Description of experiment
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Needed compounds:
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sodium sulfite : Na2SO3
potassium metabisulfite : K2S2O5
sulphuric acid : H2SO4
chrome alum : KCr(SO4)2 . 12H2O
Class:
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elem=Cr
coordination
Summary:
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Bisulfite and sulfite form coordination complexes with chromium (III). The
exact behaviour strongly depends on the pH of the solution.
When the pH is raised with sulfite and then decreased again with sulphuric
acid, then the color does not revert to the original color of aqueous
chromium (III). Probably a new compound is found, which is inert and does
not fall apart quickly in acidic environments.
Description:
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Sequence 1:
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Add a solution of potassium metabisulfite to a solution of chrome alum: The
liquid turns green.
Add some dilute sulphuric acid (appr. 2 mol/l): The liquid becomes violet
again (just as a plain solution of chrome alum).
Sequence 2:
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Add some solid potassium metabisulfite to an acidified solution of chrome
alum (acidified with some sulphuric acid): The solid dissolves, the liquid
remains violet.
Carefully add a solution of sodium sulfite, in small amounts, shaking the
solution after each small amount added: The first few amounts of sodium
sulfite do not cause a change of colour. From a certain point, however, the
liquid becomes more greenish after each small portion of sulfite added. This
can be continued, until the liquid has a deep green colour.
Add some dilute sulphuric acid (appr. 2 mol/l): The liquid becomes more
bluish, it does not become violet anymore.
Sequence 3:
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Add some sodium sulfite to a solution of chrome alum: The solid dissolves and
the liquid becomes deep green. The liquid remains clear.
Add a small amount of dilute sulphuric acid (appr. 2 mol/l): The liquid
becomes bluish green.
Add much more dilute sulphuric acid: The liquid remains bluish green.