and c. It is of general value in determining automatically, simultaneously, and rapidly certain of the data needed in fundamental polarographic studies and practical polarographic analyses. Siirnberg (8)has suggested : “This new device enables one to record quickly the electrocapillary curve simultaneously with a polarogram. This seems of major importance for the investigation of adsorption and inhibition problems, which are one of the main fields of electrochemical research in this decade.” ACKNOWLEDGMENT
This work was done under the supervision of P. F. Thomason and D . J. Fisher. John Farquharson provided the
sets of detectors and the track for the mercury-level detecting unit and advised regarding use of the unit. V.L. illaddox assisted in the fabrication of the apparatus. The illustrations mere prepared by B. S. Dunlap. The very valuable help of these persons is gratefully acknowledged. LITERATURE CITED
(1) Corbusier, P., Gierst, L., Anal. Chim. Acta 15, 254 (1956). (2) Farquharson, J., Kermicle, H. A,, Rev. Sci. Instr. 28, 324 (1957).
(3) Gierst, L., Bermane, D., Corbusier, P., Contributi teorici e Sperimentali d i Polaroarafia Vol. IT ~,
ography,”’2nd ed., V;l. I; p. 88, Interscience, New York, 1952.
(5) Lingane, J. J., IND.ESG.CHEM., ANAL.ED.16,329 (1944). (6) Lingane, J. J., Kolthoff, I. M., J . Am. Chem. SOC.61, 525 11939); esp. pp. 829-30 and Figure 4. (7) Milner, G. R. C.. “The Principles and Applications of Polarography.” p. 97, Longmans, Green and Co., Yew York, 1957. (8) Xurnberg, H. W.) Kernforschungsanlane Julich, German\.: communi- private . ca:tion, 1962. (9) Riha, J., “Advances in Polarography,” I. 8. Lonzmuir. ed.. n. 210. Vol. 1. Pergamon Tress,’ Xev,. i’ork, 1960. (10) Tsuji, IC,, Sci. Papers Znst. Phys. Chem. Res. ( T o k y o ) 54, No. 2, 223 (1960). RECEIVED for review July 18, 1962. ilccepted September 20, 1962. Oak Ridge National Laboratory is operated by Union Carbide Corp. for the .itomic Energy Commission.
The Q ua nt it a tive Se pa ration of PI utonium from Various Ions by Anion Excha nge IVAN K. KRESSIN and GLENN R. WATERBURY 10s Alarnos Scientific laboratory, University of California, 10s Alamos, N. M. b The anion exchange resin separation of plutonium has been investigated as a possible technique for the rapid quantitative separation of plutonium from various metal ions. A slurrycolumn technique provides satisfactory separation of plutonium from quite a large number of metal ions. In this method, plutoniurn(1V) is adsorbed from 7.2M nitric acid onto Dowex 1 x2 resin, and the other metal ions are washed from the column with nitric acid of this same strength. Then the plutonium is eluted with a 0.36M hydrochloric-0.0 1 M hydrofluoric acid mixture. The separation is rapid and permits an average recovery of 1 OO.OO~o of the plutonium with a standard deviation of 0.048% for known solutions, Of the 46 diverse ions investigated, only silicon reduced the recovery of the plutonium below 99.9%. Applications of this separation to various methods of analysis of plutonium-containing materials are discussed.
S
IN~ESTIGATIOSS of anion exchange resin separations of plutonium from various ions have been reported (1-7>9-16, 18). I n general these methods involve the adsorption of plutonium(1V) onto basic anion exchangers such as Dowex 1, or less frequently, Dowex 2 (18) from solutions not less than 6114 in nitric acid ( 1 , 4, 7 , 12-16) or hydrochloric acid (2, 3, 5, 8, 9, 11,18). -21~0the adsorption and elution EVERAL
1598
0
ANALYTICAL CHEMISTRY
behaviors of many other metals ions on Dowex 1 from hydrochloric acid (8) and nitric acid (4) solutions of various strengths have been reported, thus making possible the selection of the most advantageous system for a particular separation. Of these systems, the adsorption of plutonium from 6 to 8 X nitric acid onto Dowex 1 anion exchange resin reportedly is applicable to a wider range of materials,-a consequence, no doubt, of plutonium(1V) forming a highly stable anion complex in nitric acid solutions of these concentrations whereas most other metal ions do not. Investigations of this separation have been reported ( 2 , 4, 7 ) in which the metals were determined spectrochemically. However, in spite of the apparent applicability of the separation, no comprehensive investigation of the quantitative nature of this method for separating plutonium has been reported. Data giving the recoveries of plutonium following separation from various ions, and the dispositions of these ions, would permit applications of the separation method in many fields of chemistry. Several applications to analytical chemistry problems immediately are obvious in the determination of plutonium and various elements in plutonium-containing materials. The goal of the present investigation Tvas to obtain data showing the reliability of the separation of plutonium from many other ions. T o accomplish this goal efficiently, the most favorable conditions, such as
type of resin, loading solutions, and eluents, \%-eredetermined experimentally or selected according to the recommendations of past investigators. Once these \%-ereestablished, a procedure was developed for handling known solutions, and quantitative data were obtained. It is recognized that the plutonium recoveries and other data apply specifically to separations performed under conditions similar to those described. Horn-ever, the separation is sufficiently insensitive to small changes in conditions and procedure that minor modifications may be tolerated. The investigation of the separation of plutonium from 46 other ions is described here. Data showing the average recoveries of plutonium. as determined by a precise titrimetric method ( l 7 ) , are presented for each separation. In several cases, data also are presented to show the disposition of some metal ions of specific interest. A continuation of this investigation is anticipated as the need develops for information concerning separations of plutonium from other metals. EXPERIMENTAL
Special Apparatus and Reagents.
