Secondary constant current coulometry:

This method has enjoyed huger applications and is often known as a coulometric titration.  Within a coulometric titration that is one of the reactants, titrant is quantitatively generates (produced) at an electrode that then stoichiometrically reacts along with the analyte ion to be estimated.

A titration of ferrous ion versus ceric ion is an instance of a coulometric titration. Considering the oxidation of iron (II) at a platinum anode, a primary anodic reaction is

Fe²⁺     ?        Fe³⁺     +          e

Inside a constant current electrolysis, towards the completion of the reaction (since the concentration of iron (II) decreases) there is an increase within the applied cell potential. Since of concentration polarisation, the increase within potential causes the anode potential to the point whereas decomposition of water takes place

2H₂O  →        O₂ (g) + 4H⁺ + 4e

Therefore the completion of reaction, that is. oxidation of iron (II) at a platinum anode, is not achieved. The current effectiveness of such a process is less than 100 percent. Through introducing an excess quantity of cerium (III), that is oxidised at a lower potential than that of water,

Ce³⁺     →        Ce⁴⁺ + e

a coulometric determination of iron (II) could be achieved as :

Ce⁴⁺   + Fe²⁺   →        Fe³⁺   + Ce³⁺

The ceric ion produced through the oxidation of cerous ion at the anode, diffuses out and reacts quantitatively along with ferrous ion and oxidises it to ferric ion.  A net effect is an electrochemical oxidation of iron (II) along with l00% current efficiency.

Coulometric titrations are more popular since of the in situ generation of titrimetric reagents. For instance, the halogens have been found to be the most satisfactory coulometric intermediates within coulometric titrations. The reactions along with electrolytically produced bromine and iodine are meaningful for the estimation of substances such as oxine and ascorbic acid and proceed along with 100% effectiveness.