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Why does iron displace copper from one solution and not another?

I would like to have an explanation for the fact that iron displaces copper from an acid copper sulphate [on eBay or Amazon]solution but not from an alkaline cyanide solution. Is it because the electrode potential differential narrows between the two in the alkaline environment? Is there any relationship to valency? I am involved in training and I am not sure that I really do know the answer to this question.

Tony van der Spuy
Consultant - Cape Town, Western Cape, South Africa
2004

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2004

It's not a matter of alkalinity nor of valance state. You're close, but not quite on target yet :-).

The CN complexes the copper so strongly that it ionizes to only a very very limited extent. As a result, the amount of ionized copper in solution is so low that it's no longer noble to iron. The Nernst equation is what you want to look into. It relates that the nobility of one metal to another under conditions like this is not solely a function of their inherent deposition potentials, but their solution concentrations as well. No matter how much CuCN you have in solution, the complex is so tight that you have virtually zero copper in solution.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey