level of certainty. The 95y0 level of certainty, twice the standard error, was used to express the maximum likely error, which also includes the 3% error due to geometry and weighing. RESULTS A N D DISCUSSION
The results of the analyses of 18 natural diamonds are shown in Tables I and 11. Table I represents BEPO irradiations only, and no check has been made on these specimens for silicon interference. The results in Table I1 do not show a positive bias in the BEPO determinations within the limits of experimental error, thus indicating an upper limit for the silicon content of these specimens. The first BEPO determination of aluminum in R4 shown in Table I1 is well outside the experimental error, and this can only be attributed to accidental contamination. The cadmium foil irradiation of R4 suffers from low sensitivity. but also fails to show any bias. The fourth result shown for R4 used sodium bicarbonate as a flux monitor. The diamond and aluminum were irradiated separately for equal times, each with a weighed amount of sodium bicarbonate to act as a flux monitor. This approach avoids much of the necessity for automatic equipment, except the advantage of automatic precision timing. Although excellent agreement was obtained in this case,
further tests would be needed to see if this approach was universally applicable. CONCLUSIONS
A practical sensitivity to 10-8 gram has been achieved for the determination of aluminum in a noninterfering matrix, With better facilities for transferring the specimens from the DIDO reactor to the counting area, the cleaning and mounting couId be completed in under one half life, with a gain in sensitivity of a t lesst a factor of 4. ACKNOWLEDGMENT
The author is indebted to the Isotope Research Division, Atomic Energy Research Establishment, Harn-ell, for use of equipment and for providing the irradiation facilities, and to R. L. Otlet, B. Metcalfe, and H. Simpson of the above department for encouragement and discussion. J. F. Custers, Diamond Research Laboratories, Johannesburg, provided some of the diamonds, and F. C. Champion suggested the original project. LITERATURE CITED
(1) Anders, 0 . U., A N ~ LCHEM. . 33, 1706
(1961).
(2) Brindlev, K. W., At. Energy Rwearch Estab., A.E.R.E. Rept. IGR/DM089
(April 1958, declassified March 1962). (3) Brooksbank, W. A., Leddicotte, G. W., Dean, J. -4.,A N ~ LCHEM. . 30, 1785 (1958).
(4) Bunting, E. N., Van Valkenburg, A., Am. Mineralogist 43, 102 (1958). (5) Chesley, F. G., Zbid., 27, 20 (1942). (6) Cook, G. B., Duncan, J. F., “Modern
Radiochemical Practice,” Oxford Univ. Press, London, 1952. (72, Gatrousis, C., Crouthamel, C. E,.; Applied Gamma-Ray Spectrometry, C. E. Crouthamel, ed., Chap. 4, Perganion Press, New York, 1960. (8) Hughs, D. J., Schwarts, R. B., U.S. At. Energy Comm., Rept. BNL-325, 2nd ed. (1958). (9) Koch, R. C., “Activation Analysis Handbook,” Academic Press, New York, 1960. (10) Illackintosh, W. D., Jervis, R. E., ANAL.CHEW30, 1180 (1958). (11) Mellish, C. E., Payne, J. A,, Otlet, R . L.. “Radioisotopes in Scientific Research,” Vol. 1, p. 35, Pergamon Press, New York, 1955. Proceedings of UNESCO conference, Paris, September, 1957. (12) hletcalfe, B., A.E.R.E., Harwell, private communication, Jsnuary 1962. (13) Morrison, G. H., Cosgrove, J. F., ANAL.CHEM.27, 810 (1955). (14) Plumb, R. C., Lewis, J. E., Nucleonics 13 ( 8 ) , 42 (1955). 15) Raal, F. A,, ilm. Minerdogist 42, 354 (1957). 16) Robertson, R., Fox, J. J., Martin, A. E., Phil. Trans. Ray. SOC.A232, 463 (1934). 17) Worthing, A. G., Geffner, J., “Treatment of Experimental Data,” p. 196, Wiley, New York, 1943. RECEIVEDfor review May 14, 1962. Accepted July 9, 1962. Based on work for Ph.D. thesis at University of London under supervision of F. C. Champion. Supported in part by Maintenance Grant from University of London and Department of Scientific and Industrial Research.
