As a number of elements are capable of exhibiting more than one oxidation state, hence volumetric titration methods based on redox reactions are usually employed widely.
The phenomenon of oxidation may be explained in the following manner :
(i) addition of oxygen :
(ii) removal of hydrogen :
(iii) enhancement in the ratio of electronegative to the electropositive portion of the molecule :
In the same vein, the process of reduction may also be explained as stated below :
(i) addition of hydrogen :
(ii) removal of oxygen :
(iii) enhancement in the ratio of electropositive to electronegative portion of the molecule :
Example : [same as under oxidation (iii) above]
It is quite evident from the above cited examples that reduction need not always imply a reaction involving hydrogen, since HgCl2 is reduced to Hg2Cl2, and that oxidation may not essentially suggest a reaction involving oxygen, since Fe2+ is oxidized by Cl2 to Fe3+. It is, therefore, pertinent to observe here that whenever one entity undergoes oxidation, definitely some other entity undergoes reduction correspondingly and vice-versa. In other words, there always exists a transfer of electrons in oxidation-reduction reactions, because in every such reaction the charge gained or lost by one substance must essentially be lost or gained by another.
A reducing agent is the reactant that loses electrons in an oxidation-reduction reaction :
Thus, the reactant containing a constituent atom or atoms are converted to a higher state of oxidation.
An oxidizing agent is the reactant that gains electrons in an oxidation-reduction reaction :
Thus, the reactant containing a constituent atom or atoms are converted to a lower state of oxidation.
The quantitative measurement of one of the reactants may be accomplished by the reaction derived from the combination of oxidizing and reducing agents, for instance
and hence, ferrous sulphate can be estimated quantitatively by its reaction with ceric sulphate.