CONCLUSION
Meites ( 7 ) has determined as little as 0.002 peq. of zinc to +lo% by the controlled-potential coulometric oxidation of zinc amalgam under ideal conditions. To our knowledge, this is the smallest amount of material that has been determined by potentiostatic coulometry. Under similar conditions, he could determine 0.02 Feq. of zinc to + I % . We have found that to run analyses at this level under less than ideal conditions refinements in instrumentation and in cell and procedural design are required.
Several such refinements are reported in the present paper. LITERATURE CITED
( 1 ) Booman, G. L., ANAL.CHEM.29, 213 (1957). ( 2 ) Harrar, J. E., Stephens, F. B., Pechacek, R. E., Ibid., 34, 1036 (1962). ( 3 ) Kaufman, F., Ossofsky, E., Cook, H. J., Ibid.,26, 516 (1954). ( 4 ) Kelley, M. T., Jones, H. C., Fisher, D. J., Zbid.,31, 488, 956 (1959). ( 5 ) Kelley, XI, T., Jones, H. C.. Fisher, D. J.. Talanta 6. 185 (1960). ( 6 ) Lindstrom, F.; Davis, J: B., ANAL. CHEM.36, 1 1 (1964). ( 7 ) Meites, L., Anal. Chim. Acta 20, 456 (1959).
Separation of Berkelium from Other
(8) Meites, L., hIoros, S. A,, ANAL. CHEM.31, 23 (1959). (9) Merritt, L. L., Jr., Martin, E. L., Jr., Bedi, R. D., Ibid.,20, 487 (1958). (10) Philbrick Researches, Inc., “GAP/R
Electronic Analog Computers,” catalog data sheets, Dedham, MassachusettP. (11) Rockett, J. J., I’nited Kingdom At. Energy Aiith. Rept. AERE-R 3784. (12) Stephens, F. B., Harrar, J. E., “A Controlled - Potential Coiilorneter,” UCRL-7165, Jan. 21, 1963. (13) Wadsworth, S . J., Analyst 85, 673 (1960).
RECEIVED for review November 20, 1964. Accepted >larch 3, 1965. Research Sponsored by Y,S. Atomic Energy Commission under contract with 1,:nion Carbide Corp.
EIements
Application to Purification and Radiochemical Determination of Berkelium FLETCHER L. MOORE and W. THOMAS MULLINS Analytical Chemistry Division, Oak Ridge National laboratory, Oak Ridge, Tenn.
b A new, simple analytical radiochemical method for the purification and determination of berkelium tracer is based on a two-cycle liquid-liquid extraction system. Berkelium(1V) is extracted into 0.1 5 M di(2-ethylhexy1)orthophosphoric acid-heptane from 1 OM nitric acid followed by reduction and stripping into 11.8M lithium chloride-0.4M hydrochloric acid solution. Thirty per cent tricaprylamine-xylene is used in the second cycle to separate berkelium(lll) from cerium(lll) and to increase the decontamination from other elements. Excellent separation is effected from many elements including uranium, neptunium, plutonium, americium, curium, californium, lanthanide elements, cesium, strontium, barium, zirconium, niobium, ruthenium, nickel, iron, aluminum, silver, and molybdenum. Several useful analytical and process applications of this purification method are discussed.
