NOTES
760 ENRICHMENT O F Lis BY COUNTERCURRENT ELECTROMIGRATION1 BY ERNEST R. RAMIREZ~
Contribution from Institut Physikalische Chemie, Uniuersity of Zurich, Zurich, Switzerland Received January 18, 1068
Except for the case of hydrogen and deuterium, the separation of isotopes will always be an involved and expensive undertaking. I n the case of the two natural lithium isotopes, complete separation has only been achieved by electromagnetic means. A number of procedures leading t o enrichment, .however, have been successfully carried out, One of the first procedures for enriching Lis was carried out by Champetiera using fractional electrolysis on a mercury cathode. Later Holleck4 improved the enrichment process by employing a three stage electrolysis cell. At about this time, Taylor and Urey5 was able to show that Lis could be enriched by chemical exchange methods. Recently, Klemms was able to obtain a remarkable enrichment of Lis by an electrolytic migration procedure in molten LiC1. Chemical precipitation procedures have been employed7 in which Li7 waa found to be enriched as lithium carbonate, thereby showing that Li;CO3 was less soluble than Li4C03. Finally, enrichments of lithium isotopes have been carried out by employing thermal diffusion techniques in aqueous lithium nitrate solutions.8 While no important industrial use has yet been found for either of the two natural lithium isotopes, it is very likely that these isotopes may prove useful to the young nuclear industry. As far back as 1936, Budnitskii and co-workersg have demonstrated the following reaction aLi6
+
+2He4 + 1H3
It is also known that Li7 has a very low neutron cross section which lies in the range of 0.1 barn.
Theory The principle involved in countercurrent electromigration has been discussed in detail elsewhere.10J1J2 Briefly, however, the separation process is based upon the difference in mobilities of two or more ionic species. By placing these species in an electric field they are made to migrate toward the anode or cathode, depending upon the (1) The material reported herein is taken from part of the Dissertation for the Doctor of Philosophy presented by the author to the Philosophical Faculty Section I1 of the University of Zilrioh, Zurich, Switzerland in 1953. (2) General Electric Company, Metallurgical Products Dept., Detroit 32, Michigan. (3) G. Champetier and P. Regnant, Bull. SOC.Chem., [51 4, 692 (1937). (4) Ludwig Holleck, Z . Elektrochem., 44, 111 (1938). F',(5) T. I. Taylor and H. C. Urey, J . Chem. Phys., I,587 (1937). (A) A. Klemm, 2. Naturjorschuno, 6a, 512 (1951). (7) G. Wagner, A. Pelz and M. J. Higatsberger, Monatah., 85, 464 (1954). (8) G. Panson, Nuclear Sei., 8, 442 (1954). (9) D. Z. Budnitskii. V. Kurchatov and G. D. Latyshev, J. Esptl. and Theor. Phys. ( U S S R ) , I,360 (1936). (10) G. Breit and F. L. Friedman, J . Research Natl. Bur. Standards, 89, 397 (1947). (11) J. W. Westhaver, ibid., 38, 169 (1947). (12) E. R. Ramirez, J . A m . Chem. Soc., 76, 8237 (1954).
Vol. 62
charge of the ion. The mobility of each ion is an inverse square root function of the total weight of the ionic cloud undergoing movement. If the cation to be separated are metal isotopes, and if the electrolyte is made to travel in a counter direction to the cation movement simultaneously as a potential is applied across the two electrodes, then the slower cationic species will be washed toward the anode while the faster cation will be enriched at the cathode. Among other things, the rate of enrichment is a function of the magnitude of potential applied as well as the difference in masses between the two ionic species. Experimental Calibration of the Apparatus.-The details in the construction of the apparatus have been previously described.12Js Briefly, the apparatus consists of a 1.6 cm. (i.d.) Pyrex tube which is air-cooled on the outside and which is also watercooled by a concentric glass tube (0.5 cm. 0.d.) running through its center. The inside cooling tube also eerves as an anchor for closely spaced diaphragms (0.1 to 0.3 cm. apart) which w e totally permeable to electrolytic ionic flow but which are only partially permeable to the flow of the electrolyte. In order to calibrate the apparatus, an experiment was carried out by separating a mixture of sodium and potassium ions. The electrolytic separation cell employed in this initial experiment was 100 cm. long and employed 250 evenly spaced diaphragms. The anodic reservoir contained a mole ratio of K+/Na+ equal to 0.01 as hydroxides. The countercurrent flow introduced a t the cathode section consisted of doubly distilled water. The countercurrent flow, once regulated, was controlled by an automatic feed mechanism guided by the conductivity of the solution in the cathode region. The conditions present during the separation of sodium and potassium mixture are given below while the results are shown in Fig. 1. The experiment was stopped after 188 hours. (1) Initial hydroxide ion concentration was 0.012 N . (2) Average temperature of the electrolyte was 65". (3) Average countercurrent flow of distilled water was 240 om.s/hr. (4) Average voltage gradient employed was 23 volts/cm. (5) Total voltage employed varied from 2300 to 2500 volts. (6) Average cell current was 180 ma. It is of particular interest that the hydroxyl ion concentration is diminished a t the cat,hode section of the separation cell. This confirmed the enrichment of a more mobile cationic species in the cathode section. The data given in Fig. 1 was obtained by small samples (0.2 ml.) of solution taken along the separation cell. Analysis determining sodium and potassium concentrations were carried out with a flame photometer and appropriate filters.
