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Oxygen States of Vanadium

Procedure

• Place 50gm Zn amalgam in a 1 liter Florence flask.

• Add 200ml acidic ammonium vanadate solution, solution turns from yellow to green on contact with zinc amalgam.

• Stopper flask.

• Swirl flask until blue.

• Shake flask until violet.

• Decant the solution into a beaker with a stirring bar.

• Using cerium sulfate solution, titrate purple solution on a stirring motor.

Approximate amounts of CeSO₄ to add to 50ml of V²⁺ violet solution:

ml CeSO4 added	color of V soln
20ml	violet to blue
35ml	blue to green
40ml	green to yellow

Reactions

[yellow to green]    Zn(s) + VO₃^- + 8 H₃O⁺ (aq) --> VO⁺⁺ (aq) + Zn²⁺ (aq) + 12H₂O

[green to blue]        Zn(s) + VO²⁺ (aq) + 4H₃O⁺ (aq) --> 2V³⁺ (aq) + Zn²⁺ (aq) + 6H₂O

[blue to violet]        Zn(s) + 2V³⁺ (aq) --> 2V²⁺ (aq) + Zn²⁺ (aq)

[violet to blue]        Ce⁴⁺ (aq) + V²⁺ (aq) --> V³⁺ (aq) + Ce³⁺ (aq)

[blue to green]        Ce⁴⁺ (aq) + V³⁺ (aq) + 3H₂O --> VO²⁺ (aq) + Ce³⁺ (aq) + 2H₃O⁺ (aq)

[green to yellow]    Ce⁴⁺ (aq) + VO²⁺ (aq) + 6H₂O --> VO3^- (aq) + Ce³⁺ (aq) + 4H₃O⁺ (aq)

NOTE:
VO^3- is really VO₄^3- in equilib with HVO₄ 2 in basic soln. VO²⁺ should be written as a pentaaquo complex to remind p-chemists teaching the course that there is a structure change. V³⁺ also should be written as a hexaaquo complex (and remind deprotonate possible in basic media). V²⁺ is an air sensitive hexaaquo ion, it might be worth air oxidation rather than adding Ce⁴⁺. There is a structural change from tetrahedral to octahedral between V(V) and V(IV). The similarity between VO₄^3- (vanadate) and sulfate could also be mentioned.