Description of experiment
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Needed compounds:
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hydrochloric acid : HCl
sulphuric acid : H2SO4
sodium sulfite : Na2SO3
chrome alum : KCr(SO4)2 . 12H2O
potassium dichromate : K2Cr2O7
ammonium thiocyanate : NH4SCN
Class:
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elem=Cr
coordination
Summary:
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Chromium (III) can have many different colors, depending on how it is
created and with which it coordinates.
Description:
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Make a solution of NH4SCN and add some HCl (10% by weight). Add a solution
of K2Cr2O7: The liquid turns purple/pink.
Add an excess amount of a solution of NaOH to the purple/pink solution of
K2Cr2O7, reduced by NH4SCN: The liquid becomes a little opaque, milky. Even
when a lot of NaOH is used, it remains a little opaque. The color of the
liquid becomes very pale grey/green. Based on the amount of K2Cr2O7 used
for the experiment a strongly colored green solution was expected on addition
of NaOH, but only a faint color can be observed. Probably, this is due to
formation of a complex in an alkaline medium.
Dissolve some chrome alum in water: The liquid looks purple when viewed with
the light from an ordinary light bulb, it looks grey when viewed with the
light from an SL-lamp. In daylight it looks bluish-purple. Even when viewed
with an ordinary light bulb, the purple color is much more bluish, than the
color of the reduced dichromate, which tends towards pink.
Let the solution of chrome alum stand for one week: The liquid hardly has
changed color, it appears to be a little more bluish than a fresh solution.
Make a solution of Na2SO3 and add a little amount of dilute H2SO4. Add a
solution of K2Cr2O7: The liquid turns green. After a week it has become
green/blue.
Remark: Four experiments with chromium (III) show that the color of this
ion can be anywhere from pink/purple to green/blue.