TRANSPORT NUMBERS IN THE SURFACE-LAYERS OF ASYMMETRIC MEMBRANES FROM INITIAL TIME MEASUREMENTS

The initial time asymmetry potentials of two ultrafiltration membranes (cellulose acetate and polysulfone membranes) were measured in electr ochemical cells using Ag/AgCl electrodes and NaCl solutions. The conce ntration in the two electrode chambers differed slightly by a fixed co ncentration difference. Either the membranes were brought to equilibri um with the left-hand solution and subsequently exposed to the right-h and solution at the right-hand face, or the procedure;was reversed. Fr om such measurements it is possible to evaluate the transport numbers corresponding to each of the two surface layers of the membrane under conditions such that the effects of autoprotolysis of water and of for eign ions may be neglected. These measurements permit a description of each of the surface layers of the membranes and make possible an elec trochemical characterization of the asymmetry of ultrafiltration membr anes. The asymmetry is given by the difference between surface layer t ransport numbers.