Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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hydroxyl amine sulfate : (NH3OH)2 SO4
sodium hydroxide : NaOH
sodium sulfite : Na2SO3
sulphuric acid : H2SO4
vanadium pentoxide : V2O5
Class:
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elem=V,N
redox
Summary:
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The result of this experiment is remarkable. Hydroxyl amine, being a strong
reductor, appears to oxidize vanadium (IV) to vanadium (V) in alkaline
environments. Or is there another compound, which strongly resembles the
well-known yellow colour of vanadium (V) in acidic environments and the
(almost) colourless appearance of vanadium (V) in alkaline environments?
Description:
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Add some sulphuric acid to a mixture of vanadium pentoxide and excess sodium
sulfite and boil gently: All vanadium pentoxide is dissolved and a clear
bright blue liquid is formed. Boiling is continued, until no smell of SO2
can be observed anymore above the blue liquid.
Add an excess amount of a solution of NaOH, neutralizing all acid: The liquid
turns brown and remains clear. It becomes fairly dark. This is the colour of
vanadate (IV) in alkaline environments.
Add a large excess amount of solid hydroxyl amine sulphate and heat: The
liquid becomes lighter and lighter, until it is almost colorless (very light
yellow). There is a faint smell of ammonia (or is this the smell of free
hydroxyl amine, liberated in the strongly alkaline environment?). During
the reaction a lot of gas is evolved, however, when the hydroxyl amine sulfate
was added to the cooled down brown liquid, this was not yet the case.
Apparently the red/brown vanadium (IV) compound is oxidized to an almost
colorless vanadium (V) compound.
Add an excess amount of sulphuric acid: The liquid becomes intense yellow,
however, within a minute this colour shifts to green/blue and a little
while later, the liquid is almost completely blue.
Apparently the colorless alkaline vanadium (V) is transformed in yellow/
orange vanadium (V) in acidic environment, which is reduced by excess
hydroxyl ammonium ions, remaining from the excess hydroxyl amine sulfate.