In this study, the dissociation constant of acetic acid as a weak elec
trolyte in an ion-exchange membrane, (K) over bar, was estimated using
the Donnan equilibrium theory, which has been applied to a strong pol
yelectrolyte membrane. The membrane charge density and the total conce
ntration, which represents the sum of the dissociated proton concentra
tion and carboxyl concentration of the undissociated electrolyte in th
e membrane, were determined by neutral titration; the charge effective
ness was determined from membrane potential measurements; and the prot
on concentration in the external solution phase was determined by Donn
an equilibrium theory and used for the calculation of (K) over bar. Th
e dissociation constants, (K) over bar, in the cation exchange membran
e were on the order of 10(-4) and were larger than that in water (= 1.
73 x 10(-5)). The value of (K) over bar increased with an increase in
the concentration in the external solution and decreased with the fixe
d charge density increase. It is suggested that the fixed charge group
depresses the dissociation of the weak electrolyte.