12.7 Oxidation States of Vanadium
Chemical Concepts Demonstrated: Oxidation/reduction chemistry,
variable oxidation states for transition metals
Demonstration:
Three solutions are used in the demo.
- VO2+ solution: NaOH dissolved in water. Add
ammonium vanadate (NH4VO3) and 2M H2SO4.
- Zinc amalgam: Dissolve HgCl2 in DI water. Add HNO3
and 200-mesh Zn. Poured off liquid.
- Ce4+ Oxidizer: Dissolve Ce(NO3)6 in
water.
- 200 mL of water are added to a buret, and zinc amalgam is packed inside. V2+
is collected in the flask and 0.1M Ce4+ is added.
- Add a small amount of NaHSO3 to a beaker containing the VO2+ solution.
- A test tube is half-filled with 0.1M V2+, the other half with 0.1M Ce4+
and allowed to sit overnight.
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Observations:
The test tube ends up having three or four layers
inside of it. Yellow is at the top, blue is below that, green is below the blue, and
violet is at the bottom.
Explanation:
Each one of the layers represents a different oxidation state of
vanadium:
| Yellow |
VO2+ |
| Blue |
VO2+ |
| Green |
V3+ |
| Violet |
V2+ |
The oxidation reactions that take place in the demonstration are
caused by oxidizing/reducing agents of various strengths. The resulting oxidation
products separate from each other in the test tube. The reduction potentials are as
follows:
| VO2+ to VO2+ (yellow to blue) |
Eo = 1.0 V |
| VO2+ to V3+ (blue to green) |
Eo = .33 V |
| V3+ to V2+ (green to violet) |
Eo = -.26 V |
| V2+ to V |
Eo = -1.18 V |