|MadSci Network: Chemistry|
I can't think of a specific chemical for your purpose, but possibly a complexing agent (EDTA or similar) might help.
The desirable properties of this complexing agent would be:
1) Formation constant for Cu+, Cu2+, Fe2+ and Fe3+ ions is much larger than the formation constant for Cr6+
2) The Fe/Cu complex is insoluble in water (so can be filtered out)
Having a larger formation constant for Fe/Cu than Cr, indicates that the complexing agent will attach itself to the Fe/Cu ion in preference to the Cr ion.
I didn't manage to find a specific chemical with these properties, but if you have the time and patience to sift through tables of data at a local university library, and find formation constants, you might find a chemical that is of use to you.
If you need more explanation of anything, feel free to email me.
University of Technology, Sydney
Try the links in the MadSci Library for more information on Chemistry.