Description of experiment
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Needed compounds:
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ammonia : NH3
sodium hydroxide : NaOH
potassium dichromate : K2Cr2O7
sulphuric acid : H2SO4
aniline : C6H5NH2
Class:
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elem=C,N,Cr
organic
coordination
redox
Summary:
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Aniline, combined with acidified dichromate gives intensely colored compounds.
Description:
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Dissolve some aniline in excess dilute sulphuric acid. This gives a solution of
anilinium sulfate. If the solution is not colorless, use some activated carbon
to remove the color. This can be done by shaking with the activated carbon and
allowing to stand for a day. After this treatment a colorless liquid remains,
which is a solution of anilinium sulfate in dilute sulphuric acid.
Sequence 1:
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Add a small amount of a solution of potassium dichromate to an excess amount of
a solution of anilinium sulfate: The liquid at first remains orange/yellow, but
fairly quickly it becomes green. A little bit later, it becomes turbid and a
dark blue precipitate is formed. The color of the precipitate is really intense
and even a very small amount of this makes the liquid almost look black.
Add an excess amount of 5% NH3 to the liquid: The color of the precipitate
changes from deep blue/black to a beautiful deep blue/indigo.
Sequence 2:
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Mix a small amount of a solution of anilinium sulfate with a lot of water and
add a few crystals of potassium dichromate and swirl the liquid for a while:
All potassium dichromate dissolves and the liquid becomes yellow.
Wait for a few minutes: The liquid turns green. After some more time, it turns
blue and a precipitate is formed.
Add some solid NaOH and swirl again: The NaOH dissolves and the precipitate
becomes purple. The liquid also becomes purple, so the precipitate is somewhat
soluble.
Again, it is remarkable how intense the color of the compound is. Even a few
crystals of potassium dichromate cause the formation of a lot of colored
substance, sufficient for giving a deep color to several hundreds of ml of
water.
Remarks:
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The precipitate sticks to the glass and cannot be removed with alkali or acid.
Only warm concentrated nitric acid was capable of cleaning the glass. The blue
precipitate dissolves in the acid and gives a light yellow/green/brown color to
the acid.