In both cases the full advantages of the system have been realized. The Health and Safety Laboratory processes a large number of air dust samples taken on ll/*-inch diameter Whatman No. 41 paper or on glass fiber or Millipore filters (1). The diameter of the collecting area is 7/8 inch, which fits the phosphor disks. These samples can now be processed on automatic counters without cross contamination or counter contamination. Radium in urine and water is determined in this laboratory by coprecipitation with barium sulfate. I n the technique previously described (W), the sulfate precipitate was transferred as a slurry to a flat nickel disk for counting. This gave a relatively uneven deposit with variable self-absorption and one that had to be handled carefully to prevent dusting. With the present technique, the sulfate precipitate may be filtered directly on a glass fiber or Millipore filter and mounted for counting. These filters retain the
precipitate on the surface and uniform deposits are readily obtained. Jeanniaire (5) in France has successfully counted thin samples of alpha emitters mixed with ZnS, and Morken (4) in this country has counted Ra and daughters after absorption from solution on ZnS phosphor. These are similar to the method described in achieving intimate contact of the alpha emitter and phosphor. The new technique is being tried for low-level counting of plutonium to take advantage of the low background. Tests of similar mounting and counting techniques for analysis are now also being carried out with plastic phosphors, ADVANTAGES
OF TECHNIQUE
The proposed technique offers several advantages, particularly for low-level alpha counting: Samples are held flat without curling or buckling. Samples are counted with a uniform high efficiency, since the sample is in
contact with the phosphor and close to the phototube. Samples are completely enclosed, so they are readily handled and stored for recounting, without fear of loss or cross contamination. The phosphor sees only the sample and the face of the phototube,
[email protected] lower background than standard scintillation counters. Counter maintenance is reduced, since it cannot become contaminated by dust from the samples. LITERATURE CITED
(1) Alercio, J. S., Harley, J. H., Nucleonics 10, 87 (November 1952). ( 2 ) Harley, J. H., Foti, S., Ibbid.,. 18.. 45-7 (February 1952). . (3) Jeanmaire, L., Centre d'Etudes nu-
clbaires de Saclay, France. private communication (September 1959). (4) Morken, D. A., Health Phys. 2, 77-8 (July 1959). RECEIVEDfor review May 12, 1958. Resubmitted July 13, 1960. Accepted September 6, 1960. Division of Analytical Chemistry, Symposium on Radiochemical Analysis 133rd Meeting, ACS, San Francisco, Caiif., April 1958.
etermi natio ranium an Plutonium i niurn-PI utoniumement AHo R. P. LARSEN and C. A. SEILS, Jr, Chemr'cal Engineering Division, Argonne National laboratory,
b A tributyl phosphate-hydrochloric acid extraction system has been used to separate uranium and plutonium from certain fission elements and from each other prior to x-ray spectrometric determination of the uranium and spectrophotometric determination of plutonium as the nitrate complex. Coprecipitation of piutonium(lll)fluoride with lanthanum fluoride has also been utilized to effect the plutonium separation. Coefficients of variation for both the uranium and plutonium determinations have been consistently better than 1%. the plutonium-uranium core of Experimental Breeder Reactor 11 by melting the fuel in refractory oxide crucibles and recasting will, after numerous processing cycles, result in an alloy containing percentage amounts of the fission elements zirconium, molybdenum, ruthenium, rhodium, and palladium. To analyze such an alloy, separation procedures had to be devised which would permit established methods of analysis t o be used. A solvent extraction separation was perfected incorporating 30% tributyl EFISING
