Description of experiment
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Needed compounds:
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potassium ferricyanide : K3 [Fe(CN)6]
sulphuric acid : H2SO4
sodium hydroxide : NaOH
sodium citrate : COONa CH2 COH(COONa) CH2 COONa
vanadyl sulfate : VOSO4.5H2O
Class:
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elem=V
coordination
Summary:
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Vanadyl gives diverse complexes with citrate, the color of these complexes
strongly depends on the pH of the solution.
Description:
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Add a slightly acidic solution of vanadyl sulfate to a concentrated solution
of sodium citrate: Formation of an intense purple/blue complex. The liquid
remains clear and gets a beautiful purple/blue color.
Add a fairly large amount of H2SO4 (15% by weight) to the purple/blue liquid:
The liquid turns green and the color becomes much less intense. The green
color is comparable to the color of a plain nickel(II) salt, dissolved in
water.
Add a small amount of H2SO4 to the deep purple/blue liquid: No change of
color.
Add a small amount of a solution of NaOH to the deep purple/blue liquid: The
liquid turns grey and remains clear. The liquid becomes much lighter, but
it does not become colorless. When an object is observed through the liquid,
then it is observed with its original colors, except that it looks darker,
so the liquid obtains a fairly pure grey color.
Add more solution of NaOH to the grey liquid: The liquid remains grey.
Add some H2SO4 to the grey liquid: The liquid becomes deep purple/blue again.
Add more H2SO4: The liquid becomes green, like a fairly concentrated Ni2+ -
solution.
Add a solution of K3Fe(CN)6 to the green solution: Formation of a dark green
precipitate. It takes a few seconds before the liquid turns turbid.
Add a solution of K3Fe(CN)6 to the grey solution: Formation of a clear yellow
solution, which, when acidified, gives a dark green precipitate.
Add a solution of K3Fe(CN)6 to the deep purple/blue solution: Liquid remains
clear and becomes dark green/grey.
Conclusion:
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VO2+ -- citrate strongly basic : grey
VO2+ -- citrate near neutral : deep purple/blue
VO2+ -- citrate acidic : green like Ni2+, but slightly more intense.