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Oxidation States of Vanadium

Chemical Concepts Demonstrated

  • Oxidation/reduction chemistry
  • Variable oxidation states for transition metals

Demonstration

Three solutions are used in the demo.
  • VO2solution: NaOH dissolved in water. Add ammonium vanadate (NH4VO3) and 2M H2SO4.
  • Zinc amalgam: Dissolve HgClin DI water. Add HNO3 and 200-mesh Zn. Poured off liquid.
  • Ce4+ Oxidizer: Dissolve Ce(NO3)6 in water.
  1. 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.
  2. Add a small amount of NaHSO3 to a beaker containing the VO2+solution.
  3. A test tube is half-filled with 0.1M V2+, the other half with 0.1M Ce4+and allowed to sit overnight.
vana.gif

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:

 

VO2to 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


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