Acid–base Properties of Ion-exchange Resins. Dissociation and Swelling of Resin Copolymers of Methacrylic Acid,Methyl Methacrylate,Divinylbenzene, and Ethylvinylbenzene

The dissociation, capacity, swelling, and water content of crosslinked methacrylic acid—methyl methacrylate resins have been measured. Resins were prepared with different degrees of crosslinking for the same carboxylate content, and vice versa. The ionic strength of the external solution was also varied, and the behavior of commercial resins compared with that of the laboratory resins. Potentiometric titration curves were obtained, and curves were also obtained by back-titration of the salt form of the resins with acid. The capacities showed that almost all carboxyl groups are accessible in resins containing 2.5% or 4.0% divinylbenzene, but not in those containing 8% or 12%. For these highly crosslinked resins the back-titration curves differed from the forward curves. Apparent dissociation constants pK′_a = pH + n log (1 ? α)/α] decreased with increased ionic strength, increased with increased crosslinking, and showed no trend with carboxylate content. Swelling is decreased by increased salt concentration, particularly for lightly crosslinked resins. Maximum swelling is achieved at about 80% dissociation. The reciprocal of the swollen volume is proportional to the per cent of divinylbenzene. Commercial resins showed much lower swelling than laboratory prepared resins ostensibly having the same composition. The Gibbs-Donnan theory of resin dissociation was applied to calculate the intrinsic dissociation constant (pK′₀). Assuming a model of randomly kinked chains dissolved in the sorbed solution, good agreement with the expected value of 4.85 was found (calcd. pK′₀ = 4.81 = 0.14), except for the most highly crosslinked resins. For polyampholyte resins, agreement was found by using a model having a uniform potential distribution throughout the resin (pK′₀ = 4.9).