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Radiochemical Determination of Protactinium-23 1 in Environmental and Biological Materials Claude W. Sill Radiological and Environmental Sciences Laboratory, Department of Energy, Idaho Falls, Idaho 8340 I
Procedures are presented for determination of protactinium-231 in uranium ores and mill tailings, water, air dusts, bone ash, and other similar types of samples. Except for bone ash, the samples are dissolved by fusion with anhydrous potassium fluoride in a platinum dish, assuring complete dissolution of the most refractory forms. The fluoride cake is then transposed with sulfuric acid to a pyrosulfate fusion, simultaneously ellmlnatlng all fluoride and silicon by volatilization. Protactinium is removed from a hydrochloric acid solution of the pyrosulfate cake either by direct extraction into diisobutylcarbinol or by precipitation with barium sulfate prior to the extraction, depending on the interferences present. Any insoluble sulfates or acids are removed by centrifugation and treated separately. The separated protactinium can then be electrodepositedand determined by either CY spectrometry or gross CY counting, or Precipitated on barium sulfate for gross counting when equipment for either electrodeposition or a spectrometry is not available. The effect of interfering elements is discussed. Directions are also given for purification and standardization of both protactinium-231 and -233 tracers for use In determination of counting efficiencies and chemical yields.
Protactinium-231 is a n extremely toxic radionuclide produced naturally during radioactive decay as the first daughter of uranium-235. Consequently, it is present in the environment in significant quantities, particularly in uranium ores a n d in waste products of t h e uranium mining and milling industry. Federal regulations require that the concentration of soluble protactinium-231 permitted for release t o uncontrolled environments be less than 4 X lo-’‘ pCi/mL for air and less t h a n 9 X lo-’ bCi/mL for water ( I ) . T h e maximum permissible concentration (MPC) in air is lower t h a n t h a t for any other radionuclide except curium-248. A recent regulatory guide ( 2 ) suggests lower limits of detection for analytical procedures of 0.1% of the M P C s for air and 1% of t h e MPCs for water. T h e determination of protactinium-231 either in the environment or in t h e milling cycle has not received as much attention as have most of t h e other radionuclides resulting from decay of natural uranium. T h e reason generally given is t h a t its determination is too complicated, difficult and expensive. Eecause of t h e severe biological consequences resulting from this long-lived, 0-emitting bone seeker, very sensitive procedures are also required for its determination in biological materials. Protactinium has a particularly extreme tendency to hydrolyze in dilute solutions of most of the common acids except sulfuric and hydrofluoric, and t o form very refractory and intractable species t h a t are very difficult t o dissolve. T h e well-publicized tendency of protactinium to deposit on t h e walls of glass vessels a n d t o form mysterious “inextractable species” is due almost exclusively to this single characteristic ( 3 , 4 ) ,admittedly one of the most pronounced of any element in t h e entire periodic table. However, previous work in this laboratory (5, 6) has demonstrated repeatedly t h a t protactinium is just as tractahle and its chemistry is just as reliable
and predictable as t h a t of most other elements if conditions are chosen t o obtain complete conversion of refractory compounds to ionic, monomeric forms before expecting it t o undergo reproducible chemistry, and t o control its extreme hydrolytic tendencies during the subsequent analytical operations. In fact, because of the excellent complexing agents and reagents available for the extraction of protactinium (3, 4 ) , t h e determination and use of both protactinium-231 and -233 in this laboratory have been more reliable and fraught with fewer difficulties than with many other elements. Pyrosulfate fusion is one of the most effective and reliable methods available for the rapid and complete dissolution of nonsiliceous materials. It is particularly indicated for those hydrolytic elements forming refractory oxides, such as protactinium, niobium, tantalum, zirconium, etc. For siliceous refractories, a potassium fluoride fusion followed by transposition with sulfuric acid to a pyrosulfate fusion (7) is perhaps the most nearly ideal procedure for t h e complete decomposition in a single operation that is available. Both hydrogen fluoride and silicon tetrafluoride are volatilized during t h e transposition. This achievement is particularly important because silica may not be filtered off from even strongly acid solutions of protactinium or substantial losses will result. The final cake dissolves completely in dilute hydrochloric acid except when elements forming insoluble acids or sulfates are present. Proper acidity and presence of complexing agents must be maintained through all subsequent separations, including transfers from one container to another, to prevent hydrolysis or sorption of protactinium with consequent deposition on the container walls and marked deviations from the expected chemistry. Moderate concentrations of sulfuric acid and alkali sulfates produce protactinium solutions t h a t are surprisingly stable with respect to hydrolysis and yet will permit many of the analytically useful reactions of protactinium t o take place in their presence (5). Such solutions can even be boiled for reasonable lengths of time without significant hydrolysis. Use of a sulfate system t o provide continuous complexing for protactinium obviously fits in very well with t h e use of potassium fluoride and pyrosulfate fusions desired for the original sample decomposition. In particular, the presence of even moderately high concentrations of sulfuric acid and alkali sulfates does not interfere with extraction of protactinium from hydrochloric acid by diisobutylcarbinol (DIBC) (5,8,9). This reagent is undoubtedly one of t h e most rapid, efficient and selective extractants for protactinium available (3-5,8). Protactinium can be stripped from the organic phase with a solution of sulfuric acid containing potassium sulfate from which the protactinium can be precipitated quantitatively with a small quantity of barium sulfate ideally suited for gross cy counting (5,10, 11). Alternatively, protactinium can be stripped with perchloric-oxalic acid for electrodeposition and cy spectrometry.
