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I am trying to separate aqueous copper and cobalt ions. I have been told that in a certain oxidation state one of them will be able to precipitate in a certain acid. I am wondering if you have any possible approaches.
Answer 1:

My best guess is that you are trying to separate Copper(II) and Cobalt(II), which are the most common oxidation states of these ions, at least in acidic water. Copper(I) salts in general tend to be quite insoluble, While Cobalt(III) has to be stabilized in complexes with e.g. ammonia, which won't be present in acidic solution. All the other oxidation states are either very rare or unstable (in water).

There is a classic method in qualitative inorganic analysis (where one tries to find out what ions are in a sample) that involves precipitating certain metal ions (or groups of them) with hydrogen sulfide in slightlic acidic solution. Insoluble copper sulfide will form under these conditions, while cobalt(II) ions will remain in solution. Performing this type of separation was a part of first year chemistry studies in Germany when I did them, so I can tell you from personal experience that it is a very messy and smelly affair. Hydrogen sulfide gas is not only smelly but also very toxic and flammable, so it needs to be handled with extreme caution. So while this is not a chemistry experiment that I would happily recommend to anyone, I also think it likely is the method you are looking for because it has had such a long-standing place in chemical education.

Good luck with your experiments!

Answer from a few days later
I just found out that there is a compound that is used in place of gaseous hydrogen sulfide which is thioacetamide. Of course this compound still produces hydrogen sulfide in water and thus must be handled with care, but it should be easier to work with than the gaseous hydrogen sulfide.


Answer 2:

You'll want to start by looking at the solubility of different copper and cobalt metal salts. Copper (ii) and Cobalt (ii) have similar solubilities in many salts. However, Copper (i) chloride is very insoluble, whereas copper (ii) and cobalt (ii) chloride are both soluble. If you have sufficient free chloride ions (add NaCl or HCl), You can try treating your solution of mixed copper and cobalt with ascorbic acid, which will hopefully reduce the copper (ii) to copper (i), and then filter the material to get a white precipitate. This white copper (i) chloride should rapidly reoxidize in air to give you a green solid within a couple minutes. You might have to do this a couple times to separate everything. Afterwards, you can just add oxalic acid to precipitate the cobalt, which you can then filter to recover a solid.



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