9700 South Cuss Ave., Argonne, 111.
phosphate (TBP) in carbon tetrachloride as the extractant and hydrochloric acid as the salting agent. A hydrochloric acid-TBP system has important advantages over the nitratesolvent systems (4) for separating uranium and plutonium from other metals. Separation factors are considerably higher; complete separation and recovery can be effected with a minimum mmber of equilibrations. Because the medium is nonoxidizing, the lower oxidation states of plutonium and uranium, (111) and (IT), respectively, which cannot be considered in nitrate separation procedures, can be effectively employed. The final dilute hydrochloric acid solutions are readily converted t o the media required for the assays. In an alternative procedure used when only a plutonium determination is required, lanthanum fluoride is used as a coprecipitant for plutonium (111)fluoride. Separation from uranium and the fission elements is adequate for determining plutonium spectrophotometrically as the nitrate. Dissolution of the fluoride precipitate in a zirconium nitrate-nitric acid mixture removes the interference of fluoride and
oxidizes the plutonium to the quadrivalent state. For uranium analysis, the x-ray spectrometric method as applied by Flikkema, Larsen, and Schablaske (2) was chosen, since only a separation from plutonium would be required. For plutonium, the spectrophotometric measurement of the nitrate complex ion was chosen. Considerable variation in the isotopic composition of the plutonium used in the process study made radiochemical methods for plutonium impracticable. I n addition to the plutonium alpha assay, mass spectrometric and alpha pulse analyses would have been required. PROCEDURE
Unless otherwise stated, reagent grade materials are used. Reagents
for Extraction Separa-
tion. Tributyl phosphate, 30 volume yo in carbon tetrachloride, and tributyl phosphate, 30 volume % in Amsco-140 (a kerosine distillate). Dilute 300 mP. of tributyl phosphate (Commercial Solvents Gorp.) to 1 liter with carbon tetrachloride (or Amsco-140). Scrub once with 208 ml. of 0.5N sodium hydroxide Lo VOL 32, NO. 13, DECEMBER 1960
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remove traces of mono- and dibutyl phosphate. Scrub four times with distilled water and filter through a large dry filter paper to remove cloudiness. Reagents for Precipitation Separation. Lanthanum nitrate carrier, 25 mg. per ml. Dissolve 39 grams of lanthanum nitrate hexahydrate in water and dilute t o 500 ml. Kitric acid (lO-U)-zirconium nitrate (0.2111) solution. Dissolve 28 grams of zirconium nitrate (E. H. Sargent) in 150 ml. of water and make up to 180 ml. Add concentrated nitric acid to make 500 ml. Allow the mixture to stand overnight. -4dd about 1 gram of Dicalite 4200 filter aid and filter with suction through a medium porosity fritted glass funnel. Xitric acid (1.311f)-zirconium nitrate (0.075X) solution. Dissolve 21 grams of zirconium nitrate (E. H. Sargent) in water. Add $3 m!. of concentrated nitric acid and make up to I liter. Add 0.5 to 1 gram of Dicalite 4200 filter aid and filter with suction through a medium porosity fritted glass funnel. Extraction Separation. Dissolve the alloy sample, using the procedure described by Larsen (3) for this type of material, and dilute to volume. Pipet an aliquot containing 10 to 20 mg. of plutonium into a 50-ml. Erlenmeyer flask. For 20% plutonium alloys there ?vi11 be 35 to 70 mg. of uranium, more than enough for its determination. Convert to a chloride medium by evaporation to near dryness (ax) with 12M hydrochloric acid. Adjust the volume to 10 mi. and the hydrochloric acid concentration t o about 2-11. Add approximately 0.1 gram of magnesium turnings over a period of several minutes. Using a double layer of glass fiber filter paper in a filter chimney assembly, separate the precipitated Group VI11 elements by filtration, and catch the filtrate in a 60ml. cylindrical separatory funnel. Rinse the flask and filtering assembly with three 5-ml. portions of 12-11 hydrochloric acid. Add 15 ml. of 30% tributyl phosphate in carbon tetrachloride and shake for 1 minute. Allow the phases to separate and transfer the uranium-bearing organic phase to a second BO-nil. separatory funnel. Repeat the extraction n i t h 10 ml. of 307, tributyl phosphate in carbon tetrachloride and combine the uraniumbearing organic