EXPERIMENTAL Instrumentation. The 01 spectrometer using a 450-mmz surface-barrier detector and 1024-channel analyzer (12)and the scintillation technique using individual die-cut zinc sulfide
This article not subject to U.S.Copyright. Published 1978 by the American Chemical Society
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phosphor disks for gross a counting (10) have been described previously. A windowless 2-n proportional flow counter using methane as the counting gas was used in the standardization of the protactinium-231 solution. A 3-in. by 3-in. thallium-activated sodium iodide well counter was used for y-counting the protactinium-233 tracer. Reagents. Stannous Chloride, 25%. Heat 2.5 g of uneffloresced stannous chloride dihydrate with 2.5 mL of concentrated hydrochloric acid in a 30-mL beaker just to boiling to dissolve the salt. Cool and add 7.5 mL of water. Prepare fresh daily to minimize the quantity of quadrivalent tin added to the solution. Tellurous Acid, 5 mg/mL Te. Dissolve 0.625 g of tellurium dioxide in 25 mL of concentrated hydrochloric acid and dilute to 100 mL. Wash Solution, 8 M Hydrochloric Acid-4% Oxalic Acid. Heat 50 mL of water and 6 g of oxalic acid until the solid dissolves. Add 100 mL of concentrated hydrochloric acid and cool. Diisobutylcarbinol, 5070,Peroxide-Free. hfix 250 mL each of xylene and diisobutylcarbinol (2,6-dimethyl-4-heptanol, Union Carbide Chemicals Co., New York, N.Y.). Shake 25 mL of this solution vigorously for 30 s with 1 mL of a solution containing 1mg of quadrivalent titanium in 4 M sulfuric acid. A yellow color in the lower aqueous layer indicates the presence of hydrogen peroxide that invariably develops when DIBC ages in contact with air. If the color produced with 25 mL of the reagent solution is more than a very light yellow, the peroxide must be removed to avoid undesirable reoxidation of the reduced iron during extraction. Transfer the entire DIBC solution and the 1 mL of titanium test solution to a 1-L separatory funnel. Add 100 mL of 3 M hydrochloric acid and shake for 10 s to develop enough peroxytitanic acid color to serve as an indicator. Add solid potassium metabisulfite in about 0.5-g portions and shake vigorously for 30 s after each addition until all color has been removed. Allow the phases to separate and discard the lower aqueous phase. Scrub the organic phase with an additional 100 mL of 3 M hydrochloric acid to remove the last of the titanium and salts. Protactinium-231 and -233 Tracers. In a previous investigation (61, protactinium tracers were prepared in solutions containing sulfuric acid and sodium sulfate because of concern for their chemical stability in dilute nitric acid alone. As will be shown below, it has now been demonstrated that both protactinium-231 and -233 can also be prepared in pure nitric acid like the other tracers studied without significant hydrolysis or other loss of chemical stability of standard solutions over long periods of time. By eliminating the need for sulfates, sulfuric acid, or other complexing agents, even protactinium tracers can now be standardized by direct evaporation on stainless steel plates and a or y counting with a marked saving in time, and with greater precision and accuracy, than when chemical separations must be used. Extract the protactinium tracers into DIBC and wash the extract as described previously (6) with the following changes. With protactinium-231, omit the sodium metabisulfite and use the wash solution described above instead of 8 M hydrochloric acid. Add 3 drops of 20% titanium trichloride to the first scrub to ensure elimination of traces of iron, and 1 mL of 30% hydrogen peroxide to the second t o remove traces of titanium, platinum, niobium, or tantalum that might be present. With protactinium-233, make the three additional scrubs with 50 mL of the wash solution described above to remove the last traces of sulfuric acid and ammonium sulfate. Add 10 drops of 2070 titanium trichloride to the first scrub only and 2 mL of 3070 hydrogen peroxide to the third one to remove the last traces of titanium. With either nuclide, add 5 mL of the wash solution and draw off without shaking to wash the funnel stopcock and stem. Strip the purified tracer by shaking the organic phase for 1min with 50 mL of water containing 5 mL of 72% perchloric acid, 2 mL of 30% hydrogen peroxide, and 2 g of oxalic acid. Preferably, use redistilled water and acids and recrystallized oxalic acid in the strip and in making up the final solutions to minimize presence of absorbing substances. Treat the strip as described in the fourth and fifth paragraphs under “Thorium-234” (6) omitting the extra perchloric acid called for, and making up both stock and working solutions in 25% nitric acid instead of 10%. Be particularly careful with protactinium
not to heat the flask any hotter or longer than necessary during evaporation to dryness to remove perchloric acid. Boil the dried residue with concentrated nitric acid for at least 15 to 20 min to ensure complete dissolution of the protactinium before cooling and diluting to the find 25% concentration. Filter the final stock solution through a 25-mm DM-450 membrane filter (Gelman Instrument Co., Ann Arbor, Mich.) in a plastic filtering chimney (also Gelman) to remove even unsuspected traces of precipitated silica. Otherwise, highly inhomogeneous solutions will result because of sorption of protactinium on the silica. Because of similar strong sorption of protactinium on glass surfaces, even from highly acidic solutions, store all standard solutions in polypropylene bottles, and dispense with a high-precision, micropipetting system with disposable polypropylene tips (Oxford sampler, Fisher Scientific Co., Pittsburgh. Pa., catalog no. 21-205). Standardization. If gross a counting on barium sulfate is desired, standardize a freshly-purified solution of protactinium-231 in 25% nitric acid containing about 10’ dpm/mL by evaporating 200-pL aliquots on stainless steel plates and counting in a standardized 2 - 7 proportional counter for 30 min to keep the relative standard deviation less than 0.2% as