When ethanol was adsorbed on this silica, only one broad peak was observed at 0 = 0.5, two peaks were visible at 0 = 2.0, and all three peaks appeared at 0 = 6. Apparently, the silica used by Karagounis was Cab-0-Si1 or a similar product, DISCUSSION
The narrow resonance lines observed for adsorbates on Cab-0-Si1 M-5 at low surface coverages are unusual and are probably best explained by postulating that the molecules are in a highly mobile state in which local magnetic fields are averaged. The broadening at higher coverages (0 = 20 to 54) has been explained by Karagounis (6), who assumed that polarization and orientation of the first few layers by the surface causes further layers to be built up with a lattice-like order. Such an ordered structure would cause line broadening because of the nonaveraging of local magnetic fields. Above about 50 layers the ordered structure apparently breaks down, and a more mobile, gel-like structure is formed. For example, a mixture of five parts (by weight) of cyclohexane and one part Cab-0-Si1 has the consistency of a stiff paste, but its cyclohexane resonance line is no wider than that of the pure liquid. The ability to obtain NMR spectra of molecules at fractional monolayer coverages is of considerable interest to those studying adsorption processes on silica surfaces. Woessner (5), using spin-echo techniques, found that a 3/4-statistical monolayer of benzene adsorbed on a high purity precipitated silica gel had, near room temperature, a spin-spin relaxation time of about three msec, which would correspond to a halfwidth on the order of 300 Hz. Contrast this with a value
of 4 Hz (obtained in this laboratory with a high resolution instrument) for the same concentration of benzene adsorbed on Cab-0-Si1 M-5. Clearly, it would be desirable to know what properties of pyrogenic silicas make them different from other silicas with respect to NMR behavior. Studies are now in progress to determine which structural and/or compositional factors influence the line widths of substances adsorbed on pyrogenic and precipitated silicas. These results may promote the usefulness of NMR for the characterization of silica surfaces and for the study of molecules in the adsorbed state. I n addition, they should facilitate the quantitative collection and direct NMR identification of fractions from gas chromatography along the lines proposed by Amy e f a!. ( I I ) , who used liquid impregnated supports to trap fractions for mass spectrometric analysis. ACKNOWLEDGMENT
The authors thank J. W. Amy, W. E. Baitinger, and J. R. Barnes for their assistance with the preliminary measurements and for their continuing guidance and encouragement. RECEIVED for review July 26, 1967. Accepted September 13, 1967. Work supported by a National Science Foundation Traineeship (to JHP) and by U.S. Atomic Energy Commission Contract AT(11-1)-1222. (11) J. W. Amy, E. M. Chait, W. E. Baitinger, and F. W. McLafferty, ANAL.CHEM.,37, 1265 (1965).
aratbn of ) by Anion Exchange Fletcher L. Moore Analytical Chemistry Division,Oak Ridge National Laboratory, Oak Ridge, Tenn.
ONEOF THE MOST arduous tasks in transplutonium chemistry is the separation of berkelium from cerium (1-5). Methods based on liquid-liquid extraction of berkelium(1V) with di(2-ethylhexy1)orthophosphoric acid (3) or 2-thenoyltrifluoroacetone (5) are highly selective but do not separate berkelium from cerium because of their very similar oxidation potentials. Two recent papers describe the separation of berkelium(II1) from cerium(II1) by extraction chromatography using di(2ethylhexy1)orthophosphoric acid in hydrochloric acid (6) and nitric acid (7) systems. Most interestingly, during recent research studies the author discovered a simple, sharp, quantitative separation of berke(1) S. G. Thompson, B. €3. Cunningham, and 6. T. Seaborg, J. Am. Chem. Sac., 72,2798 (1950). (2) G. H. Higgins, “The Radiochemistry of the Transcurium Elements,” NAS-NS-3031 (1960). (3) D. F. Peppard, S . W. Moline, and G. W. Mason, J. Inorg. Nucl. Chem., 4, 344 (1957). (4) F. L. Moore and W. T. Mullins, ANAL.CHEM., 37,687 (1965). ( 5 ) F. L. Moore, Ibid., 38, 1872 (1966). (6) J. Kooi and R. Boden, Radiochim. Acta, 3, 226 (1964). (7) F. L. Moore and A. Jurriaanse, ANAL.CHEM., 39,733 (1967).
