Enrichment of Isotopes by Constant Current Electromigration in an Ion-exchange Membrane Temperature Effect on Enrich ment of ⁸⁵Rb by the Breakthrough Method

Abstract

The separation of rubidium isotopes (⁸⁵Rb and ⁸⁷Rb) was carried out at several temperatures by means of constant current electromigration in which a cation-exchange membrane of Hform was used as an electrophoretic supporting medium.
A membrane strip (homogeneous resin of cop olymer of styrenedivinylbenzene with sulfo groups, ashiplex CK-1) was placed in a migration apparatus (Fig.1), such that one end of the strip was immersed in an aqueous solution of sulfuric acid (catholyte) and the other end in that of rubidium salt (anolyte). A constant direct current was passed throulh the strip to move rubidium ions towards the cathode with a constant migration velocity. The sharp and stable boundary was maintained between the zones of H-form and of Rb-form during the course of the electromigration. The diffusion coefficient of rubidium ion in the cationexchange membrane was evaluated from Nernst-Einstein's equation (Eq. ( 1 )) using the data on the mobility of rubidium ion deduced from the velocity of the moving boundary and the voltage change during the migration; its values were 2.4×10^-6, 3.5×10^-6, 49×10^-6, and 6.9× 10^-6cm2/s, at 5, 25, 50, and 70°C, respectively.
When the migration distance of the bo undary reached 20 cm, the supply of the direct current was discontinued and the membrane strip was taken out of the apparatus and cut into fractions starting from the frontal end of the Rb-zone at a regular interval of 2.5 mm. For each fraction, the rubidium content and rubidium isotope ratio (⁸⁷Rb/⁸⁷Rb) were determined by flame photometric and mass-spectrometric analyses, respectively. The results of the isotopic 'analysis showed that the lighter isotope ⁸⁵Rb was enriched in the frontal part of the zone. Sv, the relative difference in migration velocity between ⁸⁵Rb and ⁸⁷Rb ions, was determ ined to be 0.77×10^-3, 0.85× 1010^-3, 1.11×10^-3, and 1.36×10^-3, at 5, 25, 50, and 70°C, respectively. The tendency of the increase in e, , with increasing temperature is consistent with the result reoprted by Bonnin et al. (Ref.3)) for the electromigration in a paper-strip medium. A theoretical equation for chromatographic separation of isotopes (Eq. ( 4 )) was applied to the Rb isotope enriched zone. The present Rb-isotopic fractionation plofiles were found to obey the theory quite closely except for the one at 70°C.