Under the conditions of their titration, above p H 12, magnesium hydroxide would be present as a precipitate and available for adsorption of calcium and, according to the suggestion of Bond and Tucker ( 2 ) , would result in interference. As no interference occurred, it is unlikely that magnesium interferes by such a mechanism. The major interfering mechanism a t high p H might be the competition between calcium and magnesium for the ammonium purpurate. Should the magnesium be competing TVith calcium for purpurate complex formation, this interference would be negligible a t p H 11.3. At this pH, an isobestic point exists a t 505 mp ( 7 ) , where the absorption spectral curves for ammonium purpurate in the presence and absence of magnesium intersect. Tammelin and Rlogensen (6) reported a n isobestic point a t 510 mp a t a p H of about 11.
An experiment run a t 505 mp and p H 11.3using a 10-7calcium aliquot yielded means for four and three replicates, respectively, of 109.4 =k 2.47, and 110.8 =k 2.47, a t magnesium to calcium ratios of 5 and 10, respectively. Although under these conditions the competition of magnesium with purpurate may have been eliminated as a source of interference, the problem mas not solved. The interference caused by the competition of magnesium with EDTA remained. ACKNOWLEDGMENT
The authors wish to thank Oscar 4. Iseri and Paul L. Alunson for perniission to incorporate the details of their analytical procedure prior to full publication. They also wish to thank RIichaela Thompson for first drawing the question of magnesium interference to their attention, Mindel C. Sheps for
advice on the statistical aspects of the paper, and Virginia h'lellor for valuable technical assistance. LITERATURE CITED
( 1 ) Blaedel, IT. J., Knight, H. T., A s . 4 ~ . CHEL 26, 743 (1954). ( 2 ) Bond, R. D., Tucker, B. hZ., Chem. Le. Ind. (London) 1954, 1236. (3) ~, Diehl. H.. Ellinrboe. J. L.. ANAL. &EN. 28. 882 T1956). Kenny, -1.D Toverud, S. U.,Zbid., 26,1059 (19b4).
MLinson, P. L., Iseri, 0. A., Kenny, A. D., Colin, V., Sheps, 11. C., J . Dental Research 34, 714 (1955). Tanimelin. L. E.. Mogensen. S..' Acta Chem. Scand: 6, 98g (1952). Williams, 11.B., Moser, J. H., ASAL. CHEM. 25, 1414 (1953).
RECEIVED for review September 12, 1958. Accepted April 9, 1958. Supported in part by research grant D-203 from the National Institute of Dental Research of the National Institutes of Health, U. S. Public Health Service.
Radiochemical Determination of Neptunium-239 and Plutonium-239 in Homogeneous Reactor Fuel and Blanket Solutions FLETCHER L. MOORE Oak Ridge National Laboratory, Oak Ridge, Tenn. ,The radiochemical determination of neptunium-239 and plutonium-239 in uranyl sulfate fuel and blanket solutions is based on the carrying of these two isotopes by lanthanum fluoride followed by liquid-liquid extraction of the neptunium-239 with 2-thenoyltrifluoroacetone-xylene. Use of a single container for all separations minimizes losses.
