Description of experiment
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Needed compounds:
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sodium sulfite hepta hydrate : Na2SO3 . 7H2O
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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Chromium (III) can be converted to chromium (VI) in strongly alkaline
environments.
Description:
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Add an excess amount of a concentrated solution of NaOH to a solution of
KCr(SO4)2.12H2O: The resulting solution is darkgreen and clear.
Add an excess amount of a 3% solution of H2O2: No visible changes occur.
Start heating of the liquid to approximately 60 C: The liquid turns bright
yellow and a slow evolution of a gas is observed. It takes approximately
1 minute to change color from darkgreen to bright yellow. After the color
change is completed, the evolution of gas continues.
Add an excess amount of a 10% (by weight) solution of hydrochloric acid:
The liquid remains yellow and clear. At those places, where the hydrochloric
acid touches the yellow liquid, the color turns to orange/yellow, but after
shaking of the liquid, it is bright yellow. The resulting acidic and yellow
liquid is called YELLOW.
Add an excess amount of a 3% solution of H2O2 to YELLOW: The liquid turns
deep blue and a gas escapes from the liquid. The deep blue color quickly
fades and the resulting liquid becomes pale bluish green (because of all
additions of liquids the concentration of chromium has dropped considerably
and hence colors are pale).
Add an excess amount of solid Na2SO3.7H2O to YELLOW: After shaking, the
liquid turns pale bluish green. Its color is almost the same as the final
color after addition of an excess amount of H2O2.