A
which confronts the analytical radiochemist currently is the purification and determination of berkelium24g. I n some respects this is more difficult than that of the other heavy elements because of its unfamiliar chemistry and weak beta particle decay. Berkelium249 ( t l / z = 314 days), the major berkelium nuclide produced by high neutron flux reactors, decays >990/, by emission of low energy beta particles (E,,, = 125 k.e.v.). Therefore, separation from other radioactivities is necessary before measureCHALLEKGING PROBLEM
ment of this isotope is possible. Fortunately, it is the only transplutonium element with a stable tetravalent oxidation state in ordinary aqueous solution. The chemistry of berkelium(1V) is very similar to that of cerium(1V) (3). Previously, no simple method existed for the radiochemical determination of berkelium in mixtures of other transuranium elements and fission products. Early investigators utilized its relative elution position from ion exchange resins to isolate berkelium. Several resin cycles are required to separate berkelium from both actinides and fission products. Such procedures are time-consuming and not satisfactory for process control. Liquid-liquid extraction is a valuable method for the rapid purification and isolation of nuclides, which are relatively solid-free and suitable for beta or alpha counting. An early patent (4) in this area describes the use of tributyl phosphate in the purification of berkelium. Yields of 92y, berkelium containing 8% americium were obtained. No separation was effected from cerium, however. A definite advance was made by Peppard ( 7 ) in the application of di(2ethylhexy1)-orthophosphoric acid to separate berkelium from a number of elements. Again, no separation from cerium is effected. T o accomplish this difficult separation and enhance the decontamination of berkelium from many other elements, an adaption of the amine extraction technique ( I , 6)
has now been successfully applied. These two solvents provide a simple two-cycle liquid-liquid extraction system for the purification and determination of berkelium. EXPERIMENTAL
Apparatus. Internal sample methane proportional counter. Voltage settings of 2900 and 4300 were used for alpha counting and beta counting, respectively. N a I well-type gamma scintillation counter 13/4 X 2 inches. Reagents. Di(2-ethylhexyl)-orthophosphoric acid-heptane, 0.ltih’. Purify di(2-ethylhexyl)-orthophosphoric acid ( H D E H P ) , available from Union Carbide Chemicals Co., New York, N. Y by mixing gently with an equal volume portion of ethylene glycol. If mixed too vigorously, the phase separation is poor. Separate the phases by centrifugation. Weigh 48.33 grams and dilute to 1 liter with n-heptane. Equilibrate with an equal volume portion of freshly prepared 10M nitric acid0.1M potassium bromate for 3 minutes just before use. 10M Nitric acid-0.1M potassium bromate solution. Prepare fresh daily. %-Heptane. 11.8X lithium chloride-0.4JI hydrochloric acid solution. 30% (w./v.) Alamine 336-S-xylene. Alamine 3 3 6 4 (tricaprylaminej is a water insoluble, symmetrical, straight chain, saturated tertiary amine. The alkyl groups are a Cs-Clo mixture with the Cs carbon chain 1)redominating. I t is available in conimwcial quantities (99-100yo tertiary amine) from General Ahfills Co., Kankakee, Ill. Ililute 300 VOL. 37, NO. 6, M A Y 1965
687
Table I.
Recovery and Decontamination of Berkelium249 Berkelium249 Deconrecovered, tamination Element 70 factor BerkeliumZ49 Cerium144 Cesiuml37 Bariuml40lanthanum >5 x 107 Ei~ropirim'j*-~ 5 , 8 2 X lo3 Zirconium95>i x . _ _ 107 niobium >7 x 107 R u t henium106 1 . 4 5 x 103 Uraniumz3* 2 . 9 8 x 103 Neptunium237 5 . 4 3 x 103 Plutonium23~ 2 x 103 Americium241 ~
Californium252 Silverl'o Iron
.4luminum 11olybdenum Nickel
~~
2 . 3 2 x 103 > 7 . 9 x 102 7 . 3 2 X lo2 > 4 . 5 x 103 > 2 . 7 x 103 1 . 4 X lo2 1 . 0 6 x 103