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lium(1V) from cerium(1V) in nitric acid solution. These elements had not been separated previously when present in the tetravalent oxidation states, EXPERIMENTAL Apparatus. An internal sample methane proportional counter was used for alpha and beta counting at voltage settings of 2900 and 4300, respectively. A NaI well-type scintillation counter, 1.75 X 2 inches, was used for gamma counting. A glass tube, 5-mm i.d. and 150 mm in length was drawn to a tip at one end. A small glass wool plug was inserted in the tube to retain the resin. Reagents. Dowex 1-x4 (100-200 mesh) strong base anion exchange resin, available from Bio-Rad Laboratories, Richmond, Calif., in the chloride form, was converted to the solunitrate form by washing several times with 5M “Os tion. Column Preparation. By use of a medicine dropper, the resin slurry was added to the glass tube to a height of 50 mm. Fifty milligrams of lead dioxide were carefully mixed into the resin bed with a platinum or stainless steel wire, being careful to eliminate air bubbles. The column should not be allowed
+
x i0 8 M HN03 O.fM NaBr03
0.5M H N 0 3
8 M HNOJ
0.5M H N O 3
-1
I I
5 IO VOLUME OF E L U A T E , rnl
i5
VOLUME OF ELUATE, ml
Figure 1. Separation of 249Bkfrom 144Ceby anion exchange in nitric acid-sodium bromate system
Figure 2. Separation of 249Bkfrom I4*Ceby anion exchange in nitric acid-lead dioxide system
Column: 5 X 50 mm., Dowex 1-x4(100-200 mesh, nitrate form), 23"C, flow rate = 5-6 drops per min
Column: 5 X 50 mm., Dowex 1 . ~ 4(100-200 mesh, nitrate form, 50 mg PbOn), 23"C, flow rate = 5-6 drops per min
to run dry. A flow rate of 5-6 drops per minute was normally attained. A small volume of 5M HNOP solution was maintained on top of the resin prior to use. Procedure. The column was conditioned by passing 5 ml of 8M H N 0 3 solution through it. The sample solution was adjusted to 8M H N 0 3 . A 0.2-ml aliquot of the sample was carefully pipetted onto the center of the column. The solution was allowed to just sorb and the micropipet was washed When the wash solution just reached once with 8M "Os. was added. The elution was the top of the resin, 8M "Os continued with 8MHN03. The first 5 ml of eluate containing the berkelium was collected in a 5-ml volumetric flask. After mixing well, suitable aliquots were prepared on stainless steel plates for beta and/or alpha measurements. If the cerium was desired, 0.5M "03 was added to the column and the next 10 ml of eluate, containing the cerium, was collected in a 10-ml volumetric flask. After mixing well, 1-ml aliquots were counted on the gamma scintillation counter. The column was cleaned with 5-10 ml of 0.5M "Os. Several milliliters were maintained on top of the resin prior to the next separation. RESULTS AND DISCUSSION The strong sorption of cerium(1V) at intermediate concentrations of nitric acid on strong base anion exchange resins has been noted (8-11); the behavior of berkelium(1V) in this system had not been explored previously. Because of its very similar chemical characteristics, the transplutonium element chemist would have predicted that berkelium(1V) would also exhibit strong sorption under the conditions used for cerium (IV). Quite surprisingly, this does not occur. In our recent studies, berkelium(1V) has been observed not to sorb appreciably from nitric acid solutions using sodium bromate (Figure I) or lead dioxide (Figure 2) as oxidant. Such a drastic
difference in their behavior forms the basis for a very practical separation of berkelium from cerium. Preliminary experiments were performed to evaluate the pertinent variables for the separation of berkelium(1V) from cerium(1V). Lead dioxide and sodium bromate were evaluated as oxidants. Roberts (11) previously demonstrated an excellent separation of cerium(1V) from several fission products on a lead dioxide-anion resin. The sorption of cerium (IV) was optimum at about 8M nitric acid in agreement with the results of other investigators (8-11). Decreased sorption of cerium(1V) occurs at nitric acid concentrations greater than about 9M because of increasing competition of nitrate ion for the resin; moreover, degradation of the anion resin becomes appreciable under such conditions. Quantitative sorption of cerium was effected on 5 X 50-mm columns of the anion exchange resin, Dowex I-x4 (100-200 mesh), containing 10, 50, or 100 mg of lead dioxide. Flow rates were 2-3 drops per minute with 100 mg of lead dioxide and 5-6 drops per minute with 50 mg of lead dioxide. The latter condition was selected because the practical flow rate eliminated the need for vacuum or pressure techniques. Elutions at elevated temperatures were not studied in view of the excellent separation achieved at ambient temperature. Sodium bromate, an excellent oxidant for berkelium(II1) (8) K. A. Kraus and F. Nelson, "Metal Separations by Anion
Exchange," Symposium on Ion Exchange and Chromatography in Analytical Chemistry, ASTM Spec. Tech. Pub. No. 195, 27
(1956). (9) J. Danon, J. Znorg. Nucl. Chem., 5 , 237 (1958). (10) D. MacDonald, ANAL.CHEM., 33, 1807 (1961). (11) F. P. Roberts, U.S.At. Energy Comm. Unclassified Report, HW-SA-3082 (1963). VOL. 39, NO.
