Determination of Plutonium in Biological Material by Solvent Extraction with Primary Amines F. W. BRUENGER, B. J. STOVER, and D. R. ATHERTON Division o f Radiobiology, Department of Anatomy, University o f Utah College o f Medicine, Salt lake City 7 2, Utah
b An accurate and simple procedure for the determination of plutonium in biological materials has been developed. Concentrated urine, or a solution of bone ash, i s made a t least 1M in HzS04, and plutonium i s extracted with a mixture of C8 I to Cia highly branched primary amines in xylene. Plutonium i s then extracted from the organic p h l x e with 8 M HCI, and measured b y alpha counting.
C
OLEMAN
et al. ( I ) have reviewed the
use of amine cxtractants. Petrow, Sohn, and Allen (6) have reported an estraction method for the radiochemical determination of tjhorium in uranium process streams. Thorium is cxtracted with a mixt'ure of amines. The great advantage of their procedure is the low degree of interference from cationic and anionic impurities, especially phosphate. This suggests its application to the determination of thorium in biological materials such as bone tissue and excreta. Furthermore, it should be applicable to elements which demonstrate similar extraction behavior, such as plutonium. hIost methods for the determination of plutonium, esplxially in urine samples, either lack reliability or involve lengthy combinationii of sensitive eoprecipitation method; and solvent estraction procedures. Although much progress has been ma de to increase accuracy and reproducibility (4, 7 , Q), there still is a need for uncomplicated but reproducible methods of high sensitivity. The method here described has been successfully applied to the study of low level plutonium metabolism but no attempt has been made, as yet, t o determine estremelj. lorn levels as iiormally encountered in routine bioassay.
Thv cztractability of plutonium was first tlctc7rmined using synthetic samples. Thf plutonium was obtained from the Argonne Sational h b o r a t o r y and its isotopic composition 'vas approximately 95% PuzSyand 57c Pu2g. The amounts C J f plutonium used were 2.3 M W C . and 2.3 mpc. in approxima,tely 100 ml. of the aqueous phase. Thc extractant used was the same as tha.; used by Petrow, Sohn, rind Allen in their thorium procedures-Primene JhI-T, a mixture of tert-alkyl primary amines. Howeyer,
instead of benzene, xylene was chofen as the solvent because of its higher boiling point and lolver toxicity. As shown in Table I, the chemical yields for both concentrations, nhich differed by 1000, were high and reproducible. EXPERIMENTAL
Apparatus and Reagents. Thc alpha-detection instrument is a 2~ proportional counter of conventional design capable of accepting 2-inch stainless steel planchettes; counter performance is checked with a U3OS standard supplied by the Kational Bureau of Standards. Primene JhI-T, a mixture of twtalkyl primary amines, 57c by volume in xylene, was purchased from Rohm 8; Haas, Philadelphia, Pa. The Primene solution is washed with half its volume of ld4 HzS04prior to use. Method of Development. The effect of the various oxidation states of plutonium and t h e possible interference of chloride a n d nitrate ions on the extraction behavior were evaluated in preliminary experiments. There exists spectrophotometric evidence (8) that excess of SO2 reduces an equilibrium mixture of plutonium to Pu(II1) and that nitrite ion oxidizes Pu(J.11) spontaneously t o Pu(IV), especially at elevated temperatures, but prevents oxidation to Pu(V1). -iccordingly, these two cheniicals were used to obtain the respective valences. An excess of KMn04 was used to oxidize plutonium to Pu(V1). Procedure. URINE~ A L Y S I S .Urine is collected in polyethylene bottles over 10 ml. of concentrated formic acid t o avoid excessive hydrolysis of urea, which mould render the specinieii basic and could result in loss of plutonium by adsorption on the wall aliquot of of the container. Li~i urine is transferred to n Kjeldahl flask containing enough 5-11 H,S04 t o attain a n acid concentration of 2J1 in the final sample. The solution is boiled without charring for about 1 hour. Thus far, volumes up to 500 ml. have been extracted. For larger volumes amounts of reagents are increased proportionally. The folloning amounts of reagents are used for 100-ml. aliquots. The sample is filtered through borosilicate glass wool into a separatory funnel and is allowed to cool to room temperature. Twenty milliliters of a 5% solution of Primene JM-T in xylene
Table 1.
