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.[1] There are two forms, rhombohedral and monoclinic, and both contain octahedral Te(OH)6 molecules.[2] Telluric acid is a weak acid,dibasic, forming tellurate salts with strong bases.[2][3]
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.[2]
- TeO2 + 2H2O2 + 2H+ → Te(OH)6
Crystallisation of telluric acid solutions below 10°C gives Te(OH)6.4H2O.[1] It is oxidizing, as shown by the electrode potential for the reaction below, although it is kinetically slow in its oxidations.[2]
- H6TeO6 + 2H+ + 2e− Template:Unicode TeO2 + 4H2O2 E
o= +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).[1]
Strong heating at over 300°C produces the α- crystalline modification of tellurium trioxide, α-TeO3.
[4]
Reaction with diazomethane gives the hexamethyl ester, Te(OMe)6.[1]
Telluric acid and its salts mostly contain hexacoordinate tellurium.[2] This is true even for salts such as magnesium tellurate, MgTeO4, which is isostructural with magnesium molybdate and contains TeO6 octahedra.[2]
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.[1]
Tellurous acid, H2TeO3 is known but not well characterised.
References
- ↑ 1.0 1.1 1.2 1.3 1.4 Template:Greenwood&Earnshaw
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Template:Cotton&Wilkinson6th
- ↑ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
- ↑ Inorganic Chemistry,Egon Wiberg, Arnold Frederick Holleman Elsevier 2001 ISBN 0123526515