phases in the second separatory funnel. Treat the plutonium-bearing aqueous phase by the procedure outlined in the next paragraph. Add 15 ml. of 0.5X hydrochloric acid to the combined organic extracts and shake for 1 minute. Allow the phases to separate and drain the stripped organic phase into a 60-ml. separatory funnel. Add 10 ml. of 0.2X hydrochloric acid and repeat the stripping operation. Discard the organic phaae and combine the aqueous strip solutions in the second separatory funnel. Add 5 ml. of carbon tetrachloride and shake 30 seconds to wash out dissolved tributyl phosphate from the aqueous phase. Discard the organic layer. Rinse the uranium solution into a 50-ml. Erlenmeyer flask and evaporate 1864
4
ANALYTICAL CHEMISTRY
to dryness (ax) on a sand bath after addition of 2-ml. portions of 16-11 nitric acid. Add 5.0 ml. of 16Jf nitric acid to dissolve the uranium. Transfer to a 50-ml. volumetric flask with water and .make up to volume. Determine uranium x-ray spectrometrically (9). For samples containing less uranium, use proportionately smaller volunies of nitric acid and volumetric flasks (down to 5 ml.). Add approximately 100 nig. of sodium nitrite to the aqueous raffinate from the uranium separation to oxidize the plutonium to the quadrivalent state. Add 20 ml. of tributyl phosphateAmsco-140 and equilibrate I minute. (dmsco-140 is used as the inert diluent to give a light organic phase.) Discard the lower aqueous raffinate. Add 15 ml. of 0.2X hydrochloric acid and equilibrate 1 minute. Allow the phases to separate and transfer the aqueous phase to a 50-ml. Erlenmeyer flask. Add 10 nil. of 0.2M hydrochloric acid to the organic phase and repeat the stripping operation. Combine the aqueous strip solutions, add 2 ml. of 12M hydrochloric acid, and evaporate on a sand bath to reduce the volume to approximately 2 ml. Transfer the solution to a 15-ml. glass centrifuge cone with a transfer pipet and dilute to 7 ml. with water. Add 0.5 ml. of 20% hydroxylamine hydrochloride and let stand 15 minutes with occasional mixing. 'CThile mixing with a platinum stirring wire, add 10M sodium hydroxide dropwise until plutonium hydroxide precipitates. Add 10 drops of sodium hydroxide solution in excess and let stand for 5 minutes. (Evaporation is not a satisfactory volume-reduction step, as it does not remove chloride. With the nitrate introduced in the next step oxidation of the plutonium to the sesivalent state would occur.) Centrifuge for 5 minutes and discard the clear supernate. mash the precipitate with water, centrifuge for 3 minutes, and discard the wash solution. Add 2.0 ml. of 1 6 X nitric acid and stir to dissolve the precipitate. Heat the nitric acid solution in a boiling water bath for 20 minutes, (This treatment will destroy any polymeric plutonium which may be present and ensure complete oxidation of the plutonium to the quadrivalent state. If the precipitate were dissolved in hot 351 nitric acid, some oxidation to the sexivalent state would occur.) Allow the solution to cool and dilute to volume in a 10-ml. volumetric flask with water. Read the absorbance vs. a reagent blank in 1-em. cells at 475 m,u and a slit width of 0.02 mm. Calculate the plutonium present from a calibration factor prepared from a series of standards carried through the hydroxide precipitation step only. Precipitation Separation. Pipet an aliquot containing 3 t o 10 mg. of plutonium into a 15-ml. polyethylene centrifuge cone. Add 0.4 ml. of lanthanum nitrate solution and adjust the volume to about 7 ml. with 1:Il nitric acid. Add 0.5 ml. of a 20% hydroxylamine hydrochloride solution and mix with a platinum stirring wire. Let stand for 15 minutes with occasional
stirring. $dd 0.5 ml. of 27-11 hydrofluoric acid dropwise with constant stirring. Let stand for 20 minutes with occasional stirring. Centrifuge 5 minutes and decant the supernate. Wash the precipitate with about 4 ml. of 1X nitric-1K hydrofluoric acid wash solution, stirring vigorously to disperse the solid phase. Centrifuge and remove as much of the supernate as possible without disturbing the precipitate. Add 2.0 ml. of 1 0 X nitric acid-0.2M zirconium reagent and stir to dissolve the precipitate. Heat in a 90' C. water bath for 15 minutes to dissolve the fluoride precipitate and oxidize the plutonium to the quadrivalent state. Cool the solution and transfer to a 10-ml. volunietric