Osmotic and donnan equilibria in polyacrylic acid–sodium bromide solutions

Osmotic and Donnan equilibria were measured in aqueous solutions of ionized polyacrylic acid and of sodium bromide. The results served to examine the validity of the previously formulated, “empirical” and “theoretical” formulas for the activities in mixed salt–polyelectrolyte systems (c.f. the preceding article). A very good agreement between the experiment and both formulas is found in case of the osmotic factor. The results obtained for the activity coefficient of the salt show a more limited agreement, and the validity of the theory seemingly does not extend—except as a very rough approximation—to the case of concentrated polymer solutions to which relatively small amounts of salt are added. Donnan and osmotic equilibria were also measured at a constant concentration of the external salt solution and the results obtained in the presence of an excess of salt interpreted in terms of the well known binding ratios ( m ′ s – m s )/ m polymer and second virial coefficients, respectively. The former are found to increase in concentrated salt solutions, in agreement with the theory (which in this case differs largely from the empirical formula), whereas the latter are described adequately enough by the simpler empirical formulas. The limited nature with respect to both the validity and significance of constant binding ratios and second virial coefficients, is discussed.