paired spins in the sample. The value of n for Dorrance anthracite has been reported previously (12). Substitution of this value (5.3 X l O I 9 g-l) into the equation yields a theoretical linewidth about twice that observed experimentally at room temperature. It is unlikely that unresolved chemical shifts play an important part since these are known to cover only a small range in the case of polynuclear aromatics (15-17). It is possible, however, that chemical shift anisotropy in the solid may be important. Although it is not possible to estimate this effect, it should again be noted that anisotropies do exist in other magnetic properties of anthracites.
shown the importance of proton dipolar line broadening in the proton spectrum of solid adamantane and found that motional narrowing of the adamantane resonance first appears - 130 “C. 13CNMR measurements at low temperatures would be needed to assess the importance of proton dipolar line broadening on the carbon resonance in adamantane. The linewidth in Dorrance anthracite is not easily explained. Possible contributors to the linewidth include proton dipolar interactions, unresolved carbon chemical shifts, chemical shift anisotropy, and line broadening by paramagnetic impurities (free radicals) which are known to be present in the anthracite (12). The paramagnetic contribution to the linewidth can be approximated using the Equation (13, 14)
-
AH
=
RECEIVED for review October 5, 1970. Accepted November 17, 1970. Reference to trade names is made for identification only and does not imply endorsement by the Bureau of Mines.
3.8yhn
where y is the magnetogyric ratio of the electron, h is the modified Planck constant, and n is the concentration of un(15) T. D. Alger, D. M. Grant, and E. G. Paul, J. Amer. Chern(12) H. L. Retcofsky, J. M. Stark, and R. A. Friedel, ANAL.
CHEM., 40, 1699 (1968). (13) A. Abragam, “Principles of Nuclear Magnetism,” Oxford Univ. Press, London, 1961, p 128. (14) M. N. Alexander,Phys. Reo., 172, 331 (1968).
Soc., 88, 5397 (1966). (16) H. L. Retcofsky, J. M. Hoffman, Jr., and R. A. Friedel, J . Chem. Phys., 46,4545 (1967). (17) R. J. Pugmire, D. M. Grant, M. J. Robins, and R. K. Robins, J. Arner. Chem. Soc., 91,6381 (1969).
New Method for Separation of Americium from Curium and Associated Elements in the Zirconium Phosphate-Nitric Acid System Fletcher L. Moore Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. To ACHIEVE THE SEPARATION of americium from curium in an all-inorganic system is the subject of a continuing study at this laboratory. Current methods based on ion exchange resins or organic solvents suffer from resin degradation, excessive gassing, and solvent instability. Although several inorganic systems (1-3) exist for this separation, they either provide inadequate decontamination, require excessive manipulations, or they are too time-consuming for analytical and process applications. At present the most widely-used method is based on the precipitation of Am(V) as the double carbonate (1). It is unsatisfactory because it requires multiple precipitations involving the use of concentrated potassium carbonate solutions. Various workers have found the subsequent removal of coprecipitated curium and large amounts of carbonate salts to be tedious and difficult. In addition, the carbonate precipitation method for americium cannot be used for the final removal of small amounts of americium from curium. The cation exchanger, zirconium phosphate, was selected as a promising candidate for the separation of americium from curium because it is stable in oxidizing media, strong (1) R. A. Penneman and T. K. Keenan, “The Radiochemistry of
Americium and Curium,” NAS-NS-3006 (1960). (2) F. L. Moore, ANAL.CHEM., 35,715 (1963). (3) H. P. Holcomb, ibid.,36,2329 (1964).
