Telluric acid

Telluric acid

Ball-and-stick model of telluric acid
Names
IUPAC name Hexahydroxidotellurium
Other names Orthotelluric acid
Identifiers
CAS Number	7803-68-1 N
ChEBI	CHEBI:30463 Y
ChemSpider	55517 Y
Jmol 3D model	Interactive image
PubChem	61609
InChI InChI=1S/H2O4Te/c1-5(2,3)4/h(H2,1,2,3,4) Y Key: XHGGEBRKUWZHEK-UHFFFAOYSA-N Y 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	H6O6Te
Molar mass	229.64 g/mol
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)	7.68, 11.0 at 18 °C
Structure
Molecular shape	octahedral
Dipole moment	0 D
Vapor pressure	{{{value}}}
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).
N verify (what is YN ?)
Infobox references

Telluric acid is a chemical compound with the formula Te(OH)₆. It is a white solid made up of octahedral Te(OH)₆ molecules which persist in aqueous solution. There are two forms, rhombohedral and monoclinic, and both contain octahedral Te(OH)₆ molecules. 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.

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 + 2H₂O → Te(OH)₆

Crystallization of telluric acid solutions below 10 °C gives Te(OH)₆.4H₂O. It is oxidizing, as shown by the electrode potential for the reaction below, although it is kinetically slow in its oxidations.

H₆TeO₆ + 2H⁺ + 2e⁻ ⇌ TeO₂ + 4H₂O E^o = +1.02 V

Chlorine, by comparison, is +1.36V and selenous acid is +0.74V 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 (H₂TeO₄)₃(H₂O)₄).
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 TeO₄²⁻ 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(OMe)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 (H₂TeO₄)₃(H₂O)₄, 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.