Plutonium Recoveries from Synthetic Samples Plutonium added, Recovery: c
AI
PW.
2.3 2.3
x
103
100.4 96.2
3.1 f 3.4 =I=
is added to the sample and the mixture is shaken for 15 minutes. After separation of the two phases, the aqueous phase is put aside for a second extraction. The organic phase is washed twice with 25 mi. of 1114 &SO4. Plutonium is removed from the organic phase with tivci 20-1111. portions of 8M HCI. The aqueous phase is again extracted with 20 nil. of 5% Primene, and plutonium is back-extracted from the Primenc with 20 mi. of 8.11 HCl. The combined HC1 fractions are dried under a heat lamp and then the organic contaminants are destroyed by heating over an open flame or in a furnace ?t 500" C. The plutonium re-'d 31 ue is dissolved in concentrated HNOD and transferred to a 2-inch-diameter stainless steel planchette for alpha counting. Evaporation of the HC1 and elimination of the transferred organic material are the points where losses most frequently occur. Acid evaporation is safely done under a heat lamp: careful use of hydrogen peroxide faditates the combustion of the organic material when a n open flame is used. BONEh A L Y S I S . Bone is ashed for 4 hours at 600" C. and, after cooling, as possible is dissolved in as little "Os and diluted with H1O to a known volume, and a suitable aliquot of this solution is taken for analysis. The amount of hone ash should not exceed 250 mg. for every 76-ml. volume of the aqueous phase. The aliquots arc c'vaporated to dryness under a h ~ a tlamp to minimize the amount of H S 0 3 and then put in solution with 4 ml. of concentrated HCOOH, and a 60-ml. portion of 2.5-11 H2S04 is added. This solution is heated in a water biith until it is clear, and then transferred t,o a separatory funnel. Transfer is coinpleted with a 10-mi. rinse of distilled m t e r . Any CaS04 that forms does not interfere. Plutonium is extracted as described above. PREPARATION O F SYKTHETIC SAMPLES.
Urine samples containing a known VOL. 35,
NO. 11, O C T O B E R 1963
1671
Table 11.
Comparison of PuZ3’ in Bone Determined Directly and by Extraction with Primene
0
0
0.2
0.4 0.G MOLARITY
0.8
1.0
Figure 1. Effect of chloride and nitrate ions on plutonium extraction
amount of plutonium were prepared in the following way. Urine from a beagle was collected over 10 ml. of concentrated formic acid in a bottle to which a known amount of plutonium had been added. A day’s output was made up to 500 ml. and aliquots were taken for analysis. Bone samples containing a known amount of plutonium were prepared by dissolving bone ash containing no plutonium in as little nitric acid as possible and then adding a known amount of plutonium. This was diluted to a known volume and aliquots were taken for analysis.
Mean count rates, c.p.m.
% ’ Difference
KO.
