Vanadium Chemistry (1): From +5 To +4

 Vanadium is the 23rd element, lies in period 4 and group VB. Being an early transition metal it is somewhat similar to main group elements, especially P, but can also be reduced to the +3 and +2 states, common in transition metals. Its +4 state mainly exist as [VO]2+ which is a stable terminal oxo complex. It is somewhat expensive and toxic, so use carefully.

Before exploring more, I wanna make V3+ and V2+ as double salts.

I obtained this element as sodium metavanadate (NaVO3), which makes it a little troublesome to remove unwanted Na+. If you can get NH4VO3 and you want an ammonium salt product, just add acid and reduce it.

NaVO3 is a fine white powder that dissolves slowly in water. Heat to dissolve it. Then add sulfuric acid carefully until pH becomes 2~3. Now the solution is dark yellow and much red stuff precipitated.

The solid is V2O5, and is very sticky. Filtering is quite hard and sick, so just pour off the liquid carefully, add water, then pour again, to wash it. You may need 3 times or so. The liquid is very deep colored, which is expected and does not indicate a serious loss.

Now, add excessive sulfuric acid to dissolve it. It is now [VO2]+ or cis-[VO2(H2O)4]+. Here "excessive" means more than 1.5 mol H2SO4 for 1mol NaVO3, as further reduction also needs it.

This ion is pale yellow. Now, add calculated amount of oxalic acid. V(V) is very oxidizing ([VO2]+/[VO]2+ = 1V) so it is able to oxidize oxalic acid. Nothing will happen until you heat it, when it gradually becomes green, then blue just like Cu2+, and you get there. This is [VO]2+ or [VO(H2O)5]2+.

Formula: 2[VO2]+ + H2C2O4 + 2H+ = 2[VO]2+ + 2CO2 + 2H2O

Further reduction requires electrolysis, so make sure you have devices! Later I may write about it.