Description of experiment
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Needed compounds:
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potassium iodide : KI
sodium thiosulfate : Na2S2O3 . 5H2O
nitric acid : HNO3
bismuth : Bi
Class:
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elem=S,I,Bi
coordination
Summary:
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Bismuth forms a yellow complex with thiosulfate. In the presence of
thiosulfate, it still forms a deep orange complex with iodide, just as without
thiosulfate.
Description:
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Prepare a solution of bismuth nitrate by dissolving some bismuth metal in
nitric acid. This takes quite some time. Initially the reaction is vigorous
with evolution of a lot of brown NO2, but lateron the reaction is slow. Dilute
the solution with water, such that the solution remains clear.
Dissolve quite some sodium thiosulfate in water and then add the clear and
fairly acidic solution of bismuth nitrate: A light yellow precipitate is formed
at once. It very much looks like sulphur, same type of yellow, same brightness.
Above the yellow precipitate there still is colorless solution of sodium
thiosulfate. The sodium thiosulfate was present in large excess amount.
Shake the liquid with the yellow precipitate: The precipitate dissolves again
and the liquid becomes clear and bright yellow. There also is a faint smell of
SO2, probably due to decomposition of thiosulfate.
Add some solid potassium iodide to the liquid: Around the crystals, the liquid
becomes bright orange, when all iodide is dissolved, the liquid becomes bright
yellow, even brighter than before. Still, the liquid is clear. This reaction is
quite much like the reaction of excess iodide with bismuth, without thiosulfate
being present.