a metalfluorechromic indicator is somewhat offset in the present case. Both indicators are, structurally, rather alike, although thymolphthalexon is a metallochromic, not metalfluorechromic, indicator, containing no fluorescent moiety in its make-up. I n both cases, however, two iminodiacetic acid groups are joined to be dyestuff molecule, fluorescein and thymolphthalein, respectively, both substituent groups being ortho to each of the phenolic hydroxyls. Such compounds chelate (like EDTA) with metal ions, the chelation occurring across the imino nitrogens and the phenolic oxygens. As a result, the intensity or the position of the spectral absorption maxima of the dyestuff nucleus of the indicator is altered. I n the case of the thymolphthexon, this is due most probably, as in the case of phthaleins in general, to the opening of the lactone ring, allowing the triphenylmethane part of the molecule to exist in a form capable of resonance. I n the case of thymolphthalexon, this results in the formation of a deep blue color. When chelation does not occur, the color of the free indicator a t pH 10 is a very faint blue or smoky gray. I n the case of fluorexon,
experiments are still in progress to elucidate the mechanism of the reaction from the standpoint of the type of chelation involved ( I S ) . Practically, however, the end points using this indicator are marked by an appearance or a quenching of fluorescence. Both of the above indicators are stable to oxidizing agents such as nitrate and chlorate in alkaline media. The separation of the manganese as dioxide by the use of potassium chlorate in concentrated nitric acid results in only partial decomposition of the chlorate in the form of chlorine dioxide. However, at the pH’s a t which the titrations are carried out, neither the chlorate nor the nitrate has anv noticeable effect on either of the inkcators. Up to 1 gram of postassium chlorate added directly to the solutions to be titrated was without effect on the colors or end points. ACKNOWLEDGMENT
The author thanks Rudolf Pribil, Czechoslovakian Academy o f Science, for gifts of fluorexon and thymolphthalexon, P. H. Potgieter for carrying out several analyses, and J. P. Coetzee
and the directors of Feralloys, Ltd., for permission to publish this paper. LITERATURE CITED
(1) ANAL.CHEW31, Yo. 8, 39 A (1959). (2) H., Elfingboe, J. L., Ibid., 28, 882 (lg5@. (3) Hildebrand, G. P., Reilley, C. N., Ibid., 29, 258 (1957). (4) Hillebrand, W. F., Lundell, G. E. F.,
Bright, H. A., Hoffman,J. I., “Applied Inorganic Analysis,” 2nd ed.7 LvileY, New York, 1953. (5) Kolthoff, 1. lt., sandell, E. B,, “Textbook of Quantitative Inorganic Analysis,” Macmillan, Nen- York, 1937. (6) Korbl, J., Pribil, R., Chem. c% Ind. (London) 1957,233. ( 7 ) KBrbl, J , , pribil, R.,Chena. listy sl, i~n4 ii 9.57). ---\---.,. (8) Leks, L. L., LIelnick, L. LI., ANAL.
CHEM.32,38 (lgGo). (9) Lott, P. F., Cheng, K. L., Chemist
~~~l~~~ 46,30 (1957).
(10) Pribil. R.. “Komolexometrie.” o. I
1
47, Chemapol; Prague,’1954. (11) Pribil, R., Kopanica, M., Chemist Analyst 48,35 (1959). (12) Reillev, C. N., ANAL. CHEM.32, 2 (1960):’ (13) Vydra, F., Pribil, R., Korbl, J., Collection Czechoslov. Chem. Communs. ‘
24, 2633 (1959).
RECEIVEDfor review June 9, 1960. Accepted Sovember 30, 1960.
The Radiochemical Determination of Promethium-147 in Fission Products R. D. BRITT, Jr. Savannah River laboratory, E. I. du font de Nemours &
b A solvent extraction method was developed for the determination of promethium-1 47 in solutions of mixed fission products derived from irradiated natural uranium. Di(2-ethylhexy1)orthophosphoric acid diluted with an inert diluent (Ultrasene) was used as the extractant. After separation, the promethium-147 activity was determined b y liquid scintillation counting. The precision of the method, based on six determinations using promethium1 4 7 tracer, was 3.2% a t the 95% confidence limit.
