Telluric acid, or more accurately orthotelluric acid, is a chemical compound with the formula Te(OH)6, often written as H6TeO6. It is a white crystalline solid made up of octahedral Te(OH)6 molecules which persist in aqueous solution.[3] In the solid state, there are two forms, rhombohedral and monoclinic, and both contain octahedral Te(OH)6 molecules,[4] 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.[4][5] It is used as tellurium-source in the synthesis of oxidation catalysts.
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| Names | |
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| IUPAC name
Hexahydroxidotellurium | |
Other names
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| Identifiers | |
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| ECHA InfoCard | 100.029.334 
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CompTox Dashboard (EPA) |
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| Properties | |
| Te(OH)6 | |
| Molar mass | 229.64 g·mol−1 |
| Appearance | White monoclinic crystals |
| Density | 3.07 g/cm3 |
| Melting point | 136 °C (277 °F; 409 K) |
| 50.1 g/(100 ml) at 30 °C[1] | |
| Acidity (pKa) | 7.5, 11, 14[2] |
| Conjugate base | Tellurate |
| Structure | |
| octahedral | |
| 0 D | |
| Hazards | |
| Occupational safety and health (OHS/OSH): | |
Main hazards |
corrosive |
| Related compounds | |
Other anions |
Hydrotelluric acid Tellurous acid Hydrogen telluride |
Related compounds |
Teflic acid Sulfuric acid Selenic acid |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). | |
Telluric acid is formed by the oxidationoftelluriumortellurium dioxide with a powerful oxidising agent such as hydrogen peroxide, chromium trioxideorsodium peroxide.[4]
Crystallization of telluric acid solutions below 10 °C gives telluric acid tetrahydrate Te(OH)6·4H2O.[3] It is an oxidising agent, as shown by the electrode potential for the reaction below, although it is kinetically slow in its oxidations.[4]
Chlorine, by comparison, is +1.36 V and selenous acid is +0.74 V in oxidizing conditions.
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 3·H2TeO4·4H2O).[6][3]
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 TeO2−4 has been confirmed in the solid state structure of Rb6[TeO5][TeO4].[7] Strong heating at over 300 °C produces the α crystalline modification of tellurium trioxide, α-TeO3. [5] Reaction with diazomethane gives the hexamethyl ester, Te(OCH3)6.[3]
Telluric acid and its salts mostly contain hexacoordinate tellurium.[4] This is true even for salts such as magnesium tellurate, MgTeO4, which is isostructural with magnesium molybdate and contains TeO6 octahedra.[4]
Metatelluric acid, H2TeO4, the tellurium analogue of sulfuric acid, H2SO4, is unknown. Allotelluric acid of approximate composition 3·H2TeO4·4H2O, is not well characterised and may be a mixture of Te(OH)6 and (H2TeO4)n.[3]
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.
