Description of experiment
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Needed compounds:
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sodium sulfide : Na2S . 3H2O
potassium dichromate : K2Cr2O7
Class:
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elem=Cr,S
coordination
redox
Summary:
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When dichromate is added to sulfide, then a transient highly coloured
compound is formed, immediately followed by reduction to green chromium (III)
in alkaline environment.
Description:
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Add a solution of potassium dichromate to an excess amount of a solution of
sodium sulfide: As soon as the orange solution enters the colorless solution,
a deep red/brown compound is formed, which quickly disappears again (it only
exists for appr. 1 second) and then the liquid becomes yellow/green and a
little turbid. A flocculent precipitate is formed which makes it turbid.
It looks as if part of the chromium (VI) is reduced to chromium (III) and
that another part remains in the hexavalent state as yellow chromate.