Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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hydrogen peroxide : H2O2
potassium ferrocyanide : K4 [Fe(CN)6] . 3H2O
hydrochloric acid : HCl
sodium thiosulfate : Na2S2O3 . 5H2O
ferrous sulfate : FeSO4 . 7H2O
ferric chloride : FeCl3 . 6H2O
Class:
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elem=Fe,S
redox
coordination
Summary:
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Thiosulfate gives a purple coordination complex with iron (III). This
complex, however, is not stable. With iron (II) no complex is formed.
Iron (III) is reduced by thiosulfate after the initial formation of
the purple coordination complex. This is shown by adding ferrocyanide,
which does not result in formation of an intense dark blue precipitate.
Description:
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Sequence 1:
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Add a slightly acidified solution of FeCl3.6H2O (just a few drops of
solution of HCl added in order to make the solution of ferric chloride
clear) to a solution of sodium thiosulfate: The liquid turns dark
purple. The liquid slowly looses its color and after appr. 2 minutes it
is almost colorless. After a few more minutes the liquid becomes slightly
turbid, due to formation of finely divided sulphur particles. The liquid
also has a smell (albeit not very strong) of sulphur dioxide.
Add a solution of potassium ferrocyanide: A precipitate is formed. Its
color is pale blue/sky-blue.
Sequence 2:
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Add some solid ferrous sulfate to a solution of sodium thiosulfate: The
ferrous sulfate dissolves. The liquid becomes pale yellow and a little
turbid (which also happens when ferrous sulfate is dissolved in plain
water).
Add a little amount of citric acid (no strong acid, like HCl is added,
otherwise the thiosulfate dissociates). The citric acid is added in order
to reduce the turbidity of the liquid because of the presence of the
ferrous sulfate: The addition of the citric acid has no visible effect.
Add a solution of potassium ferrocyanide: Formation of a milk-like liquid,
whose color is pale blue/sky-blue.
Add some H2O2 (3% solution): The liquid instantaneously turns intense
dark blue (almost black) and it becomes completely opaque. A dark blue
precipitate is formed (ferrous ferric hexacyanoferrate complex, Prussian
blue).