Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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chrome alum : KCr(SO4)2 . 12H2O
sulphuric acid : H2SO4
hydrochloric acid : HCl
sodium sulfite : Na2SO3
sodium metabisulfite : Na2S2O5
hydrogen peroxide : H2O2
sodium persulfate : Na2S2O8
potassium permanganate : KMnO4
potassium dichromate : K2Cr2O7
p-aminophenol HCl : NH2 C6H4 OH . HCl
Class:
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elem=C
redox
Summary:
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When p-aminophenol is oxidized in an acidic environment, then a compound
is formed, with a deep indigo/purple color.
Description:
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Sequence 1:
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Dissolve some p-aminophenol HCl in HCl (appr. 3% by weight): The liquid
becomes pale brown and clear.
Add a little amount of a solution of potassium dichromate: The liquid turns
intense purple.
Add some sodium metabisulfite: The liquid immediately turns dirty yellow.
Stronmg smell of SO2.
Sequence 2:
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Add a solution of chrome alum to a solution of p-aminophenol HCl in dilute
HCl (3%): No visible reaction, the liquid remains pale violet.
Sequence 3:
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Add some solid potassium permanganate to a solution of p-aminophenol HCl
in dilute HCl (3%): The solid dissolves, the liquid becomes intense purple.
Add some sodium metabisulfite: The liquid turns bright yellow and clear.
Strong smell of SO2.
Sequence 4:
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Add a few drops of H2O2 (30%) to a solution of p-aminophenol HCl in dilute
HCl (3%): No visible changes.
Heat to appr. 60 C: Slowly the liquid turns indigo/purple. The final color
of the liquid is very intense. On cooling down, the intense indigo/purple
color does not disappear.
Add an excess amount of sodium sulfite: The liquid turns yellow. Strong
smell of SO2. A noticeable amount of heat is produced (probably due to
reaction between sulfite and excess H2O2).
Sequence 5:
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Add some p-aminophenol HCl and some sodium persulfate to dilute sulfuric
acid (10% by weight): The solids dissolve, no further visible reactions.
Heat to appr. 60 C: Slowly the liquid turns intense indigo/purple.
Remark:
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The intense indigo/purple compound is an oxidation product of p-aminophenol.
It is not a coordination compound (e.g. chrome alum does not give a strongly
colored compound with p-aminophenol HCl).
The colored compound is not very stable. On longer standing (many hours or
days), the indigo color becomes more and more dull. Finally the liquid
becomes brown, due to formation of a brown precipitate.