Ammonium vanadate powder is extremely toxic (health hazard rating 4). Work with dry vanadate powder in the hood. Protect yourself from inhalation, contact and ingestion.
The ammonium meta vanadate solution in 3M sulfuric acid is toxic and corrosive.
Chemicals and Solutions:
0.1 M Ammonium meta-vanadate solution
sodium (meta) bisulfite solid
0.5% Hydrogen peroxide (make fresh) - about 100 ml
Three 250 ml glass stoppered Erlenmeyer flasks
Short stem funnel with glass wool
Cover the bottom of one flask with zinc (excess!). Add about 50 ml YELLOW NH4VO3 stock solution. PATIENCE! Hold up and swirl so that students can see color change from YELLOW to BLUE to GREEN to PURPLE. Be sure that reaction is complete. Set aside for a few minutes before doing step 3. Meanwhile go to step 2.
Add 50 ml of YELLOW NH4VO3 stock solution to next flask. Reduce with solid sodium bisulfite until the color turns BLUE.
Pour a little of the PURPLE solution though glass wool (to avoid getting zinc granules in) into the next flask. Oxidize with dilute (0.5%) H2O2 to get GREEN. If color is too intense to see the color change, add water.
VO2+ or VO(H2O)52+ (+4)
V3+ or V(H2O)63+ (+3)
V2+ or V(H2O)62+ (+2)
0.1M ammonium m-vanadate: dissolve 5.8 g NH4VO3 in 500 mL of 3M sulfuric acid.
Step 1: 2VO3- (aq) + Zn (s) + 8H+(aq) ---> 2VO2+(aq) + Zn2+(aq) +4H2O
2VO2+(aq) + Zn + 4H+(aq) ---> 2V3+(aq) + Zn2+(aq)+ 2H2O
2V3+(aq) + Zn ----> 2V2+ (aq) + Zn2+ (aq)
Step 2: 2VO3- (aq) + SO32-(aq) + 6H+(aq) ---> 2VO2+(aq) +SO42-(aq) + 3H2O
Step 3: 2V2+ (aq) + H2O2 (aq) + 2H+(aq) ---> 2V3+(aq) + 2H2O