T
HE radiochemical determination of neptunium-239 and plutonium-239 is required in connection rvith the homogeneous reactor program to determine the physical and chemical behavior of these elements in uranyl sulfate fuel and blanket solutions under pile irradiation. Previous radiochemical methods for the determination of neptunium239 (3) and plutonium-239 (4) were developed for application to nitrate or chloride systems, with free sulfate being a serious interference. Although the lanthanum fluoride technique ( 1 ) may be applied to solutions containing free sulfuric acid, the separation of neptunium and plutonium is inadequate. This paper describes the recently de-
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ANALYTICAL CHEMISTRY
veloped methods for the radiocheniical determination of neptunium-239 and plutonium-239 in pile-irradiated uranyl sulfate fuel and blanket solutions. These methods combine the better features of previous techniques (1, 3, 4). As depleted uranium was used, essentially all gamma radioactivity was due to neptunium439 nhich decays with a 2.33-day half life t o the alpha emitter, plutonium-239. The methods are based on the carrying of neptunium-239 and plutoniuni239 on lanthanum fluoride (1). dissolution of the precipitate, and liquidliquid extraction of the neptunium-239 Fvith 2-thenoyltrifluoroacetone-xylene. 411 precipitation and extraction steps are performed in the same centrifuge cone to minimize losses. A high-speed motor stirrer (Palo Laboratory Supplies, S e w York, N.Y.) with a glass paddle gives excellent mixing of the phases. NEPTUNIUM-239
Prepare necessary dilutions in 231 nitric acid. If a yield correction is to be applied, pipet a known amount of neptunium-237 in 2111 nitric acid solution into a 1% or 15-ml. Pyrex centri-
fuge cone (Corning No. S120 or 8060, respectively). Add a suitable aliquot of the sample dilution. Add 0.4 ml. of concentrated hydrochloric acid, 0.1 ml. of zirconium holdback carrier (10 mg. per ml.), 0.1 nil. of lanthanum carrier (5 mg. per nil.), and mix well. Add 0.3 ml. of 531 hydroxylamine hydrochloride and 0.3 ml. of 27W hydrofluoric acid. Stir well with a platinum stirrer. and digest for 5 minutes a t room temperature. Centrifuge for 3 minutes in a clinical centrifuge. Add 0.05 ml. of lanthanum carrier and stir the supernatant, being careful not to disturb the precipitate. Digest for 5 minutes a t room temperature. Centrifuge for 3 minutes, and carefully remove the supernatant b y use of a transfer pipet that is attached to a vacuum trap. Wash the precipitate by stirring 11ith a 0.5-nil. portion of 1-11 nitric acid-Llf hydrofluoric acid. Centrifuge for 3 minutes and carefully remove the supernatant, leaving approximately 1drop. Dissolve the lanthanum fluoride precipitate by adding 0.2 ml. of 2 X aluminum nitrate solution and 1 ml. of 4;M hydrochloric acid. Add 0.8 ml of 5 V hydroxylamine hydrochloride and 0.5 nil. of 231 ferrous chloride, freshly prepared in 0.2X hydrochloric acid every 2 Teeks. Adjust to approxi-
mately 4 ml. by adding distilled water. Mix well, and extract for 5 minutes mith a n equal volume of 0.5N 2-thenoyltrifluoroacetone-xylene. Wash the stirrer with a small portion of xylene from a mash bottle, collecting the 11-ashes in the centrifuge cone. Centrifuge for 1 minute. Draw off the aqueous phase, being careful not to lose any of the organic phase. Wash the sides of the centrifuge tubes with approximately 2 ml. of distilled Ivater, centrifuge for 1 minute, and draw off the aqueous phase carefully. Strip the neptunium-239 from the organic phase by mixing thoroughly with a n equal volume of 10-If nitric acid for 2 minutes. Centrifuge for 1 minute, and draw off the organic phase with mild suction. Count a n aliquot of the 10M nitric acid strip solution for neptunium239 gamma radioactivity in a sodium iodide (thallium-activated) well-type scintillation counter. If a yield correction is to be applied, dry a n aliquot of the strip solution on a platinum or stainless steel disk, flame to a red heat, cool to room temperature, and count the neptunium-237 alpha radioactivity in a proportional alpha counter. PLUTONIUM-239
If a yield correction is to be applied, pipet a known amount of plutonium238 in 2iM nitric acid solution into a 12- or 15-ml. Pyrex centrifuge cone. Follow the procedure for neptunium239 through the extraction step, but omit the zirconium holdback carrier, and use 1 ml. of saturated boric acid solution in place of the 2M aluminum nitrate solution. Repeat the extraction with a fresh portion of organic solution, if necessary. Each extraction removes approximately 90% of the neptunium. Wash the stirrer with a small portion of xylene from a wash bottle, collecting the wash solution in the centrifuge tube. Centrifuge for 1 minute. Dram off most of the organic phase with mild suction and discard, being careful not to lose a n y of the aqueous phase. Add several milliliters of xylene from the wash bottle, and carefully . draw off most of the organic phase. Add 0.5 ml. of 27-11 hydrofluoric acid to precipitate lanthanum fluoride. Stir well, and digest for 5 minutes at room temperature. Centrifuge for 3 minutes, and remove the supernatant. Wash the precipitate twice n ith 0.5 ml. portions of 1111' nitric acid-lAli hydrofluoric acid. Centrifuge each time for 3 minutes. Slurry the lanthanum fluoride precipitate with several drops of distilled water, and transfer quantitatively to a platinum or stainless steel plate. Rinse the cone with three 3-drop portions of distilled water, transferring the rinses to the plate. Evaporate to dryness under an infrared heat lamp, flame the
