Description of experiment
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Needed compounds:
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sodium hydroxide : NaOH
hydrochloric acid : HCl
potassium ferrocyanide : K4 [Fe(CN)6] . 3H2O
Class:
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elem=Fe
redox
precipitation
Summary:
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When concentrated hydrochloric acid is added to a concentrated solution of
potassium ferrocyanide, then a white precipitate is formed (probably this
is KCl). The liquid slowly turns blue, but this is not due to formation
of the well known prussian blue or a similar compound.
Description:
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Add some concentrated HCl (30% by weight) to a concentrated solution of
potassium ferrocyanide: A white precipitate is formed, which quickly goes
to the bottom of the test tube.
Keep tube stoppered and store for a few days in the dark: The liquid slowly
turns blue, every day it has becomes a little bit more intense. After a week
or so, the liquid is bright blue.
Decant liquid from test tube and rinse with water: A blue compound sticks to
the glass. When rinsed with sodium hydroxide solution, this compound remains
blue, as opposed to prussian blue, which becomes brown when alkali is added.