Measurement of membrane potential and estimation of effective fixed-charge density in membranes

Summary

Electrical potentialsE _m arising across cross-linked phenolsulfonate membrane separating NaCl solutions of molality M₁ and M₂ have been measured at 25 °C. These values ofE _m have been used in the Nernst equation to calculate values for the apparent transport number$\bar t_{i(app)}$ for the counterion or the co-ion in the membrane. Values of$\bar t_{i(app)}$ together with the limiting value for the cation transport number in the aqueous phase have been used in the equation developed by Kobatake and co-workers to evaluate the membrane permselectivityP _s as a function of external electrolyte concentration. With the help of the equation relatingP _s to$\overline {\phi X}$, the effective fixed-charge density in the membrane (where$\bar \phi$ is a constant,$0< \bar \phi< 1$, and$\bar X$ is the membrane stochiometric charge density and can be evaluated by chemical analysis of the membrane phase), values for$\overline {\phi X}$ and$\bar \phi$ have been determined. Values of$\bar \phi$ were low in dilute solutions and increased with increase in the concentration of the external solution. Similar behavior was noted in the case of another membrane system, cross-linked polymethacrylic acid in contact with KOH solutions. On the other hand, the membrane system, “untreated” collodion in contact with KCl solutions, exhibited a behavior in which the values of$\bar \phi$, low in dilute solutions, increased and then decreased following a gradual increase in the external concentration. This slight divergence in its behavior was attributed to the heterogeneity of the collodion membrane structure. The reliability of this potentiometric method to estimate effective fixed-charge density in membranes has been discussed in relation to a similar but old method due to Teorell, Meyer and Sievers. Also the significance of the values derived for$\bar \phi$ has been pointed out.