involved in the Crzcit species. The state of the reacting indium is unknown also since nothing is known of its relative affinity for citrate and acetate under these conditions. One might also speculate that acetate is involved in the mixedmetal complex, but this is shown to be unnecessary from solvent extraction data (Table 111). The ratio of distribution ratios of indium in the presence and absence of chromium were compared with and without the presence of acetate and were found to be similar. CONCLUSIONS
complexing agents their presence must be taken into account. Calculations of separability, for example, which are based on formation constants for the individual metals will be significantly in error. As of yet, no formation constants of mixed-metal complexes of this type have been measured. Until this is done, corrections for mixed-metal complex formation cannot be applied to these calculations. Several systems are currently under study in this laboratory with this objective; in particular, a mixed complex of uranyl ion, indium(III), and citrate of very high stability is under detailed investigation.
The foregoing study shows that complexes of numerous hydroxycarboxylic acids with more than one kind of metal can have high stability. Quite clearly in a great deal of practical work where these acids are used as buffering or
RECEIVED for review July 11, 1968. Accepted September 18, 1968. Research performed under the auspices of the U. S. Atomic Energy Commission.
eparation of Fletcher I,.Moore Analytical Chemistry Diuision, Oak Ridge National Laboratory, Oak Ridge, Tenn. A simple, rapid method is described for the separation of americium from curium and other actinide elements. The method exploits a new oxidation technique for the preparation of americium(V) in dilute nitric acid solution with ammonium persulfate. Isolation of the americium i s achieved by extraction chromatography with small Teflon columns containing di(2-ethylhexy1)phosphoric acid as the stationary phase. The method considerably simplifies the purification and determination of 343Am in the presence of high levels of *%m. Several useful applications for both analytical and purification purposes are discussed.
IN HIS SEARCH for more highly selective methods for the isolation and determination of the transplutonium elements, the radiochemist in recent years has turned to an intriguing area-exploitation of their unusual oxidation states. Pressing demands for a faster, simpler method for 243Amin transplutoniurn process streams containing high levels of 244ern led the author to explore this concept. Current radiochemical methods for 248Am are based on the high solubility of Am(VI) in lanthanum fluoride (1,2) or calcium fluoride systems (31, sorption of Arn(V1) on extraction Chromatographic columns ( 4 ) containing di(2ethylhexy1)phosphoric acid(HDEHP), z3QNp daughter ingrowth on a purified sample (5), and the inextractability of Am@) with 0.5M thenoyltrifluoroacetone-xylene (6). These methods are either time consuming, require too many manipulations, or provide inadequate decontamination from 244Cm for many process applications. For instance, our present method based on solvent extraction with thenoyl triffuoroacetone requires 4 to 6 hours, because recycling is mandatory to remove the bulk of the 244Cm interference. Similarly, the method based on 339Np daughter ingrowth requires 24 to 72 hours. (1) R. A. Penneman and T. K. Keenan, “The Radiochemistry of Americium and Curium,” NAS-hTS-3006(1960). (2) F. L. Moore, ANAL.CHEM.,35, 715 (1963). (3) H. P. Holcomb, ibid., 36, 2329 (1964). (4) E. K. Hulet, J. h o r g . Nztcl. Chem., 26, 1721 (1964). (5) C. J. Banick, ANALCHEM., 37, 434 (1965). (6) J. R. Stokely, Jr., and F. L. Moore, ibid.,39, 994 (1967).