Liquid Scintillation Method for Measuring Low Level Radioactivity of Aqueous Solutions Determination of Enriched Uranium in Urine LESTER LEVIN’ Research Department, Socony Mobil Oil Co., Inc., Box 7 025, Princeton,
b A scintillation mixture for direct measurement of low level radioactivity of aqueous solutions has been developed in which up to 10% b y volume of sample may b e dissolved and counted a t temperatures as low as -16’ C. High voltage counting a t this lower temperature results in appreciably lower background counts than from the higher temperature conditions limited by existing aqueous counting systems. The mixture is particularly applicable, therefore, to low level counting of weak beta emitters. Counting efficiencies of 4oy0 for carbon-14 and up to 5% for tritium are reported. A procedure for de-
1402
ANALYTICAL CHEMISTRY
N. J.
termining submicrogram quantities of enriched uranium in urine is presented in which the alpha disintegrations are detected a t a level of 2 d.p.m. The uranium is separated from natural radionuclides in urine by ion exchange, This bioassay is used as a screening method in a health physics program for personnel working with enriched uranium.
L
has been widely applied to the measurement of low energy nuclides in aqueous solutions. Rapkin (7) has cited various scintillation media used
with aqueous solutions with particular emphasis on the measurement of tritiated water (HTO). The principal limitations of the existing homogeneous systems are the quenching effects of solubilizers used in the aromatic hydrocarbon solvent systems, the limited solubility of the organic scintillators and the secondary solvent in the primary solvent, and the relatively high freezing points of the water miscible systems. The choice of the primary solvent is a
IQUID SCINTILLATIONCOUNTING
1 Present address: Medical Director’s Office, Soccny Mobil Oil Co., Inc., New York 17, N. Y.
major consideration in an attempt to obviate these limitations. I n low level counting, the net counting rate may be only slightly higher than the background so that minimizing the background is especially important. I t has been cited (8) that the cooling of Cs-Sb photo. cathode tubes reduces the thermionic emission and hence the thermionic contribution to background by a factor of 2 for every 13" C. that the temperature is reduced. At high voltage settings, the thermionic background may be quite high, and appreciable reduction by low temperature counting is obviously desirable. The choice of scintillator solvent, therefore, indirectly affects the background by controlling the temperature a t which the liquid sample-and hence the multiplier phototube-may be cooled. The method here reported employs lJ2-dimethosyethane (f.p. - 71" C.) as the primary solvent. Although this compound has been reported as a solvent ( I ) n-ith a rplative efficiency of 60% compared to a standard 0.3y0 (2,5-diphenylouazole, PPO) in toluene, and some workers incorporated it in miutures such as the so-called DAM 611 cocktail (7')) the solvent has received little application othera ise. This solvent readily dissolves efficient quantities of the standard organic scintillators and the secondary solvent, naphthalene, together with as much as 10% of the aqueous component. Most importantly, homogeneity is maintained a t a counting temperature of - 16" C. The counting mixture has been used with standardized aqueous solutions of carbon-14 prepared as sodium benzoate, and of H 3 as tritiated water. Counting efficiencies of approximately 40% for the carbon-14 and up to 570 with the tritium are obtainable] depending on the aqueous content. Alpha activity in water is also measurable with this mixture thereby suggesting the application to the determination of mixed alpha and beta emitters such as enriched uranium. Based on urinary excretion values from animal exposures to known uranium dust inhalation, Neuman (6) was able to demonstrate the significance of urinary uranium analysis as an index of the exposure. The usual chemical method for determining trace amounts of natural uranium is the fluorimetric procedure. Welford and associates (9) employed an ion exchange separation in their determination of urinary uranium in nonexposed individuals prior to the fluorimetry. The enhanced radioactivity of the U235 enriched form of uranium allows for higher sensitivity for uranium detection by radiometric methods. Whitson and Kwasnoski (IO) measured the radioactivity from enriched uranium alpha emissions following an electrodeposition separation. Liquid scintil-
lation counting of the alphas from aqueous uranium-223 in a dioxane counting solution has been reported by Hayes and Ott (3). In the method reported, the anion exchange procedure of Fisher and Kunin (2) is first employed to separate uranium from all likely interfering radionuclides present in urine. An aqueous solution of the eluted uranium is then used for the counting.
