Description of experiment
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Needed compounds:
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nitric acid : HNO3
hydrogen peroxide : H2O2
sodium sulfite : Na2SO3
sulphuric acid : H2SO4
potassium permanganate : KMnO4
potassium alum : KAl(SO4)2 . 12H2O
potassium ferricyanide : K3 [Fe(CN)6]
potassium ferrocyanide : K4 [Fe(CN)6] . 3H2O
Class:
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elem=Al,Mn,Fe
coordination
redox
Summary:
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Aluminium (III) does not form colored compounds with ferrocyanide nor with
ferricyanide. Manganese (II) reacts with both of them, but a colored
compound is formed with ferricyanide only.
Description:
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Add a solution of alum to a solution of potassium ferrocyanide: No
precipitate, no change of color.
Add a solution of alum to a solution of potassium ferricyanide: No
precipitate, no change of color.
Prepare an acidic solution with Mn2+ ions by adding sodium sulfite to a
dilute sulfuric acid (appr. 10%), to which potassium permanganate is added.
Preparation is done by cautiously adding sodium sulfite to the liquid, until
all potassium permanganate is converted. The resulting liquid is colorless.
Add the solution, containing manganese (II) ions to a solution of potassium
ferricyanide: A brown precipitate is formed.
Add the solution, containing manganese (II) ions to a solution of potassium
ferrocyanide: A white precipitate is formed.
Add some dilute HNO3 (2 mol/l) and some H2O2 (3%) to the white precipitate:
The precipitate slowly turns brown, finally it has become greyish
purple/brown.
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The following experiment shows some strange results, which may be due to
the strongly acidic character of the liquid.
Add a solution of alum and some of the liquid, containing manganese (II) to
a solution of potassium ferrocyanide: No precipitate is formed. However,
after approximately half a minute, the liquid becomes turbid and a pale
white/blue precipitate is formed.
Add some H2O2 (3%): The precipitate becomes greyish blue/green.
Acidify with some dilute HNO3 (2 mol/l): The precipitate becomes grey.