Tellurous acid

Tellurous acid is an inorganic compound with the formula H₂TeO₃. It is the oxoacid of tellurium(IV).^[2] This compound is not well characterized. An alternative way of writing its formula is (HO)₂TeO. In principle, tellurous acid would form by treatment of tellurium dioxide with water, that is by hydrolysis. The related conjugate base is well known in the form of several salts such as potassium hydrogen tellurite, KHTeO₃.

Tellurous acid

Tellurous acid
Names
IUPAC name Tellurous acid
Other names Tellurium dioxide hydrate, tellurium(IV) oxide hydrate
Identifiers
CAS Number	10049-23-7 N
3D model (JSmol)	Interactive image
ChEBI	CHEBI:30465 Y
ChemSpider	23310 Y
ECHA InfoCard	100.030.145
PubChem CID	24936
UNII	IVA6SGP6QM N
CompTox Dashboard (EPA)	DTXSID90143339
InChI InChI=1S/H2O3Te/c1-4(2)3/h(H2,1,2,3) Y Key: SITVSCPRJNYAGV-UHFFFAOYSA-N Y InChI=1/H2O3Te/c1-4(2)3/h(H2,1,2,3) Key: SITVSCPRJNYAGV-UHFFFAOYAI
SMILES O[Te+]([O-])O
Properties
Chemical formula	H2TeO3
Molar mass	177.616 grams
Appearance	colorless crystals
Density	~ 3 g/cm3
Boiling point	decomposes
Solubility in water	negligible
Acidity (pKa)	pKa1 = 2.48, pKa2 = 7.70 [1]
Conjugate base	Tellurite
Structure
Crystal structure	unknown
Molecular shape	pyramidal at Te
Related compounds
Other anions	Selenous acid Sulfurous acid
Other cations	Sodium tellurite
Related compounds	Telluric acid Selenic acid Sulfuric 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

Properties

In contrast to the analogous compound selenous acid, tellurous acid is only metastable. Most tellurite salts contain the TeO²⁻
₃ ion. Oxidation of its aqueous solution with hydrogen peroxide gives the tellurate ion. It is usually prepared as an aqueous solution where it acts as a weak acid.^[1][3]

H₂TeO₃ + H₂O ⇌ H₃O⁺ + HTeO⁻
₃ K_a1 = 2×10⁻³

HTeO⁻
₃ + H₂O ⇌ H₃O⁺ + TeO²⁻
₃ K_a2 = 1×10⁻⁸