ASION EXCHASGE RESIN, Dowex 1, 2% cross-linked, 100 to 200 mesh. An analytical reagent grade of this resin was obtained from BioRad Laboratories, Richmond, Calif. The resin, received in the chloride form, was mashed in a fritted-glass funnel
with 7.2-$1 nitric acid until qualitative tests of the wash solution with silver nitrate showed no chloride. The resin was dried by drawing air through it in the funnel. 10s EXCHAKGE CoLuxss, borosilicate glass, 25-mm. i d . , 200-mm. long. These columns were made b y sealing 25-nim. tubing t o glass filtering crucibles having coarse frits 25-mni. in diameter. Below the frit, but as close as possible, a stopcock with 2-nun. capillary bore tubes was sealed. Ten grams of the washed and air-dried anion exchange resin were mixed with a fen ml. of T.2.71 nitric acid, and the slurry \\as transferred t o the column. The reqin was washed t o the bottom of the column with the same acid, and the escess acid was drained from the column. ELUENTSOLUTIOK, 0.3621 in hydrochloric acid and 0.0111f in hydrofluoric acid. Three milliliters of 48% hydrofluoric acid were mixed with 300 ml. of hydrochloric acid (sp. gr. 1.187) and diluted t o 10 liters with distilled water. The solution was stored in polyethylene. SITRIC ID SOLUTIOS,7 . 2 X . This solution was prepared by diluting 1860 nil. of nitric acid (sp. gr. 1.42) to 4 liters with distilled water. PLUTONIUU SOLUTIOS, 40 nig. of plutonium per gram. Approuimately 20 grams of high purity plutonium metal were accurately weighed and diisolved cautiously in a minimum volume to 3 V hydrochloric acid. Twenty milliliters of 70% perchloric acid were added, and the solution was evaporated t o incipient dryness under an infrared heat lamp. The residue was diwolved in water, 200 ml. of nitric acid (sp. gr. 1.42) were added, and the solution was diluted with water to about 400 ml. The solution &-as allowed t o cool to room temperature and then weighed. (Spectrochemical analysis of the plutonium showed that the total of the concentrations of detected metallic impurities was 200 p.p.m.) SOLUTIOKS OF hIET.4L I O S S . Individual solutions of all of the metals tested in this investigation were prepared b y dissolution of the metal or a suitable salt of the metal in nitric acid. If the metal was insoluble in this acid, either hydrochloric or perchloric acid, or mixtures of these three acids, was used as the solvent. Separation Procedure. A 5- to 10-gram aliquot (200 t o 400 mg. of plutonium) of t h e plutonium qolution was weighed accuratrly and transferred t o a 200-inl. tall-form beaker, and a known volume of a solution containing n specific foreign metal was added. T h e beaker was covered 11-ith a ribbed watch glass, and the solution was e\ aporated t o incipient dryness under a heat lamp and on a hot plate. The beaker, after cooling, was washed donn u-ith dilute nitric acid, and the evaporation to incipient dryness was repeated. The beaker and watch glass then were washed down with 7.2.7fnitric acid, 1 ml. of 307, hydrogen peroxide was added, and the beaker was covered with n plain watch glass during the mixing of the solutions by
swirling. Under these conditions;, the plutonium was converted t o the (IV) oxidation state, as evidenced by the color of the solution turning to green. (If large concentrations of oxidants such as chromate or permanganate are present, additional hydrogen peroxide may be added to reduce these ions.) The spray was washed from the watch glass and beaker walls with 7.2111 nitric acid, 10 grams of the air-dried anion exchange resin (Dowex-1 X 2, 100 t o 200 mesh in the nitrate form) were added, and the mixture was stirred for about 2 minutes. The slurry was transferred to the anion exchange resin colunin containing 10 grams of the same type resin, and the beaker and watch glass were washed with 7 . 2 X nitric acid, the washings being added to the column. The flo~v through the column was adjusted to about 7 nil. per minute, and the coluinn was washed with 100 nil. of T.2.71 nitric acid. The washings were retained in a beaker for subsequent determination of the foreign metal and traces of plutonium. Then the plut'onium as eluted n-ith 125 ml. of 0.36.71 hydrochloric acid containing 0.01V hydrofluoric acid. The eluate was collected in a separate beaker and retained for determinations of plutonium and traces of the foreign metal. Determination of Experimental Conditions. SELECTIONOF RESIN. The wide accept'ance b y past investigators of :i strongly basic anion eschange resin, such as Doives-1, for t'he adsorption of anionic complexes of