Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium sulfite : Na2SO3
hydrochloric acid : HCl
sodium nitrite : NaNO2
sodium hexa nitrito cobaltate (III) : Na3 [Co(NO2)6]
Class:
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elem=N,Co
coordination
redox
Summary:
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The hexanitritocobaltate (III) ion is quite stable, but at very low pH, it is
destroyed either by reducing agents, or by coordinating agents. But the nitrito
ligand is MUCH more stable than in the plain nitrite ion.
Description:
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Dissolve some sodium nitrite in water and add some 10% hydrochloric acid: A gas
is produced, and the gas mixture above the liquid becomes brown. The nitrite
quickly decomposes in the moderately concentrated acid.
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Dissolve some sodium hexanitritocobaltate(III) in water and add some 10%
hydrochloric acid: The liquid remains orange, it seems to become alightly more
reddish in appearance. No other visible changes.
Heat the liquid: The color of the liquid slowly shifts from orange/red to blue,
through shades of green. This is due to slow decomposition of the
hexanitritocobaltate(III) ion into cobalt(II) and nitrogen oxides. This
reaction is possible, because of the coordinating of the cobalt(II) to the
chloride ions in the solution. The deep blue CoCl4(2-) complex is formed.
Let the liquid cool down: The color slowly changes from blue to pale pink. The
tetrachloro-complex of cobalt(II) is changed to the pink hexaaque-complex of
cobalt(II).
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Dissolve some sodium hexanitritocobaltate(III) in water and add some 10%
sulphuric acid: The liquid remains orange, no visible changes.
Add a solution of sodium sulfite in dilute sulphuric acid: Immediate production
of a colorless gas, a lot of bubbles are produced. Apparently the
nitrito-ligands are reduced by the sulfite (actually sulphur dioxide) with
production of N2 or N2O.
Heat the solution for a while: More bubbles are produced, and the liquid turns
from fairly deep orange to very pale pink, the color of dilute aqueous
cobalt(II) ion.
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Add some solid sodium hexanitritocobaltate(III) to concentrated (30%)
hydrochloric acid: The solid quickly dissolves and a deep blue/cyan compound is
formed. The liquid finally becomes intensely green.
Heat the liquid for a while: The color shifts from green to deep blue.
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This set of experiments nicely shows that nitrite can be quite stable, even in
strongly acidic environments. The "bare" nitrite ion is not stable at all in
acid, but when it is coordinated to cobalt(III) then fairly extreme
environments must be created before the nitrite-ligands are destroyed.