Two aspects of this process should be noted, firstly, that the amount of copper transferred from one electrode o the other is exactly proportional to the current passed, one copper atom for each two electrons; secondly, that no chemical work has been done by the current, copper has merely been transferred from one electrode to the other. Everything else is as before. Thus the current density controls the rate of electroplating and only a small voltage is required to drive the current round.
1. The simple salt solutions tend to give rather coarse crystalline deposits, which are more difficult to polish.
2. When a metal is baser (more negative in the electrochemical series) than metal of the electroplating solution which it is immersed in, there is a tendency for an exchange reaction to take place, without any applied current. For example, if steel is immersed in the copper sulfate plating solution mentioned above, some of the iron dissolved and is replaced by a film of copper, but this is loose and Powderly. It is not a good foundation for electroplating.
This explains why complex salts of a metal are sometimes used for electroplating rather than simple ones.
There is one other principle of electrolysis which must be understood in connection with electroplating. The principal constituent of the aqueous electroplating solution is of coarse water (H2O). This itself dissociates to come extent to yield positively charged hydrogen ions, and negatively charged hydroxyl ions (OH-), thus H2O <====> H+ + OH-
In copper sulfate solution, although the positively charged hydrogen ions are attracted to the cathode they cannot be discharged there because copper ions are more easily discharged. In a sodium sulfate solution or in an aluminum sulfate solution, however, it is far easier for the hydrogen ions to be discharged, so sodium or aluminum cannot be electroplated from aqueous solutions. As the table of electrochemical series, that it would be impossible to electroplate lead, tin, nickel, cadmium and zinc from aqueous solutions, but fortunately there is a further overvoltage of about 0.5 to 0.7 V which is necessary to evolve hydrogen gas on these metals.