Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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potassium ferricyanide : K3 [Fe(CN)6]
sulphuric acid : H2SO4
EDTA tetra sodium : (NaOOC CH2)2 N CH2 CH2 N (CH2 COONa)2
copper sulfate penta hydrate : CuSO4 . 5H2O
Class:
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elem=Cu
precipitation
coordination
Summary:
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Copper (II) ions are complexed by EDTA Na4, but when the liquid is acidified,
then the copper ions are not coordinated anymore.
Description:
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Sequence 1:
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Add a solution of copper sulfate to an excess amount of a solution of EDTA Na4:
The liquid becomes dark blue. The color is not like the deep royal blue of
cuprammine complex, but it is a more cyan-like blue.
Add a solution of potassium ferricyanide: No precipitate is formed, the
liquid remains clear and fairly dark green.
Add a few drops of sulphuric acid: Formation of a brown/green precipitate.
Sequence 2:
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Add a solution of copper sulfate to a solution of potassium ferricyanide:
Formation of a brown/green precipitate.
Remark: In sequence 1, the copper (II) ions are 'trapped' by the EDTA. The
solubility product of copper / ferricyanide is not exceeded, due to the low
concentration of free copper (II) ions. On acidification, apparently the
EDTA is not capable of keeping the copper coordinated and the free copper (II)
forms a precipitate with the ferricyanide.