Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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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=Fe,Al
precipitation
coordination
Summary:
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Ferrocyanide and ferricyanide apparently form a coordination complex with
aluminum, but only if both the ferrocyanide and ferricyanide are present.
Description:
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Sequence 1:
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Add a solution of alum to a solution of potassium ferrocyanide: No precipitate,
no visible changes. The liquid becomes almost colorless (very light yellow).
On standing, no visible changes can be observed.
Sequence 2:
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Add a solution of alum to a solution of potassium ferricyanide: No precipitate,
no visible changes. The liquid becomes intense yellow, just as any solution
of potassium ferricyanide. On standing, no visible changes can be observed.
Sequence 3:
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Mix some solid potassium ferrocyanide and potassium ferricyanide and add
some water: Both solids dissolve, the liquid becomes yellow. The color of
the ferrocyanide is completely masked by the much more intense color of the
ferricyanide. Besides this, no visible reaction occurs.
Add a solution of alum: The liquid becomes green/yellow, there is is subtle,
but clearly visible shift of color from yellow to green/yellow (lemon-like).
After appr. 15 minutes of standing, the liquid has become turbid.
Heat liquid and boil gently: The mixture becomes darker quickly and finally
it becomes dark green, with blue 'clouds' in the liquid. At a certain moment,
the liquid becomes very viscous, it almost solidifies and behaves like a
concentrated solution of starch, which has boiled for some time. On cooling
down, the liquid becomes viscous and dark green/blue.
Remark: A possible explanation for the observations above is that when both
ferricyanide and ferrocyanide are available and both species form a complex
with aluminum, then in one ion/molecule, there is iron (II) and iron (III).
This combination, together with coordinated cyanide, gives rise to intensely
colored (mostly blue/green) complexes.