Based on the uranium isotopic distribution, the alpha disintegration rate of the standard solution was equal to 149 d.p.m./ml. SCINTILLATION MIXTURE. The composition of the scintillation solution was 7.00 grams of PPO, 0.05 gram of POPOP, and 100 grams of naphthalene all dissolved in 1 liter of dimethoxyethane. Procedure. ENRICHED URANIUM I N URINE. Evaporate 100 ml. of sample urine with 50 ml. of concentrated EXPERIMENTAL nitric acid to near dryness. Repeat Apparatus. LIQUIDSCINTILLATION the evaporation with a n additional 25 ml. of acid and finally ignite the COUNTING. Counting cells with residue to a white salt. Add 30 ml. quartz and with glass windows (1.75of 10% sulfuric acid and evaporate t o inch diameter] 2.00 inches long) were strong fumes of sulfur trioxide. Dilute employed. The counting was perwith 100 ml. of distilled water and formed with a Technical Measurethen neutralize to a p H of 1 to 1.5 with ment Corp. Dual Channel Coinci4N sodium hydroxide using thymol dence Pulse Height Analyzer and blue indicator. Filter the solution if auxilliary equipment including two necessary, and dilute with distilled matched Dumont K1234 2-inch multiwater to about 200 ml. Carry through plier phototubes. The samples, shield, both a reagent blank for the backand multiplier phototubes were conground determination and a similar tained in a freezer at -16' C. The reagent blank spiked with a known freezer unit and the compressor were microgram quantity of enriched uraseparately enclosed in Faraday cages. nium for calibration. For uranium and gross radioactivity Prepare an ion exchange column in water measurements, counts were (Amberlite IRA-400 analytical grade, taken integrally above a pulse height 40 to 60 mesh) of approximately 6 mm. base line of 5.6 volts a t a photo-multidiameter and 12 cm. length by the plier high voltage setting of 975. The method of Fisher and Kunin ( 2 ) . Pass carbon-14 and tritium measurements the sample through the column a t the were made a t high voltages of 1100 and rate of about 3 ml. per minute. Wash 1250, respectively, with the lower disthe column several times by backcriminator set at 4.2 volts and 4.5 volts flushing with distilled water. Elute respectively, and the upper discrimithe uranium with 40 ml. of warm 10% nator a t 24.5 volts. perchloric acid, followed by a distilled Reagents. The scintillator solvent water wash of about 25 ml. Evaporate was 112-dimethoxyethane (Arapahoe the eluent to dryness. Chemicals, Inc., Boulder, Colo., listed Add exactly 5 ml. of a 1% nitric acid as ethylene glycol dimethyl ether, solution to the residue, mix well, and anhydrous). Although initially colorfilter, if necessary, into the scintillation less, the solvent on standing usually cell. Make up to volume with the develops a yellow coloration possibly scintillation mixture, mix well, and due t o oxidation. If the reagent is allow to cool to the temperature in the first percolated through a n activated counter (-16' C.). Count for a t alumina column (Alcoa F-20) and least 100 minutes. subsequently stored in a dark brown Calibrate the counter with a net bottle containing iron wire, no color is reading for the spiked sample. observed after storage of 1 year. The primary and secondary scintillators were reagent grade PPO (2,5DISCUSSION AND RESULTS diphenyloxazole) and POPOP p-bis 2Scintillator Composition. Varia(5-phenyl-oxazole) benzene (Pilot tions in the amounts of the scintilChemicals, Watertown, Mass.). Naphlators were investigated with only thalene (Eastman, recrystallized from alcohol) was the secondary solvent, slight changes in the observed count COUA-TIA-G STANDARDS. Benzoic-7rates of a n aqueous enriched uranium C1' acid (New England IL'uclear Corp.). solution. At -16' C. and with The true disintegration rate of the aqueous contents up t o 15% no presolid was established in a conventional cipitation of scintillators or phase toluene scintillation medium by comseparation was observed. The adparison with p-cymene (contemporary) dition of naphthalene greatly enas the primary standard. An alkaline hanced the counting rate of the alpha solution of aqueous sodium benzoate and beta emissions of the standard enwas then prepared in which 1 ml. contained 10 .UE. riched uranium solution as indicated in - of benzoate eauivalent to 7310 d.p.m. Table I. The same scintillator mixTritiated Water (Supplied as Standture, when prepared and stored as deard Source ( *10%) from New England scribed, was used for a period of 1 year Nuclear Corp.). The diluted standard without observable changes in counting is equivalent to 1.75 X lo3 d.p.m./ml. efficiency. Enriched uranium, obtained as uraEffect of Water Content. Because nium oxide, U S O ~(Certified NBS KO. of water quenching of the light pulses, U-930). A dilute uranyl nitrate solution of the oxide was prepared in which the count rates vary inversely with 1 ml. contained 1 pg. as uranium. the amount of water present. AlVOL. 34, NO. l l , OCTOBER 1962
1403
c
8 Q
6.0
r- -
___-.___
I
-
-
--L-
--
r
a
-
__
-~
I-
a
4.0
I I
I
2.0
P I-
1.0-
L n
oo
a
Figure 1.
