Description of experiment
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Needed compounds:
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hydrochloric acid : HCl
hydrogen peroxide : H2O2
chrome alum : KCr(SO4)2 . 12H2O
sodium hydroxide : NaOH
Class:
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elem=Cr
redox
Summary:
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Oxidation of chromium (III) to chromium (VI).
Description:
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Add a concentrated solution of sodium hydroxide to a concentrated solution
of chrome alum and add an excess amount of a 3% solution of H2O2 and heat
to 60 C: Liquid turns yellow and a fairly large amount of gas is evolved.
Add dilute HCl (10% by weight), while gas is still evolved: Where HCl enters
the liquid it turns blue/indigo, but this color quickly fades again.
Remark: The deep blue/indigo color is due to the formation of a peroxo-
compound of chromium. This is used to show the presence of peroxides or
chromium (VI). This experiment shows that when the liquid is completely
yellow and a gas is evolved, that some H2O2 is left and that this decomposes,
due to the heat and the alkaline environment.