Preparation of Protactinium-233 Tracer CLAUDE W. SILL Health and Safety Division, U. S. Atomic Energy Commission, Idaho Falls, Idaho
b Three new procedures are presented for preparation of protactinium-233 tracer. One procedure employs the widely-used extraction with diisobutylcarbinol to separate the protactinium-233 from irradiated thorium. The other two procedures involve milking the protactinium-233 from a solution of its long-lived neptunium-237 parent, employing either diisobutylcarbinol extraction or precipitation on barium sulfate. The procedures are much simpler and more convenient than most others currently available and overall recovery of tracer is about 987& Sulfuric acid and sodium sulfate are used to dissolve the protactinium ond keep it in solution through the separations and in the final tracer solution. Use of hydrofluoric acid is not required. Solutions so prepared are stable for long periods of time and can be reextracted or reprecipitated without repeating other preparatory treatment. Under the recommended conditions, not a single example of the capricious and erratic behavior generally attributed to protactinium has ever been observed. In fact, the procedures should be examined as much for their contribution to the fundamental chemistry of protactinium as to the preparation of the tracer itself and are directly applicable to the radiochemical determination of both protactinium-23 l and -233.
T
chemistry of protactinium is generally more difficult and unpredictable than that of most other elements primarily because of the ease with which protactinium hydrolyzes in the absence of adequate complexing agents and the insolubility of the product formed. One author (2) has expressed the opinion that the present decade “will see much of the mystery and witchcraft eliminated from protactinium chemistry.” This objective would be facilitated greatly if a convenient radioactive tracer were more easily available to a larger number of potential investigators. A gammaemitting nuclide is desirable to permit direct counting of liquids and solids in a scintillation well counter without the time-consuming separations to eliminate absorbing materials that are necessary when pure alpha- or beta-emitting tracers are employed. HE
1458
ANALYTICAL CHEMISTRY
Protactinium-231 emits fairly energetic gamma rays in moderate abundance and has been used extensively as a tracer. However, this nuclide is a longlived alpha-emitter with a half-life of 3.25 X lo4 years and presents a significantly greater health hazard than is present with short-lived beta-gamma emitters, particularly when sufficient activity is used to give high statistical precision in short counting times. Also, the low specific activity of 22 pg./gc. is undesirable in some situations. Perhaps the best tracer and the one most widely used to date is protactinium233. This nuclide has a half life of 27.4 days and emits a gamma ray a t 0.31 m.e.v. and a uranium x-ray a t 0.103 m.e.v. that are relatively abundant and easily detected. Furthermore, it is easily prepared in good yield by irradiation of natural thorium with thermal neutrons with a cross section of 7.4 barns. However, neutron sources of sufficient intensity are not readily available to most laboratories and repeated irradiations are required to sustain a continuing research program of significant size. On the other hand, protactinium-233 is also produced by radioactive decay of alpha-emitting neptunium-237. Furthermore, one half of the maximum equilibrium activity of protactinium-233 will grow back into the parent in 27.4 days. Because the neptunium-237 parent has a half life of 2.20 x 106 years, a continuing and virtually inexhaustible supply of carrierfree, gamma-emitting tracer of high specific activity is immediately available off-the-shelf by milking a solution of the neptunium parent. Neptunium237 is available from Oak Ridge National Laboratory, Oak Ridge, Tenn., a t reasonable cost. Previous work performed in this laboratory (6, 8) has shown that probably all positive ions having a charge greater than 2 and an ionic radius larger than about 1.1 a. u. (Goldschmidt values) can be carried quantitatively in quantities up to about 1 mg. on barium sulfate if approximately 35 mg. of barium is used and high concentrations of sulfates are present in a volume of about 75 ml. Elements carried most efficiently include thorium, bismuth, lanthanum and the light lanthanides, actinium, uranium(IV), and the transuranium elements. If potassium is also present, the deleterious
