Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium hydroxide : NaOH
ferric chloride : FeCl3 . 6H2O
ammonium persulfate : (NH4)2S2O8
sodium thiocyanate : NaSCN
hydrochloric acid : HCl
potassium permanganate : KMnO4
Class:
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elem=Mn
precipitation
coordination
redox
Summary:
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Permanganate is capable of oxidizing thiocyanate in acidic environments:
The result is a pink solution, more intensely colored than manganese (II)
ions (which are almost colorless).
Description:
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Add some solid KMnO4 to a solution of NaSCN, acidified with some HCl (10%
by weight): The KMnO4 dissolves, the liquid becomes pale dirty pink.
Add some solid (NH4)2S2O8: The solid dissolves, no visible changes. After
dissolving the peroxosulfate, there still is thiocyanate in solution. This
can be shown by testing with a solution of ferric chloride (deep red
coloration).
Add more solid KMnO4 and more HCl (10%): The solid dissolves and the liquid
becomes pink more intensely colored. The color is much stronger, than one
would expect on the basis of manganese (II) alone.
Add part of the pink liquid to some acetone: Liquids mix, no further visible
changes.
Add the other part of the pink liquid to an excess amount of a solution of
NaOH: Formation of a brown precipitate, which slowly becomes darker. After
approximately 1 minute this precipitate has becomes very dark brown (MnO2?).
Apparently there still was some peroxosulfate in the solution, which oxidizes
the manganese (II) or manganese (III) compounds to the almost black MnO2.
On the other hand, a test for thiocyanate with ferric chloride still is
positive. This means that, although peroxosulfate is a stronger oxidizer
than permanganate, it oxidizes thiocyanate much slower (or not at all).