Procedure and Results Previous work12 done in enriching Rb85 indicated that impurities with a higher mobility than the cations under study are especially harmful in that they accumulate in the cathode region and reduce the effective separating length of the cell. Although the purest form of LiCl was employed (C.P.) it was found necessary to run the LiCl through the apparatus for more than a week in order to remove the last traces of sodium and potassium. The results obtained in enriching Rb85 showed that the largest part of the current was transported by the mobile hydroxyl anion. This condition is undesirable since for a given power input the number of (13) K. Clusius and E. R. Ramirez, Helu. Chim. Acta, 36, (1953).
V, 1160
r
NOTES
June, 1958 cations enriched is low. To improve this condition the relatively slow acetate ion was substituted for the hydroxyl ion and consequently the countercurrent electrolyte contained 0.01 N acetic acid. The average lithium concentration in the enrichment cell, as well as in the lithium acetate reservoir, was 0.007 N . The cell was 75 em. long and contained 210 equally spaced diaphragms. Once adjusted, the apparatus was entirely self adjusting and self correcting. A voltage input of 2300 volts was used but this value changed t o 2500 volts a t night when the power demand was lowest. The self regulating system of control proved itself invaluable in that voltage changes taken place during the day and night periods were automatically compensated for by the built in guidance mechanism. The apparatus was attended at least once every 12 to 15 hours to record data and to refill the countercurrent reservoir. Data obtained during the enrichment of the lithium isotopes over a period of 6 weeks are given in Table I.
Anoda
76 1
LENGTH OF SEPARATING TUBE in cm.
Fig. 1.-Separation
Cathode
of potassium from sodium with 2300 volts.
TABLE I EXPERIMENTAL DATAON ENRICHMENT OF Lie BY COUNTERIt can be seen that in an aqueous solution the Lis ion is about 0.14'% faster than Li.7 The results inCURRENT ELECTROMIGRATION Time, Cell ciirrent,.ma. Applied voltage, v. week Max. Min. Max. Min.
Av. countercurrent flow, cm.S/hr.
Av. cell teomp., C.
1st 88 85 2500 2400 143 42 2nd 91 86 2500 2300 140 40 3rd" 90 85 2450 2300 145 42 4th 86 2500 2350 135 41 92 88 2500 2400 139 42 5th 90 6th 88 2450 2300 92 143 40 Sampling was started after the third week. Very slow siphoning was used at the cathode section. About 5 mg. of lithium acetate was taken per day.
After three weeks of sampling, the lithium acetate which was enriched in Lis was converted to lithium chloride (80 mg.). Both the unenriched and enriched lithium were analvzed for isotonic ratios on a mass spectrograph. The results are' given in Table 11. TABLE I1
dicate that so long as the ionic isotopes are heavily hydrated, the efficiency of the separating process is seriously impeded. It follows that ideal conditions for separating ionic species by countercurrent electro migration are present when the ionic species is not attached to other molecules during the migration process. Acknowledgments.-The author is thankful t o Prof. Dr. K. Clusius of the University of Zurich, Switzerland, for his ever guiding advice during this research. Many thanks are also due to Herr Dr. H. Hintenberger of the Max Planck Institute fur Chemie of Ma& (am Rhein) Germany, for carrying out the abundance ratios of the lithium isotopes in a mass spectrometer.
SECONDARY EFFECTS IN T H E THERMAL DECOMPOSITIONS OF CYCLOPROPANE ENRICHMENT OF Li6 BY COUNTERCURRENT ELECTROMIGRAAND CYCLOBUTANE TION IN A N AQUEOUSMEDIUM Enriching time
Sample description
Lie
0 (Control) unenriched lithium 7.31 3 weeks" Enriched Li in the cathode section 7.61 Sampling was started after 3 weeks and continued for three additional weeks after which time 80 mg. of enriched LiCl was obtained.
Discussion The enrichment of Li6 has been carried out in a 0.007 N solution of lithium acetate. It is apparent from the results that the enrichment is not based upon the weight ratios of Li6 and Li7 but rather Li6.XHz0and Li'eXHBO; where X is the hydration value of the ionic species. E. W. Washburn14 has shown that X lies between 14 and 25 for lithium. By arbitrarily taking X equal to 20 as a possible value for the hydration number of both lithium isotopes, the ratio of the mobilities of the two isotopes becomes (14) E. W. Washburn, J . A m . Chem. Sac., 81, 322 (1909).
BYJ. LANGRISH AND H. 0. PRITCHARD
University of Manchester, Manchester 13, England and, California Institute of Technology, Pasadena, CaEzfornza Received January 8.4, 1968
The rate constants for the thermal decompositions of cyclopropane and cyclobutane fall markedly with pressure, in general agreement with current theories of unimolecular reactions. However, the more refined theories, in particular that due to Slater,l lead one to expect a number of effects of a smaller order of magnitude, and it is with these secondary effects that this paper is concerned. Experiments with Cyclopropane.-According to the theory which was used by Slater in his discussion of the decomposition of cyclopropane, one would expect a shift in the log klk, curve with (1) N. B. Slater, Phil. Trans. Roy. Sac. (London), A246, 57 (1953); Proc. Roy. SOC.(London), Aals, 224 (1953); Proc. Roy. Sac. (Edinburgh), 864, 161 (1955); Proc. Leeds Phil. Soc., 6, 268 (1955).
