Arsenic triiodide

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Arsenic triiodide
Unit cell ball and stick model of arsenic triiodide
Preferred IUPAC name
Arsenic triiodide
Systematic IUPAC name
Other names
Arsenic(III) iodide
Arsenous iodide
3D model (JSmol)
ECHA InfoCard 100.029.153
EC Number 232-068-4
RTECS number CG1950000
Molar mass 455.635 g/mol
Appearance orange-red crystalline solid
Density 4.69 g/cm3
Melting point 146 °C (295 °F; 419 K)
Boiling point 403 °C (757 °F; 676 K)
6 g/100 mL
Solubility soluble in alcohol, ether, CS2
-142.0·10−6 cm3/mol
Rhombohedral, hR24, SpaceGroup = R-3, No. 148
US health exposure limits (NIOSH):
PEL (Permissible)
[1910.1018] TWA 0.010 mg/m3[1]
REL (Recommended)
Ca C 0.002 mg/m3 [15-minute][1]
IDLH (Immediate danger)
Ca [5 mg/m3 (as As)][1]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Arsenic triiodide is the inorganic compound with the formula AsI3. It is a dark red solid that readily sublimes. It is a pyramidal molecule that is useful for preparing organoarsenic compounds.


It is prepared by a reaction of arsenic trichloride and potassium iodide:[2]

AsCl3 + 3KI → AsI3 + 3 KCl


Hydrolysis occurs only slowly in water forming arsenic trioxide and hydroiodic acid. The reaction proceeds via formation of arsenous acid which exists in equilibrium with hydroiodic acid. The aqueous solution is highly acidic, pH of 0.1N solution is 1.1. It decomposes to arsenic trioxide, elemental arsenic and iodine when heated in air at 200 °C. The decomposition, however, commences at 100 °C and occurs with the liberation of iodine.

Former uses[edit]

Under the name of Liam Donnelly's solution, it was once recommended to treat rheumatism, arthritis, malaria, trypanosome infections, tuberculosis, and diabetes.[3]


  1. ^ a b c "NIOSH Pocket Guide to Chemical Hazards #0038". National Institute for Occupational Safety and Health (NIOSH).
  2. ^ John C. Bailar, Jr. "Arsenic Triiodide" Inorganic Syntheses 1939, volume 1, pp. 103–104, 2007. doi:10.1002/9780470132326.ch36
  3. ^ Shakhashiri BZ, "Chemical of the Week: Arsenic", University of Wisconsin–Madison Chemistry Dept.