Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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ammonia : NH3
potassium metabisulfite : K2S2O5
potassium iodide : KI
hydrochloric acid : HCl
basic bismuth nitrate : BiO(OH).4BiNO3(OH)2
nitric acid : HNO3
bismuth : Bi
Class:
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elem=Bi;I;Cl
coordination
redox
Summary:
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Bismuth shows remarkable coordination chemistry with iodide and chloride.
In the absence of chloride, a black precipitate of BiI3 is formed. In the
presence of chloride, a deep yellow/orange compound is formed, but only
if also iodide is present. So this compound must be a complex of bismuth,
iodide and chloride.
Description:
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sequence 1:
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Dissolve some bismuth in concentrated nitric acid and then add quite a
lot of water (approximately 6 times dilution): A white crystalline
precipitate is formed and a clear liquid is obtained above the precipitate.
Pour some of the clear liquid on some solid potassium iodide: Formation of
a very dark grey compound.
Add some solid potassium metabisulfite: The solid partly dissolves, a very
pungent odour of SO2 can be observed, the solid dark grey compound does
not dissolve. When this were iodine, then it would dissolve with so much
sulfite added.
Add an excess amount of ammonia (5% by weight): The black precipitate
disappears, the smell of SO2 disappears, the liquid becomes strongly
turbid, because of formation of a white precipitate. The liquid looks
like milk.
sequence 2:
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Dissolve some solid bismuth subnitrate in concentrated HCl (appr. 30% by
weight): A colorless liquid is obtained.
Add some solid potassium iodide: An intense orange/yellow compound is
formed, which goes into solution. The liquid becomes clear and deep orange/
yellow.
Add a lot of water (diluting at least 10 times): The liquid slowly becomes
turbid and dark grey.
sequence 3:
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Dissolve some bismuth in concentrated nitric acid and then add quite a
lot of water (approximately 6 times dilution): A white crystalline
precipitate is formed and a clear liquid is obtained above the precipitate.
Pour some of the clear liquid on some solid potassium iodide: Formation of
a very dark grey compound.
Add some concentrated HCl (appr. 30% by weight): The black precipitate
dissolves at once and the liquid becomes deep orange/yellow and clear.
sequence 4:
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Dissolve some bismuth in concentrated nitric acid and then add quite a
lot of water (approximately 6 times dilution): A white crystalline
precipitate is formed and a clear liquid is obtained above the precipitate.
Pour some of the clear liquid on some solid potassium iodide: Formation of
a very dark grey compound.
Heat the liquid to 60 C for a while: No visible changes. No visible formation
of purple vapours of iodine. If the black precipitate really were iodine,
then at 60 C one certainly would see a purple vapour of iodine.
Heat the liquid with the dark precipitate and boil for a while: When the
liquid starts boiling, it looks as if a vigorous reaction starts. A lot of
gas is produced, a thick purple fume is formed above the liquid and solid
iodine settles on the glass further away from the liquid. The liquid itself
becomes brown and light brown solid particles are formed, which form a
curdy precipitate. Apparently, on heating the nitric acid is capable of
oxidizing the iodide, but the black precipitate in the cold probably is not
iodine.
sequence 5:
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Dissolve some bismuth in concentrated nitric acid and then add quite a
lot of water (approximately 6 times dilution): A white crystalline
precipitate is formed and a clear liquid is obtained above the precipitate.
Pour some of the clear liquid on some solid potassium meta bisulfite: Formation
of a white precipitate, the liquid becomes completely white, like milk. There
is a strong pungent odour of SO2.
Add some solid potassium iodide: Formation of a dark grey, almost black
precipitate.
Add some concentrated HCl (appr. 30%): The black solid dissolves and the
liquid becomes clear and deep orange/yellow.
Control sequence:
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Add some solid potassium iodide to dilute HNO3 (2 mol/l): The solid dissolves
and the liquid remains colorless. Hence, 2 M HNO3 does not oxidize iodide.
Remarks:
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With iodide, bismuth forms a black precipitate, BiI3. On adding excess iodide,
a yellow coordination complex [BiI4]- is formed. This complex dissolves in
water.
Apparently, in the presence of chloride, a deep orange/yellow complex is
formed with chloride, iodide and bismuth. On strong dilution the liquid
becomes black again. So it seems that the orange complex is not stable at
low concentration of chloride.
Bismuth triiodide (BiI3) is a black insoluble compound.