External generation of titrant:

Because internal generation of titrant sometimes interferes with the titration, it becomes essential to produce the titrant externally and deliver to the reaction/titration cell. An assembly of the type shown in Figure is commonly used. For   instance, hydrogen and hydroxyl ions are produced through the electrolysis of sodium sulphate solution.

2H₂O + 2e ?  H₂ + 2OH^-                        (cathode reaction)

H₂O ? ½ O₂+ 2H⁺ + 2e                                  (anode reaction)

Figure: A cell for the external coulometric generation of acid and base