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Electrical transport phenomena in a cation-exchange membrane

1968; Elsevier BV; Volume: 28; Issue: 1 Linguagem: Inglês

10.1016/0021-9797(68)90213-0

1095-7103

P. Meares, Alice Sutton,

Membrane Separation Technologies

The transport numbers of the mobile ions have been determined in a phenol-sulfonic cation-exchange membrane under conditions of constant chemical composition. Measurements have been made at 25°C and at a number of external concentrations in the range 0.01–1.00 equiv/l with the membrane in the Na+, Cs+, and Sr2+ states. In all cases the external solutions were bromides. Measurements were also made over the whole composition range at 0.02 and 0.10 equiv/l in the Na+ + Cs+ and Na+ + Sr2+ mixed cation states. Constant composition was maintained by using radiotracers to follow the fluxes in a multicompartment cell. Results are reliable to ± 1%. The co-ion transport number was relatively much larger when the resin contained bivalent rather than univalent counterions because the mobility of the counterions was lower and the concentration of the co-ions higher. In the mixed counterion states the transport number of the thermodynamically preferred counterions was especially low when they were present in small amount. This would lower the efficiency of an electrodialytic counterion separation process. Experiments on the mixtures of uni- and bivalent counterions at low total concentration were seriously affected by polarization at the membrane/solution boundaries. This effect would further restrict an ion separation process. the mechanism of this polarization is discussed. The measurements have also provided data on the ratios of the tracer fluxes with and against the electric current. A nonequilibrium thermodynamic treatment of this flux ratio is given. It is shown to be related to the transport numbers and tracer diffusion coefficients in a simple way. This relation was tested by making use of the tracer diffusion coefficients determined previously. In general the agreement with theory was satisfactory. Failures were due to polarization, which has a greater effect on the tracer flux ratio than on the transport number, i.e., the tracer flux difference.