Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium metabisulfite : Na2S2O5
sodium bisulfate : NaHSO4
potassium ferricyanide : K3 [Fe(CN)6]
sodium sulfite : Na2SO3
oxalic acid : HOOC COOH . 2H2O
sodium hydroxide : NaOH
ferric ammonium citrate brown : Fe C6H5O7 . 2NH3 . 2H2O
Class:
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elem=Fe
coordination
redox
Summary:
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Ferric ions form coordination complexes with citrates and oxalates. These
coordination complex have completely different properties than free
ferric ions.
Description:
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Dissolve some ferric ammonium citrate brown in water: The liquid becomes
yellow brown and completely clear. This is an important difference when
compared to ferric chloride, which hydrolyses and becomes turbid.
Sequence 1
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Add an excess amount of oxalic acid and a little amount of NaOH: The solids
dissolve and the liquid becomes less intensely colored. The color shifts
from yellow/brown to yellow/green. A ferric-oxalato complex is formed, which
is yellow/green. As long as there was still solid NaOH present, the granules
were covered with a brown layer (ferric hydroxide??), but there apparently
was sufficient oxalic acid for dissolving everything and getting a perfectly
clear yellow/green solution.
Add some sodium sulfite: The liquid becomes darker and the color changes to
yellow/brown/orange. After approximately one day the color has changed to
pale yellow and a white precipitate is formed. When a little more water is
added, the white precipitate dissolves again (this white precipitate probably
is sodium sulfate).
Sequence 2
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Add a solution of potassium ferricyanide to the solution of ferric ammonium
citrate brown: The liquid remains clear and gets and intense yellow color.
Add a large amount of solid NaHSO4 (strong acidification of liquid): The
solid dissolves and the liquid becomes green/yellow.
Add a little amount of Na2S2O5: The liquid turns dark blue/green. Where the
solid is in contact with the liquid, a dark blue compound is formed.