Under experimental conditions similar to those in the elution system of the rare earth with a dilute HEDTA eluant, the Er-Tm separation factor has been found in the system of HEDTA and Dowex 50W, X-8, 50–100 mesh. The values for this pair, 1.57 at 25°C and 1.63 at 60°C were in agreement with the values calculated from the stability constants of the rare earth-HEDTA chelates. Further, an equimolar mixture of Er and Tm has been eluted with a 0.015m HEDTA solution buffered with ammonium hydroxide. Under the present experimental conditions, the adsorption band of each rare earth reached a steady state after it had moved down through the hydrogen-retaining bed to a distance several times the initial band length. Several series of experimental data were used for analyses of the adsorption band from the point of view of rate theory. It was concluded that the present ion-exchange process with a HEDTA eluant was controlled by film diffusion as the rate-determining step. The plate-height values were also found to be functions of the flow rate.
The adsorption of HEDTA (H3Ch) on the hydrogen form of Dowex 50W, X-8, 100-200 mesh, has been studied as a function of the concentration of H3Ch in the aqueous phase. The experiments were performed by means of either the column or a combination of the column and batch methods, with the latter being employed for the adsorption of HEDTA on the resin in equilibrium with dilute solutions. In solutions up to approximately 0.07mol/l, such as H3Ch, the experimental data are consistent with the Langmuir equation expressed by:[H3Ch]/(ChT/E)=1/Bk+[H3Ch]/B. The adsorbed amount is represented in the term ChT/E, where ChT is the total amount of cationic species of HEDTA on the resin, and E, the exchange capacity of resin. A good straight line is obtained with B=0.571 and k=0.420.The results show that, because of a high concentration of hydrogen ion inside the resin, the adsorbed HEDTA is present mainly in the H5Ch2+ species and partly in the H4Ch+ species.Due to the stable chelate formation of polyamino acids with metal ions, such buffered solutions as ethylenediaminetetraacetic acid (EDTA) and N'-(2-hydroxyethyl)ethlenediamine-N,N,N'-triacetic acid (hereafter abbreviated as HEDTA and H3Ch)1,2) were used as the eluants for the ion-exchange separation of the rare-earth series. For example, various compositions of HEDTA were employed for isolating the above series by ion exchange. When the rare earth is eluted with the HEDTA eluant with a closed composition to (NH4)2HCh and with a hydrogen-retaining bed, a chelate
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