effect of each element carried on the recovery of any other is markedly reduced. From the size and charge of the ion, protactinium would also be expected to be carried efficiently if the ion is unhydrolyzed. In this investigation, this prediction is confirmed since protactinium is shown to be precipitated to about 99.9% under the same conditions used in the previous work. Moore and Reynolds (4) reported only 70 to 80% of the protactinium-233 tracer was precipitated with barium sulfate, but they used only 1 to 2 mg. of barium in 1N sulfuric acid. Thus, in addition to bismuth, lanthanum and the lanthanides, lead, and the alkaline earths, small quantities of all elements having atomic numbers above and including that of radium can be carried efficiently on barium sulfate. On the other hand, elements such as uranium, neptunium, and plutonium that form large oxygenated cations of low charge in their highest oxidation states can be prevented very effectively from precipitating by oxidizing them to the higher state. Indeed, the fact that the large protactinium ion is carried so efficiently on barium sulfate is substantial evidence that the pentavalent element is present in strong sulfate solutions as an unoxygenated sulfate complex similar to the other ions carried. Because of the pronounced tendency of protactinium to hydrolyze in the absence of suitable complexing agents, ’iydrofluoric acid is widely used to produce stable homogeneous solutions. However, the hydrofluoric acid must be removed either by evaporation with a less volatile acid or by masking with large quantities of fluoride-complexing agents such as aluminum or boric acid before other characteristic reactions can be obtained. Also, the incompatibility of hydrofluoric acid with glassware is a serious inconvenience. Although hydrolysis of protactinium is known to be prevented by appropriate concentrations of sulfuric acid, most investigators still seem to prefer hydrochloric or nitric acid solutions (t,6). The consequent complaints of inhomogeneous and unstable solutions, deposition of protactinium on all surfaces with which it comes in contact, and formation of “inextractable species” have been correctly diagnosed by Kirby (@ as being “consistent with slow hydrolysis. . .” Most of these problems can be
eliminated by using sulfuric acid. In particular, Moore (9) hrts demonstrated the effectiveness of sulfuric acid in enhancing extraction of prohctinium into diisobutylcarbinol from dilute hydrochloric acid. On the other hand, it does not appear to be widely known or used that addition of alkali sulfates results in a pronounced increase in solubility of most metallic sulfates in concentrated sulfuric acid by providing a much higher concentration of sulfate ions than is available from sulfuric acid alone. This fact is most useful in preventing precipitation of heavy-metal sulfates during fuming with sulfuric acid and increasing the dissolution rate of hydrolytic products of zirconium, niobium, tantalum, protactinium, etc. Also, alkali sulfates are probably more efficient than an equal concentration of sulfuric acid in preventing hydrolysis in dilute acid solutions. In the presence of a reasonable quantity of sulfuric acid, the sulfate complex is sufficiently stable to prevent significant hydrolysis over long periods of time and still permit many of the other normal reactions of protactinium such as extraction of the chloride complex into diisobutylcarbinol to take place without demasking. Obviously, such solutions are very desirable to permit handling in conventional glassware. Consequently, the high concentration of both sulfates and/ or sulfuric acid necessary for complete precipitation of appropriate elements with barium sulfate is particularly advantageous in working with protactinium. In addition, pyrosulfate fusion can be used to ensure complete dissolution of protactinium and other refractory and hydrolytically-inclined elements such as zirconium, niobium, tantalum, thorium, etc., initially as well as to keep them in solution subsequently. This paper describes two rapid and efficient procedures by which protactinium-233 can be separated conveniently and repeatedly from its neptunium-237 parent. The procedure preferred because of its simplicity employs the well-known