flask with the 1.3M nitric acid-0.075X zirconium reagent. The final solution concentration should be 3.0M nitric acid0.1-11 zirconium nitrate. (Because the solubility of zirconyl nitrate in 10M nitric acid is limited to about 0.2-11, zirconyl nitrate must also be present in the 1.3U nitric acid solution to secure a zirconium-fluoride molar ratio of a t least 1 when dilution to volume is made.) Measure the absorbance in 2-em. cells at 475 niN in a Beckman Model D U spectrophotometer with a slit setting of 0.02 mm. Correct the absorbance readings by carrying a reagent blank through the entire procedure. Calculate the plutonium content from a calibration factor obtained from a previously prepared set of standards. DEVELOPMENT A N D DISCUSSION
Uranium stock solution was prepared by dissolving reactor grade uranium in hydrochloric acid-hydrogen peroxide mixture, and evaporating the solution until the chloride-uranium molar ratio was 2.00. Plutonium stock solution was prepared by dissolving plutonium metal in dilute hydrochloric acid. After two precipitations as the peroxide to free it of its transplutonium element content, the final peroxide precipitate yI'as dissolved in 7-11 hydrochloric acid and heated a t 100' C. for an hour to destroy the peroxide. The plutonium was stored in this strong acid medium throughout the experimentation. Extraction Separation. In establishing t h e partition of plutonium and uranium between hydrochloric acid and a 30% solution of tributyl phosphate in carbon tetrachloride, equal volumes of t h e two phases were equilibrated for 10 minutes and allowed t o separate for 10 minutes before t h e phases were sampled. Initial aqueous concentrations of uranium were 10-z.U; plutonium concentrations were l O - 4 M . The more concentrated uranium solutions were analyzed spectrophotometrically by thiocyanate ( I ) , the less concentrated ones fluorimetrically. Plutonium analyses were done by alpha counting the aqueous raffinate and an aqueous strip of the organic.
URANIUX. The extraction of uranium(1T’) and (VI) as a function of hydrochloric acid concentration is given in Table I. The uranium(T’1) solutions were prepared by diluting the stock with the various eoncentrations of hydrochloric acid. The uranium(1T’) solutions were prepared in a similar manner from 0.1M uranium stock solution m-hich had been passed through a lead reductor. PLUTONIUM. The extractability of plutonium(II1) was determined at just two hydrochloric acid concentrations : lJ1. the acidity to be used in stripping plutonium from the organic medium, and 8 M , the acidity to be used in extracting uranium(1S’) or (VI) away from the plutonium. Ferrous chloride, 0.1.11, was used as the reductant. KO detectable extraction was observed a t the lower acidity; in 8 J f acid a distribution ratio (0 A) of 1 X l o p 3 was obtained. Considering the value, lop4, obtained for aniericium(II1) in 831 acid. it is likely that the value obtained for plutonium(II1) at this acidity is high by a factor of 10. Some air oxidation of plutonium(II1) could have occurred or plutonium(1V) hydrolytic polymer species (in the original stock) may not have been reduced. The extraction of plutonium(1V) and (VI) as a function of hydrochloric acid concentration is given in Table 11. Plutonium(S’1) solutions were prepared by oxidizing a 1 M hydrochloric acid dilution of the stock with 0.1N potassium dichromate at 100’ C. for an hour. An alpha assay of a lanthanum fluoride precipitate formed in the presence of bromate as a holding oxidant showed that more than 99% of the plutonium had been oxidized to the sexivalent state. Plutonium(1V) solutions were prepared by oxidizing the stock solution with a small amount of 0 . W potassium dichromate solution at room temperature. That less than O.lY0 of the plutonium had been oxidized to the sexivalent state by this treatment was ascertained by successive lanthanum fluoride precipitations. The first was carried out in the presence of potassium bromate as a holding oxidant, and the second was carried out under reducing conditions on the supernate from the first precipitation. The higher distribution ratios for plutonium(1V) and the relative ease u-ith which it can be formed make it the preferable oxidation state for extracting plutonium from hydrochloric acid media with tributyl phosphate. HYDROCHLORIC ACID. In contrast to nitric acid which is readily extracted by TBP, hydrochloric acid is not significantly extracted a t 6Jf or less and only moderately a t 8 M (distribution ratios 0.01 and 0.12, respectively). PLTJTONICM REDVCTION.If uranium (IV) could serve as a reductant for
