Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sulphuric acid : H2SO4
sodium nitrite : NaNO2
vanadyl sulfate : VOSO4.5H2O
Class:
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elem=N,V
coordination
redox
Summary:
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When a solution of vanadyl sulfate is mixed with excess sodium nitrite, then a
very dark liquid is obtained and some NO is formed. On acidification, this
liquid becomes green/yellow, but on boiling, it becomes blue again and it
appears that all vanadium goes to oxidation state 4 again.
Description:
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Dissolve some vanadyl sulfate in water and add this solution to a large excess
amount of a concentrated solution of sodium nitrite in water: The liquid
becomes very dark brown, almost black. A thin layer of this liquid, sticking to
the glass, has an intense yellow/ochre color. Also some colorless gas is
evolved, and the gas-mix above the liquid turns light brown. So, the gas must
be nitrogen monoxide.
Add a few drops of 20% sulphuric acid: Much more gas is evolved and the liquid
turns much lighter and becomes yellow/green.
Heat the liquid for a few minutes: More colorless gas is produced and the color
of the liquid slowly shifts from gree/yellow to bright blue, the color of a
pure solution of vanadyl sulfate.
Remark:
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It seems that at near neutral pH a complex is formed and vanadium(IV) is
oxidized to vanadium(V). On acidification, the liquid is yellow with a green
tinge, indicating that most of it is vanadium(V) with some vanadium(IV) in it.
On heating it seems that the vanadium(V) oxidizes nitrite or nitrous acid and
vandium(IV) is formed.
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The same experiment was repeated with the same amount of vanadyl sulfate, but
much less sodium nitrite was used. In this case, the liquid becomes dark again,
just as with large excess of sodium nitrite, but the main difference is that
the liquid now become turbid and a brown precipitate is formed.
On acidification with acid, the result is the same as with a larger amount of
sodium nitrite, but on heating now the liquid does not becomes purely blue.
Some green tinge remains, so some vanadium remains in oxidation state +5.