extraction of protactinium into diisobutylcarbinol from dilute hydrochloric acid solution (2) but with the addition of both sulfates and sulfuric acid as stabilizing complexing agents for the protactinium. Neptunium does not extract if present in the reduced state. In the pentavalent and/or sexivalent state, neptunium extracts rather inefficiently undoubtedly because of the difficulty of maintaining the oxidized form in the presence of the carbinol extractant. The other procedure employs precipitation of protactinium with barium sulfate from a potassium sulfate-sulfuric acid solution containing dichromate to keep neptunium in the pentavalent and/or sexivalent state. The barium sulfate is then dissolved in alkaline ethylene-
diamine tetraacetic acid (EDTA) and the protactinium is precipitated as the hydroxide in the presence of iron carrier. Although not quite as simple in the present application as the extraction with diisobutylcarbinol, the barium sulfate separation is more efficient both as to precipitation of protactinium and elimination of neptunium and permits other separations to be accomplished that are not possible with the diisobutylcarbinol extraction. Work presently under way employs the barium sulfate procedure for separation and determination of all alpha emitters, particularly protactinium-231 and the transuranium elements. An alternative procedure is also given for separating protactinium from irradiated thorium for use when high-flux neutron sources are available or when particularly large quantities of tracer are required. Most of the existing proeither use hydrofluoric acid cedures (2,6) with its attendant disadvantages or are more complicated than is necessary to accomplish the relatively simple task of preparing protactinium-233 tracer from irradiated thorium. If the irradiation time is kept relatively short, very little uranium or fission products are produced so that protactinium-233 tracer of excellent radiochemical purity can be obtained by a single extraction into diisobutylcarbinol. In the presence of significant quantities of fission products, oxalic acid should be used as recommended by Moore and Reynolds (4). However, in both methods employing diisobutylcarbinol, the tracer is stripped from the organic phase with sulfuric acid-sodium sulfate instead of hydrofluoric acid to provide a stable solution suitable for storage and use in conventional glassware without interference from fluoride. The strip solution can be reextracted after adding an equal volume of concentrated hydrochloric acid if recycling should become necessary to apply the procedure to more complex solutions-e.g., process solutions from breeding uranium-233 (4). If recycling is anticipated, ammonium sulfate should be used instead of the sodium salt to avoid precipitation of sodium chloride in the 6M hydrochloric acid used for extraction. However, for overall use, sodium sulfate is preferred in the tracer solution to avoid buffering or liberation of ammonia when the tracer is used in alkaline solution. As will be discussed below, most neptunium-237 contains sufficient plutonium, and probably transplutonium elements, to increase the health hazard of the present milking procedures manyfold in comparison to that resulting from the neptunium-237 itself. Unless the neptunium to be used is known to be free of plutonium, it is probably easier and safer to purify it than to determine whether or not
purification is necessary. The barium sulfate procedure is particularly suited to separation of both plutonium and transplutonium elements from neptunium and is being investigated in detail. Plutonium is oxidized very slowly by dichromate under conditions giving rapid and quantitative oxidation of neptunium. Approximately 98% of the plutonium and all of the transplutonium elements are precipitated by barium sulfate under the conditions described if the milking is carried out as soon as the solution is prepared. For greater decontamination from plutonium, the solution must be reduced and reoxidized with dichromate and the precipitation with barium sulfate repeated. As the solution containing dichromate is allowed to age for several days before milking, an increasingly large percentage of the plutonium becomes oxidized, either directly by the dichromate or due to disproportionation of the quadrivalent plutonium, and the solution must be reduced and freshly reoxidized before the plutonium can be precipitated efficiently with barium sulfate. Unfortunately, the procedure has the disadvantage of removing the protactinium tracer along with the plutonium and a 2-week waiting period is required for one third of the equilibrium activity to grow back. The following procedure employing extraction with triisooctylamine permits recovery of over 97% of both neptunium and protactinium with a decontamination factor of about IO5 for plutonium and the transplutonium elements. EXPERIMENTAL