plate to a red heat, allow to cool to room temperature, and count the plutonium-239 alpha radioactivity in a proportional alpha counter. If a yield correction is to be applied, perform a n alpha energy pulse analysis for plutonium-238 and plutonium-239 and apply the appropriate yield correction for plutonium-239. DISCUSSION
Although the preliminary precipitation of lanthanum fluoride to carry the neptunium-239 and plutonium-239 may not be necessary, it is desirable. The acidity of most of the homogeneous reactor solutions is in the approximate p H range from 1 t o 2, a t which there is a marked tendency for neptunium and plutonium to undergo hydrolytic polymerization. The hydrolytic species are inextractable with 0.5M 2-thenoyltrifluoroacetone-xylene. However, they may be depolymerized with hydrofluoric acid and then carried on lanthanum fluoride ( 2 ) . A preliminary precipitation of lanthanum fluoride frees the neptunium-239 and plutonium-239 from the bulk of the corrosion products such as iron, cobalt, nickel, zirconium, and niobium, rt-hich may interfere in the subsequent liquid-liquid extraction step. Also, free sulfate interference is eliminated, and the extraction acidity may be controlled more efficiently by dissolution of the small quantity of lanthanum fluoride in the appropriate acid. I n the neptunium procedure, radiozirconium is eliminated in the lanthanum fluoride precipitation and the nitric acid stripping steps. Decontamination from radiozirconium has been excellent. However, if solutions of high zirconium-neptunium ratio are to be analyzed, the last traces of radiozirconium may be removed by performing a 5-minute re-extraction of the 10M nitric acid strip solution with an equal volume of 0.5M 2-thenoyltrifluoroacetone-xylene. The methods described give neptunium-239 and plutonium-239 recoveries of 99 + 3%, !Then carefully applied to process solutions. Septunium-237 and plutonium-238 may be introduced in the first step of the respective procedures and appropriate yield corrections applied as noted in the procedures. Obviously, negligible amounts of these two alpha emitters must be shown as originally present in the solutions analyzed, if they are used to indicate yields. Typical recoveries of neptunium-239 and plutonium-239 are shon-n in Table I.
Synthetic uranyl sulfate reactor solutions were analyzed. Similar yields have been obtained on actual reactor solutions. Aluminum nitrate \vas more effective than boric acid for complexing the fluoride ion under the conditions used. Therefore, aluminum nitrate is used in the neptunium-239 procedure to prevent inhibition of the extraction of the neptunium by fluoride. However, boric acid must be used in the plutonium-239 procedure, as the carrying of plutonium on lanthanum fluoride was erratic in the presence of aluminum even in the presence of excess hydrofluoric acid. Recovery values for plutonium are approximately 2% lotver for stainless steel plates than for platinum plates (Table I). This occurs because of slight fluoride etching of the stainless steel plates, n-ith a resulting increase in self-absorption of the alpha particles along m-ith slightly less back-scattering from stainless steel plates.
Table I.
Recovery of Neptunium-239 and Plutonium-239 NeptuniumPlutonium239 239 Recovered, Recovered, % Plate used % 99.3 99.1 Platinum 98.7 98.5 Platinum 99.8 99.3 Platinum 98.4 99.2 Platinum 98.2 98.8 Platinum 98.1 99.1 Stainless steel 95.8 98.8 Stainless steel Stainless steel 97.3
ACKNOWLEDGMENT
The author gratefully acknovledges the able assistance of H. TT'. Kright in the preparation of some of the synthetic solutions and of c. E. Lamb and M.. J. Gaitanis for assistance in testing the methods described. LITERATURE CITED
(1) Koshland, D. E., U. S. Atomic Energy Comm. Declassified Rept. CN-2041
(January 1945). (2) Kraus, K. &4.,Howland, J. J., U. S. Atomic Energy Comm. Secret Rept. CN-1764 (July 1944). (3) . , Moore. F. L.. ANAL. CHEX 29. 941 i1957). ' (4) Moore, F. L., Hudgens, J. E., Jr., Ibid., 29, 1767 (1957). RECEIVED for review January 6, 1958. llccepted April 3, 1958. Oak Ridge Sational Laboratory is operated by Union Carbide Kuclear Co. for the U. S. Atomic Energy Commission.
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