-
---+
3.0 6.0 9.0 12 0 AQUEOUS VOLUME PER CENT
-
Effect of aqueous concentrationon tritium counting
though increased water content does cause some dimunition with enriched uranium alpha counting, i t does not preclude useful counting of uranium with aqueous concentrations u p to 15%. Slight variations in t h e water content of the tritium media, however, are critical. Figure l illustrates the effect of water content in quenching signals from the weak beta emissions of tritium. A somewhat similar, though reduced, effect is observed with aqueous carbon-14. Obviously, the water content must be carefully controlled for reproducible and accurate results. With a water content of 1.3%, a tritium counting efficiency of almost 5y0 was obtained. Base Line Setting. The contribution of electronic noise a t t h e low energy level is very great and t h e lower discriminator setting (base line) must be set sufficiently high to eliminate this noise without appreciable loss of true counts. I n beta counting] the pulse height distribution is broadly Qkewed due to the broad energy distribution of the beta emissions. Some of the low energy beta pulses will be in the region of the electronic noise (aut-off and, therefore, cannot be counted. An alpha spectrum, however, usually involves discrete energies which appear as distinct peaks on the spectrum. Figure 2 depicts the spectrum
Table I. Effect of Scintillator Composition on Counting Aqueous Enriched Uranium Scintillator Net count composition rat ea-c .p .m , Blank (Solvent Only) 0 Saphthalene 0 PPO 230 PPO POPOP 310 PPO + Saphthalene 1310 PPO POPOP + Xaphthalene 1380 a Enriched uranium spike of about 10 pg.
+ +
1404
ANALYTICAL CHEMISTRY
BASELINE SETTING - V O L T S
15 0
Figure 2.
(mixed alpha and beta emissions) of natural uranium plotted in intervals of two volts. Although this spectrum is not sharply resolved, a main alpha peak a t 6 volts and a much smaller one at 38 volts are discernible. This suggests that with appropriate selection of the lower discriminator setting virtually 100% efficiency for alpha detection by liquid scintillation counting can be obtained. A spectrum of the background a t each high voltage setting used was obtained; as shown in Figure 2 (shaded area) the electronic noise drops sharply and forms a plateau as the pulse height increases. The base line setting is selected on the plateau for optimum detection for the particular radiation counted. I n this manner, the lower discriminator setting was selected a t 4.2 volts for carbon-14 and 4.5 volts for tritium. Uranium can be efficiently counted a t a base line of 5.6 volts. Background Counting Conditions. For low level counting applications, several factors were investigated to determine the conditions for reducing the background to minimal and constant values. Erratic pulses due to external electrical equipment were eliminated with the erection of Farad a y shields around the detectors and t h e freezer compressor. The increased high voltage required for soft beta counting-e.g. tritiumcauses an increase in the background arising from the potassium-40 disintegrations when a glass cell is used. Unfortunately, the solvent attacks plastics so they cannot be used for counting cells. -% quartz window cell was substituted for the glass cell in tritium counting and this resulted in a reduction of the background count by 30 to 40%. The most significant innovation in reducing the background count mas due t o the scintillator solvent which permits lower temperature counting than that of existing systems. Figure 3 shows the effect on the background
Spectrum of natural uranium
count of cooling the multiplier phot,otubes. -\lthough there is no reduction in background counting a t 975 volts, the reduction is very significant for tritium counting at 1250 volts. For comparison, a scintillator mixture of the same scintillator composition but' containing dioxane as the solvent was prepared. The dioxane solution also containing the same amount of water vias counted at $22" C. and $4" C. with approximately the same background count as the new mixture. However, the dioxane system could not be cooled to lower temperatures. By reducing the volume per cent of water the dimet.hoxyethane system could be cooled below -16" C., but as Figure 3 indicates there is an apparent leveling off of background reduction a t about -12' C. The thermionic emission rate is presumably not as significant a contributor to the total background below - 16" C. Scintillation Medium and Alpha Detection. The primary scintillator (PPO) and the secondary scintillator (POPOP), when present in the optimum concentration recommended by most investigators, do not produce a n efficient system for detecting uranium disintegrations (see Table I). Alpha particles, which produce a much smaller light emission t h a n an equivalent energy beta particle, are undetectable in this system. I t has been hypothesized (4) that certain substances, notably naphthalene, while not fluorescent by themselves have the property of transforming mi inefficient solvent into a workable system. The assumption is that the solvent molecules under high energy radiation produce excited molecules of Yery short lifetime, and the usual solute composition is ineffective in ext'racting this energy. Xaphthalene has a relatively long escitation lifetime with the property, when in sufficient concent,ration, of transferring the solvent energy t o a scintillator. The sharp increase in enriched uranium detection upon the addition of
