J·K−1·mol−1
-45.6 kJ/mol
Safety data sheet Guidechem
Flash point Non-flammable
Related compounds
Related compounds
Phosphorus trichloride
Phosphorus tribromide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Phosphorus triiodide is chemical compound, a red solid which reacts violently with water, releasing phosphorous acid and hydroiodic acid. It is a powerful reducing agent, with the chemical formula PI3.



Phosphorus triiodide readily hydrolyzes in water, producing phosphorous acid (H3PO3) and hydroiodic acid (HI), along with smaller amounts of phosphine and traces of diphosphanes.

PI3 + 3 H2O → 3 HI + H3PO3

Phosphorus triiodide will react with alcohols to form alkyl iodides:

3 R-OH + PI3 → 3 RI + H3PO3

PI3 is also an exceptionally powerful reducing agent and deoxygenating agent, capable of rapidly reducing sulfoxides to sulfides, even at temperatures as low as −78 °C.[1]

Heating a 1-iodobutane solution of PI3 with red phosphorus causes reduction to P2I4.


Phosphorus triiodide is an unstable dark red solid, with a melting point of 61.2 °C and decomposes when heated to 200 °C. It is soluble in benzene, carbon disulfide and fairly soluble in hexane. Its density is 4.18 g/cm3 at standard conditions.


Phosphorus triiodide is sold by various chemical entities, though it's almost impossible for the amateur chemist to purchase it.


Although it's made from two DEA List I chemicals (phosphorus and iodine) and upon standing in air/hydrolysis releases another List I chemical (hydroiodic acid, as well as elemental iodine), phosphorus triiodide is curiously not listed in the DEA List of chemicals. However, PI3's status is covered by the same legislation that covers phosphorus halides and individuals normally cannot purchase it.


Phosphorus triiodide can be made by reacting elemental phosphorus (red preferably) with iodine, in a P:I ratio of 1:3.

2 P + 3 I2 → 2 PI3

The reaction best takes place in a solvent, such as carbon disulfide, carbon tetrachloride, dichloroethane. Since the reaction is exothermic, cooling is required to keep it under control. Due to its low boiling point, carbon disulfide is a good choice, and crystallizing the resulting PI3 from the solution is safe and gives good yield. If the reaction is done without a solvent, the resulting product is impure and the yield is poor.

Another route involves the reaction of phosphorus trichloride with hydrogen iodide:

PCl3 + 3 KI → PI3 + 3 KCl3

This reaction takes place in glacial acetic acid.[2][3]


  • Make alkyl iodides
  • Diphosphorus tetraiodide synthesis
  • Organic reductions



Phosphorus triiodide reacts with water to release hydroiodic acid and phosphorous acid, which are corrosive and harmful. Direct contact with skin will cause chemical burns. The reaction will also release small amounts of phosphine which is highly toxic, as well as diphosphanes which can be pyrophoric.


Phosphorus triiodide can be stored in glass air-tight containers, though it's not recommended to be stored for long periods of time and used as soon as it's made.


Phosphorus triiodide can be neutralized with a solution of sodium thiosulfate. This should be done slowly, since PI3 reacts exothermically with water.


  1. J. N. Denis; A. Krief (1980). "Phosphorus tri-iodide (PI3), a powerful deoxygenating agent". J. Chem. Soc., Chem. Commun. (12): 544–5
  2. Ritter, H.; Liebigs Annalen der Chemie; vol. 95; (1855); p. 208 - 211
  3. Germann, F. E. E.; Traxler, R. N.; Journal of the American Chemical Society; vol. 49; (1927); p. 307 - 312

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