Separation of Neptunium and Protactinium from Plutonium by Triisooctylamine Extraction. .\dd 5 mi. of 72% perchloric acid to 25 HC, of neptunium-237 oxide (41 mg. of NpOt) in a 250-ml. Erlenmeyer flask and boil gently until the oxide has dissolved completely and only a few drops of liquid remain. If the oxide has been ignited, treatment with hot concentrated nitric acid containing a trace of hydrofluoric acid as catalyst followed by fuming with perchloric acid is effective in many cases. Pyrosulfate fusion is very effective in obtaining complete dissolution of even high-fired oxide but should be avoided if possible. Sulfates are reduced in the treatment with hydriodic acid necessary to reduce plutonium to the nonextractable tervalent state, and the neptunium must be precipitated as the hydroxide and redissolved in hydrochloric acid before the present procedure can he applied. If metallic neptunium is iiaeci (36 mg.), add 2 ml. of concentrated hydrochloric acid and 6 drops of 307, hydrogen peroxide and evaporate unti! only a few drops of liquid remain. In either case, dissolve the remaiiiing perchloric or hydrochloric acids in 25 ml. of concentrated hydrochloric acid. Add 5 ml. of 47y0 hydriodic acid, boil for 1 VOL 38, NO. 1 1 , OCTOBER 1966
1459
minute, and let stand without further heating for 5 minutes to ensure complete reduction of plutonium to the tervalent state. Cool the solution to room temperature. Shake 50 ml. of 25% (v./v.) triisooctylamine (Archer Daniels Midland Co., Minneapolis, Minn.) in xylene vigorously for 15 seconds with 10 ml. of concentrated hydrochloric acid in a 250-ml. separatory funnel (preferably with a stopcock made of Teflon) to equilibrate the amine with hydrochloric acid. Let stand for 5 minutes and discard the lower acid layer. Transfer the prepared solution of neptunium-237 in concentrated hydrochloric acid to the separatory funnel containing the triisooctylamine with an additional 20 ml. of concentrated hydrochloric acid. Shake the funnel vigorously on a mechanical shaker for 1 to 2 minutes. Allow the layers to separate for at least 5 minutes and dispose of the lower aqueous layer, which has been found to contain as much as 1 mc. of plutonium-238, in an appropriate manner. Scrub the organic extract vigorously for 1 to 2 minutes with each of two consecutive 50-ml. portions of concentrated hydrochloric acid containing 5 ml. of 47% hydriodic acid. Allow the layers to separate for 5 minutes or more each time and dispose of the aqueous washes. Strip the neptunium-237 and protactinium-233 back into the aqueous phase by shaking the organic extract in the separatory funnel vigorously for 1 to 2 minutes with 50 ml. of water containing 6 ml. of concentrated sulfuric acid and either 5 grams of ammonium sulfate or 3 grams each of anhydrous sodium and potassium sulfates depending on whether extraction with diisobutylcarbinol or precipitation with barium sulfate, respectively, is to be used for milking the protactinium-233, Draw the lower aqueous phase into a 250-ml. Erlenmeyer flask and evaporate the solution to light fumes of sulfuric acid. Add a 1-to-1 mixture of concentrated nitric and perchloric acids dropwise while swirling the flask continuously until any black suspension of charred organic matter clears completely. Do not add nitric acid before the sulfuric acid fumes or the hydriodic acid present will be oxidized to a nonvolatile form. Heat the Concentrated sulfuric acid to boiling over a blast burner caiefully while swirling the flask continuously to decompose excess perchloric acid. Continue as described in either of the following procedures. Separation of Protactinium-233 from Neptunium-237 by Diisobutylcarbinol Extraction. -4110~the boiling sulfuric acid solution of neptunium237 containing 5 grams of ammonium sulfate from the plutonium purification step to cool in air for about 2 minutes and add about 100 mg. of solid hydrazine sulfate. Swirl the flask quickly before the hydrazine decomposes completely and allow the solution to cool to at least 40' C. Add 20 ml. of water and snirl the flask to dissolve the sulfuric acid and any salts that crystallize occasionally. Add about 100 mg. of solid sodium acid 1460