described previously (12). Fuse other 200-~Laliquots of the standardized solution with potassium sulfate and sulfuric acid in 250-mL Erlenmeyer flasks and precipitate barium sulfate as described below. Count the barium sulfate in the zinc sulfide scintillation counter for 30 min and determine the counting efficiency of the counter under the exact conditions to be used in gross a counting. If N spectrometry is to be used, a standard solution of protactinium-231 is not necessary but the tracer should have been purified relatively recently to minimize the daughters present. Electrodeposit an aliquot containing about lo4 cpm of protactinium-231 under the exact conditions to be used subsequently in the preparation of the actual samples. Count the plate in the standardized 2-7 counter to determine the activity actually present after the electrodeposition. Use this standardized plate to determine the counting efficiency of the a spectrometer to be used (12). Because of the excellent radiochemical purity of the protactinium fraction obtained in the present procedure, the samples themselves can be counted directly in the standardized 2-n or scintillation counters and eliminate the need for a spectrometer, should one not be available. I t is also potentially more accurate to determine total activity by 2-n counting and use the a spectrum only if necessary to determine the fraction of the total activity coming from the nuclide of interest. This procedure eliminates errors due to changes in counting efficiency of the cy spectrometer. However, direct counting is generally less sensitive, precise, and reliable, particularly for lower-level samples, because of the higher and more variable backgrounds of many 2-n counters over long counting periods. Evaporate a 200-gL aliquot of the purified solution of protactinium-233 in 25% nitric acid containing about lo5 cpm on a stainless steel plate for use in determining the chemical yield as described for thorium-234 (13). Reexamination of this technique using protactinium-233 showed that sources prepared by direct evaporation and counted under the conditions described gave counting rates within 0.1% of those obtained with an electrodeposited source after careful correction for losses sustained during electrodeposition. Precipitate another aliquot on barium sulfate for use in determining chemical yield when gross a counting is employed. Count both samples and standards for a t least 5 min to give a relative standard deviation of 0.5%. All significant random uncertainties incurred anywhere in the entire measurement process must be propagated to each intermediate or final result in which they are involved according to the Law of Propagation of Errors. In addition to the usual uncertainties of measuring background and sample count, individual uncertainties must be determined for the standardized tracer solutions, instrumental counting efficiencies, chemical recoveries, isotopic ratios, volumetric measurements, etc., and the significant ones propagated to the final analytical result. Uranium Ores and Mill Tailings. Fuse a 1-g sample with 3 g of anhydrous potassium fluoride (not the sodium salt) in a 50-mL platinum dish, transpose the cake with concentrated sulfuric acid, and dissolve the pyrosulfate cake in dilute hydrochloric acid as described in the first three paragraphs under
ANALYTICAL CHEMISTRY, VOL. 50, NO. 11, SEPTEMBER 1978
“Determination of Thorium-230 in Uranium Ores and Mill Tailings” (13) with two exceptions. Add 200-j~Lof protactinium-233 tracer in 25% nitric acid containing about lo5 cpm in place of the thorium-234 called for and increase the boiling time to 5 min to ensure complete hydrolysis of condensed phosphates. Leave the cover glass askew as necessary so that the volume will have been reduced to 30 mL at the end of the boiling period. If the slightest turbidity detectable with the unaided eye is present after completion of boiling, treat as described below under “Separation of Protactinium from Insoluble Sulfates and Acids”. If the solution is clear, proceed as described either for the direct extraction or the barium sulfate separation as desired. With high-grade niobium or tantalum ores, reduce the sample size to 0.25 g depending on the quantity of niobium and/or tantalum present. Color photographs (14) and a detailed description of the application of this method to the complete dissolution of 10-g samples of soil (12) have been published. Direct Extraction. Cool the hydrochloric acid solution of the pyrosulfate cake in a cold water bath for a couple of minutes to lower the temperature nearly to room temperature. Add 2 mL of concentrated sulfuric acid, 0.3 mL of tellurous acid, and a variable quantity of potassium metabisulfite depending on the quantity of iron present. With 1 g of tailings containing very little iron, 1 mL of a 25% solution will suffice. With soils and ores, add 1 g of the solid reagent, and more as necessary. Cover the beaker with a watch glass and heat the solution slowly to boiling over a period of about 5 min. Boil the solution vigorously until the iron has been reduced, the excess sulfur dioxide has been expelled, the elemental tellurium has flocculated, and the volume has been reduced to 30 mL. The reduction of iron by sulfurous acid is strongly catalyzed by tellurous acid, and the solution becomes black and turbid because of precipitation of elemental tellurium when the reduction of iron is nearly complete. If the solution is still distinctly yellow and only slightly turbid by the time most of the sulfur dioxide has been expelled, the solution should be cooled somewhat, more potassium metabisulfite added, and the boiling continued. However, uranium gives a yellow color that is not removed by sulfite or subsequently by stannous chloride and should not be mistaken for incomplete reduction of iron. When the excess sulfur dioxide has been expelled. add 1 mL of additional tellurous acid and 2 g of oxalic acid. If more than a few milligrams of thorium or other elements forming oxalates insoluble in strong acid might be present, omit the oxalic acid. Sulfurous acid does not reduce