Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium sulfite : Na2SO3
hydrochloric acid : HCl
phenolphtalein : C (C6H4OH)2 C6H4 COO
sodium hydroxide : NaOH
hydrogen peroxide : H2O2
vanadium pentoxide : V2O5
Class:
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elem=V,O
coordination
Summary:
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Vanadium (V) is capable of forming many peroxo compounds, whose appearance
strongly depends on pH.
Description:
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Add some solid V2O5 to H2O2 (appr. 30% by volume): Liquid starts foaming a
little and becomes turbid and yellow.
Dilute approximately 3 times with water and then add some solid NaOH to the
bubbling/foaming liquid: The foaming becomes much stronger. The foam first
is yellow, then it becomes grey/yellow and then it becomes darker and darker,
until it is almost black. After this, the foam becomes lighter again, until
it has become light yellow. At this point, foaming stops, and the liquid
becomes clear and yellow. The liquid continues to slowly bubble a litte.
During the foaming phase, the temperature of the liquid rises considerably.
Add a little amount of HCl (10% by weight): Where the acid touches the yellow
liquid, it becomes dark brown/red. However, on mixing with the remaining
liquid, this color quickly disappears again. Finally, the liquid is clear
and yellow again, albeit a little darker and somewhat more brown.
Add more HCl (10%): The liquid becomes intense brown/red and remains like
this. Still there is slow evolution of a gas (slow bubbling). This liquid
is called BROWNRED.
Add an excess amount of NaOH to part of BROWNRED: The liquid foams a little
again and becomes light yellow and clear.
Dilute the other part of BROWNRED and add a small amount of NaOH: Liquid
becomes yellow/brown. It does not react alkaline to phenolphtalein.
Add some HCl (10%) again: Liquid becomes brown/red again.
Wait one day: Liquid still is brown/red.
Add some solution of Na2SO3: Small of SO2. Within a few minutes, the liquid
turns light blue.
Remark:
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Peroxo complexes with vanadium (V) can have many colors:
- strongly alkaline: dark blue, almost black, very unstable.
- alkaline and strongly alkaline: yellow, stable
- acidic: red/brown, stable.
When pH increases, then a gradual change from red/brown to yellow can be
observed. The dark, almost black compound, is not stable and decomposes
quickly to the yellow compound.
The peroxo compounds can be reduced easily to vanadium (IV).