Neutralisation Titrations:

Potentiometric neutralisation titrations are particularly useful for the analysis of mixture of acids or polyprotic acids or bases because discrimination between the end-points can often be made. An approximate numerical value for dissociation constant of the reactant species can also be estimated from potentiometric titration curves in theory. This quantity can be obtained from any point along the curve as a practical matter. It is most easily calculated from the pH at the point of half-neutralisation e.g. in the titration of weak acid HA, we might ordinarily suppose that at the mid-point [HA]≡   [A¯] + [H+] and therefore:

Ka =[H⁺ ] [A^- ]/ [HA]                                                                       ... (2.42)

log K_a   =  log H⁺   +  log [A^- ]/ [HA]

log K_a   =  log H⁺   -  log [A^- ]/ [HA]

pK_a = pH - log [A^-]/[HA]

At half neutralization of the acid

[A^- ] (salt) = [HA] (acid)                                                                   ... (2.43)

Therefore,  pK_a   = pH - log 1        (log 1 = 0)

or  pK_a   = pH                                                                                   ... (2.44)

This is the midway point to the equivalence pint. Therefore, pKa values can also be directly read from the titration curves.

It is significant to remember that a dissociation constant determined from a potentiometric titration curve may differ from that shown in the table of dissociation constant in the literature by a factor of 2 or more because the latter is based upon the activities while the former is not.