Telluric acid

Telluric acid is a chemical compound with the formula Te(OH)6. It is a white solid made up of octahedral Te(OH)6 molecules and this structure persists in aqueous solution. There are two forms, rhombohedral and monoclinic, and both contain octahedral Te(OH)6 molecules. Telluric acid is a weak acid,dibasic, forming tellurate salts with strong bases.

Preparation
Telluric acid is formed by the oxidation of tellurium or tellurium dioxide with a powerful oxidising agent such as hydrogen peroxide, chromium trioxide or sodium peroxide.
 * TeO2 + 2H2O2 + 2H+ → Te(OH)6

Crystallisation of telluric acid solutions below 10°C gives Te(OH)6.4H2O. It is oxidizing, as shown by the electrode potential for the reaction below, although it is kinetically slow in its oxidations.
 * H6TeO6 + 2H+ + 2e&minus; TeO2 + 4H2O2 Eo = +1.02 V

Properties and Reactions
The anhydrous acid is stable in air at 100°C but above this it dehydrates to form polymetatelluric acid, a white hygroscopic powder (approximate composition (H2TeO4)10), and allotelluric acid, an acid syrup of unknown structure (approximate composition  (H2TeO4)3(H2O)4).

Strong heating at over 300°C produces the α- crystalline modification of tellurium trioxide, α-TeO3.

Reaction with diazomethane gives the hexamethyl ester, Te(OMe)6.

Telluric acid and its salts mostly contain hexacoordinate tellurium. This is true even for salts such as magnesium tellurate, MgTeO4, which is isostructural with magnesium molybdate and contains TeO6 octahedra.

Metatelluric acid - H2TeO4,Tellurous acid - H2TeO3
Metatelluric acid, H2TeO4, the tellurium analogue of sulfuric acid, H2SO4, is unknown. Allotelluric acid of approximate composition H2TeO4)3(H2O)4, is not well characterised and may be a mixture of Te(OH)6 and (H2TeO4)n.

Tellurous acid, H2TeO3 is known but not well characterised.