Transport number of ions during electrodialysis during intensive current regime

Abstract

Transport numbers of ions across ion-exchange membranes are the main characteristics of the passport, as they determine the efficiency of electrodialysis. Methods are known for measuring them at below the diffusion limiting current density. Transport numbers values are determined by the express method of the membrane potential of the Nernst. Analytical method Hittorf requires more time but gives the possibility to obtain more accurate results. Both methods can not be used to measure the transport numbers at current densities above the limit. In the present work a method is proposed for measuring the transport numbers of ions in the membrane when intensive regime of electrodialysis. It allows to measure the transport numbers of the electrolyte under study, the number of ion transport generated by irreversible dissociation of water molecules and the number of co-ion transport. The basis of the method is the asymmetric polarization of the cation ex- change membrane and anion exchange membrane, achieved by a significant difference in concentrations in adjacent sections of desalination. In this case, the maximum current density is exceeded on the cation ex- change membrane, and on the anion exchange membrane is not achieved, and we were able to measure the transport numbers of ions formed during irreversible water dissociation. Measurement of counter-ion trans- port numbers allowed to establish their exponentially decreasing dependence on the current density. The dependence of the transport numbers of counter-ions on the value of their charge is established. The maximum numbers of counter-ion transfer were obtained for single-charge ions, the minimum for three-charge ions.The method allowed us to establish exponentially increasing the type of saturation curve dependence of the hy- drogen ion transport numbers through the cation exchange membrane as a function of current density. Since the sum of the numbers of transfer through the membrane is equal to one, it made it possible to estimate the numbers of transfer of coions. The exponential increase of the coions transport numbers with increasing cur- rent density is shown.