Electroplating and Electronegativity

Electroplating is a widely used technique to deposit a layer of a first metal on an object made of a second metal.  The discovery of the Parthian batteries suggests that electroplating may have been developed some 1500 years ago in Mesopotamia.   However, the first solid historical reference is that of Luigi Brugnatelli in 1805.  Electroplating is used to give the object some property or properties which it lacks.  Examples are wear resistance, corrosion protection, and plating jewelry made of base metals with gold or platinum. 

Electroplating is accomplished by using an electric current to negatively charge the object to be coated while immersed in a solution which contains a salt of the metal to be deposited.  The metal ions in the solution are positively charged, so are attracted toward the object.  When these metal ions reach the surface of the object, they are reduced to metallic form by the object’s negative charge.   

A fair question is “What is the role of the electric current?”  The question is prompted by the existence of immersion plating, in which metal layers are deposited on metal objects without applying current.  For example, copper can be plated on an iron object simply by immersing the object into a solution of copper sulphate. 

However, this doesn’t work for all combinations of metals.  The copper ions in the copper sulphate solution have a positive charge, and to be reduced to copper metal must acquire electrons from the object to be plated.  The ability of a metal to attract electrons to itself is called electronegativity.  In the present example, copper has an electronegativity of 1.9 while iron is 1.83.  This means that copper is capable of pulling electrons out of the iron surface.  Essentially, the combination of a copper ion and the iron surface has higher energy than does the combination of copper coating and an iron ion.  As a result, the natural equilibrium of the system is to produce the copper coating.

But what if we want to plate a gold object with platinum?  The electronegativity of gold is larger than that of platinum (2.54 vs. 2.28), so platinum ions cannot remove electrons from gold, and immersion plating will not work.  To accomplish the desired plating, it is necessary to negatively charge the gold object sufficiently that it can give up its electrons to the platinum ions.  This is the function of the electric current. 

A platinum ion needs two electrons to be reduced to platinum metal.  The difference in electronegativity tells us that the plating voltage needs to be at least one volt to pull these electrons out of the gold to be transferred to the platinum ions.  An ampere of current provides 6.24 x 10^18 electrons per second, and can thus reduce 3.12 x 10^18 platinum atoms per second, corresponding to about a milligram per second.  This doesn’t seem like much, but it would plate a platinum layer on a gold ping-pong ball (if one was laying about) with a thickness increasing at the rate of about 0.07 microns per second.  With a little patience, a substantial platinum layer can be grown.

Having quickly covered the scientific background, let’s take a look at how to perform copper electroplating.  First find a pre-1983 penny and a nickel.  Why pre-1983?  After that time pennies were made of copper plated zinc, but earlier pennies are 95% copper, and make a good source of copper for plating.  Nickels are 75% copper and 25% nickel, but will clearly show when one is plated with a layer of copper.

Next, prepare an electrolyte.  The purpose of the electrolyte is to transport copper ions to the nickel under the influence of an applied voltage.  The electrolyte must also be able to dissolve the copper from the penny, converting it into copper ions which can be transported.  Otherwise the copper in the electrolyte will be rapidly depleted.  The electrolyte for this demonstration is made by dissolving 200 grams of copper sulphate (available in hardware stores as root eater) into half a liter of distilled water, then add 95 milliliters of battery acid (diluted sulfuric acid).  Add enough additional distilled water to make a liter of the electrolyte.  Be careful while working with the electrolyte, as it is both acidic and mildly poisonous.  Goggles and rubber gloves should be worn.  When done with the experiment, dispose of the electrolyte by flushing it down the drain with plenty of water. 

Clean the penny and the nickel, either with fine sandpaper or a solution of 3 grams of salt in 15 milliliters of vinegar.  Using a wire with alligator clips, connect the penny to the positive output of a DC power supply (which can be a 9 volt battery) and the nickel to the negative output.  Suspend the coins in a beaker containing about 200 milliliters of the electrolyte, and turn on the power.  Keep the power on for about an hour, and then look at the nickel.  It should now appear copper colored, giving you evidence of the electroplating process.  

More copper is plated than any other metal.  Following these instructions will allow you to enjoy this remarkable transformation.  Following the discussion will teach you why it works.  Enjoy!