Electroplating Theory

Electroplating is a relatively large facet of the chemical engineering world. It is used whenever a material that is being manufactured has to meet certain surface criteria that it does not possess. It serves as an economical method of imparting certain surface properties on an item without paying for the construction of the item completely out of that material. (Uh, wording is difficult here). Electroplating is also extensively used to impart finishes on a manufactured item, such as a bronze or brass coating.

The concept behind electroplating is a basic electrochemical cell. It contains both an anode and a cathode, usually submersed in a solution. A chemical reaction takes place in the cell, in which the anode is oxidized and the cathode is reduced. The general reaction at the anode is:

M --> M(n+) + ne(-)

and the general reaction occurring at the cathode is:

M(n+) + ne(-) --> M

These two reactions are referred to as half-cell reactions, and their sum gives the total reaction that is taking place.

Red1 + Ox2 <--> Ox1 + Red2

The direction of the spontaneous reaction depends on the cell potential of the two half-cell reactions. For each half-reaction, a standard electrode potential is defined for the reaction written as a reduction reaction. These standard EMFs are usually measured in reference to a standard hydrogen electrode. The half-reaction for this standard cell is:

2H(+)(aq) + 2e(-) --> H2(g)

and it has a half-cell potential of zero by convention. These standard half-cell potentials are tabulated in many common chemistry texts. In order to determine if a given reaction is spontaneous, the half cell reactions are summed, with the sign of the voltage switching for the oxidation half-cell.

Change in V = V2 - V1

where V1o is the half-cell potential for the oxidation reaction. If this sum is greater than zero, the reaction is spontaneous as written. Otherwise, the reverse reaction will take place. In order to force a reaction in the reverse direction, an external voltage can be applied to counter the emf supplied by the reaction.

The actual process of electroplating in the industrial setting is much more complicated than a simple electrochemical cell. Each item to be electroplated must be pre-treated, usually by cleaning thoroughly in acid baths along with possible activation so that the reaction will actually take place. After the item is properly prepared can plating finally take place.

In choosing the type of plating for this experiment, several factors will have to be taken into consideration. Safety is the primary factor. Freshmen should not be exposed to some of the more dangerous chemicals that can be used in electroplating. The solutions and electrodes should be chosen for their ease of use and safety. The electrodes and solutions should be easily obtained and inexpensive. Although gold or silver plating may be nice, it is fairly expensive. Also, time will play a large part in the experiment. Some plating situations may take too long to give a noticeable level of plating. A reaction will have to be chosen that takes place within a given time frame of say 15 minutes. Along with the kinetics of the reaction, other considerations must be met. Does the reaction require heating? Is the amount of power supplied to the electrodes too much for safety? Do the solutions have to be kept at a certain pH or concentration? Finally, will the actual item (I'm partial to dining hall silverware) to be plated be compatible with the given reaction? All of these factors must be considered in choosing a suitable reaction.