14, DECEMBER 1967
a
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Table I. Recovery and Decontaminath of 24QBkby Anion Exchange Per Cent found in z49Bk product 249Bk 1 4Ce Run No. 1 2 3 4
5
99.8 99.9 99.4 >99.9 >99.9
Np(0.92A) > U(0.93A) > Th(0.99 A) (12, 13). This order
A>
(12) W. E. Keder, J. C. Sheppard, and A. S. Wilson, 9. Znorg. Nuel. Clieni., 12, 327 (1960). (13) J. L. Ryan, J. Phys. Chem., 64,1375 (1960).
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correlates well with their increasing ionic radii (in parentheses). Thus, on the basis of ionic radius considerations alone, one would expect the berkelium(1V) ion (-0.86 A) to show very high anionic nitrate complex formation and hence the highest sorption of any actinide ion on anion resins. Similarly, berkelium(1V) should also sorb better than cerium (IV) (0.92 A), whose behavior is predictably similar to an actinide(1V) ion. However, our experimental results do not support this supposition. A plausible reason for the marked difference in the behavior of berkelium(1V) from that of cerium(1V) and other tetravalent actinide ions may lie in the smaller ionic radius of the berkelium(1V) ion. The small berkelium(1V) ion solvates water molecules more effectively than the larger cerium(IV), actinide(1V) ions. Consequently, formation of anionic nitrate species is considerably more difficult for the berkelium(1V) ion than for these larger ions. Ironically, for several years chemists have searched for selective oxidation techniques for berkelium and cerium in order to provide bases for their simple, rapid separation from each other. The fact is that these two elements can be easily separated whether in their trivalent oxidation states (6, 7) or in their tetravalent oxidation states (as described in this paper). APPLICATIONS
This new method is immediately useful to the analytical and process chemist for the specific separation of berkelium from cerium. It serves as a valuable step in the overall.purification of berkelium, e.g., in conjunction with di(2-ethylhexy1)orthophosphoric acid ( 3 , 4 )or 2-thenoyltrifluoroacetone ( 5 ) ; for the analyst, it makes simple beta counting for 249Bk(4, 5 ) feasible in some situations. The simple method is faster than those currently used (6, 7) because the berkelium runs through the column immediately; in the latter methods based on extraction chromatography, cerium elutes first-a fact which increases the isolation time for berkelium by a factor of about three. The most common interferences encountered are a-hydroxyisobutyric acid, hydrochloric acid, and fluoride ion. These interferences are readily removed by evaporations with concentrated nitric acid. For the rapid separation of berkelium from cerium, the method is particularly attractive for large scale process applications because it functions remotely in relatively noncorrosive nitric acid at room temperature. Practical flow rates eliminate the need for vacuum or pressure techniques. In the near future, the radioisotope engineer will be faced increasingly with the separation of berkelium from fission product cerium. Currently a problem which plagues him is the instability of bromate ion in nitric acid solutions contained in holding tanks. For the specific separation of berkelium from cerium, the lead dioxide-loaded anion resin column effectively eliminates such pre-oxidation problems. ACKNOWLEDGMENT
The author gratefully acknowledges the capable assistance of G . I. Gault in the experimental work and of W. R. Laing for some of the analyses, RECEIVED for review August 25, 1967. Accepted September 27, 1967. Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corp.