polonium to the elemental state quantitatively under these conditions so that more tellurous acid must be added as carrier before reduction by stannous chloride or elimination of polonium will be incomplete. Heat the solution back to boiling, and add 5 drops of 25% stannous chloride dihydrate in 257’ hydrochloric acid while swirling the solution continuously. Boil the solution vigorously for 2 min to complete the precipitation of polonium-210 and to flocculate the metallic tellurium. Allow the solution to stop boiling and add 1 additional drop of stannous chloride to be sure that precipitation of tellurium was complete. If additional darkening is produced, more stannous chloride should be added and the boiling repeated. Cool the solution for 2 min in a cold water bath with occasional swirling to keep the metallic tellurium from drying on the sides. Remove the beaker from the water bath while the solution is still warm and proceed without delay to reduce the chances of precipitating calcium sulfate. Filter the tellurium on a 25-mm DM-450 membrane filter in a glass Wtering chimney (Millipore Filter Corp., Bedford, Mass., catalog no. XX 10 025 00) into another graduated 100-mL beaker. Disconnect the suction, rinse the beaker with 3 mL of 0.5% sulfuric acid, and add to the filtering chimney. Draw the rinse in to the main filtrate. If much uranium is present, the filtrate will be distinctly yellow. Proceed with the extraction without delay to avoid unnecessary reoxidation of iron by air. Add 50 mL of 50% peroxide-free DIBC in xylene to a 250-mL separatory funnel, preferably one having a Teflon stopcock. Transfer the filtrate from the tellurium precipitation, having a volume not larger than about 35 mL, to the funnel and add 6 drops of 20% titanium trichloride as a holding reductant. Rinse the beaker with 60 mL of concentrated hydrochloric acid and add to the main solution in the separatory funnel. Add an additional 10 mL of concentrated hydrochloric acid for every 5 mL that the aqueous volume exceeds 35 mL.
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Shake the solution vigorously for 1.5 min to extract protactinium-231 and -233. Longer shaking merely increases reoxidation of iron. Discard the lower aqueous layer. A copious precipitate of sodium and potassium chlorides will form but does not interfere with either complete phase separation or recovery of protactinium. Shake the organic phase vigorously three consecutive times for 2 min each time with 50 mL of the wash solution and 3 drops of 2070 titanium trichloride to obtain adequate elimination of iron and aluminum for subsequent electrodeposition and a spectrometry. Rinse the funnel neck and stopper with each rinse as it is added. If mill tailings or other samples containing little soluble salts are being analyzed, or if gross cy c’ountingon barium sulfate is to be employed, one titanium scrub will generally suffice. Each wash should contain a light violet color o f tervalent titanium when viewed against a white background. Shake the organic phase vigorously for 2 min with 50 mL of the wash solution and 2 mL of 3070 hydrogen peroxide to remove the small quantities of titanium, platinum, niobium, and tantalum that will have extracted, if present. If significant color is obtained, repeat the peroxide scrub. Again, discard the aqueous washes. Because of the exothermic reaction between hydrogen peroxide and strong hydrochloric acid to produce chlorine gas, loosen the stopper immediately after shaking to vent any pressure that might have built up, and shake the separatory funnel under a stream of cold running water if the solution begins to warm up. Finally, add 5 mL of the wash solution and draw off without shaking to rinse the funnel stem and stopcock. Strip the protactinium by shaking the DIBC extract vigorously for 2 min with a solution of 5 mL of 721% perchloric acid, 2 g of oxalic acid and 2 mL of 30% hydrogen peroxide in 50 mL of water, preferably redistilled. Add two 8-mesh silicon carbide boiling stones and evaporate the solution on a hot plate until the perchloric acid begins to fume. Add 5 mL ‘of 48% hydrobromic acid and 1 drop of nitric acid, and repeat the evaporation to ensure volatilization of tin, this time evaporating most of the perchloric acid. Any significant quantity of niobium or tantalum that has come through the extraction will be indicated by a turbidity developing in the boiling perchloric acid. Add 2 mL of a 10% solution of sodium hydrogen sulfate monohydrate in 1-to-1 sulfuric acid and heat the flask over a small flame from a blast burner, keeping the sides hot enough to prevent condensation of the acid until the excess sulfuric acid has been volatilized and a clear glassy melt is ob1;ained. If the melt tends to crystallize while in the flame, remove the flask from the flame immediately and cool. The sodium hydrogen sulfate cake can then be dissolved and the protactinium precipitated on barium sulfate for gross a counting or electrodeposited by any appropriate method for analysis by Q Spectrometry and/or direct counting in a standardized 2-T counter. After electrodeposition, heat the stainless steel plate on an uncovered hot plate for 5 min to decrease the spontaneous volatility of any polonium-210 that might have escaped separation to prevent subsequent serious contamination of the solid-state detector used in obtaining the a spectra (15). Determine the chemical yield by y counting the electrodeposited plates and the protactinium-233 standard under identical conditions. The y counting rate of protactinium-231 is only 0.089 cpm per dpm so that the protactinium-:!31 from the sample will have a negligible effect on the yield when lo5 cpm of protactinium-233 tracer is employed. I t should be noted that most procedures for electrodeposition are severely affected by even microgram quantities of most elements forming insoluble hydroxides, causing decreased yields and degraded Q spectra. Consequently, much more complete elimination of iron, titanium, niobium, platinum, etc., is necessary if the protactinium fraction is to be electrodeposited for Q spectrometry than is necessary with precipitation on barium sulfate for gross Q counting. The latter method not only provides an excellent separation of many interfering elements, but is itself relatively unaffected by quantities of nonradioactive elements less than about 1 mg. The combination of DIBC extraction and precipitation on barium sulfate gives a procedure that is completely specific for protactinium. Barium Sulfate Separation. If interfering elements are absent, as discussed below, proceed with the precipitation, centrifugation, and washing of barium sulfate as described in the fourth paragraph under “Determination of Thorium-230 in Uranium Ores and Mill