ANALYTICAL CHEMISTRY
sulfite, cool the solution just to room temperature and transfer it to a 125-ml. borosilicate-glass bottle with a polyethylene-lined screw cap. If crystals of ammonium sulfate precipitate due to overcooling, warm the solution slightly to dissolve the salts or extraction of protactinium will be incomplete. Rinse the flask with 30 ml. of concentrated hydrochloric acid and add the rinse to the main sulfuric acid solution. Preserve the solution until needed for milking. When protactinium tracer is desired, transfer the prepared neptunium-237 solution without rinsing to a 250-ml. separatory funnel, preferably one with a stopcock made of Teflon, containing 50 ml. of 50% diisobutylcarbinol (2,6dimethyl-4-heptanol, Union Carbide Chemicals Co., New York, N. Y.) in xylene that has been preequilibrated with 10 ml. of 6N hydrochloric acid. Shake the funnel vigorously for 1 to 2 minutes to extract protactinium-233. Allow the layers to separate for at least 5 minutes and draw the lower aqueous layer containing the neptunium-237 parent back into the glass bottle for storage until the next milking. Shake the organic extract vigorously for 1 minute each time with two consecutive 50-ml. portions of 631 hydrochloric acid. Allow the phases to separate for a t least 5 minutes before drawing off each wash. Save the first wash to recover the small quantity of neptunium-237 (ca. 0.8%) that is present. Periodically, evaporate the combined washes from several milkings in a 250-ml. Erlenmeyer flask to a volume of about 5 ml. and add to the main solution. Shake the organic extract vigorously for 1 to 2 minutes with 50 ml. of water containing 5 ml. of concentrated sulfuric acid and 3 grams of sodium sulfate to strip the protactinium out of the organic phase. Draw the lower aqueous layer off into a 250-ml. Erlenmeyer flask, and evaporate the solution to fumes of sulfuric acid. Add 2 or 3 drops of a 1-to-1 mixture of nitric and perchloric acids as necessary to ensure complete oxidation of organic matter. Cool the solution and dissolve the cake in 25 ml. of 20% H2S04,cool again, and transfer the solution to a 60-ml. borosilicateglass bottle with a polyethylene-lined screw cap. Rinse the flask with sufficient 20% HnSOa to dilute the tracer solution to about 50 ml. For use, dilute the stock solution further with 20% sulfuric acid as desired. The organic extractant can be reused many times if desired. Separation of Protactinium-233 from Irradiated Thorium by Diisobutylcarbinol Extraction. Allow the irradiated thorium to stand for a t least 2 hours before processing to permit most of the 22.1-minute thorium-233 to decay to the desired protactinium-233. I t will generally be convenient to let the thorium stand overnight to permit most of the very short-lived fission products to decay also. Place an appropriate sample of irradiated thorium metal, oxide, or car-
bonate not larger than about 50 mg. into a 250-ml. Erlenmeyer flask. Add 5 grams of ammonium sulfate and 2.5 ml. of concentrated sulfuric acid and boil the solution slowly and carefully over a small flame with continuous swirling until the solution is completely clear. If metallic thorium is used, add 2 ml. of concentrated hydrochloric acid and 6 drops of 30% hydrogen peroxide and evaporate until only a few drops of li uid remain before proceeding with %e pyrosulfate fusion. Cool, add 5 ml. of concentrated sulfuric acid, and heat carefully until the pyrosulfate cake has dissolved. Treat the hot sulfuric acid solution with hydrazine sulfate and complete as described for the extraction from neptunium with a few changes. If the irradiation time was sufficiently long to permit formation of significant fission product activity, add 2 grams of solid oxalic acid to both the dilute sulfuric acid solution containing the irradiated lhorium before extraction and to the 6M hydrochloric acid used to scrub the organic extract. Transfer the prepared thorium solution directly to a 250-ml. separatory funnel with the 30 ml. of concentrated hydrochloric acid and extract as described. Discard the aqueous phase after extraction. Separation of Protactinium - 