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This page is intended for students, as a quick introduction. It explains the scientific principles behind electroplating, and offers two demos, but it does not cover many of the issues of the real-world industrial situation.
In the real world, things like polishing, pre-treatment, and post-treatments are usually more critical than the electroplating step itself. And it is rarely possible to get satisfactory plated coatings without buying proprietary organic additives from the patent holders. In the real world, consideration must be given to safety issues, proper waste treatment and disposal, and a host of other issues. Remember, this page explains some of the basic science of the electroplating for a student experiment, but is NOT a guide to doing practical electroplating and you will not get shiny, attractive, durable parts from this procedure.
What is electroplating?
Electroplating is the deposition of a metallic coating onto an object by putting a negative charge onto the object and immersing it into a solution which contains a salt of the metal to be deposited. The metallic ions of the salt carry a positive charge and are attracted to the part. When they reach it, the negatively charged part provides the electrons to "reduce" the positively charged ions to metallic form.
How does it work?
Look at the figure below, and then follow the written explanation.
Imagine that we have an object that is made of copper or steel, and that it has been properly cleaned, and that we now want to plate it with nickel. A wire is attached to the object, and the other end of the wire is attached to the negative pole of a battery (the wire is blue in this picture). To the positive pole of the battery we connect the red wire; the other end of the red wire we connect to a rod made of nickel.
Now we fill the cell with a solution of a salt of the metal to be plated. It is theoretically possible to use a molten salt, and in rare cases that is done, but most of the time the salt is simply dissolved in water and acid. The NiCl2 salt ionizes in water to Ni++ and two parts of Cl-
Because the object to be plated is negatively charged, it attracts the positively charged Ni++. The Ni++ ions reach the object, and electrons flow from the object to the Ni++. For each atom of Ni++, 2 electrons are required to neutralize it or 'reduce' it to metallic form.
At the anode, electrons are removed from the Nickel metal, oxidizing it to the Ni++ state. Thus the nickel metal dissolves as Ni++ into the solution, supplying replacement nickel for that which has been plated out, and we retain a solution of nickel chloride in the cell.
We used nickel chloride as the first example here for the simplicity of explanation. But we do not recommend that nickel be used for school science demonstrations.
For a school demonstration, we suggest plating copper pennies with zinc, or plating quarters with copper.
The first demonstration
For the first demonstration, the cathode is copper (the pennies), the anode is zinc, and the electrolyte (solution) is a zinc salt dissolved in vinegar and water.
One source of zinc is the shell of conventional carbon-zinc batteries (make sure not to use alkaline batteries like Duracell or Eveready Energizers, nor rechargeable nickel-cadmium batteries, but the cheap 1-1/2 volt AA, C, or D plain carbon-zinc batteries). The science teacher can cut up such batteries and remove the black glop, and give the student the clean zinc. An alternative source is zinc anodes available from any boating store. A final option is to sand down a modern U.S. penny until the copper surface is removed and the underlying zinc substrate is exposed.
For the pennies you wish to plate onto, any pennies will do, but if you start with a dull brown penny, you'll end up with a dull zinc plated penny. So, try to find shiny new pennies for best results. Immediately before plating, clean it with toothbrush and toothpaste, or a gentle scouring powder like Bon Ami or Multiscrub. Rinse well after cleaning, but use plastic gloves so you do not get fingerprints or other soils on the penny after cleaning.
A transparent plating container is best, a Pyrex beaker is excellent; but if not available, a Pyrex dessert bowl can serve well.
A recipe suggested by Tom Pullizzi, and retested by Ted Mooney and found to work is:
Fill the container about half way with vinegar, but measure how much vinegar that is. Put the zinc anode into vinegar and let it sit for several hours, allowing some of it to dissolve. We'd like to shoot for 100 g/l of dissolved zinc, although the vinegar probably will not support that much dissolution.
Add 100 g/l of Epsom Salts
and 120 g/l of table sugar.
Connect one flashlight battery (1-1/2 volts) to the penny and the zinc, and place them into the solution. Don't let them touch each other. With luck, within a few minutes you'll begin to get a bright silvery coating. Ted didn't have quite that much luck when he tried it, but did find that a reapplication of the toothbrush and toothpaste quickly polished the thin greyish coating to a fairly bright shine.
A second demonstration
Another slightly harder demonstration is plating a quarter or a brass key with copper. The key on the left was copper plated from a solution of vinegar with a pinch of salt and a pinch of sugar, again using a 1-1/2 volt flashlight battery for power.
Understanding why this is a little harder to do is a good science lesson. Remember that you can't plate a metal out of a solution if that metal hasn't been dissolved into the solution (This is why, incidentally, you can't do a student demo of silver, gold, or chrome plating--you won't be able to dissolve these metals in vinegar, you would need a strong and more dangerous acid).
Copper will not dissolve in vinegar without electricity to help it along, so it's best to get started with a small piece of scrap as your cathode and a large coil of copper wire as the anode. I stripped about 2 foot of 14 gauge wire as the anode and used a 1/2 inch length of stripped copper wire as the scrap cathode. After I ran it this way for a couple of hours the solution had a faint blue tinge to it--indicating at least a little copper was dissolved in it. Then I cut off the scrap length of cathode wire, attached the key and plated it for several hours. Vinegar is too weak an acid to support a lot of copper in solution, so there is no rushing it, you have to plate slow and for a long time so copper can slowly dissolve into solution to replace what you plate out. I found that just a pinch of salt (maybe two shakes) was enough. If you use more, what happens is you make a more conductive solution, so more electricity flows, but since there is not enough copper dissolved to support that plate-out rate, you generate a lot of hydrogen gas and "smut" --you can't plate copper out of solution faster than it goes into solution.
What is "smut", or as some students call it, "black glop that coats the coin"? When you have too much current flow, the moment each atom of copper gets to the cathode, it is "reduced" instantaneously with no opportunity for proper crystal formation. Such tiny, non adherent individual specs appear black. Use 1-1/2 volts, maximum, and take your time.
Waste Disposal
In the "real world" waste disposal is a major headache for electroplating factories. But here you have only used toothpaste, vinegar, Epsom salt or table salt, and sugar. The very small amount of zinc or copper you have dissolved into it is really not an issue. Good luck.
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From a reader: I tried electroplating a quarter with a penny, it was awsome. I have attached an image of my apparatus, so you could add this to your site as "copper quarter". -- Zeno L.
Ed. note: We don't understand using 9 volts in your experiment, since our results indicate that even 3 volts is too much. We're also not sure if you have drawn a wire connecting the salt water to the Coke, but we're glad it worked for you. |
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Please e-mail your suggestions to:
Ted Mooney, P.E.
<mooney@finishing.com>
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