grams of hlamine 336-S to 1 liter with reagent grade xylene and mix well. Pretreat by contacting for 2 minutes with a n equal volume of 1J1 hydrochloric acid solution in a separatory funnel. .After phase separation, centrifuge the organic phase for several minutes and store in a glass bottle. 13.1Jf Lithium chloride-0.1M hydrochloric acid solution. 11Jf Hydrochloric acid solution. Spreading Agent. Dilute 1 ml. of triethylene glycol to 10 ml. with ethyl alcohol. Mix well and dilute 1 : 10 with distilled mater. Procedure. The sample solution should be adjusted to 1 0 M nitric acid-0.1M p o t a k u m bromat'e before the first extraction. Although a preliminary evaporation is often unnecessary, the evaluat'ion of the procedure included this step. Pipet, a suitable aliquot of the sample solution into a 50-ml. beaker. Evaporate to near dryness. Transfer quantitatively to a 125-ml. open-end separatory funnel (16 em. long, 3.4-em. i.d.j by washing the beaker with 5 ml. of 10.U nitric acid-0.1JP potassium bromate added in several small portions. -Add 5 ml. of 0.15.U HDEHP-heptane solution and extract for 3 minutes. High speed motor stirrers with glass paddles are satisfactory. After the phases have separated, drain off and discard the aqueous phase being careful to lose no organic solution. Do not stir, but carefully rinse the walls of the estraction funnel twice with about j ml. of distilled water to eliminate nitric acid and oxidant which interfere in the nest step. Discard the aqueous wash solution. Add 15 nil. of n-heptane and 10 ml. of 118 1 1 lithium chloride-0.4M hydrochloric acid solution. Strip the berkelium into the aqueous phase by vigorous mixing for 5 minutes. Drain most of the heptane off by suction decantation, being careful to lose none of the aqueous phase. Rinse the ex688
ANALYTICAL CHEMISTRY
traction funnel twice with about 10 ml. of n-heptane, discarding the heptane. Add 1 0 ml. of 30% Alamine 336-Sxylene solution and extract for 3 minutes. After the phases have separated drain the aqueous phase into another separatory funnel. Retain the organic phase. Extract the aqueous phase again with 10 ml. of 30% hlaniine 336-S-xylene for 3 minutes. After phase separation, drain off and discard the aqueous phase. Combine the organic phases. Wash the sides of the empty funnel with 20 ml. of 13.1J1 lithium chloride0 . l M hydrochloric acid solution and drain into the first funnel containing the combined organic phases. Scrub the organic phase with 20 ml. of 13.1.11 lithium chloride-0.1M hydrochloric acid solution for 3 minuteq. After phase separation, drain off and discard the aqueous phase. Repeat the scrub step. Drain the organic phase into a 50-ml. centrifuge tube and centrifuge for 5 minutes. Pipet 10 ml. of the organic solution into a 125-ml. open-end separatory funnel. Add 10 ml. of l l J 4 hydrochloric acid solution, and strip the berkelium by mixing well for 3 minutes. Drain the aqueous phase into a 50-ml. centrifuge tube. and centrifuge for several minutes if it is not clear. Add one drop of the spreading agent to the center of a platinum or tantalum plate. Pipet a small aliquot (0.1-0.2 ml.) of the l l J 4 hydrochloric acid product solution into the spreading agent. Heat to dryness under an infrared heat lamp. Carefully flame the plate high over a burner until organic matter has volatilized. Ignite the plate to a red heat, allow to cool to room temperature, and count the berkelium249 beta particles in an internal sample gas flow proportional counter. RESULTS A N D DISCUSSION
I n Peppard's method ('7) berkelium (IV) and cerium(1V) are extracted essentially quantitatively with 0.1554 di(2-ethylhexyl)-orthophosphoric acidheptane. Hydrogen peroxide is used to strip these elements from the organic phase. The writers directed considerable effort to the possibility of developing a selective strippant for berkelium (and cerium) from di(2-ethylhexylj-orthophosphoric acid-heptane solution and of developing optimum conditions for the subsequent amine extraction, scrubbing, and stripping steps. Only the optimum conditions are reported here. These studies resulted in the finding that, after removal of the oxidizing aqueous medium, berkelium(1V) is reduced and readily stripped from 0.15.11 di(2ethylhexy1)-orthophosphoric acid-heptane with an aqueous solution of lithium chloride-hydrochloric acid. For rapid quantitative stripping it is necessary to dilute the di(2-ethylhexyl)-orthophosphoric concentration four-fold. A onehalf volume portion of 11.8M lithium chloride-0.4M hydrochloric acid solu-
tion strips the berkelium and cerium from the organic phase in three minutes. The strippant solution serves as the feed solution for the second cycle extraction with a tertiary amine. This revision simplifies the method considerably by eliminating several steps-hydrogen peroxide stripping, followed by tedious evaporation and adjustment steps for the subsequent amine extraction. In the second cycle berkelium(II1) is extracted as an anionic chloro complex into 30% hlamine 336-S-xylene; the bulk of the cerium(II1) remains in the aqueous phase. Several scrubs of the extractant with 13.1-11lithium chloride0.1JI hydrochloric acid solution further increase the decontamination from cerium. Two other commercially available tertiary amines, -1dogen 381 and hdogen 382 (.