the secondary solvent, naphthalene, is very likely due to this enhancement of the alpha detection. Ion Exchange Procedure. Several ion exchange procedures for separating trace uranium were considered, each of which depends on the formation of a complex uranium anion-Le., as a chloride (9),or an acetate ( 6 ) ,or as a sulfate ( 2 ) . The sample treatment as outlined prepares the uranium in a form convenient to adapt to the sulfate procedure. I n this separation, the only commonly occurring element which is adsorbed with uranium on the column it: iron, which-unlike the fluorimetric method-does not interfere with the uranium detection. The other natural radionuclides which could interfere rndionietrically-e.g. potassium40, thorium, and radium--are not adsorbed by the column. Tlie ion exchange separation does not, however, separate all fission products from uranium so that the method cannot be applied to persons expused to uranium and fission products. By this procedure, the total background count in the urine of unexposed individuals (see Table 11) which, of course, includes fallout, averages less than 1 c.1i.m. per 100 ml. of urine. Tlie nitcthod is applicable, therefore, as a screening test for individuals esposed solely to uranium isotopes. Calibration. If the standard enriched uranium solution is introduced directly into the scintillation medium arid counted, a count, rate is observed which is about 25 to 30% higher t h a n t h e count from t h e same quantity of enriched uranium which is utldcd as a spike t o a urine sample and subsequently ion exchanged. This dispwit'y in count rates is due to mixed alpha and beta radiations contributed by the daughter radioisotopes-i .e. thorium-231, -234, and protactinium231, -234, etc.--n.hich are present in the uranium stantlard but vhich are removed by the ion exchange separation 13s adding the spike to a reagent blank and performing the identical treatment as with the urine sample, identical count rates rcsult. Table I11 illustrates the net count rates per microgram of the "direct" arid ion exchanged 93% enriched uranium-235 spikes under different inst.rument settings. h t the instrument settings chosen, 1 c.p.ni. is equivalent to less than 0.009 fig. of the enriched uranium standard. Based on the known alpha disintegration rate of the standard, the detection efficiency a t the selected settings is about 85%. Counting Time. For most tracer applications, t,he expected net count rate will be many times the background count so t h a t the gross count rate will determine the counting time required. I n low level counting where t'he net count is equal to or less than the background, t h e const,ancg and
/
:200~
t-
2
u
1 2 5 0"
O
__-
LTS 9 1 3 "V- U
501-
01
-16
I
-6
0
8
TEMPERATURE
Figure 3.
7
16
24
- 'C
Effect of cooling phototubes on background count
reliability of t h e background count are extremely important. In the control method for enriched urinary uranium, the net count is less than the background. It was important to establish the statistical variations in the background count a t prescribed instrument settings. Table IV reports the average background values of approximately 10 measurements for each of the described settings. At the settings selected for uranium counting, the background value was 30.6 f 0.1 c.p.m. with no value exceeding 3 standard deviations. Eased on the allowable cumulative lung exposure t o insoluble enriched (> 90%) uranium, \\%itson and Nwasnoski ( I O ) calculated that the tolerance limit for enriched uranium in urine was 0.016 pg. per 100-ml. sample which is equivalent to 2 c.p.m. by the liquid scintillation method. If microgram quantities of enriched uranium are present in the urine, the net count rates are high and can be determined accurately. For screening purposes, however, it is not necessary t o know the accurate submicrogram amount of uranium but rather that the net count rate is significantly greater or less than 2 c.p.m. per 100 ml. of urinp. The net count in the urine of i n d i d u n l s n-orking with enriched uranium is considered to be the net value above the normal range of activity found by this method in the urine of the same personnel in advance of actual work with uranium. -is the results of Table I1 indicate, the w i g e in net background values of unesposed individuals is from zero to less than 1 c.p.m. This minimal activity is very likely due to fission products contained in fallout which are not, separated by the ion eschange technique as w1l as natural uranium. -1net samule count. rate of 2 c.1i.m. may be considered indicative of exposure providing that the probability of wror in observing that sample rate is less than 2 c.p.m. a t a high confidence level (99%). The error (y) for a net
Table \I. Background Counts of Unexposed Individuals Sample
no.
