Explain the theory behind the potentiometric titration of acid base?

Answer 1

In potentiometric titration, the change in the concentration of H+ ions are monitored based on the emf measurement with respect to Saturated calomel electrode ( a reference electrode having a standard reduction potential of 0.2422 V). H+ ions concentration in terms of pH change is measured using primarily by quinhydrone electrode.

Based on hydrogen electrode also it can be measured.

An electrochemical cell consists of H+ ions (acid to be titrated) in contact with hydrogen gas (1 atm) or quinhydrone/ quinone should be formed and the emf (potential difference) of the cell is measured with reference to saturated calomel electrode (SCE).

Then based on Nernst equation value of standard electrode potential of hydrogen ion is,

EH = Eo + 0.0591 log [H+]

or EH = - 0.0591 pH

since Eo = 0 V and -log [H+] = pH

So EMF of the cell is Ecell = ESCE - EH

Ecell = 0.2422 - EH

The Ecell value is plotted ( Y axis) against volume of base added (X axis).

The volume corresponds to drastic Ecell change gives equivalence point of the titration. At equivalence point all the free H+ ions are replaced as H2O and this is reflected and can be inferred in the Ecell values.