The EMF has been determined in concentration cells with homogeneous cellulose acetate (CA) membranes as separators. The electrolytes studied are HC1, NaCl, KC1 and CaCl 2 in water at 25 °C. The concentration range is from KT 4 mol/dm 3 to 3mol/dm 3 .It is shown by means of Onsager's reciprocal relations that the apparent transference numbers calculated at various values of the variable concentration (from the slope of the EMF vs. log c curves) are identical to the real transference numbers of the cations at the same concentrations with a correction for water transference.Nernst-Planck equations are derived in the low concentration limit of some more general transport equations derived from friction theory and a generalization of Kohlrausch's law of independent ion mobility. Nernst-Planck equations in combination with a simplified treatment of the Donnan distribution of ions seem to be sufficient to explain the data. The water transference is apparently negligible.From the dependence of the EMF on concentration, the ratio between the ionic diffusion coefficients in the membrane and the membrane charge divided by a complex of ionic distribution constants can be determined. The measurements support the assumption that CA-membranes are weak cationic exchangers because of the presence of sparsely distributed glucuronic acid groups along the cellulose chains. The carboxylic groups are partially titrated by HC1, so the charge density becomes lowered. The charge density is also lowered by specific binding of Ca + + ions. The cations K + , Na* and a fortiori Ca + + are much more restricted in their motion than QT, probably due to electrostatic interactions Brought to you by | Purdue University Libraries Authenticated Download Date | 6/16/15 1:50 AM 2 T. Smith S0rensen, J. Birger Jensen with the negative fixed charges and the oxygen lone pairs. The mobility of H + seems to be almost unaffected by the presence of the membnme.The findings explain, why salt rejection in desalination experiments is strongly dependent on pH at low concentrations of NaCl. Today reverse osmosis with CA-membranes is only economically feasible for brackish waters, but we give some suggestions how to change the properties of CA-membranes, so that they might be used for sea water desalination.Inserting Eq. (23) into (22), we obtain an equation for the laboratory flux / x . The equation for J 2 is derived similarly, and we have: