|
|
Department of Chemistry and Biochemistry | |
| DEMONSTRATIONS |
Faculty & Research |
| |
Academic Programs |
| |
Industrial Relations |
| |
Research Facilities |
| |
Department Administration |
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:
VO3- is really VO43- 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 VO43- (vanadate) and sulfate could also be mentioned.