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Tailings” (13). When actual samples are being analyzed, more barium is used to provide greater capacity for interfering ions and the solution is centrifuged hot to reduce precipitation of calcium sulfate than when pure standards or purified protactinium fractions are precipitated on barium sulfate for gross o( counting as described below. Add 5 mL of 72% perchloric acid to the centrifuge tube containing the washed barium sulfate and heat the tube very carefully over a small flame from a blast burner while swirling the solution continuously to prevent bumping or cracking the tube until any excess water has been expelled and the barium sulfate has dissolved. Heating longer than necessary increases the quantity of protactinium sorbed on the walls of the tube. Note that small permissible quantities of thorium, lanthanides, or other elements carried in the barium sulfate, if present, form complexes with the potassium sulfate also present that are very insoluble in even hot 72% perchloric acid, making it appear that the barium sulfate does not dissolve completely. However, these complexes are easily soluble in the more dilute acid used in the next step. Cool the solution to about 40 “C and pour into a 250-mL separatory funnel containing 50 mL of wash solution while swirling the solution rapidly and continuously to prevent reprecipitation of barium sulfate. Rinse the centrifuge tube with 10 mL of concentrated hydrochloric acid and add the rinse to the separatory funnel. Add 25 mL of wash solution to the centrifuge tube and place the tube in a beaker of boiling water for 5 or 10 min until needed to redissolve most of the sorbed protactinium from the walls. With as little delay as possible to minimize the possibility of reprecipitation of barium sulfate, add 3 drops of 2070 titanium trichloride to ensure elimination of iron, cerium, etc., and 50 mL of 50% DIBC in xylene to the separatory funnel and shake vigorously for 2 min to extract the protactinium. If the supernate has to be extracted because of interferences with the barium sulfate separation, use the same DIBC to extract the barium sulfate fraction. Discard the lower aqueous phase. Cool the wash solution in the centrifuge tube to room temperature in a beaker of cold water and add to the separatory funnel. Rinse the tube with another 25 mL of wash solution and add to the funnel. Shake the funnel vigorously for 2 min and discard the aqueous phase. Repeat the scrub with another 50 mL of wash solution containing 2 mL of 30% hydrogen peroxide to remove titanium, platinum if present, and the last traces of barium. Add 5 mL of wash solution and draw off without shaking to rinse the funnel stem. Strip the protactinium and finish as described above. This procedure also permits protactinium to be recovered from barium sulfate precipitates used in gross a counting if electrodeposition and 01 spectrometry should subsequently become desirable. Because protactinium can also be extracted very efficiently from strong solutions of aluminum nitrate by Aliquat-336, particularly if the nitrous acid is omitted (121,the determination can also be completed as described in the fifth paragraph under “Determination of Thorium-230 in Uranium Ores and Mill Tailings” (13). If much barium sulfate is present, some scaleup from the directions given might be necessary. In the absence of interferences, barium sulfate precipitates small quantities of all large multivalent cations to better than 99.99%, separating them from all mono-, di-, and small ter- and quadrivalent cations (10, 11). The precipitate is highly crystalline and easy to handle quantitatively, and is formed best in dilute acid solutions containing high concentrations of both potassium and sulfate ions. However, it must be emphasized that precipitation of large multivalent cations with metallic sulfates is very incomplete if more than about 1 mg of thorium, cerium, lanthanum, or other elements that are also carried efficiently in the sulfate lattice ( 1 2 ) are present in the aliquot taken for analysis. When such interferences are present, the supernate from the centrifugation must be treated as described above under Direct Extraction in addition to treating the insoluble sulfates and/or earth acids as described below. Whether or not the supernate needs to be treated can easily be determined by y counting the supernate in a well counter. In 50 mL of solution in a 3-in. by 3-in. sodium iodide well counter, 1 g of pitchblende containing 1%UBOsgives only about 7000 7 cpm for the entire system in secular equilibrium, most of which comes from lead-214 and bismuth-214. Two hours after sample decomposition and consequent loss of radon-222, over 9570 of the