Ket c.p.m./100 ml. urine
06
-1-1
0 4
-1-2
A-6
0 1 Average = 0 .?I 0 37 c p.ni 0 0 A. O G
R-1
0.9
9-3 A-4
A-5
c-1
0 . 4 ilvixrngc = 0 . 8 0.60 c.p.m.
1)-1
0 . 5 B.C.D.E.F. 0.4
E-1
F-1
Table I l l .
Count Rates of Enriched Uranium Ion exIlirect changed soh. net
solution net c.p.m.:
voltage
Rase linevolts
pg.
Y6.
950 975 97 5
6.3
130
104 1 2G 113 106
High
148 1XS 134
5.6 6.3
7.0
Table IV.
c.p.ln./
Background Count Rates s o . of
values exStand- ceeding ard devia3 tion, c p.m.
std
36 2 38.7 29.7 28 3 2; 1
0 6 0 3 0.2 0.1 0.2
1 3
r r
,a
33
X0
30 G
0 3 0 1 0 1 0 1
1 0
Base line 60 60 80 90 100
90
100
VOL. 34,
\
28 2 28 0
NO. 11, OCTOBER 1962
der. 0 0 0
0
0
1405
counting rate is given by the following expression :
K N,,
= Nb
ta, ta
= =
Probability of error constant (Gaussian distribution) Count rates of sample and background respectively, c.p.m. Counting time of sample and background respectively, minutes.
As the ratio of the sample and background counting rates is close to unity, the most efficient sampling time is when t, = tb. The background averaged 30.6 c.p.m. i 0.1 c.p.m. so that N , and Ns are approximately 31 and 33 respectively. For a confidence level of 99%, K = 2.58. Thus, on substituting:
+
Scientific Laboratory Report LA-2095, 31 33 ‘/p 2.58 ( 7 ) September 1957. (4) Kalman, H., Furst, M., “Basic Processes Occurring In The Liquid Scinor, t is approximately 100 minutes. I n tillator. Liquid Scintillation Countother words, a net count of as much as ing,” C. G. Bell, F. N. Hayes, eds., 2 c.p.m. is indicative of a uranium exPergamon, New York 1958. (5) Korkiuh, J., Thiard, A., Hecht, F., posure no greater than the tolerance Mikrochim. Acta 1950, 1422-30. limit. Because the alpha detection of (6) Neuman. W. F.. I n d . Mcd. 19. 185 ~, uranium by the liquid scintillation (April 1950). method approaches loo%, a lower limit (7) Rapkin, E., “Determination of Radioactivity in Aqueous Solutions,” Packard of detection can be obtained by using a Instrument Co., Inc. Technical Bullelarger urine sample. tin., April 1960. (8) Swank, R. K., “Limits of Sensitivity LITERATURE CITED of Liquid Scintillation Counters. Liquid Scintillation Counting,” C. G. Bell, (11,Davidson, J. TI., Round-Table on F. N. Hayes, eds., Pergamon, New Homogeneous Counting Systems,” LiYork. 1958. quid Scintillation Counting, C. G. Bell, (9)Weiford,-G. A., Morse, R. S., Alercio, F. N. Hayes, eds., Pergamon, New J. S., Am. I n d . Hyg. Assn. J . 21, 6s-70 York, 1958. (1960). (2) Fisher, S., Kunin, R., Proc. Infern. (10) Whitson, T. C., Kwasnoski, T., Zbid., Conf. Peaceful Uses A t . Energy, Geneva, 20, 169-74 (1959). Vol. 8 , p. 291 United Nations, New RECEIVEDfor review February 27, 1962. York, 1956. Accepted July 24, 1962. ( 3 ) Hayes, F. N., Ott, I). G., J,os Alamos 2
=
Solvent Extraction Method for Radiocerium S. FREDERIC MARSH, WILLIAM J. MAECK, GLENN L. BOOMAN, and JAMES E. REIN Afomic Energy Division, Phillips Petroleum Co., Idaho Falls, Idaho
b In this method, radiochemical cerium, in fission product mixtures, is oxidized to the quadrivalent state with bivalent silver and extracted as the tetra-npropylammonium nitratocerate ion-association complex into nitroethane. Cerium is then stripped from the organic phase with hydrogen peroxide-hydrochloric acid and precipitated as cerous oxalate. This method is rapid, safe, and requires a minimum of laboratory technique. It is applicable to aluminum, thorium, zirconium, and stainless steel nuclear fuel matrices and tolerates up to 1 ml. of concentrated nitric, sulfuric, perchloric, hydrochloric, and hydrofluoric acids, and 0.25 ml. of concentrated phosphoric acid. The relative standard deviation is 1 %.
