Arsenic triiodide

  • Formula: AsI3
  • Hill system formula: As1I3
  • CAS registry number: [7784-45-4]
  • Formula weight: 455.635
  • Class: iodide
  • Colour: red
  • Appearance: crystalline solid
  • Melting point: 141°C
  • Boiling point: 400°C; 424°C
  • Density: 4730 kg m-3; 4390 kg m-3

The following are some synonyms of arsenic triiodide:

  • arsenic triiodide
  • arsenic(III) iodide
  • arsenic iodide

The oxidation number of arsenic in arsenic triiodide is 3.


AsCl3 + 3KI → AsCl3 + 3KCl

Arsenic(III) triiodide can be made by the direct reaction of the elements, but is also conveniently made by the reaction between arsenic(III) chloride and iodide salts. The arsenic(III) chloride is formed simply enough by dissolving the oxide As2O3 in concentrated hydrochloric acid. This is mixed with a solution of potassium iodide in water. The yield is about 90%. Arsenic(III) triiodide is orange-red and melts at 149°C. Its boiling point is 400°C and it is slightly water-soluble.

Solid state structure

  • Geometry of arsenic:
  • Prototypical structure:

Crystal structure of arsenic triiodide

Element analysis

The table shows element percentages for AsI3 (arsenic triiodide).

Element %
As 16.44
I 83.56

Isotope pattern for AsI3

The chart below shows the calculated isotope pattern for the formula AsI3 with the most intense ion set to 100%.


The data on these compounds pages are assembled and adapted from the primary literature and several other sources including the following.

  • R.T. Sanderson in Chemical Periodicity, Reinhold, New York, USA, 1960.
  • N.N. Greenwood and A. Earnshaw in Chemistry of the Elements, 2nd edition, Butterworth, UK, 1997.
  • F.A. Cotton, G. Wilkinson, C.A. Murillo, and M. Bochmann, in Advanced Inorganic Chemistry, John Wiley & Sons, 1999.
  • A.F. Trotman-Dickenson, (ed.) in Comprehensive Inorganic Chemistry, Pergamon, Oxford, UK, 1973.
  • R.W.G. Wyckoff, in Crystal Structures, volume 1, Interscience, John Wiley & Sons, 1963.
  • A.R.West in Basic solid state chemistry Chemistry, John Wiley & Sons, 1999.
  • A.F. Wells in Structural inorganic chemistry, 4th edition, Oxford, UK, 1975.
  • J.D.H. Donnay, (ed.) in Crystal data determinative tables, ACA monograph number 5, American Crystallographic Association, USA, 1963.
  • D.R. Lide, (ed.) in Chemical Rubber Company handbook of chemistry and physics, CRC Press, Boca Raton, Florida, USA, 77th edition, 1996.
  • J.W. Mellor in A comprehensive treatise on inorganic and theoretical chemistry, volumes 1-16, Longmans, London, UK, 1922-1937.
  • J.E. Macintyre (ed.) in Dictionary of inorganic compounds, volumes 1-3, Chapman & Hall, London, UK, 1992.

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