nitric acid, and elevated temperatures; moreover, it exhibits superior resistance to ionizing radiation over the organic ion exchangers and organic solvents. The new method described here is based on the negligible sorbability of Am(V) on zirconium phosphate from dilute nitric acid solutions. Under the conditions used, Cm(II1) and a number of other elements are strongly sorbed. For analytical applications the isotope dilution technique in conjunction with alpha spectrometry is used for an accurate determination of 243Am. EXPERIMENTAL
Apparatus. An internal sample methane proportional counter was used for fission, alpha, and beta counting at voltage settings of 2100, 2900, nnd 4300, respectively. A NaI well-type scintillation counter, 13/4 X 2 inches, was used for gamma counting. A silicon diode detector (3 cm2) coupled to a 256-channel analyzer was used for alpha spectrometry. A glass tube, 5 mm i.d. and 180 mm in length was drawn to a tip at one end. A small glass wool plug was inserted in the tube to retain the support. To prevent disruption of the column, a small glass wool plug was placed on top of the column. Reagents. “OB, 0.01M; “Os, 10M; and (NH&S20s, 0.5M were used. Zirconium phosphate cation exchange crystals, Bio-Rad ZP-1, 50-100 mesh, hydrogen form is available from Bio-Rad Laboratories, 32nd and Griffith Ave., Richmond, Calif. ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
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Table I. Decontamination of Americium in the Zirconium Phosphate-Nitric Acid Method Element Decontamination factor 1Wesium > 5 . 5 x 106 162-4Europiurn > 2 x 106 23Wranium 3 x 102 8.5 28QPlutonium 2 Wurium > 2 . 5 X 106 2 . 1 x 102 4vBerkelium > 4 . 5 x 106 26PCalifornium
1
2 x105
1
i 15
20
25
VOLUME OF ELUATE, m l
Figure 1. Separation of americium from curium by cation exchange in the zirconium phosphate-nitric acid system Column: 5 X 30 mm, zirconium phosphate (50100 mesh), 23" C, flow rate = -0.3 ml per minute
To 10 grams in a 1-oz screw cap bottle, add 25 ml of 0.1M HN03. Shake well for about 30 seconds and allow the crystals to settle for about 15 seconds. Pour off the supernatant solution and repeat the procedure five times. This removes most of the fines from the zirconium phosphate crystals. Store in 25 ml of 0.1M "03. Column Preparation. Fill the glass tube with 0.01M "03. By use of a medicine dropper, add to the glass tube the slurry of zirconium phosphate exchanger, 5-6 drops at a time. Tap the column after each addition, being careful to eliminate air bubbles. Pack the column to a height of 30 mm and allow the nitric acid to drain to near the top of the column. Do not allow the column to run dry. Condition the column by passing 5 ml of 0.01M H N 0 3 through it. A flow rate of 7-8 drops per minute (-0.3 ml) is attained. Procedure. For analytical purposes add to the sample solution an aliquot of a s;andardized solution of 241Am (5.48 mev) contained in 0.1M HN03. The alpha radioactivity of the 241Amadded should be about one half the estimated alpha radioactivity of the 243Am(5.27 mev) in the sample aliquot. For general purification purposes omit the above paragraph. Adjust the sample solution to 0.01MHN03~.1M(NH,)2S20s. Mix gently by swirling. Oxidize the americium by heating the tube in a water bath at 80-90 O C for 10 minutes. Remove the tube and cool for 5-10 minutes in a beaker of water at room temperature. Place a 10-ml volumetric flask under the column to collect the americium fraction. Transfer the oxidized solution to the column and elute with 0.01M H N 0 3 at 7-8 drops per minute until 10 ml are collected. Mix well. Evaporate 100 ~1 or less on tantalum plates for alpha measurements. If the curium fraction is desired, elute it from the column with 10 ml of 10M HN03. Mix well and prepare suitable aliquots on stainless steel plates for alpha measurements. RESULTS AND DISCUSSION The conditions used for the oxidation-reduction sequence, Am(II1) + Am(V1) -P Am(V), were similar to those previ488
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
ously developed ( 4 ) . Ammonium persulfate oxidizes Am(II1) to Am(V1) in dilute nitric acid solution. In the absence of a holding oxidant or stabilizing anion, Am(V1) reduces to Am(V). The optimum conditions selected for the preparation of Am(V) are 0.01-0.1M nitric acid, 0.05-0.1M ammonium persulfate, 10-minute oxidation at 80-90 "C followed by a 5-10 minute cooling period at room temperature. Early batch type experiments indicated that the large Am02+ ion showed little tendency to sorb on zirconium phosphate from dilute nitric acid; Am(II1) and Cm(II1) sorbed strongly. The following column conditions were selected for the separation of americium and curium : 5 X 30 mm column of zirconium phosphate (50-100 meshj, room temperature operation at a flow rate of 7-8 drops (-0.3 ml) per minute; eluents, 10 ml of 0.01M H N 0 3for Am and 10 ml of 10M H N 0 3 for Cm. Typical elution curves are shown in Figure 1. Am(V) elutes rapidly from the column with 0.01M HN03. If desired, the Cm(II1) may be eluted with 10M HN03. The curium elution curve shows some tailing, a characteristic of inorganic exchangers. However, about 97% of the curium elutes in 5 ml of 10M "OB; quantitative elution is achieved with 10 ml. The recovery of americium and its decontamination from curium is excellent. Americium yields average about 90% with curium decontamination factors >2.5 X lo5. The method requires only about 11/2 hours. Ammonium persulfate concentrations of 5 0.2M resulted in slightly premature elution of curium and europium. Presumably, persulfate, sulfate, or bisulfate ions complex the trivalent ions, thereby lowering their distribution coefficients for zirconium phosphate. Several attempts were made to oxidize Am(III), previously sorbed on the zirconium phosphate column, to the nonsorbable Am(V) oxidation state. Although Am(II1) sorbed tightly at 0.01-0.1M "OB, subsequent yields of Am(V) were very low. In addition, at the elevated temperature required for oxidation, mechanical difficulties associated with gas bubble formation prevented smooth column operation. Although the major interest in this study was the separation of americium from curium, some observations were made of the behavior of several other metal ions often associated with americium. Table I shows that high decontamination is achieved from these elements. Decontamination factor equals the total amount of element in the feed solution divided by the total amount found in the americium product. The zirconium phosphate-nitric acid system described affords an impressive separation of americium not only from curium but also from other actinide elements, lanthanide elements, and cesium. The trivalent ions of the transcali(4) F. L. Moore, ANAL.CHEM., 40, 2130 (1968).