250 pl. aliyuotsof*
Extracted aliquotsb
in amounts of Puz39 foundc
2- 1 2-2 2-3 6- 1 6-2 6-3 7- 1 7-2 7-3 9-1 9-2 9-3 10-2 10-3 11-1 11-2 11-3 12-2 12-3 31-3 35-1 35-2 38-1 -_ 38-2 38-3
391 =k 7 1276 f 2 0 . 6 SO9 f 1 3 . 4 836 f 11.6 2713 f 4 2 . 4 1229 f 19.8 1662 f 1 3 . 4 2190 =t100 1317 f 2 0 . 4 539 f 8 . 4 1403 f 20.2 609 f 8 2080 f 28.2 743 f 1 0 . 6 692 f 11.2 3150 f 5 6 . 6 1831 f 29.6 2240 f 106.4 695 f 11.2 2550 f 42.4 666 f 11.2 3010 Et 4 2 . 4 480 f 7 . 2 2340 f 36 1155 f 18.6
3s5 f 4 1272 f S SO4 f 6 797 f 9 2525 f 16 1133 i= 34 1649 f 13 2142 f 14 1317 f 11 550 f 7 1395 f 12 615 f 8 2216 =t15 691 f 8 671 f 8 3303 f 18 1751 f 13 2316 f 15 710 f 8 2553 f 16 720 f 8 3148 f 18 485 f 7 2 2 i i i 15 1168 f 11
1 . 5 f. 2 . 3 0.3 f 1.7 0.6 f 1.8 4 . 7 f 1.7 6.9 f 1.6 7.5 f 3.1 0.8 f 1.1 2.2 f4.5 f 1.8 -2.0 f2.0 0.6 f1.7 -1 f 1.8 - 6 . 5 f. 1 . 4 7 f 1.8 3.0 f 2 -4.8 f 1.8 4.4 f 1.7 -3.4 f4.6 -2.2 f 2 -0.1 =I=1 . 8 -8.1 f 1.9 -4.6 f 1.4 -1.0 f 2 . 1 - 5 . 5 f 1.6 - 1 . 1 i 4.17
Sample
~
Arithmetic mean 0 . 4 2 f. 4 . 1 7 Count rates of 250-pl. rtliquots adjusted t o same volume as extracted aliquots. b Standard deviation of mean is given. Pu by extraction x 100. - Pu directly 4
RESULTS AND DISCUSSION
)
The extraction behavior of the various oxidation states of plutonium was similar. The extractability decreases in the order: Pu(1V)
> Pu(II1) > Pu(V1)
Since other constituents of the urine may influence the ease of extraction, it is necessary to state the extractability under definite conditions in terms of the amount of plutonium extracted over the total plutonium, instead of in terms of the more general extraction coefficient. The following yields were obtained in a single extraction of 75 ml. of spiked urine with 20 ml. of Primene JM-T 5%: Pu(II1) 90.6%, Pu(1V) 98.9%, and Pu(V1) 85%. Plutonium could be extracted from aqueous phases having acid concentrations as high as 8M for HAC, 711iI for HCOOH, and 6M for H2S04, but extraction was not feasible with HC1, “Os, HClO,, or CC1,COOH. Since C1- and NOS- are the ions most frequently encountered in practical work, an upper limit for their concentration was determined as shown in Figure 1. Phosphate interfered in formic and acetic acid solutions and to a lesser, but significant, degree for concentrations of sulfuric acid less than I M . In 0.5M &SO4 acid, Pu(II1) still behaves as a cation, but above 1M I-IzS04 an anion complex is formed which is probably stronger than the positively charged phosphate complex. This suggests that it is probably a negatively charged sulfate complex which is extracted with Primene when 1672
ANALYTICAL CHEMISTRY
phosphate is present. Pu(1V) and Pu(V1) form negatively charged complexes readily a t lower acid concentrations (8,6). Although an HzS04 solution seemed undesirable as the aqueous phase for the extraction of plutonium from biological material because of the presence of varied quantities of calcium, it proved to be the only acid tested that could be successfully applied. As long as H2SOa was added to an already dissolved sample, precipitating calcium sulfate did not affect the recovery of plutonium. I t can either be filtered off with insignificant loss of activity, or left in the solution. In addition to the work with the svnthetic samples, the chemical yield of Puz39 by the Primene-extraction method was determined for bone samples from dogs injtcted with PuZs96 weeks t o 5 years before death (2). The samples were analyzed by this method and by direct plating. The bones contained sufficient Pu239 that 25O-pl. aliquots of the solutions could be plated directly for alpha counting to determilie Pu239. These solutions mere sufficiently diluted to make mass absorption negligible. Then 5 or 10 ml. of each of these solutions was extracted with Primene. This romparison has the disadvantage that the precision of the estraction method is greater than that of the direct
plating, since the counting rates, of course, are much greater for the larger aliquots. The results are presented in Table 11. The count rates of the 250-p1. aliquots have been adjusted to the same volumes as those used in the extraction procedure. Both the direct platings and the extractions mere done in duplicate. First is given the mean count rate and standard deviation obtained by the direct plating method, followed by the mean count rates and standard deviations obtained by the extraction method. The last column shows the percentage difference in amounts found by the two methods. The standard deviation of this quantity is calculated from the measured standard deviations of the mean count rates of the samples done by the two methods. The difference in amounts of Pu239 found by the two methods is small. The mean of the differences for the 25 samples studied is only 0.42% with a standard deviation of 4.17Q/,. The method described above is proving to be valuable in the accurate analysis of biological materials for small amounts of Pu*39. The absence of a coprecipitation step results in a higher and more reproducible chemical yield and a more effective separation from contaminants. The risk of mechanical loss through transfers is also decreased.
