REPRESENTATION OF POLYGALACTURONIC ACID BY SUBUNITS IN THE ANALYSIS OF ITS POTENTIOMETRIC TITRATION CURVES

The potentiometric titration curves of aqueous solutions of polygalact uronic acid were determined in the presence of either tetraalkylammoni um or alkaline ions in order to explain the effects of the size and th e nature of the counterion on the degree of dissociation. In the evalu ation of potentiometric curves, the approach proposed by Strauss was u sed and successive ionization constants of appropriately chosen oligom eric subunits of the polymer molecule were determined. The partition f unctions and Gibbs free energies of ionization per mole of polygalactu ronic acid at the ambient chemical potential of hydrogen ions, and the Gibbs free energies per mole of polyacid related to the hydrogen ion in its standard state, were calculated and are given as a function of hydrogen ion activity or of the degree of ionization. From the above f unctions the differences in the counterion specificities were observed and are discussed on the basis of the formation of contact ion pairs between an alkaline ion and the carboxylate group. In addition, the va lues of the entropy and enthalpy changes per mole of repeating unit (h ydrogen ion at standard state potential) accompanying the stepwise dis sociation reactions were calculated on the basis of the temperature de pendence of the stepwise dissociation constants via the Gibbs-Helmholt z relation, and the thermodynamic data were correlated with the struct ural properties of the investigated polymer solute.