Antimony trioxide

Antimony trioxide is the chemical compound with the formula Sb2O3. It is the most important commercial compound of antimony.

Preparation
As the primary oxide of antimony, Sb2O3 is prepared by burning elemental antimony in air:
 * 4Sb + 3O2 &rarr; 2Sb2O3

Alternatively, antimonide minerals can be roasted to give the same products.

Structure
The structure of Sb2O3 depends on the temperature of the sample. Only at very high temperatures does the molecule Sb2O3 exist. At moderate temperatures, the gas consists of Sb4O6. These molecules are bicyclic cages, similar to the related oxide of phosphorus, phosphorus trioxide. The cage structure is retained in a solid that crystallizes in a cubic habit. The Sb-O distance is 1.977 Å and the O-Sb-O angle of 95.6°. This materials exists in nature as the mineral senarmontite. Below 606 °C, the more stable form of is orthorhombic, consisting of pairs -Sb-O-Sb-O- chains that are linked by oxide bridges between the Sb centers. This form exists in nature as the mineral valentinite.

Antimony trioxide is an amphoteric oxide, dissolving in alkaline solution to give antimonites and in acid solution to given a range of polyantimonous acids. It can be readily oxidized to antimony pentoxide or other antimony(V) compounds, but is also easily reduced to antimony, sometimes with production of stibine.

Uses
The annual consumption of antimony trioxide in the United States is approximately 10,000 tonnes.
 * Flame retardant for textiles, polymers, and coatings
 * Opacifying agent for glasses, ceramics and enamels
 * Pigment antimony white

Toxicology
Antimony trioxide is only weakly absorbed by the digestive system, and the main route of exposure is by inhalation of the dust. The elimination of antimony from the body is slow, leading to a risk of chronic toxicity in the form of pneumoconiosis with repeated inhalation exposures. Acute poisoning is very rare, and the signs are fairly non-characteristic (vomiting, abdominal pain, irritation of the mucous membranes, diarrhea, cardiac irregularities). These symptoms are more often associated with ingestion of other more water soluble compounds.

Chronic poisoning by antimony trioxide is also rare. The main signs are irritation of the respiratory tract and of the skin and a characteristic pneumoconiosis, which is visible on chest X-rays.

Antimony trioxide is known to pass into breast milk and to transverse the placenta only in very small amounts. One study of exposed female workers suggested a higher incidence than usual of menstrual problems and of late-term miscarriages; also their children may have developed slower than usual during the first twelve months of life, although this study is inconclusive. A more recent developmental study in rodents did not confirm an effect on mammalian development.