Tin (IV) oxide

Tin(IV) oxide, also known as stannic oxide, is the inorganic compound with the formula SnO₂. The mineral form of SnO₂ is called cassiterite, and this is the main ore of tin. With many other names, this oxide of tin is an important material in tin chemistry. It is a colourless, diamagnetic, amphoteric solid.

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.

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 kilotons. SnO₂ is reduced industrially to the metal with carbon in a reverberatory furnace at 1200–1300 °C.

Amphoterism

Although SnO₂ is insoluble in water, it is amphoteric, dissolving in base and acid.''Inorganic & Theoretical chemistry'', F. Sherwood Taylor, Heineman, 6th Edition (1942) "Stannic acid" refers to hydrated tin (IV) oxide, SnO₂, which is also called "stannic oxide."

Tin oxides dissolve in acids. Halogen acids attack SnO₂ to give hexahalostannates, such as nI₆sup>2−. One report describes reacting a sample in refluxing HI for many hours.
:SnO₂ + 6 HI → H₂SnI₆ + 2 H₂O
Similarly, SnO₂ dissolves in sulfuric acid to give the sulfate:
:SnO₂ + 2 H₂SO₄ → Sn(SO₄)₂ + 2 H₂O

SnO₂ dissolves in strong bases to give "stannates," with the nominal formula Na₂SnO₃. Dissolving the solidified SnO₂/NaOH melt in water gives Na₂ n(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

Tin(IV) oxide has long been used as an opacifier and as a white colorant in ceramic glazes.’The Glazer’s Book’ – 2nd edition. ''A.B.Searle.The Technical Press Limited.'' London. 1935. This has probably led to the discovery of the pigment lead-tin-yellow, which was produced using tin(IV) oxide as a compound.Hermann Kühn, 1967, "Blei-Zinn-Gelb und seine Verwendung in der Malerei", ''Farbe und Lack'' 73: 938-949 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.

SnO₂ has been used as pigment in the manufacture of glasses, enamels and ceramic glazes. Pure SnO₂ gives a milky white colour; other colours are achieved when mixed with other metallic oxides e.g. V₂O₅ yellow; Cr₂O₃ pink; and Sb₂O₅ grey blue.

Dyes

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.

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. Tin(IV) oxide for this use is sometimes called as "putty powder" or "jeweler's putty".

Glass coatings

SnO₂ coatings can be applied using chemical vapor deposition, vapour deposition techniques that employ 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 is used in 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.
Room temperature gas sensors are also being developed using reduced graphene oxide-SnO₂ composites(e.g. for ethanol detection).

Doping with various compounds has been investigated (e.g. with CuO). Doping with cobalt and manganese, gives a material that can be used in e.g. high voltage varistors. Tin(IV) oxide can be doped with the oxides of iron or manganese.

References

Further reading

* Technical discussion of how SnO₂:F is used in low-emissivity (low-E) windows. The report includes reflectance and transmittance spectra.
* Information on chemical safety and exposure limits

{{DEFAULTSORT:Tin (IV) oxide
Oxides
Tin(IV) compounds
Semiconductor materials