|Molar mass||229.64 g/mol|
|Appearance||White monoclinic crystals|
|Melting point||136 °C (277 °F; 409 K)|
|50.1 g/100 ml at 30 °C|
|Acidity (pKa)||7.68, 11.0 at 18 °C|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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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 which persist in aqueous solution. There are two forms, rhombohedral and monoclinic, and both contain octahedral Te(OH)6 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.
- TeO2 + H2O2 + 2H2O → Te(OH)6
Crystallization 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− ⇌ TeO2 + 4H2O E
o= +1.02 V
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 (approximate composition (H2TeO4)10), and allotelluric acid, an acid syrup of unknown structure (approximate composition (H2TeO4)3(H2O)4).
Typical salts of the acid contains the anions [Te(O)(OH)5]− and [Te(O)2(OH)4]2−. The presence of the tellurate ion TeO42− has been confirmed in the solid state structure of Rb6[TeO5][TeO4]. 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.
Other forms of telluric acid
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.
Other tellurium acids
Tellurous acid (H2TeO3), 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.
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