Oxidation States of Vanadium

Hazards

• 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
• Granular zinc
• 0.5% Hydrogen peroxide (make fresh) - about 100 ml
• Distilled water

Solutions Prep

0.1M ammonium m-vanadate: dissolve 5.8 g NH₄VO₃ in 500 mL of 3M sulfuric acid.

Materials

• Three 250 ml glass stoppered Erlenmeyer flasks
• Short stem funnel with glass wool
• Eyedropper

Procedure

1. 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.
2. Add 50 ml of YELLOW NH₄VO₃ stock solution to next flask. Reduce with solid sodium bisulfite until the color turns BLUE.
3. 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.

• Yellow
  $\mathrm{VO}{\phantom{A}}_3{\phantom{A}}^{-}(+5)$
• Blue
  $\mathrm{VO}{\phantom{A}}_2{\phantom{A}}^{+}\mathrm{or}\mathrm{VO}(\mathrm{H}{\phantom{A}}_2\mathrm{O}){\phantom{A}}_{52}{\phantom{A}}^{+}(+4)$
• Green
  $\mathrm{V}{\phantom{A}}_3{\phantom{A}}^{+}\mathrm{or}\mathrm{V}(\mathrm{H}{\phantom{A}}_2\mathrm{O}){\phantom{A}}_{63}{\phantom{A}}^{+}(+3)$
• Purple
  $\mathrm{V}{\phantom{A}}_2{\phantom{A}}^{+}\mathrm{or}\mathrm{V}(\mathrm{H}{\phantom{A}}_2\mathrm{O}){\phantom{A}}_{62}{\phantom{A}}^{+}(+2)$

Discussion

Reactions

Step 1

$\underset{\text{yellow}}{2\mathrm{VO}{\phantom{A}}_3{\phantom{A}}^{-}{\phantom{A}}_{(\mathrm{aq})}+\mathrm{Zn}{\phantom{A}}_{(\mathrm{s})}+8\mathrm{H}{\phantom{A}}^{+}{\phantom{A}}_{(\mathrm{aq})}}⟶\underset{\text{blue}}{2\mathrm{VO}{\phantom{A}}_2{\phantom{A}}^{+}{\phantom{A}}_{(\mathrm{aq})}+\mathrm{Zn}{\phantom{A}}_2{\phantom{A}}^{+}{\phantom{A}}_{(\mathrm{aq})}+4\mathrm{H}{\phantom{A}}_2\mathrm{O}}$

$\underset{\text{blue}}{2\mathrm{VO}{\phantom{A}}_2{\phantom{A}}^{+}{\phantom{A}}_{(\mathrm{aq})}+\mathrm{Zn}+4\mathrm{H}{\phantom{A}}^{+}{\phantom{A}}_{(\mathrm{aq})}}⟶\underset{\text{green}}{2\mathrm{V}{\phantom{A}}_3{\phantom{A}}^{+}{\phantom{A}}_{(\mathrm{aq})}+\mathrm{Zn}{\phantom{A}}_2{\phantom{A}}^{+}{\phantom{A}}_{(\mathrm{aq})}+2\mathrm{H}{\phantom{A}}_2\mathrm{O}}$

$\underset{\text{green}}{2\mathrm{V}{\phantom{A}}_3{\phantom{A}}^{+}{\phantom{A}}_{(\mathrm{aq})}+\mathrm{Zn}}⟶\underset{\text{purple}}{2\mathrm{V}{\phantom{A}}_2{\phantom{A}}^{+}{\phantom{A}}_{(\mathrm{aq})}+\mathrm{Zn}{\phantom{A}}_2{\phantom{A}}^{+}{\phantom{A}}_{(\mathrm{aq})}}$

Step 2

$\underset{\text{yellow}}{2\mathrm{VO}{\phantom{A}}_3{\phantom{A}}^{-}{\phantom{A}}_{(\mathrm{aq})}+\mathrm{SO}{\phantom{A}}_{32}{\phantom{A}}^{-}{\phantom{A}}_{(\mathrm{aq})}+6\mathrm{H}{\phantom{A}}^{+}{\phantom{A}}_{(\mathrm{aq})}}⟶\underset{\text{blue}}{2\mathrm{VO}{\phantom{A}}_2{\phantom{A}}^{+}{\phantom{A}}_{(\mathrm{aq})}+\mathrm{SO}{\phantom{A}}_{42}{\phantom{A}}^{-}{\phantom{A}}_{(\mathrm{aq})}+3\mathrm{H}{\phantom{A}}_2\mathrm{O}}$

Step 3

$\underset{\text{purple}}{2\mathrm{V}{\phantom{A}}_2{\phantom{A}}^{+}{\phantom{A}}_{(\mathrm{aq})}+\mathrm{H}{\phantom{A}}_2\mathrm{O}{\phantom{A}}_2{\phantom{A}}_{(\mathrm{aq})}+2\mathrm{H}{\phantom{A}}^{+}{\phantom{A}}_{(\mathrm{aq})}}⟶\underset{\text{green}}{2\mathrm{V}{\phantom{A}}_3{\phantom{A}}^{+}{\phantom{A}}_{(\mathrm{aq})}+2\mathrm{H}{\phantom{A}}_2\mathrm{O}}$