Description of experiment
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Needed compounds:
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hydrogen peroxide : H2O2
hydrochloric acid : HCl
ammonium persulfate : (NH4)2S2O8
sodium hydroxide : NaOH
chrome alum : KCr(SO4)2 . 12H2O
Class:
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elem=Cr,S,O
redox
Summary:
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Chromium (III) can be oxidized to chromium (VI) by persulfate in alkaline
environments.
Description:
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Add some solid chrome alum to a concentrated solution of NaOH: The chrome
alum dissolves, the liquid becomes green and clear.
Add a fairly large amount of ammonium persulfate: Some gas is evolved, the
ammonium persulfate dissolves, no further visible changes.
Heat to appr. 60 C: The color of the liquid changes, it becomes yellow/green.
Even after many minutes, the greenish color does not disappear completely.
Add an excess amount of HCl (10% by weight): The liquid turns yellow/brown
with a greenish shade. The liquid is clear.
Add some H2O2 (3% solution): The liquid turns deep blue and a gas is evolved.
The deep blue color quickly fades, finally the liquid is clear and green.
Conclusion: Persulfate is able to convert chromium (III) to chromium (VI),
but this reaction is slow and not complete.