Tin dioxide

Tin dioxide, also known by the systematic name tin(IV) oxide and stannic oxide in the older notation, 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 (see infobox), this oxide of tin is the most important raw material in tin chemistry. This colourless, diamagnetic solid is amphoteric.

Structure

It crystallises with the rutile structure, wherein 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 in the past as stannic acids, although such materials appear to be hydrated particles of SnO₂ where the composition reflects the particle size.

Preparation

Tin dioxide occurs naturally but is purified by reduction to the metal followed by burning tin 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 an amphoteric oxide, although cassiterite ore has been described as difficult to dissolve in acids and alkalis. "Stannic acid" refers to hydrated tin dioxide, SnO₂, which is also called "stannic hydroxide."

Tin oxides dissolve in acids. Halogen acids attack SnO₂ to give hexahalostannates, such as [SnI₆]^2âˆ'. One report describes reacting a sample in refluxing HI for many hours.

SnO₂ + 6 HI â†' H₂SnI₆ + 2 H₂O

Similarly, SnO₂ dissolves in sulfuric acid to give the sulfate:

SnO₂ + 2 H₂SO₄ â†' Sn(SO₄)₂ + 2 H₂O

SnO₂ dissolves in strong base to give "stannates," with the nominal formula Na₂SnO₃. Dissolving the solidified SnO₂/NaOH melt in water gives Na₂[Sn(OH)₆]₂, "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 dioxide has long been used as an opacifier and as a white colorant in ceramic glazes. Its use 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.

Polishing

Tin dioxide can be used as a polishing powder, sometimes in mixtures also with lead oxide, for polishing glass, jewelery, marble and silver. Tin dioxide 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.

Gas sensing

SnO₂ wires are commonly used as the detecting element in carbon monoxide detectors.

SnO₂ is used in sensors of combustible gases. 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. 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 dioxide can be doped into the oxides of iron or manganese.

References

Further reading

• "How Pilkington Energy Advantageâ„¢ Low-E Glass Works". Pilkington Group Limited. 18 July 2005. Retrieved 2012-12-02.  Technical discussion of how SnO₂:F is used in low-emissivity (low-E) windows. The report includes reflectance and transmittance spectra.
• "NIOSH Pocket Guide to Chemical Hazards - Tin(IV) oxide (as Sn)". Centers for Disease Control and Prevention. 4 April 2011. Retrieved 2013-11-05.  Information on chemical safety and exposure limits