TRANSPORT NUMBERS FROM INITIAL TIME AND STATIONARY STATE MEASUREMENTSOF EMF IN NONIONIC POLYSULPHONIC MEMBRANES

Initial membrane potentials of two non-ionic polysulphonic (PS) membra nes, used in hyperfiltration experiments, have been measured in an ele ctrochemical cell filled with NaCl solution using Ag/AgCl electrodes. The concentration in one chamber (left chamber of the cell) was fixed to be 0.01 M. In the other chamber of the cell the concentration was v aried between 10(-4) and 0.5 M. The two membranes were (1) a dense, al most uniform membrane, and (2) a porous, asymmetric membrane. It is po ssible from these initial time EMF measurements to evaluate the transp ort numbers of the sodium ion corresponding to each of the two surface layers of the membrane. From these transport numbers, an electrochemi cal characterization of the asymmetry in hyperfiltration membranes can be made. A comparison with the mean transport number obtained by stat ionary state EMF measurements is also performed. This transport number is always greater than the transport numbers obtained by the initial state method. The increased asymmetry of the membrane No. 2 is reflect ed in the initial time transport numbers (especially at low salt conce ntrations) but only to a minor extent in the stationary state mean tra nsport numbers.