P
such as ion exchange (2, S), for the determination of promethium-147 in the presence of other fission products are time consuming and not readily adaptable to routine work. Gravimetric methods of analysis (a), in which a precipitate is counted for beta activity, are subject to errors when the beta energy of the emitting nuclide is low, as is the case for promethium-147. Peppard et al. (6, 7 ) reported the use of UBLISHED METHODS,
602
ANALYTICAL CHEMISTRY
Co., Aiken, S. C.
di(2 - ethylhexy1)orthophosphoric acid (HDEHP) for the separation of the lanthanides and yttrium. The distribution coefficient for an individual lanthanide was shown to be directly proportional to the third power of the H D E H P concentration and inversely proportional to the third power of the hydrogen ion concentration. The separation factor between adjacent lanthanides was reported to be 2.5. illcCown and Larsen (6) used H D E H P for the separation of cerium from fission products. A solvent extraction method was developed that uses H D E H P diluted with an inert diluent, Ultrasene, to separate promethium-147 from nitric acid solutions of fission products derived from irradiated natural uranium. The uranium was irradiated for the purpose of producing plutonium-239 and was cooled between 120 days and 5 years. However, it should be possible to use the method for the analysis of promethium-I47 in older samples or in other fissionable material providing
the difference in fission product composition is considered. The method consists of four extraction steps in which the nitric acid and H D E H P concentrations are varied in order to separate promethium from cerium, yttrium, and other fission products. Under the conditions of the first extraction step, yttrium, with a distribution coefficient of 25, is extracted into the organic phase while cerium(II1) and promethium, whose distribution coefficients are 0.02 and 0.05, respectively, remain in the aqueous phase. I n the second extraction step, a separation from cesium and strontium is obtained by extracting cerium(II1) and promethium into the organic phase after the acidity of the aqueous phase is reduced by the addition of sodium hydroxide. The distribution coefficients for cerium and promethium in this step are > l O 3 . I n the third extraction step, promethium is stripped from the organic phase into IOM nitric acid, while cerium is oxidized with sodium bromate to cerium(1V) and retained quan-
titatively in the organic phase. The organic phase is diluted with additional Ultrasene in order to lower the distribution coefficient of promethium. The last extraction step removes any cerium that had been entrained in the aqueous phase during the previous step and gives a better decontamination from cerium. Promethium-147 is determined in an aliquot of the final aqueous phase by liquid scintillation counting. EXPERIMENTAL
Apparatus and Reagents. Promethium-147 p counting was done with a Packard Tri-Carb liquid scintillation counter, Model 314, which is a two-channel p pulse height analyzer. The scintillator solution mas prepared by adding 110 grams of recrystallized naphthalene, 4 grams of 2,5-diphenyloxazol, 0.05 gram of p-bis(2,5-phenyloxazol) benzene, and 70 ml. of distilled water t o 1 liter of dioxane. The di(2-ethylhexy1)orthophosphoric acid was obtained from Union Carbide Chemicals Co. Dilutions of this material XTere made with Ultrasene. In step 1, use 0.75M H D E H P that has been equilibrated three times with equal volumes of 3% hydrogen peroxide prepared by diluting 30y0 hydrogen peroxide with 1.65J1 nitric acid. The peroxide-scrubbed H D E H P is stable and may be prepared in advance. In step 2, use 1.5M H D E H P that has been scrubbed twice with equal volumes of 10X nitric acid that is 1M in sodium bromate, and then scrubbed once with distilled watw. Procedure. STEP 1. Pipet 2 ml. of 1.65M nitric acid into a 15-ml. conical centrifuge tube and add a n aliquot of the sample containing an estimated 5 X 106 d.p.m. (disintegrations per minute) of promethium-147. Add 2 ml. of the 0.7531 H D E H P , mix the two phases for 2 minutes, and centrifuge. Transfer the aqueous phase to a 15-ml. conical centrifuge tube. STEP 2. Add 3 ml. of l N NaOH. Add 5 ml. of 1.5111 HDEHP, mix the two phases for 2 minutes, and centrifuge. Discard the aqueous phase and wash the organic phase rvith 3 ml. of 0.05M nitric acid. STEP 3. Dilute the organic phase with 5 ml. of Ultrasene. Add 10 ml. of 101V1 nitric acid and 1 ml. of 1 A f sodium bromate. Mix the phases for 2 minutes and centrifuge. Transfer the ' aqueous phase to a 40-ml. conical centrifuge tube. STEP4. Add 5 ml. of 0.75111 H D E H P that has been scrubbed twice with equal volumes of 1OM nitric acid that is 1-11 in sodium bromate. Centrifuge and transfer the aqueous phase to a 50-ml. centrifuge tube. STEP 5 . Add 500 pl. of 30% hydrogen peroxide to reduce the excess bromate. (Bromate causes a large decrease in counting efficiency.) Heat to near boiling and allow to stand for 5 minutes. Heat to near boiling again in order t o drive off bromine. Cool
A000
3000
-
= s,
..
2000
s
1000
n
Figure 1 . Beta spectra of promethium-1 47 and rhodium106 in a liquid scintillator
and add 25 pl. of this solution to 15 ml. of scintillator contained in a '/Z-ounce polyethylene bottle. Count as described below. Preparation of Calibration Sources. RUTHENIUM.The source was prepared in order to correct for the ruthenium interference. This interference will be discussed under Results. The ruthenium was distilled from a sulfuric acid-sodium bismuthate mixture into sodium hydroxide, and ruthenium oxide was precipitated with ethyl alcohol (4). The precipitate was dissolved in 6JI nitric acid and an aliquot of this solution was added t o the scintillator and counted. The correction factors for ruthenium were obtained as described in the section on Counting. PROMETHIUM. The efficiency of the scintillator for promethium counting was determined by combining 10 ml. of 10-If nitric acid with 1 ml. of 1M sodium bromate and following step 5 of the procedure. Before counting, a 25pl. aliquot of a promethium-147 standard was added to the scintillator. This aliquot was 0.1M in nitric acid. Counting. The spectra of promethium-147 and rhodium-106 are shown in Figure 1. These spectra were obtained by using the analyzer, with discriminators B and C set 10 volts apart, as a single-channel pulse height analyzer. The fraction of each nuclide t h a t appears in the red and green channels can be expressed by two linear simultaneous equations.
The solution of these tn-o equations for promethium-147 is:
where 111,M2 = mixture c.p.ni. in red and green channt.ls, respectively, - background Fii
-
Pm std. c.p.m. in red channel - background Pm std. c.p.m. in both channels - background
Fgi
= 1
- F11
F??
-
Ru std. c.13.m. in nreen channel 2 backgGund Ru std. c.p.m. in both channels - background
F12
=
E
-
1 - Fzz Pm std. c.p.m. in both channels - background Pm std. d.p.m.
With the settings given below, the value for E in Equation 1 was 30y0 and the values for F11, Fzl, F 2 2 , and FIZ, were, respectively, 0.80, 0.20, 0.90, and 0.10. The value obtained from Equation 1 was multiplied by 1.05 to correct for losses in the first extraction step due to the promethium distribution coefficient. VOL. 33, NO. 4, APRIL 1961
603
The settings of the analyzer controls used in this work were as follows: High voltage, 900 volts A and B amplifiers, position 1 AA' discriminator, 10 volts B discriminator, 30 volts C discriminator, 100 volts Analyzer mode switch, position 2 Freezer temp., 5' C. RESULTS
Di (2-ethylhexyl) ort hophosphori c acid gave distribution coefficients similar to those obtained with highly purified H D E H P . Three different batches of H D E H P were used during the development of this method and all three batches gave the same separation of promethium from yttrium. The distribution coefficients for cerium. promethium, and yttrium under the various conditions of acid and HDEHP concentration found in the method are listed in Table I. Table I. Distribution Coefficient for Ce(lll), Ce(lV), Pm, and Y between Nitric Acid and HDEHP Diluted with Ultrasene
Step 1, 1.65M HNOg
Step 2, 0.1M "03-
Steps 3 and 4, 10M HKOr
0.75M 1.5M 0.75M Element HDEHP HDEHP HDEHP Ce(II1) 0.02 >io3 ... Ce(1V) ... ... > 103 Pm 0.05 >io3
