Vanadium is the only element which possesses four adjacent oxidation states (+5, +4, +3, +2) that are all stable in aqueous solution. Moreover, they all show very different colors. Starting from yellow V(V) obtained from ammonium metavanadate in acidic solution, all the oxidation states of vanadium can be produced with suitable reducing agents: blue V(IV) with sulfite, dark green V(III) with tin, violet V(II) with zinc amalgam. The sequence of steps from V(V) to V(II) can also be performed in the opposite direction, oxidizing V(II) to V(V) stepwise with cerium(IV) ammonium nitrate: the same sequence of colors in reverse order is displayed.
