Description of experiment
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Needed compounds:
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ammonia : NH3
formaldehyde : CH2O
hydrochloric acid : HCl
potassium dichromate : K2Cr2O7
Class:
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elem=Cr
coordination
redox
Summary:
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The color of chromium (III) depends on how it is part of coordination
complexes.
Description:
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Dissolve some K2Cr2O7 in dilute HCl (appr. 10% by weight): The liquid
becomes orange.
Add an excess amount of a solution of CH2O: The liquid slowly turns green.
It takes a few minutes and goes through a range of colors: orange -->
orange/brown --> brown --> brown/green --> mossy green --> bluish green.
The final color is a bluish green. All chromiun (VI) is converted to
chromium (III).
Add NH3 (appr. 5% by weight), until the liquid becomes strongly turbid:
The liquid remains turbid, due to formation of chromium hydroxide.
Add HCl (appr. 10% by weight), until the liquid becomes clear again: Now
the liquid is purple instead of bluish green. Apparently the chromium now
builds another coordination complex, than when it first appears after
reduction of the dichromate.