Definitions

# Tin dioxide

Tin dioxide 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 Table), 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 6 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 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 reverbatory 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" (CAS RN 13472-47-4) refers to hydrated tin dioxide, SnO2, which is also called "stannic hydroxide."

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

$mathrm\left\{SnO_2 + 6 HI longrightarrow H_2\left[SnI\right]_6 + 2 H_2O\right\}$
Similarly, SnO2 dissolves in sulfuric acid to give the sulfate:
$mathrm\left\{SnO_2 + 2 H_2SO_4 longrightarrow Sn\left(SO_4\right)_2 + 2 H_2O\right\}$

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 dyeing 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.

Throughout history it has been used as an opacifier in the ceramic industry (where it is just known as tin oxide), especially in earthenware. Tin oxide does not go into solution in the glaze melt, generally amounts of 4-8% are needed. Zircon compounds are also used for this purpose.

SnO2 coatings can be applied using CVD, 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. 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. SnO2 has been used as a polishing powder and is sometimes known as "putty powder", SnO2 is used in sensors of combustible gases. In these the sensor area is heated to a constant temperature (low 100s °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 + 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

CeramicMaterials.Info

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