Letter 19022

Copper plating issues [California] 

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I am a seventh grade student who is doing a science project on electroplating. I have used Vinegar, sugar, and salt. I used a 6V battery as my power source. First I placed both anode and cathode with Copper wire and then once the solution had some copper dissolved in it(color of solution changed to blue), I changed the cathode with a brass washer for plating.

I am getting a black colored and I am wondering why I am not getting the bright color of copper. Also I would like to know how sugar and salt help in the plating process. Can you also help me in getting the chemical reactions (equations) for this experiment.

Thank you,

Anna
- San Jose, CA, America

Anna, switch to a 1-1/2 volt battery. Vinegar is a very mild acid capable of holding only a small amount of copper in solution. There isn't enough copper in solution to keep up with the electricity flow from a higher voltage battery, so much of the electricity will cause water to separate into hydrogen and oxygen instead of depositing copper. This results in the smuttiness and "burning" you are seeing.

Your copper plating will not be shiny because in a school experiment the grain size of the copper crystals is too large and the surface too rough to make a mirror-like surface. But I did this experiment myself and photographed the results I got for comparison (see http://www.finishing.com/faqs/school.html). Take your time, plate for an hour, then rub off any smut.

Salt helps by making the solution more conductive. A tiny pinch of salt is plenty. We have not proven that sugar does anything at all. Theoretically it is a "brightener" that deposits along with the copper, interrupting the copper crystals, so more and smaller crystals have to form. But we have no feedback from students about whether it really works.

The exact formulas are beyond 7th grade level--it is too complicated to describe how and why vinegar (acetic acid) ionizes in the manner it does. But at the anode the battery steals some electrons from the copper and it becomes a somewhat different material, a positively charged copper ion. These positively charged ions repel each other while also seeking negatively charged ion that can neutralize their charge. The acid is the negatively charged ion and the copper plus the acid form a water soluble copper acetate salt. At the cathode (the item you are plating), electrons coming out of the battery replace the electrons the copper ions are missing, reducing the charge on each copper atom back to zero so it can reform as metal rather than ions.

 
Ted Mooney, P.E.
finishing.com Inc. - Brick, NJ

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