Antimony trioxide
Encyclopedia
Antimony trioxide is the inorganic compound
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...

 with the formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....

 Sb2O3. It is the most important commercial compound of antimony
Antimony
Antimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...

. It is found in nature as the minerals valentinite
Valentinite
Valentinite is an antimony oxide mineral with formula Sb2O3. Valentinite crystallizes in the orthorhombic system and typically forms as radiating clusters of euhedral crystals or as fibrous masses. It is colorless to white with occasional shades or tints of yellow and red. It has a Mohs hardness of...

 and senarmontite. Like most polymeric oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....

s, Sb2O3 dissolves in aqueous solutions only with hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...

.

Production and properties

Global production of antimony trioxide in 2005 was 120,000 tonnes, an increase from 112,600 tonnes in 2002. China produces the largest share (47 %) followed by US/Mexico (22 %), Europe (17 %), Japan (10 %) and South Africa (2 %) and other countries (2%).

Antimony trioxide is mainly produced via the smelting of stibnite
Stibnite
Stibnite, sometimes called antimonite, is a sulfide mineral with the formula Sb2S3. This soft grey material crystallizes in an orthorhombic space group. It is the most important source for the metalloid antimony...

 ore, which is oxidised to crude Sb2O3 using furnaces operating at approximately 850 to 1,000 °C. The transformation is described as follows:
2 Sb2S3 + 9 O2 → 2 Sb2O3 + 6 SO2

Crude Sb2O3 is purified by sublimation, which allows it to be separated from the more volatile arsenic trioxide
Arsenic trioxide
Arsenic trioxide is the inorganic compound with the formula As2O3. This commercially important oxide of arsenic is the main precursor to other arsenic compounds, including organoarsenic compounds. Approximately 50,000 tonnes are produced annually...

. This step is relevant because antimony ores commonly contain significant amounts of arsenic.

Antimony oxide is also obtained via antimony trichloride
Antimony trichloride
Antimony trichloride is the chemical compound with the formula SbCl3. The soft colorless solid with a pungent odor was known to the alchemists as butter of antimony.-Preparation:...

, which can be obtained from stibnite.
2 Sb2S3 + 3 CaCl2 + 6 O2 → 4 SbCl3 + 3 CaSO4

After fractional distillation to separate it from arsenic trichloride
Arsenic trichloride
Arsenic trichloride is an inorganic compound with the formula AsCl3, also known as arsenous chloride or butter of arsenic. This poisonous oil is colourless, although impure samples may appear yellow. It is an intermediate in the manufacture of organoarsenic compounds.-Structure:AsCl3 is a...

, SbCl3 can be hydrolyzed to the oxide:
2 SbCl3 + 3 H2O → Sb2O3 + 6 HCl

Intermediates in the hydrolysis include the oxychlorides SbOCl and Sb4O5Cl2.

Although impractical for commercial purposes, Sb2O3 can be prepared by burning elemental antimony in air:
4 Sb + 3 O2 → 2 Sb2O3

Properties

Antimony trioxide is an amphoteric oxide, dissolving in alkaline solution to give antimonite
Antimonite
In chemistry, an antimonite refers to salts of antimony, such as NaSb4 and NaSbO2 which can be prepared by reacting alkali with antimony oxide, Sb2O3...

s and in acid solution to given a range of polyantimonous acids. It can be readily oxidized to antimony pentoxide and related antimony(V) compounds, but it is also easily reduced to antimony, sometimes with production of stibine
Stibine
Stibine is the chemical compound with the formula SbH3. This colourless gas is the principal covalent hydride of antimony and a heavy analogue of ammonia. The molecule is pyramidal with H–Sb–H angles of 91.7° and Sb–H distances of 1.707 Å...

.

Structure

The structure of Sb2O3 depends on the temperature of the sample. Dimeric Sb4O6 is the high temperature (1560 °C) gas. Sb4O6 molecules are bicyclic cages, similar to the related oxide of phosphorus(III), phosphorus trioxide
Phosphorus trioxide
Phosphorus trioxide is the chemical compound with the molecular formula P4O6. This compound was discovered by Neil G. Mehta . Although it should properly be named tetraphosphorus hexoxide, the name phosphorus trioxide preceded the knowledge of the compound's molecular structure, and its usage...

