Description of experiment
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Needed compounds:
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hydrochloric acid : HCl
potassium hydroxide : KOH
potassium ferricyanide : K3 [Fe(CN)6]
Class:
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elem=Fe
redox
Summary:
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This experiment nicely demonstrates the effect of light on a chemical compound.
An alkaline solution of potassium fericyanide (K3Fe(CN)6) is stable in the
dark, but in sunlight it decomposes in a few hours.
Description:
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Dissolve some potassium hydroxide in water and let the liquid cool down (quite
some heat is evolved when potassium hydroxide is dissolved in water).
In a separate test tube, dissolve some potassium hexacyanoferrate (III), a.k.a.
red prussiate of potash, in water. This gives a clear deep yellow solution.
Mix the two solutions. This results in a clear yellow solution.
Divide the solution over two test tubes and stopper both test tubes.
Put one test tube in a totally dark place and put the other test tube in a
sunny place (direct sunlight is best). Leave the test tubes in their place for
one day or so.
After one day, the test tube which was put in a dark place still contains a
clear yellow liquid, while the test tube, which was put in a sunny place
contains a turbid brown/orange liquid and quite some precipitate has settled at
the bottom.
Add a small amount of the turbid brown/orange liquid to excess dilute
hydrochloric acid: The liquid turns green. A small amount of iron(II) is
present, leading to formation of prussian blue, but the amount is very small,
hence the green color, due to mixing with the strong yellow color.
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Remark: The experiment also works with sodium hydroxide instead of potassium
hydroxide.
Remark 2: Ferricyanide ion is stable in the dark, but when light shines on it,
then it slowly decomposes. The decomposition probably is some internal redox
reaction, which in the strongly alkaline conditions of this experiment leads to
formation of iron(III)hydroxide. Any iron(II), formed in the process most
likely is oxidized back to iron(III) by oxygen from the air, or by some species
in solution.