Description of experiment
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Needed compounds:
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ascorbic acid : C6H8O6
anhydrous chromic acid : CrO3
potassium dichromate : K2Cr2O7
Class:
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elem=C,Cr
coordination
redox
Summary:
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Hexavalent chromium oxidizes ascorbic acid. This reaction is fast and even
proceeds without adding additional acid. The acidity of the ascorbic acid
suffices to make the reaction proceed quickly.
The resulting chromium(III) forms an intensely colored dull-green complex
(green/grey, almost black at moderate concentration).
Description:
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Put a pile of ascorbic acid in a petri dish and add a drop of 50% solution of
chromium trioxide to this: As soon as the solution of CrO3 hits the pile of
ascorbic acid, a violent reaction starts, with hissing and production of white
fumes (probably steam of boiling water). The resulting material is very dark
brown and solid.
Rinse with a lot of water: The solid material dissolves, giving a dark brown
solution, with many insoluble dark particles in it.
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In a test tube, dissolve some potassium dichromate: A bright orange solution is
obtained.
In another test tube, dissolve some ascorbic acid. Use an amount, which is
clearly much more than the amount of dichromate used, so that the ascorbic acid
is present in excess amount.
Mix the two solutions: As soon as the liquids are mixed, the liquid turns very
dark green/grey, nearly black.
Dilute the liquid with a lot of water: The liquid appears to be green, but the
color is not bright. It is a dull greyish green.
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Repeat the above experiment with a little chromium trioxide instead of
potassium dichromate (a solution of chromium trioxide is more acidic): Again,
an orange solution is obtained, which on mixing with the colorless solution of
ascorbic acid gives a very dark grey/green solution. The color of the resulting
solution is the same as that obtained with potassium dichromate.