Arsenic trichloride

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Arsenic trichloride
IUPAC name
Arsenic(III) chloride
Other names
Arsenic(III) chloride
Arsenous chloride
Butter of arsenic
de Valagin's solution
Molar mass 181.28 g/mol
Appearance Colorless liquid
Odor Acrid, acidic
Density 2.163 g/cm3 (20 °C)
Melting point −16.2 °C (2.8 °F; 256.9 K)
Boiling point 130.2 °C (266.4 °F; 403.3 K)
Solubility Reacts with alcohols
Miscible with carbon tetrachloride, chloroform, diethyl ether, hydrochloric acid, phosphorus trichloride, THF, toluene, xylene
Vapor pressure 8.78 mmHg at 20 °C
Safety data sheet Sigma-Aldrich
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
48 mg/kg (rat, oral)
80 mg/kg (rat, dermal)
Related compounds
Related compounds
Nitrogen trichloride
Phosphorus trichloride
Antimony(III) chloride
Bismuth chloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Arsenic(III) chloride or arsenic trichloride is an inorganic compound with the formula AsCl3. It is highly corrosive and poisonous.

As arsenic is in group 15, it can also form arsenic pentachloride (AsCl5). However, this compound is unstable at temperatures above −50 °C, meaning it has no uses in chemistry except for research.



Arsenic trichloride will rapidly hydrolyze in contact with water, forming 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.

AsCl3 undergoes redistribution upon treatment with As2O3 to give arsenic oxychloride, AsOCl.

AsCl3 + As2O3 → 3 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 + 3 KBr → AsBr3 + 3 KCl
AsCl3 + 3 KI → AsI3 + 3 KCl


Arsenic trichloride is a colorless liquid, that fumes in moist air. Impure samples may appear yellowish or cloudy. It reacts with water and alcohols, but it's miscible with ethers.


Arsenic(III) chloride is sold by chemical suppliers, however due to its hazards, and the fact that arsenic compounds are highly regulated in most countries, it's practically impossible for the hobby chemist to acquire it.

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.


There are several preparation routes:

Reacting arsenic trioxide with an excess of hydrogen chloride will yield the compound, while the excess HCl will prevent hydrolysis:

As2O3 + 6 HCl → 2 AsCl3 + 3 H2O

Disulfur dichloride can be used as substitute for HCl, and has the advantage of destroying any water produced by the reaction:[1]

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

Thionyl chloride, while more expensive, can also be used:[2]

2 As2O3 + 3 SOCl2 → 2 AsCl3 + 3 SO2

In all these cases, the reaction takes place during reflux. The final product is distilled.

A water-free route, which gives a very pure product, involves direct chlorination of arsenic at 80–85 °C, but this method requires elemental arsenic, which is volatile and more hazardous:

2 As + 3 Cl2 → 2 AsCl3


  • Make organoarsenic compounds (don't you have better things to do, though?)
  • Make arsenic bromides and iodides
  • Compound collecting



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


Arsenic trichloride must be kept away from moisture. Schlenck flasks or ampouling are recommended.


The neutralization of arsenic halides is complicated not only by their volatility, but also by the high toxicity of arsenic compounds. Any neutralization must be done in a special installation, designed to scrub any traces of arsenic wastes.


  1. R. C. Smith, "Manufacture of Arsenic trichloride" The Journal of Industrial and Engineering Chemistry 1919, volume 11, pp. 109–110
  2. S. K. Pandey, A. Steiner, H. W. Roesky, S. Kamepalli, A. H. Cowley, "Arsenic(III)chloride" Inorganic Synthesis 1997, volume 31, pp. 148-150

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