A theoretical study of the bi-ionic potential (BIP) has been carried out us
ing the extended Nernst-Planck equation in the case of a mixed control of t
he interdiffusion process by the ion-exchange membrane (IEM) and the diffus
ion boundary layers (DBLs), a non-zero co-ion flux, a non-zero water flux,
a variable selectivity coefficient and an affinity coefficient different fr
om unity. The numerical integration of the two coupled differential transpo
rt equations allowed, for a given common concentration C-0, the computation
of the BIP from the values of 12 parameters required.
Three of these 12 parameters are taken from the literature, the others are
determined from independent experiments except the DBL thickness and the co
-ion diffusion coefficient in the membrane. We have developed a procedure t
o deduce these two parameters from the experimental curves of the BIP vs. C
-0.
For the NaCl/CM2/LiCl bi-ionic system, the DBL thickness changes from 59 mu
m at high stirring rate to 196 mu m in the absence of stirring. The chlori
de diffusion coefficient in the CM2 membrane has been estimated to be equal
to 2.1x10(-7) cm(2) s(-1).
Using these values, we have studied theoretically the influence of each of
the three parameters: affinity coefficient, selectivity coefficient and wat
er flow. We have shown that the affinity coefficient has the most important
contribution but the selectivity coefficient and the water flow influence
only the BIP at high common concentrations. (C) 1999 Elsevier Science B.V.
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