plutoniuiii(1Y) at'tests t o t h e superior qualities of the resin for this applicat,ion and led to its selection as the adsorbing medium in the present investigation. Following this selection of the type of resin, the effects of particle size (mesh size) and percentage cross-linkage on the ease of handling the resin aiid on the efficiency and speed of the separat,ion of plutonium were investigated to determine optimum d u e s for these factors. Dowex-1 resins having cross-linkages between 1 and 87, and mesh sizes between 50 and 400 were tried in columns 25-mm. in diameter containing 20 grams of resin. A resin haring 27, cross-linkage and 100 to 200 mesh !vas optimum for a highly quantitative aiid rapid separation. Flow rat'es ryere too slow for resins having a greater percentage cross-linkage or a smaller particle size (larger mesh). A resin having 3 mesh size of 50 to 100 has a small capacity, and a resin having only 1% cross-linkage swells excessively during elution of plutonium from the columns. SELECTIOS OF SLURRY-COLUUS TECHSIQCE. Three techniques for adsorbing the plutonium ont'o the resin were investigated with respect t o rapidity and efficiency: a slurry technique in which the resin is niised successively FT-ith the plutonium-containing solution and wash solutions, and the supernatant solutions are decanted from the resin follolving centrifugation, a column technique in which the solutions are passed through the resin held in a colunin 25 nim. in diameter; and a combination of the
first and second techniques which is called the slurry-column technique and involves mixing the plutonium-containing solution with about half of the resin to form a slurry, transferring the slurry to a column containing the remainder of the resin, and then continuing the separation as in the column technique. The slurry technique was the most rapid but did not adsorb t h e plutonium quantitatively. The column technique, while highly efficient, cvas time consuming. The slurry-column technique yielded quantitative separations in the shortest operator time and was used in this n.ork. The greater speed of this technique is accomplished in t n o m y s . First, the bulk of the plutonium is adsorbed rapidly b y mixing the resin with the plutonium-containing solution to expose simultaneously all particles of the resin to the plutonium. Second, the small quantity of plutjonium remaining in the solution is quantitatively adsorbed by the short column of resin even a t high flow rates. SELECTIOSOF ACIDITYCOXDITIOXS. Although plutonium. as either the chloride or nitrate anion complex, may be adsorbed onto anion exchange resin, previous investigators (4, 8)have shown that adsorption from a nitric: acid system should separate plutonium from a greater variety of ions, that is, fewer metal ions should accompany the plutonium. For this reason, this investigation was limited to a study of the adsorption from nit'ric acid solutions. The concentrations of nitric acid recommended have varied betLI-een 6 M and lOJl (1, 4,7 , 12-16]. Ryan and Wheelwright (16 ) , follon-ing a n extensive investigation of this separation on a pilot plant scale, recommended a nitric acid concentration between 7.0 and 7.5JI for separations carried out a t temperatures between 25' and 60" C. T o determine the optimum nitric acid concent'ration for the present application, either the percentages of the plutonium adsorbed from solutions of various nitric acid concentrations can be measured, or the efficiency of the various concentrations of nitric acid as eluents for plutonium adsorbed on a column mag be determined. The latter method was used in the following manner. A 7JI nitric acid solution containing plutonium(1V) was passed through a Doivex 1 X 2 anion exchange resin column until the capacity of the resin was exceeded, a condition which was observed by measuring the plutonium radiochemically in the effluent. Then t,he column was washed successively with three column volume$, each of the solutions having various nitric acid concentrations between 3 and 12.7f. Plutonium \vas measured radiochemically in portions of the third-colunin volume of each was11 solution. The minimum concentration of plutonium \vas found in the wish solution having a nitric acid concentration of 7.2Jf. The plutonium concentrations were all very low in the wash solutions containing nitric acid Concentrations in the range of 6 to 8.71, but increased significantly for acid concentrations outside this range. VOL. 34,
NO. 12, NOVEMBER 1962
1599