fornium elements would be predicted to sorb at least as efficiently as californium from 0.01M "OB. Europium, californium, and uranium elute essentially quantitatively in the 10M H N 0 3 fraction, as expected for the trivalent and hexavalent ions. The elution curves of Am(III), Cm(III), and Eu(II1) are essentially identical. Plutonium showed the lowest decontamination factor of the actinide elements. The small loss of plutonium to the americium product probably is a reflection of the strong sulfate complexation, which lowers the distribution coefficient for Pu(1V). About 82% of the initial plutonium eluted in the 10M "0, fraction. About 90% of the initial berkelium eluted in the 10M HNOI fraction. The separation of cesium from americium is striking. Less than 10% of the cesium added to the column eluted with 10M HNOa. Cesium exhibits a stronger affinity for zirconium phosphate than many multivalent ions. It is noteworthy that no separation of cesium from americium is attained in the extraction chromatographic method ( 4 ) recently developed. A number of other elements not evaluated here are known to sorb efficiently from 0.01M H N 0 3 like curium. Among these elements are strontium, ruthenium, zirconium, niobium, and iron.
without the necessity to resort to vacuum or high pressure techniques. The method is simple, fast, and requires few manipulations-advantages for glove box or hot cell work. Other attractive features of the zirconium phosphate-nitric acid method are that it is considerably less corrosive than the earlier fluoride systems developed (1-3), and it is more selective for americium than the lanthanum fluoride (2), calcium fluoride (3), or extraction chromatographic (4) methods. We have used the new method as a satisfactory tool for the purification and isolation of americium in solutions containing other actinide (111, IV, VI) ions, lanthanide (111, IV) ions, cesium, and various other ions. A practical problem which often arises in the final purification of curium nuclides is the removal of small amounts of contaminating americium. The zirconium phosphate method described here offers a promising approach for achieving that separation. In process work the most common interferences encountered are chloride ion and a-hydroxyisobutyric acid. These interferences may be eliminated by evaporations with concentrated nitric acid. ACKNOWLEDGMENT
APPLICATIONS
The author gratefully acknowledges the help of F. Nelson and H. 0. Phillips for useful discussions of inorganic exchangers.
The zirconium phosphate cation exchanger for the separation of americium from a number of other elements is valuable both to the analytical and preparative chemist. The column operates at room temperature with relatively high flow rates
RECEIVED for review September 25, 1970. Accepted November 20, 1970. Research sponsored by U. S. Atomic Energy Commission under contract with Union Carbide Corporation.
I AIDS FOR ANALYTICAL CHEMISTS Rapid and Efficient Method for Removing Viscous Polymer Solutions from Nuclear Magnetic Resonance (NMR) Sample Tubes Alex Jakab Research Laboratories, The Goodyear Tire & Rubber Co., Akron, Ohio 44316 WE ROUTINELY analyze polymers by NMR and we have found that the use of solvents, strong acids, cleaning solution ( I ) or pyrolysis to clean the sample tubes is not always successful. We have devised a method for removing these viscous polymer solutions from the sample tubes which is simple and relatively rapid, and increases the lifetime of the sample tubes. We utilize a nonsolvent to precipitate the polymer in situ, in conjunction with a stirring rod made from a copper wire formed into a closed-loop on one end. The diameter of the loop is slightly smaller than the diameter of the sample tube. The copper wire facilitates the mixing of the viscous solution and the nonsolvent, and at the same time provides a nucleation site for the precipitation of the polymer. Our experience has been limited to polybutadienes and polyisoprenes in CCll or benzene, and our procedure is as follows : (1) N. W. Jacobsen, J. Chern. Educ., 47, 507 (1970).
The polymer solution usually fills the bottom inch of the cylindrical sample tube. Sufficient methanol is added to fill the sample tube. The copper loop is inserted and the mixture gently agitated by an up-and-down motion of the stirring wire. The precipitated polymer, which adheres to the copper loop, and any polymer clinging to the wall of the tube are removed by lifting the copper wire loop slowly out of the tube. The liquid remaining in the tube now has a much lower viscosity-essentially a CC14 (or benzene)-methanol mixture, so it can be poured easily from the sample tube. If the precipitation is incomplete, as indicated by only a slight change in the viscosity of the original sample, the nonviscous upper layer is decanted and fresh methanol re-introduced. A distilled water rinse, followed by a half dozen acetone rinses of both inside and outside of the tube, and drying in an air-oven at 70 "C completes the procedure. RECEIVED for review August 26, 1970. Accepted October 8, 1970. ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
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