Arsenic trichloride

Arsenic trichloride

Arsenic(III) chloride in glass ampoules
Names
IUPAC name Arsenic(III) chloride
Other names Arsenic(III) chloride Arsenous chloride Butter of arsenic de Valagin's solution Trichloroarsine
Properties
Chemical formula	AsCl3
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 in water	Reacts
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
Hazards
Safety data sheet	Sigma-Aldrich
Flash point	Non-flammable
Lethal dose or concentration (LD, LC):
LD50 (Median dose)	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 a highly corrosive and poisonous liquid that fumes in air.

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.

Properties

Chemical

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

Physical

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.

Availability

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.

Preparation

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:

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

Thionyl chloride, while more expensive, can also be used:

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

Projects

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

Handling

Safety

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

Storage

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

Disposal

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.

References

Sciencemadness Library

• A TEXT-BOOK OF INORGANIC CHEMISTRY. VOLUME XI. ORGANOMETALLIC COMPOUNDS. PART II. DERIVATIVES OF ARSENIC.

Relevant Sciencemadness threads

• Preparation of Arsenic trichloride