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Electroplating in Basic Solution/How pH affects electroplating

We are doing a final project for our 10th grade chemistry class and have successfully plated zinc onto a penny. Interestingly, we were only successful in getting a nice silvery plate on the penny when we used a KOH solution. In acidic solution and water the electrolytic cell produced a bizarre black deposition on our penny. Our anode was copper, our solution was a zinc salt, and our cathode was a copper wire holding a penny. How can this be explained at a chemical level?

Noah
- Albany , California, USA
2003

A guess- Your copper anode will dissolve in an acid media and give you a strange copper zinc alloy. If it was black, you applied far too much voltage. The alkaline solution does not dissolve the copper as fast and plates out at a higher voltage, so you got a better plate. Go online and look up Metal Finishing. I think that it is metalfinishing.com You can find old zinc plating formulations and voltages in their guidebook.

James Watts
- Navarre, Florida
2003

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If you check your electrochemical series, you'll see it is easier to plate copper from solution than zinc. So when the copper anode starts dissolving in the acid, it plates out with the zinc. Acid zinc platers know that just a few parts per million of copper in a bath with 20,000 ppm of zinc will mess up their parts. Zinc is "amphoteric", dissolving in caustics or acids, but copper is not very soluble in caustics. So when KOH was the basis of your electrolyte, any copper stripped from the anode would have quickly dropped out of solution as Copper Hydroxide, no longer available for plating! Craig Haseltine
- St Charles, Illinois USA
2003