lead-214 and bismuth-214 will have decayed, and the thorium-234 and protactinium-234 will have been removed in the barium sulfate separation. Actual tests showed less than 600 cpm of total 1 activity remaining, of which 130 cpm cumes from the potassium-40 in the potassium fluoride fusion. Lower concentrations of uranium will give proportionately lower levels of residual y activity. When 10’ cpm of protactinium-233 tracer is used as recommended, any substantial loss of protactinium will be indicated by a marked increase in y activity in the supernate compared to that expected from the above considerations. A similar argument can be made with respect to thorium ores except that the lead-212 and bismuth-212 daughters will not decay off appreciably in a couple of hours. Separation of Protactinium from Insoluble Sulfates a n d Acids. Assuming that botn the potassium fluoride and pyrosulfate fusions were completely clear, both original sample and silica will be known to have been eliminated completely. Consequently, if the hydrochloric acid solution of the pyrosulfate cake is not completely clear, either earth acids, double rare earth sulfates, or alkaline earth sulfates are indicated, even small quantities of which will cause severe loss of protactinium by coprecipitation if not treated ( 5 , 10). Some ores might contain any combination of these elements simultaneously. Cool the solution thoroughly in a bath of cold running water but do not allow to stand for long periods which increases the chance of precipitation of calcium sulfate. Transfer the solution to a 40-mL conical centrifuge tube and rinse the beaker with two 1-mL portions of 0.5% sulfuric acid. Do no? transfer the boiling chips. Centrifuge at 2000 rpm for 5 min. Decant the supernate into anuther graduated 100-mL.beaker. The subsequent treatment necessarily depends on the identity of the insoluble material. Barium Sulfate Only. If a small quantity of barium sulfate (and/or strontium sulfate) is the only insoluble material present, and other interfering elements are absent, it will generally be simpler to add enough more barium to make the separation complete, and analyze the barium sulfate only, than to analyze both original precipitate and filtrate. Before centrifuging, add more barium and finish as described above under Barium Sulfate Separation. If the quantity of barium already present exceeds about 10 mg! the additional barium should be omitted. Double Rare Earth Sulfates and/or Calcium Sulfate. Although most of the protactinium will he coprecipitated with the double rare earth sulfates, neither barium nor additional rare earth can be used to recover the remainder from the filtrate efficiently because of interference by the lsrge quantity of thorium usually associated with rare earths. Precipitation of protactinium in the double earth sulfates is not quantitative for the same reason. The filtrate must be extracted. The precipitate of double rare earth sulfates andlor calcium sulfate can be dissolved easily in dilute hydrochloric acid after the alkali metal sulfates have been removed and extracted with the same portion of DIBC used to extract the filtrate. Add 25 mL of 8 M hydrochloric acid to the centrifuge tube containing the double rare earth sulfates without washing, suspend the precipitate with a stirring rod, and place the centrifuge tube in a beaker of boiling water until the solution is nearly boiling. If the solution becomes completely clear, place the tube in a beaker of cold water until the solution has cooled nearly to room temperature. Add 3 drops of 20% titanium trichloride to reduce any tervalent iron or quadrivalent cerium present, both of which will otherwise extract. Transfer the solution to the 250-mL separatory funnel containing the 50 mL of 50% DIBC used to extract the main filtrate without scrubbing. Rinse the centrifuge tube with another 25 mL of 8 R.1 hydrochloric acid, and extract, scrub, and strip as described above, beginning with the last half of the third paragraph under Direct Extraction. If the precipitate does not dissolve completely, alkaline earths are probably present. In this case, recentrifuge the solution before decanting into the separatory funnel for extraction. Dissolve the barium sulfate in perchloric acid and finish as described above, beginning with the second paragraph under Barium Sulfate Separation. Use the same 50-mL portion of DIBC to extract the original aqueous supernate, the rare earth fraction, and the barium sulfate fraction sequentially before proceeding with the scrubs. Earth Acids. When present in significant quantitites, both niobium and tantalum precipitate more or less completely on
ANALYTICAL CHEMISTRY, VOL. 50, NO. 1 l., SEPTEMBER 1978
boiling the pyrosulfate cake in dilute hydrochloric acid, carrying the protactinium in roughly similar proportion. The beaker is generally coated with milky stains that cannot be rinsed out completely with water. A test for niobium and/or tantalum can be made while cleaning the beaker simultaneously. Warm a few milliliters of dilute sulfuric acid in the beaker and add a few drops of 30% hydrogen peroxide. Stains caused by niobic or tantalic acids dissolve rapidly only on addition of the peroxide. Add 25 mL of 1-to-1 sulfuric acid to the centrifuge tube containing the earth acids and disperse the precipitate thoroughly with a stirring rod. Add 5 mL of 3 0 7 ~hydrogen peroxide, mix thoroughly, and place the tube in a beaker of boiling water. As soon as the solution clears completely, place the tube immediately in a beaker of cold running water and cool the solution thoroughly to avoid undesirable decomposition of the hydrogen peroxide. Transfer the cold solution to a 250-mL separatory funnel, rinse the tube with 60 mL of concentrated hydrochloric acid, and shake vigorously for 2 minutes with 50 mL of 50% DIBC. Loosen the stopper immediately after shaking to avoid pressurizing the funnel due to formation of chlorine and/or oxygen gases, and swirl the funnel in a stream of cold running water to dissipate the heat that will have been generated. Discard the aqueous phase. Cool a mixture of 75 mL of the wash solution and 10 mL of concentrated hydrochloric acid thoroughly in a cold water bath and add to the separatory funnel. Add 5 mL of 30% hydrogen peroxide and shake vigorously for 2 min. Loosen the stopper immediately and cool the funnel in running water if necessary. Discard the aqueous phase. Shake the organic phase for 10 s with 50 mL of the wash solution containing 0.5 g of solid potassium metabisulfite to reduce residual peroxide. Add 20% titanium trichloride 3 drops at a time with shaking for 10 s each time until all yellow color in the organic extract has been removed. Add 3 drops excess and continue shaking for 2 min. Repeat the scrub with another 50 mL of the wash solution containing 3 drops of 20% titanium