H2TeO3 has been synthesized as a solid but it is not
well characterized. If tellurium dioxide, TeO2, is added
to water, tellurous acid does not easily form, but it can
be made other ways. Its CAS number is 10049-23-7.
Tellurous acid can be formed from the chlorides. Two
chlorides are known, the dichloride, TeCl2, and the tetrachloride,
TeCl4. They are both obtained by passing chlorine
over tellurium, the product being separated by
distillation (the tetrachloride is the less volatile). The
dichloride is an amorphous, readily fusible, almost black
solid. It is decomposed by water with formation of tellurium
and tellurous acid:
2TeCl2+ 3H2O ? Te+H2TeO3+ 4HCl
However, pure tellurous acid, H2TeO3, can be
obtained when the tetrachloride is decomposed by
water, or on dissolving tellurium in nitric acid and pouring
the solution into water. It is a colorless solid and
behaves as a dibasic acid. The alkali tellurites are soluble
in water. They also gives rise to super acid salts, such as
KHTeO3·H2TeO3.
White, amorphous substance of varying composition; H2O content
is frequently less than that corresponding to the formula HsTeO3;
loses water when dry but can be stored under H2O for several days
without change.
On heating above room temperature, extensively loses water
with conversion to TeO3. Very slightly soluble in water.
