Tin(IV) oxide
Chemical compound known as stannic oxide, cassiterite and tin ore
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Through Chemical formula, Oxide and Diamagnetism
Structure
Tin(IV) oxide crystallises with the rutile structure. As such the tin atoms are six coordinate and the oxygen atoms three coordinate. SnO2 is usually regarded as an oxygen-deficient n-type semiconductor.
Hydrous forms of SnO2 have been described as stannic acid. Such materials appear to be hydrated particles of SnO2 where the composition reflects the particle size.
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Preparation
Tin(IV) oxide occurs naturally. Synthetic tin(IV) oxide is produced by burning tin metal in air. Annual production is in the range of 10 thousand metric tons (11 thousand short tons). SnO2 is reduced industrially to the metal with carbon in a reverberatory furnace at 1,200–1,300 °C (2,190–2,370 °F).
Reactions
The reaction from tin(IV) oxide with hot carbon monoxide is practiced on a large scale as this carbothermal reduction is used to obtain tin metal from its ores:
- SnO2 + 2 CO → Sn + 2 CO2
Some other reactions relevant to purifying tin from its ores are:
- SnO2 + MgCl2 + CO → SnCl2 + MgO + CO2
- 4 SnO2 + 6 FeCl2 → 2 SnCl2 + 2 SnCl4 + 2 Fe3O4
SnO2 converts to the monoxide at 1,500 °C (2,730 °F):
- 2 SnO2 → 2 SnO + O2
SnO2 is insoluble in water. It dissolves in sulfuric acid and in molten sodium hydroxide. It is not amphoteric. Like rutile, it is not attacked by solutions of acid or base.
Dissolution of SnO2 in sulfuric acid gives the sulfate:
- SnO2 + 2 H2SO4 → Sn(SO4)2 + 2 H2O
The latter compound can add additional hydrogen sulfate ligands to give hexahydrogensulfatostannic acid.
SnO2 dissolves in molten alkali to give "stannates," with the nominal formula Sodium stannate (Na2SnO3). Dissolving the solidified SnO2/NaOH melt in water gives Na2[Sn(OH)6], "preparing salt," which is used in the dye industry.
Uses
In conjunction with vanadium oxide, it is used as a catalyst for the oxidation of aromatic compounds in the synthesis of carboxylic acids and acid anhydrides.
Ceramic glazes
SnO2 is used as pigment in the manufacture of glasses, enamels and ceramic glazes. Thousands of tons of SnO2 are produced annually for this application. Pure SnO2 gives a milky white colour; other colours are achieved when mixed with other metallic oxides e.g. vanadium(V) oxide (V2O5) yellow; Chromium(III) oxide (Cr2O3) pink; and antimony pentoxide (Sb2O5) grey blue. This use probably led to the discovery of the pigment lead-tin-yellow, which was produced using tin(IV) oxide as a compound. The use of tin(IV) oxide has been particularly common in glazes for earthenware, sanitaryware and wall tiles; see the articles tin-glazing and Tin-glazed pottery. Tin oxide remains in suspension in vitreous matrix of the fired glazes, and, with its high refractive index being sufficiently different from the matrix, light is scattered, and hence increases the opacity of the glaze. The degree of dissolution increases with the firing temperature, and hence the extent of opacity diminishes. Although dependent on the other constituents the solubility of tin oxide in glaze melts is generally low. Its solubility is increased by Na2O, K2O and B2O3, and reduced by CaO, BaO, ZnO, Al2O3, and to a limited extent PbO.
Glass coatings
SnO2 coatings are valued as transparent conducting oxides (TCOs). Like other TCOs, SnO2 has significant electrical conductivity but is transparent, an unusual combination of properties. Windows coated with SnO2 also reflect infrared radiation, which is relevant to temperature control for smart windows. Coatings can be applied using chemical vapor deposition, vapour deposition techniques that employ tin(IV) chloride (SnCl4) or organotin trihalides e.g. butyltin trichloride as the volatile agent. This technique is used to coat glass bottles with a thin (<0.1 μm) layer of SnO2, which helps to adhere a subsequent, protective polymer coating such as polyethylene to the glass.
Thicker layers doped with Sb or F ions are electrically conducting and used in electroluminescent devices and photovoltaics.
Gas sensing
SnO2 has been evaluated as sensors of combustible gases including carbon monoxide detectors. In these the sensor area is heated to a constant temperature (few hundred °C) and in the presence of a combustible gas the electrical resistivity drops.
Historical uses
This oxide of tin has been utilized as a mordant in the dyeing process since ancient Egypt. A German by the name of Kuster first introduced its use to London in 1533 and by means of it alone, the color scarlet was produced there.
Tin(IV) oxide for this use is sometimes called "putty powder" or "jeweler's putty".
Polishing
Tin(IV) oxide can be used as a polishing powder, sometimes in mixtures also with lead oxide, for polishing glass, jewelry, marble and silver.