Redox Chemistry of Transition Metals

Iron exists is 2 states, Fe2+ and Fe3+

Fe2+ is oxidised easily in aqueous solutions:

Fe2+ àFe3+ + e-

This means Fe2+ is a reducing agent.

The concentration of Fe3+ is a solution can be determined by titrating with an oxidising agent.

Manganese exists commonly as Mn2+, Mn4+ and MnO4-, 

MnO4- can be reduced to Mn2+ by Fe2+ in acidified conditions:

8H+ + 5e- + MnO4- à Mn2+ +4H2O
Fe2+   àFe3+ + e-

8H+ + 5 Fe2+ + MnO4- --> Mn2+ + 4 H2O + 5 Fe3+

Vanadium exists in 4 common ions in aqueous solution: VO₂⁺ VO²⁺ V³⁺ V²⁺

All vanadium compounds can be reduced to the +4, +3 and then +2 oxidation state by strong reducing agents such as zinc in acid solution:

 VO₂⁺(aq) + 4H⁺(aq) + 3e- --> V²⁺(aq) + 2H₂O(l)
Zn(s) --> Zn²⁺(aq) + 2e

2VO₂⁺(aq) + 8H⁺(aq) + 3Zn(s) --> 2V²⁺(aq) + 3Zn²⁺(aq) + 4H₂O(l) 

The colours of this reaction are:

yellow(VO₂⁺) --> green (VO₂⁺ and VO²⁺) --> blue (VO²⁺) --> green (V³⁺) --> violet (V²⁺)

Mild oxidising agents can reduce V5+ to V4+ but no further.
Air slowly oxidises V2+ to V3+