Tin dioxide

Structure

It 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. "Stannic acid" refers to hydrated tin (IV) oxide, SnO₂, which is also called "stannic hydroxide."

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

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

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 (IV) oxide has long been used as an opacifier and as a white colorant in ceramic glazes. This has 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.

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 (IV) oxide can be used as a polishing powder, sometimes in mixtures also with lead oxide, for polishing glass, jewelery, 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.

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. 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.