Chemical Properties
Formula weight 229.66. Colorless crystals, stable in air. May
occur in a monoclinic modification (space group C
52h) and a cubic
modification (space group O^3
); the large crystals are generally
monoclinic while the microcrystalline powder frequently consists
of a mixture of both forms.
Heating H6TeO6 between 100 and 220°C converts it to solid,
water-insoluble polymetatelluric acid; the latter decomposes above
220°C into TeO3, and at 400°C or higher into TeO2 and O2. Melts
in a sealed tube at about 136°C, forming a concentrated aqueous
solution of polymetatelluric acid and some polyorthotelluric acid,
d (monoclinic) 3.071; d (cubic) 3.17.
Synthesis
(1) The starting materials are very fine Te powder and aqueous
HClO3 solution. The acid required for the oxidation of 0.1 mole
(that is, 12.75 g.) of Te is prepared by adding a lukewarm mixture
of 40 ml. of H2O and 7.2 ml. of concentrated H2SO4 to a solution
of 24 g. of Ba(ClO3)2·H2O in 100 ml. of H2O. After about five
hours, the solution is decanted from the precipitated BaSO4 through
a filter, and if desired the residue is extracted once with H2O. To
oxidize the Te, it is added to a 500-ml., round-bottom flask placed
under the hood. It is then moistened with 5 ml. of 50% HNO3 and
about one fourth of the HClO3 solution. With thorough agitation
(by swirling) the flask contents soon boil and the reaction proceeds at boiling with strong Cl2 evolution to form H6TeO6
.
(2) The process is somewhat more cumbersome than method I since
it requires, among other things, separate preparation of TeO2 as
well as repeated recrystallization of the acid to quantitatively re -
move the simultaneously formed salts. The yield is 75-85%. For
detailed description of the procedure, see the literature below.
(3) Reaction of Te or TeO2 with 30% H2O2 in sulfuric acid or alkaline solution and subsequent precipitation of H6TeO6 with concentrated nitric acid.
(4) Oxidation of Te with HNO3 and CrO3. The product must be re - crystallized several times in order to remove the Cr(NO3)3 byproduct.