trichloride to remove the last traces of iron and quadrivalent cerium. Add a cooled mixture of 50 mL of the protactinium wash solution and 10 mL of concentrated hydrochloric acid, and 5 mL of 3070 hydrogen peroxide to the separatory funnel and shake vigorously for 2 min. Repeat the peroxide scrub to remove the last traces of titanium, niobium, and tantalum. Strip the organic phase and finish as described above. Any significant quantity of niobium or tantalum carrying through the extraction will produce a noticeable turbidity when the solution has been evaporated to strong fumes of perchloric acid. If the precipitate does not dissolve completely in the sulfuric acid-peroxide solution, rare earths and/or alkaline earths are indicated. Cool the solution thoroughly and recentrifuge before decanting into the separatory funnel. Add 60 mL of concentrated hydrochloric acid directly to the separatory funnel for the extraction, and treat the precipitate as described above for rare earths and/or barium. Precipitation of Protactinium on Barium Sulfate for Q Counting. If gross cr counting of either the pure tracer or the purified protactinium fraction of a sample is desired, add 2 mL of concentrated sulfuric acid to the protactinium in a 250-mL Erlenmeyer flask, and evaporate to fumes to volatilize the nitric or perchloric acids, respectively. Add 3 g of anhydrous potassium sulfate (not the sodium salt) and evaporate to a pyrosulfate fusion. Cool the cake and add a mixture of 20 mL of water and 3 mL each of concentrated hydrochloric acid and 30% hydrogen peroxide, premixed immediately before use. Heat the solution to boiling. Precipitate barium sulfate with two 1-mL portions of 0.45% barium chloride dihydrate, centrifuge, wash, mount the precipitate on a 47-mm DM-450 membrane filter, and count in a zinc sulfide scintillation counter as described in the third and fourth paragraphs under “General Procedure” in the previous publication ( 1 1 ) . The counting efficiency of the zinc sulfide scintillation counter has been found to be highly dependent on the contact obtained between the phosphor and the very soft precipitate of barium sulfate (161, and must be determined carefully for the exact conditions to be used in mounting samples. In the present work, a counting efficiency of 45.2% was obtained when the phosphor was placed gently over the barium sulfate and locked in place with the retaining ring without allowing any pressure to be exerted
1563
on the phosphor. When the phosphor was pressed into the barium sulfate precipitate by rubbing firmly and repeatedly over the entire area with the thumb, the counting rate increased progressively to a maximum at about 52.4%, a relative increase of about 16%. The effect is due apparently to an increase in geometry when the barium sulfate containing a activity is pressed into the small pits in the phosphor surface. The reproducibility is probably better if all pressure on the mounted sample is avoided meticulously than by expending the considerable a:mount of effort necessary to obtain a reproducible maximum. When precipitating a standard solution to determine the efficiency of the scintillation counter, a correction should be made for a small loss of 1 to 3%, due largely to decantation of barium sulfate in both supernate and wash. Decant the supernate and wash from the barium sulfate precipitation back into the same Erlenmeyer flask in which the precipitation was made. When filtered through a membrane filter, the alcohol transfer liquid has never been found to contain barium sulfate and can be discarded. Add 2 mL of concentrated sulfuric acid to the centrifuge tube used and heat carefully to prevent bumping until the refluxing acid cleans the walls of the tube. Cool and add to the other solutions in the flask. Evaporate the solution to a pyrosulfate fusion and repeat the precipitation, mounting, and counting of barium sulfate. Add the activity obtained to that from the main precipitation. Hydrochloric acid is used in the pres8entprocedure to prevent precipitation of lead, bismuth, and polonium isotopes, particularly polonium-210. Precipitation of barium sulfate is slowed down detectably compared to a straight sulfate system but does not affect recovery of protactinium significantly. The barium sulfate is much more crystalline, giving a thin translucent appearance instead of the milky appearance usually obtained. It also packs much more loosely on centrifugation, giving slightly larger losses on decantation than in the absence of hydrochloric acid. However, if even 0.1 mg of barium is present during the pyrosulfate fusion. as in the reclaimed solutions above, t.he solution becomes noticeably turbid with the first few drops of barium chloride during the subsequent precipitation, and the final suspension appears several times as heavy and has the usual milky appearance. This effect is due to the fact that barium sulfate is extremely finely divided when precipitated from a pyrosulfate fusion cake on dissolution in water. The large number of condensation nuclei available from a quantity of barium sulfate barely visible in the Tyndall beam from a flashlight causes the subsequent precipitation to take place rapidly and in a much more finely divided condition. If counting times of several hours are to be used to obtain maximum sensitivity, each phosphor should be precounted for approximately the same length of time before use to determine its individual background. Similarly, if 1;he highest precision and accuracy on high-counting samples is desired, each phosphor should also be screened with a common radioactive source, and phosphors of similar counting rates used for both samples and standards. A blank containing the same quantity of protactinium-233 tracer should also be precipitated on barium sulfate and counted for the same length of time to determine any contribution from either tracer or reagents. If desirable, the protactinium can be stripped directly from the DIBC extract by shaking vigorous1.y for 2 rnin with 50 mL of water containing 3 g of anhydrous po.tassium sulfate, 3 mL of concentrated sulfuric acid, and 