233 from Neptunium-237 by Barium Sulfate. Allow the boiling sulfuric acid solution of neptunium-237 containing 3 grams each of anhydrous sodium and potassium sulfates from the plutonium-purification step to cool in the air for 2 minutes. Swirl the flask vigorously for 15 seconds to remove sulfur dioxide from the decomposition of sulfuric acid, and add about 100 mg. of solid potassium dichromate. If the solution is too hot, the dichromate will be rapidly decomposed. Swirl the flask to dissolve most of the dichromate. Cool the solution to about 50' C., add 60 ml. of water, and mix thoroughly. The solution must have a distinct yellow color of excess dichromate. Stopper the flask and preserve the solution until needed for milking. When protactinium tracer is desired, remove the stopper from the Erlenmeyer flask and warm the solution to about 50' to 70' C. Add 1 ml. of a 6.6% solution of barium chloride dihydrate dropwise while swirling the solution vigorously and heat to boiling. Transfer the solution without rinsing to a heavy-duty 90-ml. round-bottomed borosilicate centrifuge tube and centrifuge a t 2000 r.p.m. for 2 minutes. Decant the supernate carefully back into the glass-stoppered Erlenmeyer flask and allow the tube to drain thoroughly. Stopper the flask and preserve the solution until needed for the next milking. Add 3 ml. of concentrated sulfuric acid and 0.5 gram each of anhydrous sodium and potassium sulfates to the centrifuge tube containing the barium sulfate and boil the solution gently until all barium sulfate has dissolved. Cool the solution for 2 minutes, swirl vigorously for 15 seconds, and add 4 or 5 small crystals of solid potassium
dichromate, just enough to give the solution a distinct light yellow color. Cool to about 50' C., add 25 ml. of water, and place the centrifuge tube in a beaker of boiling water for 2 minutes. Centrifuge at 2000 r.p.m. for 2 minutes and decant the supernate into another 250-ml. Erlenmeyer flask. Although not generally necessary, the neptunium-237 content of the protactinium tracer can be reduced conveniently to any level desired by additional reprecipitations of the barium sulfate in the presence of dichromate. Save the supernates from the first reprecipitation to recover the small quantity of neptunium-237 present (ca. 0.5% each). Periodically, evaporate the combined supernates from several milkings until excess sulfuric acid has been volatilized, dissolve the cake in water, and preciuitate the neDtunium with sodium hydroxide. Add 3 ml. of 10% disodium ethvlenediamine tetracetic'jcid dihydrateand 2 ml. of 8111 sodium hydroxide to the centrifuge tube containing the barium sulfate. Swirl the tube vigorously to disperse the barium sulfate as finely as possible. Complete dissolution of the barium sulfate will be speeded up considerably if all small lumps are smeared against the side of the tube with a stirring rod. hlso, do not allow the barium sulfate to stand overnight or complete dissolution is almost impossible. Add 25 ml. of water and heat the centrifuge tube in a beaker of boiling water until all barium sulfate has dissolved. Add 2 ml. of ferric perchlorate solution containing 1 mg./ml. of iron, mix thoroughly and digest in the hot water bath for 2 minutes to precipitate ferric hydroxide completely. Cool to room temperature and centrifuge at 2000 r.p.m. for 2 minutes to pack the soft precipitate adequately. Discard the supernate. -4dd 1 nil. of 72% perchloric acid and heat the tube gently with continuous swirling over a blast burner until the slight turbidity of residual barium sulfate clears. Cool to room temperature and add in order, 20 ml. of water, 3 ml. of 10% disodium EDTA solution, and 3.5 ml. of 8.11 sodium hydroxide. Heat the tube in the beaker of boiling \vater for 2 minutes as before. Cool and centrifuge at, 2000 r.1i.m. for 2 minutes and discard the supernate. .4dd 10 nil. of concentrated sulfuric acid, 3 grams of anhydrous sodium sulfate, 3 drops each of concentrated nitric and perchloric acids to the centrifuge tulle, and heat carefully over a blast burner to strong fumes to oxidize traces of EDT.4 and to dissolve the protactinium. Cool the solution to room temperature, add 20 nil. of water and mix. Transfer the solution to a 60-ml. borosilicate bottle with a polyethylenelined screw cap and rinse the tube with sufficient 20y0 sulfuric acid to dilute the tracer to about 50 ml. -4fter allowing it to stand overnight, centrifuge and decant the solution from the small quantity of barium sulfate that will have settled out,. Prepare a working solution for use by diluting the stock solution as desired with 2070 sulfuric
