ABSTRACT
Transport numbers have been extensively used over the last decades to characterize ion transport in charged membranes. In spite of their conceptual simplicity, a number of difficulties arise in the interpretation of experimental results because of the influence of the operating conditions. The meaning of the transport numbers measured with different methods is here explained by relating them to the local migrational transport numbers and the concentration profiles. The important effects of the diffusion boundary layers and the electric current density used (in the case of Hittorf s method) are then analyzed. Although electromembrane processes often involve multicomponent systems, the characterization of the transport properties of the membrane is often made in terms of binary electrolyte systems. The definitions of potentiometric transport numbers in ternary systems are worked out and these numbers are computed from the solution of the transport equations. These transport numbers are compared to the apparent potentiometric transport numbers used in the literature (i.e., those obtained by assuming that the ternary system behaves as a binary system), and the benefits of their use are clearly shown. Finally, a brief account of the difference between transport numbers in the membrane-fixed and solvent-fixed reference systems is given, and a recent modification of Hittorf s method that makes use of countercurrent convective flow is reviewed.