ACKNOWLEDGMENT
We thank D. S. Buster for assistance with the statistical calculations. LITERATURE CITED
( I j Coleman, C. I., Brown, Ti. Lc., Moore, J. G o ,Grouse, D. J., I n d . Eng. Chem. 50,
1756 (1958).
(2) Dougherty, T. F., Stover, R. J., Dougherty, J. H., Jel:, W.6. S.,Mays,
C. W., Rehfeld, C. E., Christensen, W. R., Goldthorpe, H. C., Radiation Res. 17, 625 (l9V2). (3) Hindman, J. C., Anies, D. P., “The Transuranium Elenients,” U. S. At. Energy Comm. Paper 4.2, IV-14B (1949j. (4) Holstein, V., Hoogmn, A. H. AI., Kooi, J., Health Phys. 8, 1 (1962). ( 5 ) McLane, C. IC., Dixon, J. S., Hindman, J. C. , “The Transuranium Elements,” IJ.S.At. Energy Comni., Paper 4.3 IV-14B (1949).
(6) Petrow, H. G., Sohn, Bernard, Allen,
R. J., ANAL.CHEM.30, 1301 (1961). (7) Sanders, 8. M., Leidt, d. C., Health Phys. 6 , 189 (1961). (8) Stover, B. J., Atherton, I). It., Bruenger, F. W., Buster, D., l b d , 8 , 589 (1962). (9) Weiss, H. V., Shipman, W. S , ASAL. CHEM.33,37 (1961). RECEIVED for review February 11, 1963. Accepted July 8, 1963. ork supported by the U. S. Atomic Encrgg Conmiissicin under Contract AT( I 1-1)-11‘3.
Am pe rometric Tit rat io n of PIutonium(VI) with Iro n(II) C. A. SEILS, Jr., R. J. MEYER, and R. P. LARSEN Chemical Engineering Ilivision, Argonne National laboratory, Argonne, 111.
b A method has been developed for the precise titration of plutonium(V1) with iron(ll) in which the end point is detected amperometrically with a rotating platinum electrode. The plutonium is oxidized to the sexivalent state in a sulfuric acid medium with argentic oxide, the excess oxidant destroyed by heating, and the plutonium titrated b y the addition of standcird iron(ll) sulfate from special weight bLrets. For 15-mg. samples, precisions of &0.0670 relative standard deviaticn are obtainable with no significant bicis. For 0.2-mg. samples, the relative standard deviation is 0.4% with a 0.6% negative bias.
