wrote:
Wilco,
The best chelating agent for copper is EDTA
(ethylenediaminetetraaacteic acid whose di-sodium salt is usually
used.
The best pH for copper chelation is around 2.0. Once the complex is
formed at this pH it is stable in highly alkaline and acidic media.
EDTA is stable, perhaps easily available, non-toxic and stable with
respect to air-oxidation.


Farooq,

I actually have some EDTA-Na2 and EDTA-Na4. Both of these perfectly
chelate with copper (II), but as soon as a few drops of 1 M H2SO4 are
added, then the chelate is destroyed. I tried this, by mixing a
solution of CuSO4 and K3Fe(CN)6. Without EDTA this mixture gives a
precipitate. With EDTA (either -Na4 or -Na2) the solution remains
clear. However, when the solution is somewhat acidified, then a
precipitate is formed again.
One can also see that the complex is destroyed without the
ferricyanide. The complex has a deep blue color. On acidification with
even a small amount of H2SO4, the deep blue color disappears and the
color of plain aqueous Cu(2+) appears again.

As you write, the complex should be stable. Did I miss something?
Should the complex be prepared in a special way? Is the blue complex I
have not the same as what you are referring to?

What I want is that Cu(2+), [VO2](+) and [Fe(CN)6](3-) all are in
solution at the same time. The pervanadyl only can exist at low pH. I
can have the pervanadyl and ferricyanide in solution at the some time,
but adding the copper is a problem. When I manage to have all these
three in solution, I probably would have created a bright orange toner,
which is quite special in photography. If you are interested in this
aspect of chemistry, the idea behind metal-ferricyanide toning is
explained in

http://www.woelen.nl/photo/toner.pdf

which I wrote approximately half a year ago. The second part of this
document may be most interesting for you.

Wilco