Colloidal titration: from the perspective of stability constants between oppositely charged polyelectrolytes

Abstract

Colloidal titration is a unique method to measure the concentration of charged groups based on the association between cationic and anionic colloids. Titrimetry must involve a unidirectional reaction with a large formation constant. However, the underlying polyelectrolyte complex (PEC) for colloidal titration lacks sufficient data regarding the stability constants (KI). To date, the stoichiometry of colloidal titration has not been discussed in relation to KI. Thus, in this paper, we reviewed PEC studies by focusing on KI and discuss colloidal titration comprehensively. The topics include the history of its establishment, the method to calculate KI, and the special characteristics of PEC compared with ion association reactions, the practical use of colloid titration, and the condition by which the stability constant can be increased. In terms of analytical reagents, polyelectrolytes are similar to (but positioned as a subspecies of) ion association reagents. Ionic strength strongly affects KIs values. Excluding coaggregation effects, the maximum values of log10 KI obtained from reliable data were found to decrease linearly with as ionic strength increases logarithmically. The critical line connected with the maximum KI value at each ionic strength can be used as an empirical reference value and facilitate the analytical design of colloidal titration.