Telluric acid


Telluric acid is a chemical compound with the formula Te6. It is a white solid made up of octahedral Te6 molecules which persist in aqueous solution. There are two forms, rhombohedral and monoclinic, and both contain octahedral Te6 molecules.
Telluric acid is a weak acid which is dibasic, forming tellurate salts with strong bases and hydrogen tellurate salts with weaker bases or upon hydrolysis of tellurates in water.

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.
Crystallization of telluric acid solutions below 10 °C gives Te6.4H2O.
It is oxidizing, as shown by the electrode potential for the reaction below, although it is kinetically slow in its oxidations.
Chlorine, by comparison, is +1.36V and selenous acid is +0.74V in oxidizing conditions.

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, and allotelluric acid, an acid syrup of unknown structure 3.

Typical salts of the acid contains the anions − and 2−. The presence of the tellurate ion TeO42− has been confirmed in the solid state structure of Rb6.
Strong heating at over 300 °C produces the α- crystalline modification of tellurium trioxide, α-TeO3.
Reaction with diazomethane gives the hexamethyl ester, Te6.
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.

Other forms of telluric acid

Metatelluric acid, H2TeO4, the tellurium analogue of sulfuric acid, H2SO4, is unknown. Allotelluric acid of approximate composition 34, is not well characterised and may be a mixture of Te6 and n.

Other tellurium acids

, containing tellurium in its +4 oxidation state, is known but not well characterised.
Hydrogen telluride is an unstable gas that forms hydrotelluric acid upon addition to water.