| 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
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
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
|