Description of experiment
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Needed compounds:
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oxalic acid : HOOC COOH . 2H2O
sodium sulfite : Na2SO3
hydrochloric acid : HCl
potassium dichromate : K2Cr2O7
Class:
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elem=Cr
coordination
redox
Summary:
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Chromium (III) has many different colors, depending on coordinated ligands.
Description:
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Prepare a solution of potassium dichromate (K2Cr2O7) in dilute hydrochloric
acid (appr. 5% by weight). This solution is called (1).
Sequence 1:
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Add an excess amount of sodium sulfite to (1):
The crystals of sodium sulfite dissolve and the solution turns dark green
with a bluish shade.
Add a fairly large amount of oxalic acid to the green/bluish solution: The
oxalic acid dissolves. The color of the solution hardly changes.
Heat to appr. 60 C: The solution does not change noticably, it remains
green/bluish.
Let solution cool down to appr. 15 C: No visible change.
Sequence 2:
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Add a fairly large excess amount of oxalic acid to (1): Solution hardly
changes, it remains orange. The oxalic acid dissolves.
Heat to appr. 60 C: The color of the solution changes from orange to orange/
brown. No further changes, not even after 10 minutes.
Add a crystal of solid sodium sulfite to the still warm solution: The sodium
sulfite quickly dissolves, the liquid starts foaming. The color quickly
changes from orange/brown to deep purple. After cooling down to 15 C the
liquid remains purple and clear.
(The foaming probably is due to formation of SO2 in the warm solution, which
escapes as a gas).
Final results:
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After sequence 1 there is a green/bluish liquid, after sequence 2 there
is a purple liquid.
One hour later:
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The purple liquid still is purple.
The green/bluish solution is slightly more bluish.
Two hours later:
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The greenish solution has turned less green, its color is less distinctive.
One day later (appr. 24 hours):
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The greenish solution has become purple as well.