Description of experiment
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Needed compounds:
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sulphuric acid : H2SO4
hydrogen peroxide : H2O2
potassium ferricyanide : K3 [Fe(CN)6]
potassium ferrocyanide : K4 [Fe(CN)6] . 3H2O
cobalt sulfate : CoSO4.7H2O
Class:
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elem=Co,Fe
precipitation
Summary:
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Cobalt (II) gives a green precipitate with ferrocyanide and a dark red/
purple precipitate with ferricyanide. The green precipitate cannot be
converted to the red one by means of oxidation by hydrogen peroxide, but
another, dark blue, compound is formed.
Description:
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Sequence 1:
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Add a solution of cobalt sulfate to a solution of potassium ferricyanide: A
dark red/purple precipitate is formed.
Add some dilute sulphuric acid (appr. 1 mol/l): No visible change.
Sequence 2:
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Add a solution of cobalt sulfate to a solution of potassium ferrocyanide: An
olive-green precipitate is formed.
Add some hydrogen peroxide (appr. 10% by weight): The precipitate slowly
changes color. It first seems to become grey, but within a few minutes it
turns fairly dark grey/blue and a slow evolution of a gas can be observed.
Add some dilute sulphuric acid (appr. 1 mol/l): No visible change.