Arsenic trichloride


Arsenic trichloride is an inorganic compound with the formula AsCl3, also known as arsenous chloride or butter of arsenic. This poisonous oil is colourless, although impure samples may appear yellow. It is an intermediate in the manufacture of organoarsenic compounds.[4]

Arsenic trichloride
Ball-and-stick model of arsenic trichloride
Other names
Arsenic(III) chloride, Arsenous trichloride, Butter of arsenic, de Valagin's solution
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3D model (JSmol)
  • Interactive image
  • 22974 checkY
ECHA InfoCard 100.029.144 Edit this at Wikidata
EC Number
  • 232-059-5
  • 24570
RTECS number
  • CG1750000
  • 5XW39M1300
UN number 1560
  • DTXSID3042556 Edit this at Wikidata
  • InChI=1S/AsCl3/c2-1(3)4 checkY
  • InChI=1/AsCl3/c2-1(3)4
  • Cl[As](Cl)Cl
Molar mass 181.28 g/mol
Appearance colourless oily liquid
Density 2.163 g/cm3, liquid
Melting point −16.2 °C (2.8 °F; 256.9 K)
Boiling point 130.2 °C (266.4 °F; 403.3 K)
Solubility soluble in alcohol, ether, HCl, HBr, chloroform, CCl4[1]
-79.9·10−6 cm3/mol
Viscosity 9.77 x 10−6 Pa s
Occupational safety and health (OHS/OSH):
Main hazards
Very toxic, carcinogen, corrosive, decomposes on contact with water releasing HCl
GHS labelling:
GHS05: CorrosiveGHS06: ToxicGHS08: Health hazardGHS09: Environmental hazard
H301, H310, H314, H331, H350, H410
P201, P202, P260, P261, P262, P264, P270, P271, P273, P280, P281, P301+P310, P301+P330+P331, P302+P350, P303+P361+P353, P304+P340, P305+P351+P338, P308+P313, P310, P311, P321, P322, P330, P361, P363, P391, P403+P233, P405, P501
NFPA 704 (fire diamond)
Lethal dose or concentration (LD, LC):
48 mg/kg
100 mg/m3 (cat, 1 hr)
200 mg/m3 (cat, 20 min)
338 ppm (rat, 10 min)[3]
NIOSH (US health exposure limits):
PEL (Permissible)
[1910.1018] TWA 0.010 mg/m3[2]
REL (Recommended)
Ca C 0.002 mg/m3 [15-minute][2]
IDLH (Immediate danger)
Ca [5 mg/m3 (as As)][2]
Related compounds
Other anions
Arsenic trioxide, Arsenic trifluoride
Other cations
Antimony trichloride
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


AsCl3 is a pyramidal molecule with C3v symmetry. The As-Cl bond is 2.161 Å and the angle Cl-As-Cl is 98° 25'±30.[5][6] AsCl3 has four normal modes of vibration: ν1(A1) 416, ν2(A1) 192, ν3 393, and ν4(E) 152 cm−1.[7]


This colourless liquid is prepared by treatment of arsenic(III) oxide with hydrogen chloride followed by distillation:

As2O3 + 6 HCl → 2 AsCl3 + 3 H2O

It can also be prepared by chlorination of arsenic at 80–85 °C, but this method requires elemental arsenic.[4]

2 As + 3 Cl2 → 2 AsCl3

Arsenic trichloride can also be prepared by the reaction of arsenic oxide and sulfur monochloride. This method requires simple apparatus and proceeds efficiently:[8]

2 As2O3 + 6 S2Cl2 → 4 AsCl3 + 3 SO2 + 9 S

A convenient laboratory method is refluxing arsenic(III) oxide with thionyl chloride:[9]

2 As2O3 + 3 SOCl2 → 2 AsCl3 + 3 SO2


Hydrolysis with water gives arsenous acid and hydrochloric acid:

AsCl3 + 3 H2O → As(OH)3 + 3 HCl

Although AsCl3 is less moisture sensitive than PCl3, it still fumes in moist air.[10]

AsCl3 undergoes redistribution upon treatment with As2O3 to give the inorganic polymer AsOCl. With chloride sources, AsCl3, forms salts containing the anion [AsCl4]. Reaction with potassium bromide and potassium iodide give arsenic tribromide and arsenic triiodide, respectively.

AsCl3 is useful in organoarsenic chemistry, for example triphenylarsine is derived from AsCl3:[11]

AsCl3 + 6 Na + 3 C6H5ClchlorobenzeneAs(C6H5)3triphenylarsine + 6 NaCl


Arsenic compounds are highly toxic, and AsCl3 especially so because of its volatility and solubility.

It is classified as an extremely hazardous substance in the United States as defined in Section 302 of the U.S. Emergency Planning and Community Right-to-Know Act (42 U.S.C. 11002), and is subject to strict reporting requirements by facilities which produce, store, or use it in significant quantities.[12]


  1. ^ John Rumble (June 18, 2018). CRC Handbook of Chemistry and Physics (99th ed.). CRC Press. pp. 4–41. ISBN 978-1138561632.
  2. ^ a b c NIOSH Pocket Guide to Chemical Hazards. "#0038". National Institute for Occupational Safety and Health (NIOSH).
  3. ^ "Arsenic (inorganic compounds, as As)". Immediately Dangerous to Life or Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
  4. ^ a b Sabina C. Grund, Kunibert Hanusch, Hans Uwe Wolf "Arsenic and Arsenic Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, VCH-Wiley, 2008, Weinheim.doi:10.1002/14356007.a03_113.pub2
  5. ^ P. Kisliuk; C. H. Townes. "The Microwave Spectra and Molecular Structure of Phosphorus and Arsenic Trichloride". J. Chem. Phys. 1950, 18.
  6. ^ Jean Galy; Renee Enjalbertl Pierre Lecante; Andrzej Burian "AsCl3: From the crystalline to the liquid state. XRD (176< T (K) < 250) and WAXS (295K) studies" Inorg. Chem 2002, volume 41, pp. 693–698.doi:10.1021/ic0102788
  7. ^ Klapoetke, Thomas M. "The vibrational spectrum of arsenic trichloride" Main Group Metal Chemistry 1997, volume 20, pp. 81–83.
  8. ^ R. C. Smith, "Manufacture of Arsenic trichloride" The Journal of Industrial and Engineering Chemistry 1919, volume 11, pp. 109–110. doi:10.1021/ie50110a009
  9. ^ S. K. Pandey, A. Steiner, H. W. Roesky, S. Kamepalli, A. H. Cowley, "Arsenic(III)chloride" Inorganic Synthesis 1997, volume 31, pp. 148-150. doi:10.1002/9780470132623.ch24
  10. ^ Holleman, A. F.; Wiberg, E. Inorganic Chemistry Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
  11. ^ Shriner, R. L.; Wolf, C. N. (1963). "Tetraphenylarsonium Chloride Hydrochloride". Organic Syntheses.; Collective Volume, vol. 4, p. 910. Describes the preparation of As(C6H5)3.
  12. ^ "40 C.F.R.: Appendix A to Part 355—The List of Extremely Hazardous Substances and Their Threshold Planning Quantities" (PDF) (July 1, 2008 ed.). Government Printing Office. Archived from the original (PDF) on February 25, 2012. Retrieved October 29, 2011. {{cite journal}}: Cite journal requires |journal= (help)