SELECTION OF ELUEKT.The plutonium must be freed from the nitrate complex to permit rapid elution with a minimum of tailing. This can be accomplished : by reducing the plutonium to the (111) oxidation state which has only a slight tendency to form complexes, by reducing the nitrate concentration of the solution, or by adding an agent that forms with plutonium a neutral or cationic complex that is more stable than the plutonium nitrate complex. I n addition, the acidity of the eluent must be high enough to prevent hydrolysis and subsequent precipitation loss of the plutonium; also the eluent should contain no substance that would interfere with the subsequent determination of plutonium. Possible eluent solutions, therefore, are dilute acids n-hich may contain a n easily destroyed reductant or simple complexing agent. although the efficiency of a dilute acid eluent increases with decreasing acidity, an acid concentration of about 0.35-11 is necessary t o prevent hydrolysis of the plutonium (16). Several solutions of approsimately this acidity were investigated, including 0.39M nitric acid, 0.39M nitric acid-0.06M hydroxylamine, 0.39 M nitric acid-0.01X hydrofluoric acid, 6% sulfurous acid, 0.36M perchloric acid, and 0.36M hydrochloric acid0.01M hydrofluoric acid. Of the four eluents giving rapid elution rates (Figure l), elution was most rapid with a solution 0.3631 in hydrochloric acid and 0.01-!!4 in hydrofluoric acid. Sixty milliliters of this solution will quantitatively elute 200 mg. of plutonium from 20 grams of Doa-ex 1 X 2 a t a flow rate of 7 ml. per minute. An increase in the hydrofluoric acid concentration much greater than 0.01.11 did not increase this rate of elution. Therefore, a solution 0.36;M in hydrochloric acid and 0.01N in hydrofluoric acid mas selected as the eluent in this investigation.
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ANALYTICAL CHEMISTRY
80
IO3
120
140
V O L U M E OF ELUENT,ml
Figure 1 . Elution of plutonium by various solutions A 0.39M 6 0.39M C 0.39M D 0.36M Estimated plutonium
HNOj "03-0.01 M HF HN03-0.06M N H 2 0 H HCI-0.01 M HF relative error in measurement of concentrations i s &SO%
Table I. Efficiency of Anion Exchange Resin Separation of Plutonium from Diverse Ions (250 to 350 mg. of plutonium taken for each separation) L4v.fraction of added element Av. fraction of plutonium Wash rash Element
added, mg. Al, 120 Sb, 100 As, 100 Ba, 100 Bi, 100
Eh, :1:
Ca, 100 Ce. 100 Cr, 100 co, 100 c u , 100 Ga, 100
Au, 1 I n , 100 Ir, 2
Fe, 100 La, 100 Ph, 100 Li, 100 Mg, 100 Mn, 100 Hg, 100 hIo, 100 Nd, 100 Si, 100 os, 1 Pd, 10 Pt, 1 Rh, 1
RESULTS A N D DISCUSSION
Efficiency. The efficiency of the anion exchange resin separation of plutonium was determined first in t h e absence of other metal ions. This was accomplished b y determining the recovery of a known quantity of plutonium taken through the separation procedure in the following manner. The concentration of the plutonium solution was verified b y titration of weighed aliquots. Then several weighed aliquots of this solution were adsorbed on the anion exchange resin columns as described above, eluted, and the plutonium again was determined by titration. For these measurements, the highly precise method described by Waterbury and Metz (17') was used. This method consists of the oxidation of plutonium to the (VI) oxidation state in fuming perchloric acid, addition of a slight excess of iron(I1) solution from a weight buret, and titration of the excess iron(I1) with cerium(1V) to a potentiometric end point. The average value
63
43
for nine titrations of aliquots of the plutonium solution was 41.488 mg. of plutonium per gram of solution n i t h a relative standard deviation of 0.039%. Follon ing the adsorption on the resin and elution, an average value of 41.486 mg. of plutonium per gram of solution n a s obtained with a relative standard deviation of 0.048%. N o significant difference between the results of the two bets of determinations was observed, showing that the separation is quantitative. Effects of Diverse Ions. The effects of various ions on the efficiency of this separation were determined by adding the quantities of these ions shown in Table I to weighed aliquots of t h e plutonium solution, and then determining the concentrations of the plutonium and other added ions
Ru, 11 sc, 10 Si, 100
Ag, 100
Sr,
70
s,
100 Ta, 10 T1, 100 Sn, 100 Ti, 100 w, 100
u, v, Y,
Q
100
100 100
Zn, 100 Zr, 100 Not determined.
Eluate, % 99 99 99.91 99.98 99 97 99 97 100.02 99.99 99.97 99.96 99.99 99.97 100.00 99.98 100.02 100.01 100.02
solution, % 0.003
0.01
0
0.01 0.02 0.01 0.01
0.01
100 00
0 01
0.01