0‘ 500
480
460
440
,
I
420
400
WAVE LENGTH
Figure 1 . Absorption spectra of 0.004M plutonium(1V) nitrate in 3M nitric acid A. No fluoride B. 0.1 M fluoride, 0.1 M$irconium C. 0.1M fluoride, 0.1 M aluminum
plutonium(1V). the possibility existed of effecting a separation without the introduction of some extraneous extractable constituent, such as iron. Uranium(1V) chloride was found to be very satisfactory for this purpoee. After 10 mg. of plutoniuni(1V) had been treated with 10 mg. of uranium (IT’) in 2JI hydrochloric acid a t room temperature, 0.45% of the plutonium was extracted from 8 X hydrochloric acid. By heating a t 90’ C. for 10 minutes, the amount of extractable plutonium was reduced to 0.3%. These losses of plutoniuni in the extraction are again indicative of refractory plutonium(1V) ion species. To reduce plutonium to the trivalent state with uranium(IV), in samples to be analyzed for both plutonium and uranium, magnesium is a satisfactory reductant. When 10 ml. of a 2 M hydrochloric acid solution containing 10 mg. of uranium and 8 mg. of plutonium was treated with 0.5 gram of magnesium turnings at 75” C., less than 0.6% of the plutonium was extracted from 8M acid. Fluoride and nitrate must be removed by fuming with perchloric acid before this reduction can be effected. In the absence of uranium, magnesium is ineffective as a reductant for plutonium. During the reduction with magnesium metal, ruthenium, rhodium, and palladium, highly colored in solution and interferences in the plutonium analysis, are reduced to the elemental form and separated by filtration or centrifugation. Molybdenum, which is reduced to the trivalent state, and zirconium extract quantitatively with the uranium(1V). Precipitation Separation. Although plutonium(II1) fluoride is a relatively insoluble salt and can be precipitated
quantitatively from pure solution a t t h e 10-mg. level, recovery was incomplete when separation from uranium and the fission elements was attempted. The use of lanthanum fluoride as a carrier for plutonium in t h e radiochemical determination and early production processes made i t an
Table I. Distribution of Uranium(lV) and Uranium(V1) between Hydrochloric Acid and 30% Tributyl Phosphate in Carbon Tetrachloride
Initial uranium (as). -10-2M. Phaee ratio. 1 HydroDistribution Ratio (O/A)chloric Uranium (IV) Uranium(V1) Acid, :1.I 0.008 ... 1 .o 2.0 3.0 4.0
5.0 6.0
7.0 8.0
...
... ...
0.5 15
50 102
0.9 0.54 2.5 3.6 9.0 15 21
Table 11. Distribution of Plutonium(lV) and Plutonium(V1)between Hydrochloric Acid and 30% Tributyl Phosphate in Carbon Tetrachloride Initial plutonium (as). - 10-4M. Phase ratio. 1
Hvdrocfiloric Acid, M 1.0
Distribution Ratio (O/A) Plutonium Plutonium(IV) (S71)
...
0.0001
3.0
VOL. 32, NO. 13, DECEMBER 1960
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1865
obvious choice for use in separating plutonium from the other alloy conA lanthanum-plutonium stituents. weight ratio of a t least one was necessary to ensure quantitative precipitation. I n the presence of 50 mg. of uranium and in 2iU hydrochloric acid coprecipitation of 10 mg. of plutonium with 10 mg. of lanthanum was quantitative, but 11% of the plutonium remained in the supernate when 5 mg. of lanthanum was used. Nitric acid solutions of both zirconium nitrate (6) and aluminum nitrate were tested as media for dissolving the lanthanum fluoride-plutonium fluoride precipitate and converting the plutonium to the quadrivalent nitrate complex. At zirconiumfluoride molar ratios greater than 1 the absorbance curve obtained with samples which had been carried through the precipitation procedure was identical to that obtained with a direct standard (Figure 1). Aluminum was also tested a t the same compiexant-fluoride ratio, but proved to be less effective. Absorbance measurements a t 475 mp were 30% low. When the aluminum concentration was increased by a factor of 10, from 0.1 to 1.OM absorbance measurements were 4% low. Although there is no significant sensitivity advantage in using zirconium rather than aluminum, small variations in the fluoride content of the precipitates result in slightly erratic absorbance-concentration plots when aluminum is used. When IO mg. of plutonium is co-
