Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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potassium ferricyanide : K3 [Fe(CN)6]
ammonia : NH3
sodium persulfate : Na2S2O8
nitric acid : HNO3
thallous nitrate : TlNO3
Class:
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elem=Fe,Tl
redox
Summary:
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Thallium(I) ion is fairly easily oxidized to thallium(III). In neutral aqueous
solutions, this ion hydrolyzes to a dark hydrous oxide, which forms a compact
precipitate. The dark oxide easily can be dissolved in nitric acid, such that a
colorless solution of thallium(III) nitrate is formed in nitric acid.
Thallium(III) ion forms an ochre/yellow color with ferricyanide ion, which is
stable at low pH, but at high pH this decomposes, giving a yellow solution of
ferricyanide and a dark brown suspension of hydrous thallic oxide.
Description:
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Dissolve some thallium(I) nitrate in water and add a solution of sodium
peroxodisulfate: The mixed solution is colorless and clear. No precipitate is
formed.
Slightly heat the colorless solution (to 50 C or so): Slowly, the liquid
becomes turbid and a dirty brown color develops. The brown color also sticks to
the glass. On further heating the liquid remains turbid and very dark brown,
nearly black. Here, Tl(+) ions are oxidized to Tl(3+) ions and these hydrolyse
in the neutral solution to something like Tl2O3.nH2O, which has a very dark
brown color.
Heat the liquid more strongly and in the meantime swirl the liquid with the
dark brown suspension: The precipitate becomes more compact, no further changes
can be observed.
Pour the liquid with the nearly black precipitate into another test tube: The
test tube in which the precipitate was formed has a thin layer of brown
material sticking to the glass, which is not rinsed away with water. However,
when dilute nitric acid (appr. 2 M) is added, then it can be removed fairly
easily, especially if the acid is slightly heated. A colorless solution
remains.
In the other test tube, allow the dark precipitate to settle at the bottom and
then decant the colorless liquid above the dark precipitate and add a small
amount of fairly concentrated nitric acid (appr. 50% by weight) to the dark
precipitate: The precipitate does not dissolve at once in the nitric acid.
Heat the nitric acid with the dark precipitate: The precipitate now fairly
quickly dissolves and a perfectly colorless liquid is obtained.
Dilute the colorless solution in 50% nitric acid with 5 times its volume of
water: The liquid remains colorless and clear.
Add a solution of potassium ferricyanide: A flocculent ochre/yellow precipitate
is formed at once. This precipitate does not quickly settle at the bottom of
the test tube.
Pour the liquid with the ochre flocculent precipitate in ammonia (5% by
weight): The precipitate turns dark brown at once. This most likely is due to
decomposition of a ferricyanide precipitate. Such precipitates are known to be
unstable at high pH, with formation of the free ferricyanide ion and the metal
oxide or metal hydroxide.