Dissociation of weak polymeric acids and bases

TLDR

The electrolytic dissociation of polymeric acids and bases in aqueous solutions is characterized by potentiometric behavior described by equations relating intrinsic dissociation constants and ionization degrees. The study derives the electrostatic surface potential ψ₀ from the derivative of electrostatic energy with respect to ionized groups, provides equations for randomly kinked and stretched polyelectrolytes, and demonstrates that ψ₀ can be independently measured by electrophoresis, linking potentiometry and electrophoresis. Potentials obtained by potentiometry, electrophoresis, and theory agree within 3 %, validating the combined approach, which yielded pK₀ values of 3.40 for pectic acid, 3.53 for polyaspartic acid, and 10.44 for polylsine.

Abstract

Abstract The electrolytic dissociation of several polymeric acids and bases in aqueous solutions has been investigated. The potentiometric behavior is well described by the following equations, For polyacids: and for polybases: pK 0 is the intrinsic dissociation constant of the monomeric unit, α and β are the degrees of ionization of the polyacid and the polybase respectively, ψ 0 is the electrostatic surface potential of the polyion. It is shown that ψ 0 is equal to (1/ϵ) (δ F e /δ v ) for polyacids and (1/ϵ) (δ F e /δζ) for polybases where F e is the electrostatic energy of the polyion and v or ζ the number of negative or positive ionized groups respectively. Equations for the calcualtion of ψ 0 in the cases of randamly kinked and stretched polyelectrolytes are given and discussed. As the potential ψ 0 can be obtained independently from electrophoretic measurements, the above equations correlate potentiometry and electrophoresis. The potentials of polymethacrylate ions were calculated from theory and obtained from electrophoretic and potentiometric measurements. The potentials obtained by the three methods agree within 3% thus justifying the use of the combined potentiometric and electrophoretic measurements for the evaluation of pK 0 . Application of this method to the potentiometric analysis of pectic acids gave good agreement with experiment and lead to pK 0 = 3.40. Combined potentiometric‐electrophoretic analysis for polyaspartic acid gave pK 0 = 3.53 and for polylsine pK 0 = 10.44.