TELLURIC ACID

Telluric acid, H6TeO6, is obtained in the form of its salts when tellurium is fused with potassium nitrate plus carbonate, or by the oxidizing action of chlorine on a tellurite in alkaline solution. The free acid may be obtained by decomposing the barium salt with sulfuric acid and concentrating the solution.
where tellurium is in an octahedral coordination. Telluric acid, Te(OH)6, has the CAS number of 7803-68- 1 and a molecular weight of 229.6425 g/mol. It is a white, monoclinic crystal with a density of 3.07 g/cm³. Its melting point is 136°C and its solubility in water is 50.1 g/100 ml.
It is also formed when tellurium dioxide is oxidized by hydrogen peroxide in caustic solution (A. Gutbier, Zest. Anorg. Chem., 1904, 40, p. 260), and perhaps best of all by oxidizing tellurium with a mixture of nitric and chromic acids. It crystallizes as prisms, which lose their water of hydration at 160°C. The tellurates of the alkaline earth metals are more or less soluble in water, those of the other metals being very sparingly or almost insoluble in water. Some tellurates exist in two forms, a colorless form soluble in water and acids, and a yellow form insoluble in water and acids.

White, heavy crystals. Soluble in hot water and alkalies; slightly soluble in cold water.

White crystals; dimorphic solid; exists in both cubic and monoclinic crystalline forms; density 3.07g/cm³; melts at 136°C; tends to polymerize (similar to stannic acid); forms polymetatelluric acid (H2TeO4)n on strong heating; soluble in water, about 33g/100 mL at 30°C; the solubility decreases as the molecule polymerizes and becomes colloidal; a very weak dibasic acid, pKa1 7.68 and pKa2 11.0 at 18°C; soluble in dilute nitric acid and alkalies.

Chemical reagent.

Telluric acid can be prepared by reducing barium tellurate with sulfuric acid: BaTeO₄ + H₂SO₄ + 2H₂O → H₆TeO₆ + BaSO₄ Also, telluric acid can be prepared by oxidation of tellurium or tellurium dioxide with a strong oxidizing agent such as hydrogen peroxide, sodium peroxide, chromic acid, or potassium permanganate in nitric acid. Molecular equations for overall reactions are shown below: Te + 3H2O2 → H6TeO6 TeO₂ + H₂O₂ + 2H₂O → H₆TeO₆ Te + 2CrO₃ + 3H₂O → H₆TeO₆ + Cr₂O₃
At cold temperatures at about 1°C, telluric acid crystallizes as tetrahydrate.

Telluric acids exist in both cubic and monoclinic crystalline forms.1 It may be prepared by treating tellurium and tellurous acid with strong oxidants.

As for tellurium.

Poison by ingestion and intravenous routes. When heated to decomposition it emits toxic fumes of Te. See also TELLURIUM COMPOUNDS.

Crystallise it once from nitric acid, then repeatedly from hot water (0.4mL/g). [Fehér in Handbook of Preparative Inorganic Chemistry (Ed. Brauer) Academic Press Vol I pp 451-453 1963.]