From: ahahma@utu.fi (Arno Hahma)
Subject: Re: net.chemist wanted
Organization: University of Turku

In article <1992Aug24.181113.19481@cbnewse.cb.att.com>
sfaber@ihlpb.att.com (Steven R Faber) writes:

#If you have a mixture of metal ions, the one with the lowest potential
#will plate out in preference to the others, until the concentration
#of the first is lowered to such an extent that the voltage across the cell
#increases to the point it can plate out the metal with the next higher
#reducing voltage.   In the case of Zn and Cu, the voltages are so

Thermodynamically correct, but not in practice. Electroplating is not
a reversible process - ever heard about overvoltage at an electrode?
That is, by applying enough voltage both metals can be plated
simultaneously.  There are numerous recipes for plating brass, bronze,
etc. in the books about electroplating. Check any encyclopedia about
chemical technology, say that of Kirk&Othmer.

#could get a mixture of both to plate.   In fact in the case of Zn, I
#don't think you could plate it at all from an aqueous solution, since

Again, hydrogen overvoltage in action causing hydrogen to be evolved
after Zn.  Nails, sheet metal, etc. is being zinc plated
electrolytically and it is done from aqueous solution.

#you would start reducing hydrogen first.  Then to prevent that, you
#could make the solution more basic, reducing the concentration of H+,
#but then you would precipitate out zinc hydroxide.

Unless you add something that complexes the Zn, like cyanide. However,
there are also acid baths for zinc plating.

#Steve Faber

ArNO
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