\reher Daniels Midland Co., Minneapolis, Minn.), are as efficient as Alamine 336-S for this separation. Diethylbenzene is a useful alternate diluent for the amine. I Separation of Berkelium from Other Elements. Using the procedure described above, typical recovery a d decontamination data for berkelium2q9 tracer (6 X lo4 beta c.1i.m.) are shown in Table I. Decontamination factor equals the total amount of element in the feed solution divided by the total amount found in the berkelium product. Excellent separation of berkelium is effected from other heavy elements, fission products, and elements often associated with it in processing. Although the purification involves concentrated solutions of lithium chloride, the berkelium product contained less than 4 p.p.m. of lithium. Occasionally, it may be desirable to isolate and concentrate berkelium from solutions containing various possible interferences prior to applying the purification method described above. Preliminary coprecipitations of berkelium with lanthanum hydroxide employing excess ammonium hydroxide and/or sodium hydroxide followed by a lanthanum fluoride precipitation will remove many cations and anions. Cerium is the limiting nuclide in regard to decontamination. I n solutions containing very high cerium/berkelium ratios, further scrub steps niay be used in the second cycle for greater decontamination (Table 11). If much additional chemistry is performed, the analytical radiochemist may desire to use a yield correction. In this case, an internal standard or spike method is valuable. Duplicate experiments are run in which a large excess of berkelium249is added to the sample solution. By comparison with the unspiked samples, the yield under these specific conditions may be determined. Cesium, barium, strontium, ruthenium, trivalent rare earths, americium
( ] I t ) , cwriuni(III), and californium of 10.11 nitric acid-0.1.11 potassium (111) do not estract appreciably into the bromate solution prior to the second d i ( 2 - e t h y l h ~ ~ s y l ) - o ~ t h o ~ ~ h o sacid~ ~ h o r i c cycle estraction. The cui,iumz44 dehri)tanc solution. Elements of Z greater than 98 were not available for testing. but are known to behave yuite similarly to californium. Zirconium ( I V ) , niobiuni(V), uranium(V1). neptuniuin(VI), and plutonium(V1) extract esswtially quantitatively along with the bcrkelium; fortunately, these elements do not strip with the berkelium into 1 1.%.I[ lithium chloride-0.4.U hydrochloric acid solution. In addition to q ) a r a t i n g the tierkrlium from cerium in the second q-cle of the method, many other elements which form chloro complexes (6) remain in the amine phase when the berkelium is striliped into 11-11 hydrochloric acid solution. Thus, the decontamination factor for zirconiumg5-niobium wab 333 before the striiiping step but 1 X lo7after stripping. I3ecau.e the berkelium isolation will be done on relatively long-cooled solutions. the behavior of short-lived fission Ixoducts in the met’hod is of less interest. I t is known, however, that molybdenum: technetium, iodine, and trlliiriuni do not extract under t’he conditions used. h high degree of flexibility is possible in the second cycle of the method. For instance, 11.11 hydrochloric acid solution i:, used to strip the berkelium from the amine because of the inherent high selectivity of that berkelium can be readily stripped from the amine \vith any reagent which does not contain high concentrations of lithium chloride. ’Thus, for sllecial purposes, the berkelium can be stripped into distilled water or any concentration of various mineral acids-e.g., hydrochloric, nitric, sulfuric. The small amount of silver following through the procedure may be further reducrd to a negligible value by st’ri1)ping the amine with 0.1J1 hydrochloric acid solution rather than 11M hydrochloric acid solution. Under this condition, silver remains in the organic phase and berkelium strips into the 0.1.11 hydrochloric acid solution. Known anionic interferences are chloride and fluoride ions. Chloride ion should be reduced to less than 0.1.11 by evalloration or dilution. Fluoride ion must’ be removed or effectively complesed prior to the first extraction stel’. Recoveries averaged 95.8y0 berkewith a relative standard deviation of 1%. Several experiments were performed in which the HI)EHI’-heptane’solut,ion was scrubbed for 3 minut,es with P m l .
contamination factor wab increased to 4.9 X lo4, but the berkelium yield dropped to about 907,.