acid. If sulfate is objectionable for the use anticipated, the ferric hydroxide can be dissolved in other acids but the solutions will be relatively less stable with respect to protactinium unless hydrofluoric acid is used (2). RESULTS
Solutions are counted in a 3-inch by 3-inch thallium-activated sodium iodide well crystal in volumes up to 75 ml. contained in polystyrene counting bottles as described previously (7'). If the protactinium-233 is in equilibrium with its parent-Le., over 5 or 6 months have elapsed since the last milkingapproximately 25 X lo8 gamma c.p.m. will be present in 25 pc. of neptunium237, or about 5 X l o 5gamma c.p.m. per milliliter in 50 ml. of solution. A fivefold dilution of the stock solution gives a convenient working solution containing lo5 gamma c.p.m. per milliliter. The total tracer a t equilibrium is sufficient for 250 experiments using lo5 c.p.m. per run. However, because the protactinium-233 tracer grows back to 30y0 of its equilibrium level in 2 weeks, an average of 7.5 runs per day for 5 days per week can be maintained indefinitely a t the very liberal level employed. This quantity of activity per run gives a relative standard deviation of only 0.6% a t the 95% confidence level in a counting time of only 1 minute. For preliminary or less exacting work, much lower activities can be used, thus greatly extending the number of experiments that can be accommodated. The efficiency of the protactiniumneptunium separation was determined quantitatively for both milking procedures using 2 x lo5 gamma c.p.m. of protactinium-233 and up to 4 x l o 6 gamma c.p.m. of neptunium-239 tracers in separate runs. The distribution of each element in each fraction obtained was determined by gross gamma count-
Table I.
ing in a sodium iodide well crystal under identical conditions. As shown by the data of Table I, 99.5% of the protactinium-233 tracer is removed in a single extraction with diisobutylcarbinol with an overall recovery for the procedure of about 99%. Even in the presence of 2 grams of oxalic acid, recovery is better than 98.5%. In the barium sulfate procedure, 99.9% of the protactinium is coprecipitated with barium sulfate under the conditions described while less than 0.3% of the neptunium is carried when in the pentaor sexivalent state. The overall recovery is nearly 98% with most of the loss being due to either incomplete precipitation of protactinium hydroxide and/or failure to separate the soft hydroxide precipitate completely during centrifugation. The neptunium activity of the final protactinium product from either procedure was not detectable by gamma counting unless very large quantities of tracer were employed. Both milking procedures were also checked using 25 pc. of neptunium-237 as described. Each separated fraction was examined carefully by multichannel gamma spectrometry to determine the distribution of both the neptunium-237 parent and its protactinium-233 daughter. The results were entirely consistent with the quantitative values obtained by gross gamma counting of each tracer separately. The neptunium activity of each fraction was also determined with greater sensitivity by separation and direct alpha counting. The results for the extraction procedure agreed well with those given in Table I. Although not determined specifically, separation of neptunium should be even better in the barium sulfate procedure, particularly if two reprecipitations are made. Thus, approximately 2 X l o 3 d.p.m. of neptunium-237 will be present in the final tracer solution
Distribution of Protactinium and Neptunium in the Recommended Procedures
Fraction Separation using diisobutylcarbinol Aqueous hase after extraction First was1 of main organic phase Second wash of main organic phase Aqueous sulfate strip containing Pa233 Residual organic phase Material balance Separation using barium sulfate" Supernate from fint BaS04 precipitation Supernate from second Bas04 precipitation Supernate from first Fe(OH)$precipitation Supernate from second Fe(OH), precipitation Final protactinium-233 solution Material balance a Neptunium oxidized with dichromate.
Quantity present, % Np239
Pa233
0.5
0.1 0.1 99.0
0.5
98.9 0.8 0,004
0.003
