Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium dithionite : Na2S2O4 . 2H2O
potassium ferrocyanide : K4 [Fe(CN)6] . 3H2O
ferrous ammonium sulfate hexa hydrate : Fe (NH4)2 (SO4)2 . 6H2O
potassium ferricyanide : K3 [Fe(CN)6]
ferric ammonium sulfate hydrate : Fe NH4 (SO4)2 . 12H2O
Class:
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elem=Fe
coordination
redox
Summary:
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Ferric ions give a dark brown complex with ferricyanide. No precipitate is
formed. This precipitate is very easily converted to the dark blue prussian
blue.
Ferrous ions give a light yellow precipitate with ferrocyanide. It is,
however, very difficult to get this precipitate. The slightest amount of
oxygen makes the precipitate blue.
Description:
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Sequence 1:
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Dissolve some ferric ammonium sulfate in water and add a solution of potassium
ferricyanide: The liquid becomes dark brown and remains clear. When the liquid
is left alone for a while, then it still is dark brown and clear.
Add a small amount of a solution of sodium dithionite: The vapour of the
sodium dithionite solution (containing SO2, smell!) already is capable of
making the dark brown liquid blue. A little of the dark brown liquid, sticking
to some glass quickly turns intense blue, when it is kept above the dithionite
solution, without actually touching it.
When the dithionite solution is mixed with the dark brown liquid, then a dark
blue precipitate is formed at once.
Sequence 2:
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Dissolve some ferrous ammonium sulfate in water and add a solution of potassium
ferrocyanide: A non-uniform light blue precipitate is formed. At some places
it is very pale blue, at some other places a little bit darker. At the surface
of the liquid, the intensity of the blue color quickly increases.
Add a very small amount of a solution of sodium dithionite and shake: The blue
color disappears and a light yellow precipitate is formed. Probably that is
the real color of ferrous ferrocyanide. The blue color is due to oxidation
products, causing the formation of mixed ferro/ferri cyanide compounds, which
have a very strong blue color.