Radiochemical Determination of Molybdenum by Solvent Extraction WILLIAM J. MAECK, MAXINE ELLIOTT KUSSY, and JAMES E. REIN Atomic Energy Division, Phillips Petroleum Co., Idaho Falls, Idaho
b This method for the determination of molybdenum-99 is based on the extraction of the a-benzoinoxime complex into ethyl acetate with subsequent stripping into dilute hydrochloric acid. The extraction circumvents the inconvenient perchloric acid wet oxidation of the precipitated complex as used in prevailing methods. Advantages of the proposed method are excellent decontamination, decreased analysis time, and greater safety. The half life value of molybdenum-99 based on this method is 66.55 5 0.22 hours.
F
molybdenum-99 is often determined in young fission product midures to measure the number of fission events that have occurred. It is used to monitor fuel element fission breaks in reactor coolant waters. Generally, molybdenum-99 is preferred over other fission products for measuring fissions because a sufficiently long half life (66.5 hours) minimizes decay corrections; beta counting is not too difficult due to a decay by emission of moderately energetic beta particles; a well known decay scheme allows the usc of gamma spectrometry in preference to beta counting; a nongaseous precursor; a high and well known yield of 6.06% ( 4 ); and the absence of other long lived molybdenum isotopes which interfere in counting. A discussion of the radiochemistry of molybdenum by Scadden and Ballou ( 8 ) indicates that while several solvent extraction methods are available for small quantities of natural molybdenum in various matrices such as soils, steels, and alloys, limited information is available concerning the extraction of radioactive molybdenum. Extractions of radioactive molybdenum as the thiocyanate ( I O ) and chloride (9) conipleves have been reportc,d. The high specificity of oc-benzoinoxime as a gravimetricreagent for molybdenum has long hem knon-n (6). One of the most commonly used radiochemical determinations of molybdenum-99 is based on multiple precipitations of the oxime complex ( 7 ) . followed by nitric and perchloric acid wet oxidation of the complex. This procedure is inconvenient and lengthy, and can be hazardous. ISSIOX PRODUCT
A solvent extraction procedure for radiochemical molybdenum which retains the selectivity of the molybdenum-a-benzoinoxime complex and avoids a perchloric acid wet oxidation is reported. APPARATUS
Precipitates mere collected on 25-nini. hlillipore filters, type HA, pore size of 0.45 micron, and mounted on 3 X 4 inch aluminum plates. A 3 x 3 inch NaI(T1) cylindrical scintillation crystal or a 3 x 3 inch NaI(T1) well crystal coupled to a 256channel pulse height analyzer was used for all gamma spectrum analyses. Beta counting was done with a gas flov proportional counter. REAGENTS
Reagent grade inorganic chemicals and Eastman Kodak Co. White Label a-benzoinoxime were used without purification. Fission product mixtures were prepared by irradiating highly enriched uranium in the Materials Testing Reactor. Prepare the molybdenum carrier by dissolving 18.5 grams of (~H4)&10& . 4H10 in 100 ml. of water. adding. 1 ml. of 0.5X NaBr03, and diluting to-1 liter with 6M HC1. Standardize the carrier bv adding. 5 ml. of the carrier solution t i a 250-&l. beaker containing 50 ml. of water and 25 ml. of SOYo ammonium acetate. Heat the solution to boiling and add a Pb(N03)* (50 mg./ml.) solution dropwise until precipitation is complete. Boil 2 to 3 minutes and check for complete precipitation by adding more Pb(S03)2. -4djust to p H 4.5 and quantitatively filter through a tared medium-porosity crucible. Transfer and wash with 15 ml. of 3% ammonium nitrate. Dry to constant weight a t 110' C. in an oven. PROCEDURE
High Activity Samples Such a s Fission Product Mixtures. Transfer an aliquot of the sample to a 50-ml. centrifuge tube. Add 3 ml. of molybdenum carrier, 1 ml. of iodine holdback carrier (10 mg.jm1. KI), 1 ml. of 0.05M K X n 0 4 , 10 mg. of iron(II1) scavenger, dilute to 20 ml., and heat to boiling. Add NHIOH to precipitate iron and manganese. Centrifuge. Decant or filter into a 125-m1. separatory
funnel, add 5 ml. of 1 O N HCl, 10 ml of a-benzoinoxime (2% in 95% ethyl alcohol), 30 ml. of ethyl acetate, and extract for 2 minutes. Discard the aqueous phase, scrub for 2 minutes with 25 ml. of 1N HC1, and again discard the aqueous phase. Add 15 ml. of 4N KH40H and strip for 5 minutes or until the organic phase is clear. Transfer the aqueous strip to another 125-mi. separatory funnel and add 10 ml. of ION HCl. Repeat the extraction starting with the addition of 10 ml. of a-benzoinoxime reagent. Again strip and transfer the strip phase to a 50-ml. centrifuge tube. Add 5 ml. of concentrated HN03, 3 ml. of Pb(KOJ2 (50 mg./ml.) solution and cool. ildd a few drops of methyl red indicator and N H 4 0 H until just yellow to methyl red, followed by 1 ml. of glacial acetic acid. Centrifuge and decant the supernatant solution. Wash the precipitate with water and filter on a tared hlillipore filter. Wash with water and dry for 10 minutes in an oven at 110' C. Low Molvbdenum Concentration Water Samples. Transfer 200 ml. of sample to a 400-ml. beaker. Add 3 ml. of molybdenum carrier, 1 ml. of iodine holdback carrier (10 mg 'ml. K I ) . 