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A New Model of Counterion Condensation in Polyelectrolyte Solutions II. Application to Potentiometric Titration of Weak Polyacids

The apparent dissociation constants, pKa, of weak polyacids in aqueous salt solutions are estimated as a function of the degree of dissociation, α, by our new model for counterion condensation (Intermediate Model, IMM) and our corrected previous model (CPM). Both models predict smooth titration curves free from a break point which is a defect in the prediction by Manning theory. Titration curves obtained by CPM show a maximum, indicating that CPM overestimates the degree of condensation with increasing α. Sigmoidal titration curves are predicted by IMM; the underestimation for a lower α region of poly(acrylic acid) is corrected through effective charge densities estimated from the counterion activity data, and that observed for the higher α region is interpreted as caused by hydration changes of the polyion with α.