Description of experiment
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Needed compounds:
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potassium dichromate : K2Cr2O7
copper sulfate penta hydrate : CuSO4 . 5H2O
sodium hydroxide : NaOH
ferric chloride : FeCl3 . 6H2O
tannin : C76H52O46
Class:
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elem=Fe;Cu;Cr
redox
coordination
Summary:
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Tannine (a polyphenolic compound of large molecular weight of indeterminate
composition) reacts with many metal ions, forming highly coloured complexes.
It also shows some other reactions. The exact type of reactions is not
always clear.
The tannine, used in these experiments, was brown. It's intended use is
as an additive for making wine.
Description:
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Add a solution of ferric chloride to a solution of tannine: The liquid becomes
black and turbid. No clear precipitate is formed.
Add a solution of NaOH to a solution of tannine: The liquid becomes slightly
darker. At the surface it becomes much darker. When it is shaken, then the
entire liquid becomes darker and turbid. Is the darkening due to reaction
with oxygen from the air?
When the dark liquid is diluted with a lot of water, then the liquid becomes
bordeaux-red and clear.
Add a solution of copper sulfate to a solution of tannine: The liqud becomes
turbid and green/brown.
Add some dilute H2SO4 (appr. 2 mol/l) to the turbid green/brown liquid: It
becomes clear and becomes green, with a brownish hue.
Add some solid K2Cr2O7 to a solution of tannine: The liquid becomes dark and
it is covered by a thin solid layer, as sometimes can be observed on old tea.
The liquid also has the same smell as old strong tea.