1 ml. of 0.05M K111n04. 10 mg. of iron(II1) scavengei, heat to boiling, and transfer to a 250-ml. centrifuge tube. -$dd concentrated ?;H40H to precipitate the iron and manganese. Centrifuge and filter through coarse paper into another 250ml. centrifuge tube. Add a few drops of methyl red indicator and make just acid with HSOs. Add 3 ml. of Pb(NO& (50 mg. 'ml.) solution, make just alkaline to methyl red with T\",OH, then add 1 ml. of glacial acetic acid. Centrifuge and discard the supernatant solution. Dissolve the precipitate in 5 ml. of 10Ar HCl and dilute to 25 ml. with water. Transfer to a 125-ml. separatory funnel, add 10 ml. of CYbenzoinoxime (2% in 95% ethyl alcohol) 30 ml. of ethyl acetate, and continue as stated above for high activity samples. EXPERIMENTAL AND DISCUSSION
Extraction of the molybdenum-abenzoinoxime complex into chloroform followed by either an in situ spectrophotometric analysis ( I ) or a perchloric acid m-et oxidation of the organic phase (3) has been reported for the determination of nonradioactive molybdenum. Because this extraction is made from strong acid, an alkaline VOL. 33, NO. 2, FEBRUARY 1961
237
I o5
Table I. Half Life of Molybdenum-99 Obtained by Proposed Method
Sample 1 2
3 a
Half Life (Hours) 66.53 =!= 66.44 =!= 66.69 Av. 66.55 =!=
0.18' 0.17 0.29 0.22
ANALYTICAL CHEMISTRY
Zr, Nb
I o4
Absolute standard deviation.
solution strip was postulated to strip the molybdenum. This proved valid. However, the reagent itself stripped as a n insoluble precipitate which interfered with subsequent chemical treatments and probably achieved little decontamination. The substitution of ethyl acetate for chloroform overcame this problem. Pretreatment before extraction involves adding permanganate to the sample plus carrier to ensure chemical identity, iodine holdback to decrease the extraction of iodine activity, and an iron(II1) scavenging. The double extraction gives a high purity molybdenum fraction as shown in Table I and Figure 1. If speed of separation is paramount and small amounts of iodine activity can be tolerated, the second extraction may be omitted. Molybdenum is finally isolated as lead molybdate. The over-all yield of the method is 75%. A solid precipitate was employed for the yield determination to facilitate simple beta proportional counting; however, if sufficient activity is available, it is reasonable to gamma count the final strip phase in a well crystal measuring the 750-k.e.v. gamma peak. Gamma transitions through this level are reported as 14 per 100 disintegrations (6). A simple spectrophotometric yield determination would decrease errors inherent in a gravimetric yield determination. The procedure was evaluated by carrying three aliquots of 18-day-old fission products through the method. The gamma spectrum of the original sample and the molybdenum fraction 18 hours after the final separation as lead molybdate are shown in Figure 1. The decay of the separated molybdenum mas followed by gross beta counting for approximately 11 half lives, and the data were subjected to a regression analysis with results as given in Table I.
238
TC- MO
SAMPLE
> k I I-
'V
5: 103 W
I Ia 1 W LL
Mo-PRODUCT 18 HOURS AFT E R SEPARATION I d
IO
02
Figure 1.
0.4
0.6
08 1.0 1.2 ENERGY -(M.E,Y)
1.4
1.6
1.8
2.0
Distribution of Fission Product Activity in Method
The mean half life value of 66.55 i 0.22 hours can be compared to recent
literature values of 66.00 ( 2 ) and 66.96 hours (11). Precision of the method was established by carrying six aliquots of a 5day-old fission product mixture through the procedure and analyzing the 750k.e.v. gamma peak. A standard deviation of 2.4y0 for a single determination was obtained. Time required for the single analysis of a high activity sample is appro\imately 1.2 hours. ACKNOWLEDGMENT
The authors appreciate the statistical evaluation by Lewis B. Hansen. LITERATURE CITED
( 1 ) Goldstein, G., Manning, D. L., Menis, Oscar, A i v . 4 ~ . CHEST.30, 539 (1958).
( 3 ) Gunn, S. R., Hicks, H. G., Levy, H. B., Stevenson, P. C., Phys. Rev. 107, 1642 (1957). (3) Jeffrey, P. G., Analyst 81, 104 (1956). 14) Katcoff. Sevmour. Nucleonics 16, No. 4, 78 (1958). (5) Knowles, H., Bur. Standards J . Research 9, 1 (1932). (6) Miller, C. F., U. S. Atomic Energy Comm. Rept. USNRDL-TR-160 (1957) (7) Scadden, E. X,A'ideonzcs 15, No. 4, 102 (1957). (8) Scadden, E. M.,Ballou, X. E., U. S. Atomic Comm. Rept. NAS-NS-3009 (1960). (9) \Tiles, D., Coryell, C., Phys. Rev. 96, 696 (1954). (10) Wilkinson, G., Grummitt, W., S u cleonics 9, 90. 3, 52 (1951). (11) Kright, H. W., Wyatt, E. I., Reynolds, S. A , , Lyons, W. S., Handley, T. H., Suclear Scz. Eng. 2,427 (1957).
RECEIVEDfor review August 15, 1960. Accepted Sovember 7, 1960. Division of Analytical Chemistry, 138th Meeting, ACS, Kew York, N. Y., September 1960. Rork performed under Contract No. AT( 10-1)-205, for the U.9. ;Itomic Energy Commission.
