# Tin(IV) oxide

Tin(IV) oxide, also known as stannic oxide, is the inorganic compound with the formula SnO$_2$. The mineral form of SnO$_2$ 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. SnO$_2$ is usually regarded as an oxygen-deficient n-type semiconductor.
Hydrous forms of SnO$_2$ have been described as stannic acid. Such materials appear to be hydrated particles of SnO$_2$ 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. SnO$_2$ is reduced industrially to the metal with carbon in a reverberatory furnace at 1200–1300 °C.

## 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:
    SnO$_2$ + 2 CO → Sn + 2 CO$_2$
Some other reactions relevant to purifying tin from its ores are:
    SnO$_2$ + MgCl$_2$ + CO → SnCl$_2$ + MgO +CO$_2$
    4 SnO$_2$ + 6 FeCl$_2$ → 2 SnCl$_2$ + 2 SnCl$_4$ + 2 Fe$_3$O$_4$
SnO$_2$ converts to the monoxide at 1500 °C:
    2 SnO$_2$ → 2 SnO + O$_2$
SnO$_2$ 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 SnO$_2$ in sulfuric acid gives the sulfate:
    SnO$_2$ + 2 H$_2$SO$_4$ → Sn(SO$_4$)$_2$ + 2 H$_2$O
The latter compound can add additional hydrogen sulfate ligands to give hexahydrogensulfatostannic acid.
SnO$_2$ dissolves in molten alkali to give "stannates," with the nominal formula Na$_2$SnO$_3$. Dissolving the solidified SnO$_2$/NaOH melt in water gives Na$_2$[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

SnO$_2$ is used as pigment in the manufacture of glasses, enamels and ceramic glazes. Thousands of tons of SnO$_2$ are produced annually for this application. Pure SnO$_2$ gives a milky white colour; other colours are achieved when mixed with other metallic oxides e.g. V$_2$O$_5$ yellow; Cr$_2$O$_3$ pink; and Sb$_2$O$_5$ 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 Na$_2$O, K$_2$O and B$_2$O$_3$, and reduced by CaO, BaO, ZnO, Al$_2$O$_3$, and to a limited extent PbO.

### Glass coatings

SnO$_2$ coatings are valued as transparent conducting oxides (TCOs). Like other TCOs, SnO$_2$ has significant electrical conductivity but is transparent, an unusual combination of properties. Windows coated with SnO$_2$ 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 SnCl$_4$ 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 SnO$_2$, 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

SnO$_2$ 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 as "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.