Results of osmotic and of Donnan equilibria measurements in polymethacrylic acid–sodium bromide solutions. Part II

Abstract

AbstractThe osmotic and Donnan equilibria were measured in solutions of polymethacrylic acid in the presence of sodium bromide. The primary aim of these measurements was to apply a previously proposed thermodynamical interpretation of the experimental data. The Donnan distribution was determined by titration of the bromide, and was used to calculate the activity coefficient of the salt (in the presence of PMA). The osmotic equilibrium was determined by the previously described osmotic concentration method, and served to evaluate the osmotic coefficient of the solvent. The measurements were carried out at several degrees of ionization in solutions in which the concentration of both solutes was varied a thousandfold. The graphical differentiation of the above coefficients and the use of certain thermodynamical relations led to the calculation of the partial derivatives of the activity coefficients of the polyelectrolyte itself. The results obtained for the activity coefficients of salt, solvent, and polyelectrolyte could be described by equations of a very simple form, dependent only of the composition of the solution and on the experimentally determined osmotic coefficients of the pure solutions of the polyelectrolyte (i.e., with no salt added). These equations permit an unamiguous calculation of the activity of the various components in a salt‐containing polyelectrolyte solution of any given composition. Furthermore, their simple form should facilitate the theoretical interpretation of the properties of polyelectrolytes.