Electroplating -- How It Works

What is electroplating?

Electroplating is depositing a metallic coating by putting a negative charge on an object and exposing it to a solution containing a metal salt. The positively charged metal ions in the salt solution are attracted to the object and reduced to metallic form upon it.

How does it work?

Look at the figure above: We have a metallic object we want to plate with a metal. First we fill a cell with a solution of a salt of the metal to be plated. Most of the time the salt (nickel chloride in our example) is simply dissolved in water and a little acid.

The NiCl₂ salt ionizes in water into Ni⁺⁺ ions and two parts of Cl^- ions.

A wire is attached to the object, and the other end of the wire is attached to the negative pole of a battery (with the blue wire in this picture) and the object is immersed in the cell. A rod made of nickel is connected to the positive pole of the battery with the red wire and immersed in the cell.

Because the object to be plated is negatively charged (by being connected to the negative pole of the battery), it attracts the positively charged Ni⁺⁺ ions. These Ni⁺⁺ ions reach the object, and electrons flow from the object to the Ni⁺⁺ ions. For each ion of Ni⁺⁺, 2 electrons are required to neutralize its positive charge and 'reduce' it to a metallic atom of Ni⁰. Thus the amount of metal that electroplates is directly proportional to the number of electrons that the battery provides.

This relationship is a reflection of Faraday's Law of Electrolysis. If you are advanced enough in chemistry (a high school student), that you've heard terms like gram molecular weight, mole, valence and Avagadro's number, but it's all a hodepodge to you instead of a cohesive whole, don't despair!  Study Faraday's Law, and suddenly all of these disparate wacky terms will come together in a moment of enlightenment.

Meanwhile back at the anode, electrons are being removed from the Nickel metal, oxidizing it to the Ni⁺⁺ state. Thus the nickel anode 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.

As long as the battery doesn't go dead, nickel continues to dissolve from the anode and plate out onto the cathode.

We used nickel chloride in the example chiefly for simplicity of explanation. First, because nickel always dissolves in the "+2" oxidation state (Ni⁺⁺), whereas many other metals like copper and zinc can dissolve in either the "+1" or "+2" state and add some confusion; secondly because chloride is a simple one-atom anion whereas most anions like sulphate or acetate are far more complex. But we do not recommend that nickel be used for school science demonstrations because -- while the explaining is simple -- the plating is difficult :-)

For school demonstrations, we suggest plating copper pennies with zinc, or plating quarters or brass keys with copper.

The first demonstration -- Zinc plating a penny

For the first demonstration, the cathode will be copper (pennies), the anode will be zinc, and the electrolyte (solution) will be zinc dissolved in vinegar and water.

With a hacksaw a teacher or group can cut many slices from one anode.

A second option is to sand down a modern U.S. penny (1983 or later) until the copper surface is removed and the underlying zinc substrate is exposed.

A third possible 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 -- just 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 cleaned zinc.

For the pennies that 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. Try to find shiny new pennies for best results! Immediately before plating, clean the penny with toothbrush and toothpaste, or a gentle scouring powder like Bon Ami or Multiscrub, and rinse well after cleaning. Your hands are oily, so wear 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 a glass dessert bowl can serve well.

A recipe suggested by Tom Pullizzi, and found to work is:

Fill the container about half way with vinegar (vinegar is mild acetic acid). Put the zinc anode into vinegar and let it sit for several hours, allowing some of the zinc to dissolve.

Add 100 g/l of Epsom Salts (this salt helps make the solution conductive)
and 120 g/l of table sugar (this is called a "brightener")

Connect one flashlight battery (1-1/2 volts) to the penny and the zinc anode, 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 Mooney 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 he got to a fairly bright shine.

A second demonstration -- Copper plating a key or a quarter

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 table 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: You can't plate a metal out of a solution until you can get that metal dissolved into the solution (this is why we don't do a student demo of nickel, silver, gold, or chrome plating; you won't be able to dissolve these metals in vinegar, you would need a stronger 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 and crunched together about 2 foot of 14 gauge wire to use as the anode (wire is very pure copper) 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 acquired a faint blue tinge to it -- indicating that 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 hold much 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 table 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 in the solution to support that current flow, you generate a lot of hydrogen gas and deposit a lot of black "smut" -- you can't plate copper out of solution faster than it goes into solution!

What happens if your solution is too conductive due to too much table salt? The electricity is flowing through the solution, so electrons are flowing into it from the cathode. But if there are no copper ions there to pull out of solution, the electricity will pull hydrogen ions out of the water per this equation:

2H₂0 --> H₂⁺⁺ + 2OH^-

This will cause bubbles of hydrogen gas to accumulate on your key or quarter, and the OH, the hydroxide, will neutralize your vinegar so you'll have no acid left. What is "smut", or as some students call it, "black glop that coats the coin"? When you have too much current flow, what also happens is the moment an ion of copper gets to the cathode, it is "reduced" instantaneously with no opportunity for proper crystal growth, forming a powder of tiny, non adherent individual specs of metal which appear black. So use 1-1/2 volts maximum, very little salt, 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, salt and sugar. The very small amount of zinc or copper you have dissolved into it is really not a significant issue. But let's learn environmental responsibility, rather than practicing talking ourselves out of it! With the wires disconnected, put a good size chunk of steel wool in your dish. Any copper, and some of the zinc, will "immersion deposit", plating out onto the steel wool, removing the waste from the solution. Copper metal is not a pollutant, but dissolved copper is. Good luck!