Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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potassium dichromate : K2Cr2O7
sulphuric acid : H2SO4
hydrochloric acid : HCl
sodium hydroxide : NaOH
ascorbic acid : C6H8O6
Class:
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elem=C
redox
Summary:
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Ascorbic acid is oxidized in a strongly alkaline environment, probably by
oxygen from the air. The oxidation product can be oxidized further, but
this only occurs slowly, compared to the speed with which fresh ascorbic
acid can be oxidized.
Description:
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Add some ascorbic acid to an excess amount of NaOH and dissolve both solids
in water: The liquid becomes very pale yellow/green, it is just not colorless.
The liquid becomes clear.
Leave in contact with air for 24 hours: Liquid has become pink and a small
amount of a pink precipiate is formed with a brownish hue.
Decant approximately 75% of the liquid and set this apart. This part is used
in sequence 2.
Sequence 1:
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Take the remaining liquid, containing the precipitate and add an excess
amount of HCl (10% by weight): The liquid becomes yellow and clear. Besides
this some gas is evolved (this probably is CO2, which was taken from the
air by the alkaline solution).
Add a little amount of K2Cr2O7 and shake well, until all of it is dissolved:
The liquid becomes yellow/orange/brown. It does not get the bright color of
a plain dichromate solution. After approximately 10 minutes the liquid is
brown and has become slightly darker.
Add some fresh ascorbic acid to part of the brown liquid: The solid dissolves
and the liquid becomes greyish immediately, all remaining dichromate is reduced
at once.
The remaining part of the brown liquid is left to stand for two days. After
these two days it has become greyish/blue/violet, depending on the type of
lamp, used for observing the liquid. The dichromate is reduced, but it takes
a long time to reduce all of it.
Sequence 2:
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The 75% of the liquid, not containing anything from the precipitate is
acidified with an excess amount of sulphuric acid (appr. 2 mol/l): The
liquid becomes pale yellow and remains clear. A small amount of a colorless
gas is evolved (probably CO2, taken up from the air by the alkaline liquid).
Add a little amount of solid K2Cr2O7 and dissolve this by shaking: The
liquid becomes yellow/brown. After 15 minutes it has become a little darker
and its color has shifted to brown.
Wait a few days: The liquid has become bluish, all dichromate is reduced by
the ascorbic acid.
Remark: Apparently ascorbic acid is oxidized by oxygen from the air in a
strongly alkaline solution. This oxidation brings the compound to a state,
which can be further oxidized, but not very easily. Dichromate is capable
of oxidizing this compound, but it takes a fairly long time.