Arsenic trioxide

Arsenic trioxide
General
Other names	Arsenic(III) oxide Arsenic sesquioxide Arseneous oxide Arseneous anhydride
Molecular formula	As2O3
Molar mass	197.84 g/mol
Appearance	white solid
CAS number	[1327-53-3]
EINECS number	215-481-4
Properties
Density and phase	3.74 g/cm3, solid
Solubility in water	2 g/100 ml (25 °C) see text
Melting point	278 °C
Boiling point	465 °C
Acidity (pKa)	9.2
Basicity (pKb)	?
Structure
Molecular shape	tetrahedral molecules, As4O6, in α-form and in liquid and gas phases
Crystal structure	cubic (α) <180°C monoclinic (β) >180°C
Dipole moment	zero
Thermodynamic data
Standard enthalpy of formation ΔfHosolid	−657.4 kJ/mol
Standard molar entropy Sosolid	? J.K–1.mol–1
Hazards
MSDS	External MSDS
EU classification	Very toxic (T+) Carc. Cat. 1 Dangerous for the environment (N)
NFPA 704
R-phrases	R45, R28, R34, R50/53
S-phrases	S53, S45, S60, S61
Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Other anions	Arsenic trisulfide
Other cations	Phosphorus trioxide Antimony trioxide
Related compounds	Arsenic pentoxide Arseneous acid
Except where noted otherwise, data are given for materials in their standard state (at 25°C, 100 kPa)

Arsenic trioxide is the most important commercial compound of arsenic, and the main starting material for arsenic chemistry.

Preparation

• Burning arsenic in air
• Hydrolysis of arsenic trichloride
• Roasting of arsenide minerals (main industrial route)

Crystal structure

Chemical properties

Arsenic trioxide is an amphoteric oxide which shows a marked preponderance for its acidic properties. It dissolves readily in alkaline solutions to give arsenites. It is much less soluble in acids, but will dissolve in hydrochloric acid to give arsenic trichloride or related species. It reacts with oxidizing agents such as ozone, hydrogen peroxide and nitric acid to give arsenic pentoxide, As₂O₅: the reaction with hydrogen peroxide can be explosive. It is also readily reduced to arsenic, and arsine (AsH₃) may also be formed.

Uses

• Starting point for the manufacture of arsenic-based pesticides (sodium arsenite, sodium arsenate, sodium cacodylate).
• Starting point for the manufacture of certain arsenic-based pharamaceuticals (Neosalvarsan) and vetinary products.
• Decolorizing agent for glasses and enamels.
• Preservative for wood.
• Starting point for the preparation of elemental arsenic, arsenic alloys and arsenide semiconductors.

Toxicology

See also: arsenicosis.

Arsenic trioxide is readily absorbed by the digestive system: toxic effects are also well known after inhalation of the dust or fumes and after skin contact. Elimination is rapid at first (half-life of 1–2 days), by methylation to cacodylic acid and excretion in the urine, but a certain amount (30–40% in the case of repeated exposure) is incorporated into the bones, muscles, skin, hair and nails (all tissues rich in keratin) and eliminated over a period of weeks or months.

The first symptomes of acute arsenic poisoning by ingestion are digestive problems: vomiting, abdominal pains, diarrhea often accompaied by bleeding. Sub-lethal doses can lead to convulsions, cardiovascular problems, inflammation of the liver and kidneys and abnormalities in the coagulation of the blood. These are followed by the appearance of characteristic white lines (Mees stripes) on the nails and by hair loss. Lower doses lead to liver and kidney problems and to changes in the pigmentation of the skin.

Cases of acute arsenic poisoning are known after inhalation and after skin contact with arsenic trioxide. The first signs are severe irritation, either of the respiratory tract or of the exposed skin, followed by longer term neurological problems. Even dilute solutions of arsenic trioxide are dangerous on contact with the eyes.

Chronic arsenic poisoning is known as arsenicosis: it is found after professional exposure (for example, in metal smelters), in populations whose drinking water contains high levels of arsenic (0.3–0.4 ppm) and in patients treated for long periods with arsenic-based pharmaceuticals.

Arsenic trioxide has been shown to be a human carcinogen. Studies on workers exposed in copper foundries in the U.S., Japan and Sweden indicate a risk of lung cancer 6–10 times higher for the most exposed workers compared with the general population. Long-term ingestion of arsenic trioxide either in drinking water or as a medical treatment can lead to skin cancer. Reproductive problems (high incidence of miscarriage, low birth weight, congenital deformations) have also been indicated in one study of women exposed to arsenic trioxide dust as employees or neighbours of a copper foundery.

Bibliography

Institut national de recherche et de sécurité (INRS), Fiche toxicologique nº 89 : Trioxyde de diarsenic, 1989.

External links

• International Chemical Safety Card 0378
• NIOSH Pocket Guide to Chemical Hazards
• IARC Monograph – Arsenic and Arsenic Compounds
• NTP Report on Carcinogens – Inorganic Arsenic Compounds
• European Chemicals Bureau
• Institut national de recherche et de sécurité, France (in French)
• NIST Chemistry WebBook

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