The Basic Principles of Electrochemistry

 

There are two types of electrical conductor which are of interest in electrochemistry, they are distinguished from electronic conductors such as metals by the fact that that the transfer of current is accompanied by the transfer of matter. One type concerns pure substances such as fused salts and the other involves solutions, usually solutions of acids, bases or salts in water. There are a few substances that have both characteristics e.g. b-silver sulphide in which 80% of the current is conducted electrolytically the remainder electronically. Solutions of alkali metals in liquid ammonia and certain molten alloys are also examples of substances that conduct by both mechanisms simultaneously.

 

The transport of matter accompanying the transfer of current is simply demonstrated by immersing two platinum electrodes in a dilute sulfuric acid solution and connecting them to the poles of a battery. The electrode connected to the positive pole of the battery is called the anode and that connected to the negative pole is called the cathode, it is seen that hydrogen is evolved at the cathode and oxygen at the anode. If the sulfuric acid is replaced with copper sulfate, it is found that copper is deposited at the cathode instead of evolving hydrogen.   

 

Electrical charge is transferred in solution by means of charged ions. Consider a salt (AB) made up of an acid (A) and a base (B). Now the salt will dissociate into acid ions and base ions and some of the salt will remain undissociated e.g.’

 

                                                

 

Applying the law of mass action in its simplest form, in which concentrations (c) are employed instead of activities,

 

                                                                                        ₍₁₎

 

where (k) is the dissociation constant.

 

 Assume the solution contains (c) mol per liter of electrolyte (AB) and let (a) be the degree of dissociation then the concentrations, (c_A) and (c_B) will be (c) and the concentration of the undissociated molecules, (c_AB), (1-a)c.

 

        Thus,                                                                (2)

 

 

Equation (2) is known as Oswald’s Dilution Law. It is only approximate but is accurate enough to be practically useful. Before proceeding further it is necessary to discuss the electrical conductivity of solutions.