. The cage structure is retained in a solid that crystallizes in a cubic habit. The Sb-O distance is 197.7 pm and the O-Sb-O angle of 95.6°. This form exists in nature as the mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...

 senarmontite. Below 606 °C, the more stable form 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
Valentinite
Valentinite is an antimony oxide mineral with formula Sb2O3. Valentinite crystallizes in the orthorhombic system and typically forms as radiating clusters of euhedral crystals or as fibrous masses. It is colorless to white with occasional shades or tints of yellow and red. It has a Mohs hardness of...

.

Sb4O6
senarmontite
valentinite


Uses

The annual consumption of antimony trioxide in the United States and Europe is approximately 10,000 and 25,000 tonne
Tonne
The tonne, known as the metric ton in the US , often put pleonastically as "metric tonne" to avoid confusion with ton, is a metric system unit of mass equal to 1000 kilograms. The tonne is not an International System of Units unit, but is accepted for use with the SI...

s, respectively. The main application is for flame retardant
Flame retardant
Flame retardants are chemicals used in thermoplastics, thermosets, textiles and coatings that inhibit or resist the spread of fire. These can be separated into several different classes of chemicals:...

s in combination with halogenated materials. The combination of the halides and the antimony being key to the flame-retardant action for polymers, helping to form less flammable chars. Such flame retardants are found in electrical apparatus, textiles, leather, and coatings.

Other applications:
  • Antimony trioxide is an opacifying agent
    Opacifier
    An opacifier is a substance added to a material in order to make the ensuing system opaque. An example of a chemical opacifier is tin dioxide , which is used to opacify ceramic glazes and milk glass; bone ash is also used....

     for glass
    Glass
    Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

    es, ceramic
    Ceramic
    A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...

    s and enamel
    Vitreous enamel
    Vitreous enamel, also porcelain enamel in U.S. English, is a material made by fusing powdered glass to a substrate by firing, usually between 750 and 850 °C...

    s.
  • Some specialty pigments contain antimony.
  • Antimony trioxide is a useful catalyst
    Catalysis
    Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....

     in the production of polyethylene terephthalate
    Polyethylene terephthalate
    Polyethylene terephthalate , commonly abbreviated PET, PETE, or the obsolete PETP or PET-P, is a thermoplastic polymer resin of the polyester family and is used in synthetic fibers; beverage, food and other liquid containers; thermoforming applications; and engineering resins often in combination...

     (PET plastic) and the vulcanization
    Vulcanization
    Vulcanization or vulcanisation is a chemical process for converting rubber or related polymers into more durable materials via the addition of sulfur or other equivalent "curatives." These additives modify the polymer by forming crosslinks between individual polymer chains. Vulcanized material is...

     of rubber.
  • Flame retardant
    Flame retardant
    Flame retardants are chemicals used in thermoplastics, thermosets, textiles and coatings that inhibit or resist the spread of fire. These can be separated into several different classes of chemicals:...

     for textiles, leather, polymers, and coatings.

Safety

The toxicity of Sb2O3 is topical because it is a likely byproduct of the combustion of some materials "fireproofed" with antimony compounds. The oxides of arsenic, antimony, and bismuth are comparable in their toxicity, but their volatilities differ widely. Antimony trioxide has suspected carcinogenic potential for humans. Its TLV
Threshold Limit Value
The threshold limit value of a chemical substance is a level to which it is believed a worker can be exposed day after day for a working lifetime without adverse health effects. Strictly speaking, TLV is a reserved term of the American Conference of Governmental Industrial Hygienists...

is 0.5 mg/m3, as for most antimony compounds.

Further reading

  • Institut national de recherche et de sécurité (INRS), Fiche toxicologique nº 198 : Trioxyde de diantimoine, 1992.
  • The Oxide Handbook, G.V. Samsonov, 1981, 2nd ed. IFI/Plenum, ISBN 0-306-65177-7

External links

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