Electroplating..

What is electroplating?

Electroplating is the deposition of 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.