|Molar mass||150.71 g·mol−1|
|Density||6.95 g/cm3 (20 °C)
6.85 g/cm3 (24 °C)
|Melting point||1,630 °C (2,970 °F; 1,900 K)|
|Boiling point||1,800–1,900 °C (3,270–3,450 °F; 2,070–2,170 K)
|Solubility||Soluble in hot concentrated alkalis, concentrated acids
Insoluble in alcohol
Refractive index (nD)
|Crystal structure||Rutile tetragonal, tP6|
|Space group||P42/mnm, No. 136|
|Point group||4/m 2/m 2/m|
|Lattice constant||a = 4.737 Å, c = 3.185 Å|
|Lattice constant||α = 90°, β = 90°, γ = 90°|
Trigonal planar (O2−)
Std enthalpy of
Gibbs free energy (ΔfG˚)
LD50 (Lethal dose)
|> 20 g/kg (rats, oral)|
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
|what is: / ?)(|
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.
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.
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.
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."
- 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 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.
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.
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".
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.
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.
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