precipitated with 10 mg. of lanthanum fluoride in the presence of 50 mg. of uranium, about 1 mg. of uranium is also precipitated. This amount of uranium does not interfere in the subsequent determination, since uranium becomes a significant interference only when the uranium concentration is half the plutonium concentration. When 8.2 mg. of plutonium was determined in the presence of 22.1 mg. of uranium, the result was 670 low. Of the group VI11 elemental constituents of the alloys, none interfered when the lanthanum fluoride precipitation was used to separate them from plutonium. Each was tested a t a weight ratio to plutonium of 1.5 (10 or more times the amount to be encountered in the analysis of the alloys) and only rhodium was not completely separated. Under these conditions the plutonium absorbance was 7% high. Absorbance of Plutonium(1V) Nitrate. The absorbance is independent of nitric acid concentration in the range 2.3 to 3 . 5 M . At 1.6iW and Q.8M the absorbances were down 5 and lo%, respectively. The effect of higher nitric acid concentrations was not investigated. RESULTS
The reliability of the method was ascertained through the analysis, along with the regular samples, of a synthetic solution prepared from standard uranium, plutonium, and fission element solutions. The nominal composition
was 75YG uranium, 20% plutonium, 2.5% molybdenum, 2% ruthenium, 0.3% rhodium, and 0.201, palladium. By the solvent extraction separation, the coefficients of variation obtained in eight determinations of uranium and plutonium were 0.5 and 0.8%, respectively. h negative bias of 0.5% for the plutonium analysis is probably attributable t o incomplete reduction to the trivalent state prior t o the uranium extraction. Using the lanthanum fluoride separation when only plutonium analyses were required, the coefficient of variation was 0.7% with no apparent bias. ACKNOWLEDGMENT
The authors express their sincere appreciation to R. k1. Clarke for his numerous careful analyses. LITERATURE CITED
( 1 ) Crouthamel, C. E., Johnaon, C. E., ARAL.CHEY.24, 1780 (1952). (2) Flikkema, D. S., Larsen, R. P.,
Schablaske, R. V., U. S. At. Energy Comm., Rept. ANL-5641 (Kovernber lQS6). ~ _
_ .
(3) La;sen, R. P., ANAL. CHEM.31, 545 (19.59\. \----,-
(4) Metz, C. J., Ibid., 29,1748 (1957). (5) Seaborg, G. T., Katz, J. J., “The
Actinide Elements,” McGrsw-Hill, New York, 1954. RECEIVED for review December 14, 1959. Accepted August 11, 1960. Work performed under the auspices of the U. S. Atomic Energy Commission, Contract W-3 I-109-eng-38.
on Exchange Separation Radioeleme W. 1. BLAEDEL and EUGENE D. OLSENI Chemistry Department, University o f Wisconsin, Madison, Wis.
R. 6.. BUCHANAN Argonne National Laboratory, Lemonf, 111.
lp An ion exchange separation scheme Is described by which tracer amounts of over 35 metallic radioelements may b e separated into six groups. fn the rocedure, the sample is pretreated to put several elements into particular oxidation states and to complex some elements into anionic form, then adsorbed onto a column of Dowex 56W cation-exchangerl and finally eluted with a series of complexing eluents, each of controlled pH and ionic strength. The elements in each group are obtained in 15 Po 3 0 ml. OS solutions ~on~aining only ammonium
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ANALYTICAL CHEMISTRY
salts, organic acids, or hydrochloric acid. With this procedure, each of 36 eiements falls predominantly into its own group, with less than 1% falling into any other group. Four elemenis (Pd, Au, Hg, and Ag) cannot b e tolerated in the scheme. In designing the procedure, less importance was given bo high yields than to clean separations; nevertheless, the yields of 3 1 elements are over 90%, and three more are recovered with yields above 80%. The conditions necessary fer these separations were studied in detail.
in mixtures of only a few activities are identified easily by simple decay and energy measurements (5), or by the more retined techniques of scintillation spectrometry (7). However, for samples containing many elements, direct identification is rarely possible, and must usually be preceded by a chemical separation. For such complex mixADIOELEMENTS
1 Present address, Chemistry Department, Franklin and Marshall College, Lancaster, Pa.