T
HE trend in radiochemical analysis is to faster and more specific separations. -4 simple technique for achieving this goal is solvent extraction. Extraction methods developed to date for radiocerium are generally tedious, requiring several additional steps for adequate specificity. McCown and Larsen (3) report the quantitative extraction of cerium(1V) from nitric acid as the chelate of 2-ethylhexyl phosphoric acid into n-heptane, followed by two perchloric acid fumings for the removal of ruthenium. For young fission product samples, a second extraction from 9M hydrochloric acid is required for the Separation of molybdenum. Glendenin, Flynn, Buchanan, and Steinberg ( I )
1406
ANALYTICAL CHEMISTRY
extract cerium into methyl isobutyl ketone from strong nitric acid, forming a potentially hazardous mixture. For specificity, repeated oxalate precipitations are required. Smith and Moore ( 5 ) report greater than 98y0 extraction of tracer levels of radiocerium with 0.5M 2-thenoyltrifluoroacetone-xylene; however, chloride, fluoride, and phosphate interfere. Recently, the extraction of the elements as quaternary ammonium complexes into methyl isobutyl ketone has been studied (4). The extraction coefficients were reported to be high for those elements that formed univalent anionic complexes. The negligible distribution of cerium(IV), postulated to exist as [HCe(h’O&]- in strong nitric acid, was attributed to its reduction to nonextractable cerium(II1) by the ketonic solvent. The substitution of nitroethane for methyl isobutyl ketone resulted in the complete distribution of the tetrapropylammonium cerium(1V) nitrate complex, forming the basis for the separation of radiocerium reported in this paper. APPARATUS AND REAGENTS
The solvent extractions were performed in 30-ml. separatory funnels. Glass fiber filter paper, type 934-AH (H. Reeve Angel &- Co., Clifton, N. J.) is recommended ( 2 ) for filtering the cerous oxalate precipitate. Reagent grade chemicals were used without purification. Bivalent silver oxide (Handy & Harman, New York) was used in the solid form.
Tetra-n-propylammonium i nitrate (TPAN) reagent. Add 78 ml. of concentrated nitric acid to 500 ml. of 10% ( 0 . 5 M ) tetra-n-propylammonium hydroxide (Eastman Kodak) and dilute to 1 liter with distilled water. Cerium carrier. Dissolve 33 grams of cerous nitrate hexahydrate in 1 liter of 0.01M nitric acid. Standardize as follows: Pipet a 3-ml. aliquot into a 250-ml. beaker and dilute to 150 ml. with distilled water. Add 1 ml. of pyridine, 2 drops of 0.2% cresol red indicator, and sufficient 1:100 arsenazo: sodium chloride mixture to give a reddish purple color to the solution. Titrate with standard 0.05X (ethylenedinitrilo) tetraacetic acid (EDTA) to the yellow-orange end point (6). Nitroethane-n-hexane solvent. Mix 9 volumes of nitroethane (Eastman Kodak practical grade) and 1 volume of n-hexane (Eastman Kodak White Label). PROCEDURE
Chloride - Containing Samples. Pipet a n aliquot of t h e sample into a 50-ml. test tube. Add 1 ml. of cerium carrier and a volume of concentrated nitric acid equal t o t h e sample aliquot. Fume over a burner until evolution of nitrogen dioxide ceases. Continue fuming, if necessary, t o a final volume of 1 ml. Transfer t o a 30-ml. separatory funnel and continue with the General Procedure. Other Aqueous Samples. Pipet 1 ml. of the sample into a 30-ml. separatory funnel. Add 0.5 ml. of concentrated nitric acid. If the sample contains fluoride, add 1 ml. of satu-
