Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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hydrogen peroxide : H2O2
sodium hydroxide : NaOH
tartaric acid : HOOC CHOH CHOH COOH
sulphuric acid : H2SO4
vanadyl sulfate : VOSO4.5H2O
Class:
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elem=V
coordination
redox
Summary:
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Vanadyl ions do not form a special complex with tartaric acid in strongly
acidic environments.
When hydrogen peroxide is added, a complex is formed, but is
this due to the presence of tartaric acid?
When tartaric acid is added to a solution of vanadyl sulfate in an
environment, which is only slightly acidic, then a green complex is
formed. On making the liquid more basic, a vague sequence of color
changes occurs, through grey/blue, finally going to brown.
Description:
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Sequence 1:
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Add some tartaric acid to a solution of vanadyl sulfate, acidified with
sulphuric acid: No visible reaction, the tartaric acid dissolves.
Add a solution of sodium hydroxide: The alkaline layer becomes light brown,
when this is poured on the blue liquid very carefully. When the liquid is
shaken, it becomes light blue, the expected colour for a dilute vanadyl
solution.
Add some hydrogen peroxide (3% by weight): Liquid becomes dark red/brown, a
gas is evolved and the temperature rises noticeably. After 5 minutes the
liquid has become moss green and fairly dark. When the liquid is shaken,
still some gas is evolved.
Heat the liquid for approximately half a minute, such that it is just about
to boil: The liquid becomes blue again.
Remark:
Apparently, the moss green colour was due to a mix of the red/brown color
and blue. The dark red/brown color is due to a coordination complex with
hydrogen peroxide. This experiment makes the assumption that vanadyl does
not form coordination complexes in acidic environments with tartaric acid
very plausible.
Sequence 2:
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Add a solution of vanadyl sulfate to a concentrated solution of tartaric acid:
The liquid becomes green.
Add a solution of sodium hydroxide: The liquid becomes grey, on addition of
more sodium hydroxide the liquid appears to becomes more bluish. On adding
even more sodium hydroxide, the liquid becomes green/brown.