Effect of Dissociation Equilibria on Ion-Exchange Processes of Weak Electrolytes

Ion-exchange processes involving weak electrolytes are influenced strongly by the pH via the occurrence of association/dissociation equilibria. This influence was investigated quantitatively through binary displacement experiments with different electrolytes to gain better insight in the role of the pH and to test a fixed-bed ion-exchange model used for the description of multicomponent processes. The two major factors determining the ion-exchange behavior of weak electrolytes are the actual exchange of counter-ions and the sorption of neutral species. The sorption of neutral species resulted in concentration fluctuations superimposed on the normal ion-exchange profiles, whereas pH fluctuations inherent to ion-exchange processes in general also influenced concentration through modified ionic fractions. The relative importance of these contributions is determined by the magnitude of parameters such as resin capacity, distribution coefficients of neutral species, and selectivities of ions, as well as by operating conditions such as concentrations of all species.