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"Electroplating Zinc onto a Penny"

April 22, 2010

Hi all, my name is Ray and I'm a middle school science teacher in Santa Monica, CA. I've always had a passion for chemistry and thought I'd have some fun showing the kids how to make "silver" and "gold" pennies and copper quarters and dimes following instructions I found on the internet.

Before doing anything with the kids, I set out to do them on my own as tests. First I plated copper onto a quarter. Using copper sulfate alone and a 1.5 V battery, I got terrible results. After several tries and reformulations, I got a great plate. Specifically I ended up adding acid to the copper sulfate, making the anode pure copper wire, and agitating the solution by putting it on an electric stirrer.

On to the pennies. After successfully making "silver" pennies with a solution of sodium hydroxide and zinc dust, I wanted to try doing it electrochemically. I have met with very limited success. I searched the archives but couldn't find anyone getting the results I'm getting so I thought I'd post and see if anyone can help.

I started by making a solution of zinc sulfate by mixing 50 g of ZnSO4 with 495 ml of distilled water. I then added 5 ml of concentrated sulfuric acid. I did this because when electroplating copper onto quarters, I had needed to add sulfuric acid to the copper sulfate solution.

After making the zinc sulfate solution, I cleaned some pennies by first using toothbrush and toothpaste, and then dipping them into a vinegar/salt solution to remove the corrosion. I then rinsed them with tap water and dried them and connected them to alligator clips to be the cathode in the solution. For the anode, I sanded down a penny. Here is where my mistake could be--I didn't get all the copper off the penny. I assumed that the zinc would dissolve preferentially because it's a more active metal than copper.

I used a 1.5 V battery and put the electrodes in the solution. Something definitely plated onto the penny but instead of it being a nice shiny silvery color like I got from the immersion plate, it was more like an bright, slightly bluish material that did not look metallic.

I think these might be possible reasons:
1. Using tap water to rinse the pennies before using them as electrodes.
2. Adding acid to the zinc sulfate.
3. Not removing all copper from the anode.
4. Not having a concentrated enough zinc sulfate solution.

So that's it. Any help or ideas would be greatly appreciated.

Ray Avedian
hobbyist, science teacher - Santa Monica, California, USA

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