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

Abstract

Electrical potentials Em arising across cross-linked phenolsulfonate membrane separating NaCl solutions of molality M1 and M2 have been measured at 25 degrees C. These values of Em have been used in the Nernst equation to calculate values for the apparent transport number ti(app) for the counterion or the co-ion in the membrane. Values of ti(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 permselectivity Ps as a function of external electrolyte concentration. With the help of the equation relating Ps to phiX, the effective fixed-charge density in the membrane (where phi is a constant, 0 less than phi less than 1, and X is the membrane stochiometric charge density and can be evaluated by chemical analysis of the membrane phase), values for phiX and phi have been determined. Values of 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 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 phi has been pointed out.

Estimation of surface charges in some biological membranes

“…the value of q~ tends to increase with increase in the external electrolyte concentration. Similar behavior is also shown by the parameter • (see last column of Table 1 of Lakshminarayanaiah, 1975a) in the case of ion exchange membranes used by Lakshminarayanaiah (1975 a). In the case of other ion exchange membranes in which the membrane undergoes little dimensional change (i.e.…”

“…In the case of model systems, the low value of q~X in solutions of low ionic strength has been attributed to the close association of counterions with the ionogenic groups (Lakshminarayanaiah, 1975a). There will be few co-ions in the membrane phase and thus the double layer formed of negative groups of the membrane and the associated counterions will be compact and clear.…”

“…Recently, it was shown by Lakshminarayanaiah (1975a) that in the case of cross-linked sulfonated phenol-formaldehyde and polymethacrylic acid, membranes whose density of charges were determined both analytiCally (values of X) and potentiometrical]y (values of 0-X), the values of both X and qS-X increased with increase in the concentration of the surrounding electrolyte solutions. At any given concentration, the value of 0-X was less than the value ofX.…”

“…In a recent paper, Lakshminarayanaiah (1975a) applied the theory of membrane potential developed by Kobatake and colleagues (see Kobatake & Kamo, 1973 for a review) to derive values for the effective fixedcharge density of artificial ion exchange membranes. It was shown that this theory (K-theory) gave more reliable values for the effective fixedcharge density of membranes than an older theory (TMS) due to Teorell (1953) and Meyer and Sievers (1936) whose theoretical and technical limitations have been discussed elsewhere (Lakshminarayanaiah, 1974).…”

Transport studies of ions across polystyrene based composite membrane: Evaluation of fixed charge density using theoretical models

“…they change in the same manner [53]. This low value of X was attributed to the heterogeneity of the membrane structure [54].When ions enter membrane; the ion can interact with the fixed charge in the membrane owing to Donnan potential. With increasing the concentration of electrolyte, more cations will interact with single fixed ion.…”