Telluric acid
Template:Short description Template:Chembox
Telluric acid, or more accurately orthotelluric acid, is a chemical compound with the formula Template:Chem2, often written as Template:Chem2. It is a white crystalline solid made up of octahedral Template:Chem2 molecules which persist in aqueous solution.[1] In the solid state, there are two forms, rhombohedral and monoclinic, and both contain octahedral Template:Chem2 molecules,[2] containing one hexavalent tellurium (Te) atom in the +6 oxidation state, attached to six hydroxyl (–OH) groups, thus, it can be called tellurium(VI) hydroxide. 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.[2][3] It is used as tellurium-source in the synthesis of oxidation catalysts.
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]
Crystallization of telluric acid solutions below 10 °C gives telluric acid tetrahydrate Template:Chem2.[1] It is an oxidising agent, as shown by the electrode potential for the reaction below, although it is kinetically slow in its oxidations.[2]
- Template:Chem2, E
o= +1.02 V
Chlorine, by comparison, is +1.36 V and selenous acid is +0.74 V 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 (approximate composition Template:Chem2), and allotelluric acid, an acid syrup of unknown structure (approximate composition Template:Chem2).[4][1]
Typical salts of the acid contains the anions Template:Chem2 and Template:Chem2. The presence of the tellurate ion Template:Chem2 has been confirmed in the solid state structure of Template:Chem2.[5] Strong heating at over 300 °C produces the α crystalline modification of tellurium trioxide, α-Template:Chem2. [3] Reaction with diazomethane gives the hexamethyl ester, Template:Chem2.[1]
Telluric acid and its salts mostly contain hexacoordinate tellurium.[2] This is true even for salts such as magnesium tellurate, Template:Chem2, which is isostructural with magnesium molybdate and contains Template:Chem2 octahedra.[2]
Other forms of telluric acid
Metatelluric acid, Template:Chem2, the tellurium analogue of sulfuric acid, Template:Chem2, is unknown. Allotelluric acid of approximate composition Template:Chem2, is not well characterised and may be a mixture of Template:Chem2 and Template:Chem2.[1]
Other tellurium acids
Tellurous acid Template:Chem2, 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.
References
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Template:Tellurium compounds Template:Hydrogen compounds Template:Hydroxides Template:Authority control