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Telluric acid

Names
IUPAC name Hexahydroxidotellurium
Other names Orthotelluric acid Hexahydroxytellurium Hexahydroxy-λ-tellane Tellurium hexahydroxide Tellurium(VI) hydroxide
Identifiers
CAS Number	7803-68-1
3D model (JSmol)	Interactive image
ChEBI	CHEBI:30463
ChemSpider	55517
ECHA InfoCard	100.029.334
PubChem CID	62686
UNII	WE1KZR49WU
CompTox Dashboard (EPA)	DTXSID5064887
InChI InChI=1S/H2O4Te/c1-5(2,3)4/h(H2,1,2,3,4)Key: XHGGEBRKUWZHEK-UHFFFAOYSA-N InChI=1/H2O4Te/c1-5(2,3)4/h(H2,1,2,3,4)Key: XHGGEBRKUWZHEK-UHFFFAOYAT
SMILES O(O)(O)(O)(O)O
Properties
Chemical formula	Te(OH)6
Molar mass	229.64 g·mol
Appearance	White monoclinic crystals
Density	3.07 g/cm
Melting point	136 °C (277 °F; 409 K)
Solubility in water	50.1 g/(100 ml) at 30 °C
Acidity (pKa)	7.5, 11, 14
Conjugate base	Tellurate
Structure
Molecular shape	octahedral
Dipole moment	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 , 100 kPa). N verify (what is  ?) Infobox references

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. In the solid state, there are two forms, rhombohedral and monoclinic, and both contain octahedral Te(OH)₆ molecules, 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. 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.

TeO₂ + H₂O₂ + 2 H₂O → Te(OH)₆

Crystallization of telluric acid solutions below 10 °C gives telluric acid tetrahydrate Te(OH)₆·4H₂O. It is an oxidising agent, as shown by the electrode potential for the reaction below, although it is kinetically slow in its oxidations.

Te(OH)₆ + 2 H + 2 e ⇌ TeO₂ + 4 H₂O, E = +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).

Typical salts of the acid contains the anions [Te(O)(OH)₅] and [Te(O)₂(OH)₄]. The presence of the tellurate ion TeO2−4 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(OCH₃)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 3·H₂TeO₄·4H₂O, 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.

References

1. Lide, David R. (1998), Handbook of Chemistry and Physics (87 ed.), Boca Raton, Florida: CRC Press, ISBN 0-8493-0594-2
2. Perrin, D. D., ed. (1982) . Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution. IUPAC Chemical Data (2nd ed.). Oxford: Pergamon (published 1984). Entry 219. ISBN 0-08-029214-3. LCCN 82-16524.
3. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
4. ^ Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999), Advanced Inorganic Chemistry (6th ed.), New York: Wiley-Interscience, ISBN 0-471-19957-5
5. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
6. 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.
7. 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.

• Hydroxides
• Tellurates
• Oxidizing acids
• Chalcogen oxoacids