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Telluric acid
Names
	IUPAC name Hexahydroxidotellurium
	Other names Orthotelluric acid; Hexahydroxytellurium; Hexahydroxy-λ6-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[Te](O)(O)(O)(O)O;
Properties
Chemical formula	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)
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 [77 °F], 100 kPa). 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.^[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.

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.^[4]

TeO₂ + H₂O₂ + 2 H₂O → Te(OH)₆

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]

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).^[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]

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.^[3]

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

^ Lide, David R. (1998), Handbook of Chemistry and Physics (87 ed.), Boca Raton, Florida: CRC Press, ISBN 0-8493-0594-2
^ Perrin, D. D., ed. (1982) [1969]. 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.
^ ^a ^b ^c ^d ^e Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
^ ^a ^b ^c ^d ^e ^f 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
^ ^a ^b 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.