Measurement
of
Berkelium 24Q. alpha branching ratio of berkeliumY4Q is very low; a value of 1.41 x 10-5 was recently reported ( 2 ) . When lat,ger amounts of berkelium249beconic available, as in concentrated process product solutions, count,ing the 5.42-m.e.v. alpha particle will afford a valuable method for its measurement. In addition, alpha counting will allow less stringent chemical purification. This situation is a t least several years in the future, however. An alternative method for alpha counting is to measure the californium249 daughter (5.8 m.e.v., tli2 = 360 years) after a suitalile decay period of purified berkelium24Q. Because of the long time interval. t’his method is not attractive. BETA COUNTING.Beta counting is the most practical method for measuring small amounts of berkeli~m24~. An internal gas flow prol)ortional counter was found adequate for this purpose. Platinum or tantalum plates are necessary for mounting aliquots of t,he hydrochloric acid product solution. Most of our work was done with the cheaper tantalum plates. Because berkelium24gis a weak beta emitter, the method of counting sample preparation is important. I t was necessary to use a spreading agent to obtain good reproducibility. Thus, in the absence of a spreading agent the hydrochloric acid product solution tended to dry in a very small area, resulting in reduced counting rates by as much as 20%. This problem n-as essentially eliminated by use of dilute alcoholic solution of triethylene glycol. Zapon laquer (Glidden Co., Cleveland, Ohio) was a satisfactory substitute. An attractive alternative measurement technique is that, of beta liquid scintillation counting. We plan to study this method in the near future. . ~ L P H ACOUXTING.The
APPLICATIONS
Analytical. T h e method described above i s ’ u s e d currently a t this laboratory for the isolation and determination of berkelium nuclides in various solutions produced during research studies. I t is also iiromising as a n analytical radiochemical method for the control of transuranium element processes. I n addition, it is useful for the preliminary separation
~~
Table II. Effect of Multiple Scrubs“ on the Separation of Berkelium24g and Cerium144
KO.of scrubs
BkZ49
recovered,
c&
Ceriiim’44 decwn t amiriatiorr factor
95 8 b0 87 2 570 8.5 3 .5000 a Scrub solution was 1:3.1.11 IJiCl-~).lA\l HC1 for 3 min. 2 3 4
of berkelium nuclides. prior to study by other methods, such as mass spectrometry, spectrophotometry. and electroanalytical investigations. General Purification Work. ‘This method possesses the desirable features of speed, ready adaptability to remote control and continuous counter current procmsing a t room temperature, rclatively high radiation stability, and high x)paration factors from other elrments. 130th th(, radioisotopesTprocessor and heavy clclment chemist should find, profitable allplications for this purification in the isolation and recovery of berkelium isotopes produced in various transplutonium processes. ACKNOWLEDGMENl
The author5 are indebted to A. Chetham-Strode for ,-haring hi< herkelium tracer. The capahle awstance of G. I. Gault in bome of the experimental work is gratefully acknowledged. LITERATURE CITED
( 1 ) Baybarz, 11. I)., Weaver, R., Leiize, It. E., ~Vucd.Scz’. Eng. 17, 4.57 (196:3). ( 2 ) Chetham-Strode, A., Silva, 11. J., c‘. S . A t . Energy Commission C.nclassi$ed Report ORNL-3679 ( 1964). ( 3 ) Higgins, G. H., “The Radiochemistry of the Transcriritim Elements,” NAS-NS-3031 (1960), available from the Office of, Terhnical Services, Ilept. of Commerce, LTashirigtori 25, I>. C. ( 4 ) H d e t , E. K., U. S. Patent 2,909,405 ( t o U. S. Atomic Energy Commission) Oct. 20, 1059. ( 5 ) lloore, F. I,., A X A I , .CFIEM.33, 748 ( 196 1 ).’ ( 6 j,AIoiire, F. IJ., “Liqriid-Liqriiti Kxtractioir with High-lIolec.rilar LVeight Amines,” NAS-NS-3101 ( 106(l), available from the. Office, of Tecahriic.al Servires, I)ept. of Commerce, \$-ashingtoii 2 5 , I). C. ( 7 j I’epprtrd, I ) . F., lloline, S. \V.,l l a s o n , (;. W., J . Innrg. .\-iirl. Cheni. 4 , 344 (1057). RECTIVEDfor review .Jaririary 7, 196.5. Accepted Fehriiary 18, 1065. Research sporrsored b y t h e I’. S. Atomic Energy Commission tinder cwntracat with the
Vriion Carbide Chrporation.
VOL. 37, NO. 6, M A Y 1965
689
