Chromium, cobalt, vanadium and manganese chemistry

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VANADIUM

Oxidation states of vanadium

Species Colour Oxidation state of metal
VO2+   Yellow +5   vanadate (V) ion
VO2+ Blue     [VO(H2O)5]2+ +4   vanadyl (IV) ion
V3+ Green   [VCl2(H2O)4]+ +3
V2+ Violet   [V(H2O)6]2+ +2

Vanadium forms stable compounds in 4 different oxidation states, +2, +3, +4 and +5.

All vanadium (V) compounds can be reduced by strong reducing agents such as zinc in acid solution:

VO2+ + 4H+ + 3e– → V2+ + 2H2O
Zn→ Zn2+ + 2e–

2VO2+ + 8H++ 3Zn → 2V2++ 3Zn2++ 4H2O
(yellow)                     (violet)

As reduction occurs, the following colour changes occur when using acidified Zn:
yellow (VO2+) → green (mixture of VO2+ and VO2+) → blue (VO2+) → green (V3+) → violet (V2+)
If exposed to air, the violet +2 complex will slowly oxidize to the green +3 complex.

Analysing redox potential values of the reducing agent against vanadium oxidation states will determine the final reduction state of vanadium, being complete (+5→+2) or selective (e.g. +5→+4 or +5→+3).

Examples of selective reduction
Acidified H2O2 :     2VO2+ + 2H++ H2O2  → 2VO2++ O2 + 2H2O
Acidified Tin    :           VO2+ + 4H++ Sn → V3++ Sn2++ 2H2O

Alkaline hydrogen peroxide may be used to oxidise any lower to any higher vanadium oxidation state.
Half-equation for alkaline H2O2:       H2O2 + 2e− ⇌ 2OH−

Factors determining the colour of solutions containing vanadium complexes

Furthermore, in addition to oxidation state impacting on the colour of the complex, so does the type of acid used.
For example, if acidification of Zn is achieved through HCl, the +3 complex is green [VCl2(H2O)4]+ as the aqueous chloride ion exerts a ligand substitution effect. However, if H2SO4 is used, the +3 complex is grey-blue [V(H2O)6]3+.
This supports the observation that the colour of a complex depends on both its oxidation state and type of ligand.

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