21 30
e
ANALYTICAL CHEMISTRY
The classical separation of americium from curium is based on the oxidation of Am(II1) to Am(V) in potassium carbonate or potassium bicarbonate solution ( I ) . Am(V) precipitates as a double carbonate, K3AmQ2(G03)2;curium (111) remains in solution as a carbonate complex. Successful industrial processes (7, 8) are based on this principle. Inasmuch as micro levels of Am(V) do not precipitate in this alkaline system ( I , 9), attempts were made to utilize the TeflonHDEHP system (10) for the separation of Am(V) from Cm (111). Such a system is most effective for the strong sorption of Cm(II1); one would expect Am(V) to show very low sorption in this method. This approach proved impractical because of the very low yields and difficulties associated with the alkaline precipitation of various metal ions; moreover, excess solids in the eluates precluded direct measurements of alpha particles. A particularly attractive alternative is to utilize an acidic solution for both the preparation of Am(V) and its subsequent separation from Gm(II1). Until recently such an approach remained unexplored, because chemists regarded the AmQ2+ ion too intractable to exploit for separations in acid systems. Early work in this area is described in one recent paper (6) relating to the inextractability of Am(V) with thenoyltrifluoroAttempts to adapt the aqueous acetone-xylene at pH -5. conditions cited therein to extraction chromatography (10) were unsuccessful. Further studies by the author led to the discovery of a simple technique for the preparation of Am(V) with high yield in nitric acid solution. Most importantly, the conditions used are optimum for exploiting the chemical behavior of Am(V) in an extraction chromatographic system based on Teflon-HDEHP-HNQ3. For analytical applications, the (7) W. D. Burch, Oak Ridge National Laboratory Unclassified Report, ORNL-3880, p 28 (1966). (8) G. A. Burney, Nucl. Appl., 4,217 (1968). (9) H. P. Holcomb, 155th National Meeting, ACS, Div. of Nuclear
Chemistry and Technology, Paper 16, San Francisco, Calif., (Mar. 31-Apr. 5 ; 1968). (10) F. L.Moore and A. Jurriaanse, ANAL.CHEM., 39, 733 (1967).
Table I. Effect of Variables on Oxidation and Recovery of Americium by Extraction Chromatography z41Amtracer AgNOs, M Temp., OC Time, min. recovered, "Os, M ("4)2SzOs, M 23 10 0.10 0.03 IO6 were effected on the 5 X 70 mm columns, larger columns were not evaluated. On the basis of our studies, the following conditions were selected for the extraction chromatographic isolation of americium, curium, and californium : 5 X 70 mm column of Teflon-6 powder (60-70 mesh) containing 1M HDEHPheptane as the stationary phase, room temperature operation at a flow rate of 5 to 6 drops per minute; eluents, 5 ml of 0.01M "OB for Am, 10 ml of 0.3M H N 0 3 for Cm, 10 ml of 4M HNOa for Cf and heavier elements. Typical elution curves are shown in Figure 1. The pH range of the americium product is 1.9-2.1. The recovery of americium and its decontamination from 244Cm is excellent. Yields average 85-95z with a relative standard deviation of 3.6% (n = 12). z4rCmdecontamination factors average 2.8 X lo5. The method may be performed in about 1 hours. The elution data for Cm(II1) and Cf(II1) are presented to demonstrate their sequential behavior in this system. It is similar to our current method (IO) for the separation of Cf(II1) from Cm(III), Am(II1) except that liM HDEHPheptane (instead of 0.5M) is used as the stationary phase.
To minimize tailing of the curium at the higher solvent concentration, the nitric acid concentration in the eluent has been increased from 0.2 to 0.3M. Recoveries of the z44Cm and z W f under the conditions described are typically >99 %. Any Am(II1) present in the oxidized solution elutes with the Cm(II1). Persulfate ion in the americium fraction corrodes stainless steel plates, thereby decreasing the resolution of alpha particle measurements. To avoid this problem, one may use tantalum or platinum plates, because they provide excellent sources for ordinary process control measurements. Also the need for an additional electrodeposition step is eliminated, but for more elegant studies, electrodeposition is still recommended. A plausible explanation for the experimental results is the following: persulfate oxidizes Am(II1) to Am(V1). In the absence of a holding oxidant, like silver ion, or a strong complexing anion, like fluoride ion (2),Am(V1) readily reduces to Am(V). Further reduction is more difficult because of the necessity to break oxygen bonds. Am(V) disproportionates to form Am(II1,VI) slowly-an observation which is successfully exploited in this new method. Inasmuch as the large Am02+ ion exhibits negligible tendency to complex with HDEHP and sorb on the column, it elutes rapidly. Under the conditions used, the AmOz+ ion is relatively stable for some time, even in the presence of a large excess of hydrogen peroxide. For instance, in the first exploratory experiments, it was thought that a mild reductant, like hydrogen peroxide would be required to provide high yields of Am(V) by reduction of Am(V1). Interestingly, the yields of Am(V) were essentially equal, whether excess hydrogen peroxide (as high as 5M) was present or not. In a brief study, after the persulfate oxidation in the standard method, the solution was adjusted to 1.8M hydrogen peroxide and allowed to sit for various periods of time at room temperature (-23 “C) prior to sorption on the column. Recoveries of Am(V) were 87% after 5 minutes, 7 4 z after 1 hour, and
