| Verifiedfields | changed |
|---|---|
| Watchedfields | changed |
| Verifiedrevid | 470481521 |
| Iupacname | Hexahydroxidotellurium |
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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.[1] In the solid state, there are two forms, rhombohedral and monoclinic, and both contain octahedral Te(OH)6 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]
- TeO2 + H2O2 + 2 H2O → Te(OH)6
Crystallization of telluric acid solutions below 10 °C gives telluric acid tetrahydrate Te(OH)6*4H2O.[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]
- Te(OH)6 + 2 H+ + 2 e- ⇌ TeO2 + 4 H2O, 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 (H2TeO4)10), and allotelluric acid, an acid syrup of unknown structure (approximate composition 3*H2TeO4*4H2O).[4][1]
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 TeO4(2−) has been confirmed in the solid state structure of Rb6[TeO5][TeO4].[5] Strong heating at over 300 °C produces the α crystalline modification of tellurium trioxide, α-TeO3. [3] Reaction with diazomethane gives the hexamethyl ester, Te(OCH3)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]
Other forms of telluric acid
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}.[1]
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.
References
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- ^
- ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
- ^ Loub, J.; Haase, W.; Mergehenn, R. (1979). "Structure of an adduct of orthotelluric acid and urea". Acta Crystallographica Section B: Structural Crystallography and Crystal Chemistry. 35 (12): 3039–3041. Bibcode:1979AcCrB..35.3039L. doi:10.1107/S0567740879011286
- ^ Catherine E. Housecroft & Alan G. Sharpe (2008). "Chapter 16: The group 16 elements". Inorganic Chemistry, 3rd Edition. Pearson. p. 526. ISBN 978-0-13-175553-6.