Oxidation States of Vanadium (V5+ --> V2+)

Materials

1 liter Florence Flask with rubber stopper

Zink Amalgam

Bottle of Acidic Ammonium Vanadate Solution

Magnetic Stirplate

600 mL beaker with a stirbar

Rinse Bottle with Cerium Sulfate Solution

Gloves and Goggles

    
Procedure

Before class starts, place 50gm Zn amalgam in a 1 liter Florence flask.  To perform this demo, add 200ml acidic ammonium vanadate solution to the Florence Flask.  The solution will turn from yellow to green on contact with zinc amalgam.  Stopper the Flask.  Then swirling the Flask turns the solution blue, and shaking the flask will turn the solution  violet.  Then decant the solution into a beaker with a stirring bar. Using cerium sulfate solution, titrate purple solution on a stirring plate.

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

  
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.