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Abstract:
The concentration dependence of the membrane potential of a film of a polyelectrolyte gel (Nafion) separating two aqueous KCl solutions is measured. Thus, the range of electrolyte concentrations is determined in which the membrane potential is given by the difference of the two equilibrium potential drops (the “Donnan potentials”) across the phase boundaries between the Nafion film and the electrolyte solutions. The results are compared with the concentration dependence of the Volta potential difference between electrodes coated with the same Nafion matrix and a reference metal (Kelvin's vibrating capacitor plate). For these experiments the coated electrodes are withdrawn under potential control from the electrolytes. It is demonstrated that both experimental approaches yield similar information on the equilibrium Donnan potential across the membrane/electrolyte interface, as long as the electrolyte concentration remains below the fixed-charge concentration. At higher electrolyte concentrations, at which the measured membrane potential includes a diffusion potential developing within the membrane phase, the corresponding Volta potential measurements show a measurable change in the surface dipole potential of the withdrawn membrane surface.