A
R ~ ~ C E N T L Yas 1957 a review by Metz (8) of the analytical chemistry of plutonium revealed that only one precise method was available for the determination of plutonium. This method, which was h t reported by Boaz, et al. ( 1 ) was Lie potentiometric titration of plutonium JII) to (IV) with cerium(1V) sulfate. By the use of weight burets for both aliquoting and the addition of titrant a precision of *0.05% is obtainable (8). Iron, the most common contaminant of plutonium metal and its compour,ds, interferes and a correction for the iron by means of a spectrophotometric determination is necessary. Several tariations in the titration of the (111-1.V) couple, such as photometric end point detection (d), and use of potassium dichromate as the titrant (IO),have sine€ been introduced. More recently several new highprecision analytical methods for plutonium have been reported (11, 13, 14) in which the interference of iron has been either eliminated completely or significantly reduced. Scok and Peekema (11) and Shults (13) used controlled potential coulometry to titrate Plutonium(II1) to (IV). At the 20-mg. level, a precision of *0.06% is obtainable. At iron to plutonium molar ratios of one or greater (an unusual situation) iron
s
does interfere, but otherwise the method was remarkably free of interferences. In a method developed by Waterbury and Metz (14) plutonium(V1) (obtained by perchloric acid oxidation) is reacted with excess standard iron(I1) sulfate to form plutonium(1V) and iron(TI1). The excess iron(11) is then back titrated potentiometrically with standard cerium(1V). With 500-mg. samples, a precision of *0.02% can be obtained. Shults (1.2) demonstrated that the excess iron(I1) could be determined equally well by controlled potential coulometry. Chromium, manganese, and vanadium interfere. Sulfate cannot be tolerated as it inhibits the preliminary oxidation of plutonium to the sexivalent state. Plutonium(V1) can be titrated directly with iron(I1) by incorporating an amperometric end point detection of the type used by Kolthoff and May (4) in the chromium(V1)-iron(I1) titration. In both the plutonium and chromium titrations essentially no current is passed by the rotating platinum microelectrode until the end point is reached and thereafter the current is proportional to the amount of excess iron(I1) titrant. Argentic oxide, the reagent introduced by Lingane and Davis ( 7 ) for the oxidation of chromium, manganese, and cerium, has also been found to be particularly satisfactory for oxidizing plutonium to the sexivalent state. The precision and accuracy of the method are comparable to those obtained in other titrations. During the preparation of this paper for publication, it came to the authors’ attention that Helbig (3) had also titrated plutonium(V1) amperometrically with iron(I1). I n principle the titration which Helbig reports is nearly identical to the one reported herein, but the titrations are quite different in practice. Helbig carried out his titrations a t the 0.1- and 1-fig. levels with relative standard deviations of k 6
and =k3’%, respectively; titrations carried out in our laboratory were a t the 0.2- and 15-mg. level with standard deviations of 2~0.4and 0.06%, respectively. h thorough and comprehensive review of this phase as well as the other phases of plutonium analytical chemistry c m be found in the recently published treatise by hletz and Waterbury (9). EXPERIMENTAL
Apparatus. The titration assembly used in this work consisted of a n E. 1%. Sargent and Co. “Ampot” with the microammeter replaced by the more sensitive Ealing Corp. “Scalamp” (KO.29-222), a 600-r.p.m. synrronous rotator for the platinum microelectrode, and a low resistance mercurymercurous sulfate reference electrode. Saturated potassium sulfate solution was the electrolyte used in the reference electrode and salt bridge. Iliffrision of the electrolyte from thc bridge into the solution being titrated was prevented by a closure of unfused vyror (Corning So. 7930). Operation of the assembly is 1)ropcr if a plot of current us. weight of 0.01 meq. per gram of iron(I1) (corrected for volume changes) is linear in the ranre 0 to 50 kta. When not in use the platinum microelectrode is stored in 1.Y sulfuric acid. Before use it is operated for 2 minutes a t +1.2 volts us. the reference electrode. Without this treatment, the residual current in the first titration may be himh. ?he weight buret used in this analysis is shown in Figure 1. Its particular uses and advantages are covered in the “lliicussion” section. Reagents. The plutonium, iron(lI), and chromium(V1) solutions used in this a n 11) sis are prepared and aliquoted by w i g h t rather than volume. Concentrations are evpressed in milliequivalents per gram of solution. Plutonium chloride, 0.05 meq. per gram, is prepared by dissolving National 13ureau of Standards (NBS) high purity plutonium metal in 3N hydrochloric acid. VOL 35, NO. 11, OCTOBER 1963
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