Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium metaperiodate : NaIO4
iodic acid : HIO3
perchloric acid : HClO4
hydrochloric acid : HCl
nitric acid : HNO3
thallic oxide : Tl2O3
thallous nitrate : TlNO3
Class:
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elem=Tl
redox
coordination
Summary:
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This is a set of experiments, demonstrating an interesting aspect of thallium
chemistry, in combination with different iodine-containing compounds.
Description:
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Add thallium(III) oxide to solutions of nitric acid (20%), perchloric acid (20%)
and hydrochloric acid (30%): The dark brown thallium(III) oxide quickly dissolves
in hydrochloric acid. The solution is colorless and very slightly opalescent. The
opalescense almost certainly is due to the presence of a tiny amount of thallium(I)
in the thallium(III) oxide, resulting in formation of insoluble TlCl.
Thallium(III) oxide only dissolves in the nitric acid with great difficulty. It
takes many hours (appr. one day) to have the oxide dissolved in an excess of the
nitric acid. The final solution is colorless and completely clear. No opalescence
at all.
Thallium(III) oxide does not dissolve appreciably in the perchloric acid. Even after
a week of standing still nearly 100% of the solid remains undissolved under the
colorless excess amount of acid.
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The solution in nitric acid is divided over several test tubes.
To the colorless solution of thallium(III) oxide in excess nitric acid add a
solution of NaIO4: Immediately, a mustard colored (dirty yellow/brown) precipitate
is formed, which settles at the bottom in a few hours. It is interesting to see
that even in the strongly acidic solution a precipitate is formed. The mustard-
colored precipitate most likely is a mixed thallium/hydrogen orthoperiodate,
something like ThH4IO6 or Tl2H3IO6.
To the colorless solution of thallium(III) oxide in nitric acid add a solution
of HIO3: A white precipitate is formed. Again, it is remarkablle, that the
white precipitate is formed at very low pH. This most likely is something like
Tl(IO3)3 or TlH(IO3)4.
Heat the liquid with the white precipitate of thallium(III) iodate: Nothing
happens, the precipitate does not dissolve again, nor does it change color.
Even when heated to near boiling, no change of color can be observed.
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Dissolve some thallium(I) nitrate in water: This goes with some difficulty,
heating helps dissolving the solid more quickly.
To the cooled down solution add a solution of HIO3 in water: A white
precipitate is formed. This white precipitate most likely is TlIO3 or
TlH(IO3)2. The thallium(I) iodate looks very similar to thallium(III)
iodate.
Heat the liquid with the white precipitate of thallium(I) iodate: Nothing
happens, the precipitate does not change color. It becomes slightly more
compact, the particles settle at the bottom more quickly.
Store the precipitate for one week, under water: No visible changes occur.
It remains snow-white.
Add an excess amount of conc. HCl (30%): The white precipitate quickly
dissolves. The liquid turns bright yellow and bubbles of Cl2 escape from
the liquid. The liquid becomes completely clear and deep yellow. Apparently
there is a lot of excess iodate in this experiment and all thallium(I) is
oxidized to thallium(III) and only when all thallium is oxidized, then
free chlorine gas can escape. The bubbling of Cl2 is quite vigorous for
a second or two.
Remark: Be careful with this experiment. The liquid bubbles vigorously,
producing a fine aerosol of droplets of the solution. This brings thallium
(besides the chlorine) in the air. Chlorine is toxic, but inhaling thallium
is worse, due to its extreme toxicity and long-term effects. Cover the test
tube with a paper tissue when the acid is added. This allows chlorine to
pass along the tissue, while small droplets of solution, containing some
thallium, are trapped into the tissue.
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Dissolve some thallium(I) nitrate in water and add a solution of NaIO4:
A compact crystalline precipitate is formed, which quickly settles at
the bottom. The precipitate is pure white. It most likely is TlIO4.
The reaction very much looks like the reaction between solutions of KNO3
and NaIO4, resulting in formation of the nearly insoluble KIO4.
After a while, however, the precipitate is not pure white anymore. Very
slowly, the precipitate changes color: white -> pale yellow -> yellow ->
mustard -> mustard brown -> brown (like light milk chocolate).
On heating, the change of color occurs more quickly. When the liquid is
near boiling, then the change occurs in just two minutes or so.
It looks like the TlIO4 undergoes an internal redox reaction with formation
of thallium(III). The liquid turns somewhat alkaline, leading to formation
of a brown precipitate of hydrous thallium(III) oxide/hydroxide, possibly
mixed with the mustard-colored thallium(III) periodate if excess periodate
is present.