Telluric acid is a chemical compound

Chemical compound

A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

 with the formula

Chemical formula

A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....

 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

Weak acid

A weak acid is an acid that dissociates incompletely. It does not release all of its hydrogens in a solution, donating only a partial amount of its protons to the solution...

 which is dibasic, forming tellurate

Tellurate

The tellurate ion is TeO42− or TeO66−.Unlike sulfate, tellurate is a somewhat good oxidizer; it can be reduced to tellurite or tellurium....

 salt

Salt

In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...

s 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

Tellurium dioxide

Tellurium dioxide is a solid oxide of tellurium. It is encountered in two different forms, the yellow orthorhombic mineral tellurite, β-TeO2, and the synthetic, colourless tetragonal , α-TeO2...

 with a powerful oxidising agent such as hydrogen peroxide

Hydrogen peroxide

Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...

, chromium trioxide

Chromium trioxide

Chromium trioxide is the inorganic compound with the formula CrO3. It is the acidic anhydride of chromic acid, and is sometimes marketed under the same name.This compound is a dark-red/orange brown solid, which dissolves in water concomitant with hydrolysis...

 or sodium peroxide

Sodium peroxide

Sodium peroxide is the inorganic compound with the formula Na2O2. This solid is the product when sodium is burned with oxygen. It is a strong base and a potent oxidizing agent. It exists in several hydrates and peroxyhydrates including Na2O2·2H2O2·4H2O, Na2O2·2H2O, Na2O2·2H2O2, and...

.

TeO₂ + H₂O₂ + 2H₂O → Te(OH)₆

Crystallisation of telluric acid solutions below 10°C gives Te(OH)₆.4H₂O.
It is oxidizing, as shown by the electrode potential

Electrode potential

Electrode potential, E, in electrochemistry, according to an IUPAC definition, is the electromotive force of a cell built of two electrodes:* on the left-hand side is the standard hydrogen electrode, and...

 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 selenious 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).

Strong heating at over 300°C produces the α- crystalline modification of tellurium trioxide

Tellurium trioxide

Tellurium trioxide is an inorganic chemical compound of tellurium and oxygen. In this compound, tellurium is in the +6 oxidation state.-Polymorphs:There are two forms, yellow-red α-TeO3 and grey, rhombohedral, β-TeO3 which is less reactive....

, α-TeO₃.


Reaction with diazomethane

Diazomethane

Diazomethane is the chemical compound CH2N2. It is the simplest of diazo compounds. In the pure form at room temperature, it is a extremely sensitive explosive yellow gas, thus it is almost universally used as a solution in diethyl ether...

 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

Sulfuric acid

Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

, 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

Tellurous acid

Tellurous acid is an inorganic compound with the formula H2TeO3. It is the oxoacid of tellurium. The compound is not well characterized. An alternative way of writing its formula is 2TeO. In principle, tellurous acid would form by treatment of tellurium dioxide with water, i.e. hydrolysis...

, containing tellurium in its +4 oxidation state, (H₂TeO₃) is known but not well characterised.
Hydrogen telluride

Hydrogen telluride

Hydrogen telluride is the inorganic compound with the formula H2Te. The simplest hydride of tellurium, it is rarely encountered because of it decomposes rapidly to the constituent elements. Most compounds with Te-H bonds are unstable with respect to loss of H2. H2Te is chemically and structurally...

 is an unstable gas that forms hydrotelluric acid upon addition to water.