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Names | |
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IUPAC name Hexahydroxidotellurium | |
Other names
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Identifiers | |
3D model (JSmol) | |
ChEBI | |
ChemSpider | |
ECHA InfoCard | 100.029.334 |
PubChem CID | |
UNII | |
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, 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.
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. [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.