Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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ammonia : NH3
hydrogen peroxide : H2O2
sodium carbonate : Na2CO3 . 10H2O
sodium sulfite : Na2SO3
sulphuric acid : H2SO4
vanadium pentoxide : V2O5
Class:
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elem=V
coordination
redox
Summary:
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Vanadium in its +4 oxidation state forms a blue compound, vanadyl, in acidic
environments. In near neutral to alkaline environments, the situation is
less clear. Reduction of neutral or alkaline vanadium (V) compounds yields
a dark and turbid liquid.
Vanadium in its +5 oxidation state gives light yellow compounds in mildly
alkaline environments. Al lowering the pH, the color becomes more intense,
until a maximum is reached. When the pH is lowered even more, then lighter
yellow compounds are formed again.
Vanadium (IV) cannot coexist with hydrogen peroxide. In alkaline media it
is oxidized to vanadium (V), which with excess peroxide gives a yellow
peroxo complex. In acidic media, vanadium (IV) is oxidized to vanadium (V)
which gives a deep brown/red peroxo complex with excess poroxide.
Description:
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Dissolve some V2O5 in excess NH3 (15% by weight): Liquid becomes light yellow
and slightly turbid.
Slowly add an excess amount of dilute H2SO4: The liquid first becomes more and
more yellow. It even has quite an intense yellow color, when it still is a
little alkaline. On addition of more H2SO4 the liquid becomes deeper yellow.
At a certain point the liquid is bright orange. On adding even more H2SO4 the
liquid becomes lighter again and finally it is yellow again. The liquid is
completely clear at this point. The liquid also becomes warm. This is due to
the reaction between the NH3 and dilute H2SO4.
Add a slight excess amount of a solution of Na2SO3: The liquid becomes very
dark, it becomes almost black. This is a vanadium (IV) compound at mildly
alkaline conditions.
Add a little more sulphuric acid: The liquid becomes bright blue. This bright
blue color is the color of the blue vanadyl ion, [VO]2+.
Add an excess amount of sodium carbonate: A small amount of CO2 is evolved,
not much, showing that the liquid only was mildly acidic. On addition and
dissolving of more sodium carbonate, the liquid becomes green/brown, somewhat
sepia-like, but darker.
Add an excess amount of H2O2 (10% by weight): The liquid becomes yellow and
clear.
Add an excess amount of dilute H2SO4: Liquid becomes red/brown.
*** Here a branch occurs from the point, where the mildly acidic
*** blue liquid with vanadyl ions was made.
Add an excess amount of H2O2 to the blue liquid, containing vanadyl ions:
The liquid becomes yellow.
Add some dilute H2SO4: The liquid becomes red/brown.
Add an excess amount of dilute NH3. The liquid becomes light yellow. The NH3
neutralizes the acid and the brown peroxo complex is tranformed to a yellow
peroxo complex.
Add an excess amount of dilute H2SO4 again: The liquid becomes red/brown again.