2 mL of 30% hydrogen peroxide. Evaporate the strip solution to fumes of sidfuric acid and volatilize any tin that might be present with hydrc’bromic acid. Evaporate the sulfuric acid to a pyrosulfate fusion, dissolve the cake, and precipitate protactinium on barium sulfate as described above. If much tantalum is present, the solution must not be boiled vigorously or the resultant decomposition of hydrogen peroxide will permit hydrolysis of the tantalum, and severe losses of protactinium will result. Heat the solui.ion just to boiling, add 1 mL of the barium chloride solution by slow dropwise addition. heat the solution just back to boiling, and allow to stand for 1 min without further boiling. Set the flask on a clay triangle at the side of the hot plate if necessary. Add the second 1-mL portion of barium chloride and heat for 1 min in the same way. Cool the solution immediately in a cold water bath. Do not use boiling chips and minimize decomposition of hydrogen peroxide. About
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 11, SEPTEMBER 1978
10 mg of tantalum can he accommodated with only a 170loss of protactinium. Considerably larger quantities of niobium can he tolerated even with the vigorous boiling described previously (11). Soil. An excellent procedure for the determination of protact,inium-231 in 10-g samples of soil resulted inadvertant,ly from an investigation directed a t the determination of the transuranium elements (12). However, DIBC is a more selective extractant than Aliquat-336, and the final prot,actinium fraction resulting from the present procedure is somewhat purer, both chemically and radiochemically, for electrodeposition. Dissolve the sample and precipitate barium sulfate as described previously (12). Dissolve the barium sulfate and extract as described above. Water. Add 10 mL of concentrated nitric acid and 200 pL of protactinium-233 tracer containing about lo5 cpm to 250 mL of water in a 400-mL heaker and evaporate to about 5 t o 10 mT, of st,rong nitric acid to ensure complet,e elimination of chlorides to minimize subsequent dissolution of the platinum dish. Avoid letting the solution evaporate to dryness to facilitate subsequent transfer. Add 1 or 2 mL of 48% hydrofluoric acid and rinse the walls of the beaker by swirling the solution vigorously until any caked salts and/or gelatinous silicic acid from dehydration of soluble silicates have dissolved or broken loose from the walls of the beaker. With as little delay as possible to minimize attack on the beaker, transfer the solution, generally turbid with precipitated calcium fluoride, to a 50-mL platinum dish. Rinse the beaker with as little water as necessary to obtain reasonably complete transfer, and evaporate the solution to dryness. Add 3 g of anhydrous potassium fluoride and 0.25 g of silica gel to neutralize the alkali produced in the subsequent potassium fluoride fusion (14), and continue as described above for Uranium Ores and Mill Tailings. Liquid Mill Effluents. Treat an appropriate sample as described for water. Generally, smaller samples will have to he used because of the higher concentrations of calcium, aluminum, salts, etc. that will he present. Air Dusts. Collect a sample of dust from about m' of air on a 4-inch filter using suitable high-volume sampling equipment, e.g., 25 cfm for 1 day. Filters made of cellulose such as the MSA 2133, of polystyrene such as Microsorban, or of glass fiber such as type A/E or spectro-grade (Gelman) can he used. Filters made of other grades of glass fiber generally contain relatively large quantities of barium and should be avoided in the present sulfate system. Fold the filter with the sample toward the inside, tear into strips if necessary to keep the filter from unfolding, and stack in a 50-mL platinum dish. With cellulose and polystyrene filters, ignite and allow to burn slowly. Polystyrene requires external heating to keep the plastic burning smoothly. Ignite the final ash over a small flame from a blast burner if necessary to oxidize the last of the organic matter. Cool the ash, add 200 p L of protactinium-233 tracer containing about lo5 cpm and 0.23 g of silica gel, and evaporate very carefully to dryness. With glass fiber filters, moisten with water to decrease the vigor of the reaction with hydrofluoric acid, add 5 mL of hydrofluoric acid, and evaporate to dryness. Add tracer and reevaporate to dryness. Fuse the residue with 3 g of potassium fluoride and finish as described above for Uranium Ores and Mill Tailings. Bone. Add about lo5 cpm of protactinium-233 tracer and 43 mL of 72010 perchloric acid to 25 g of bone ash in a 400-mL beaker. Cover the beaker with a cover glass and heat moderately until most of the sample has dissolved. Place the beaker on the hottest part of an uncovered high-temperature hot plate and heat stronglluntil the black suspension of carbon which is generally present has been oxidized and most of the excess perchloric acid has heen volatilized. Evaporate the solution a t the highest temperature available from the hotplate until a t,hick syrupy solution is ob. tained (>250 "C.) that tends to crust over and crystallize at the maximum temperature of the hotplate. Do not permit the solution to go dry and hake, even in local spots on the bottom of the heaker. Keep the cover glass in place during the entire evaporation to prolong the digestion and ensure complete dissolution of refractory protactinium compounds. When the solution begins to crystallize, set the beaker off the plate and remove the cover glass to permit the condensing perchloric acid to escape. Allow the beaker and contents to cool to room temperature. As the pasty mass begins to harden, stir with a heavy glass stirring rod to break up the lumps as fine as possible to facilitate dissolution in the subsequent step.
Add 125 mL of concentrated hydrochloric acid to the beaker all at once and stir occasionally to facilitate dissolution of the cake. When the initial frothing has ceased, place the beaker on a moderate hot plate and warm gently. When the solution begins to froth vigororisly, remove the beaker from the hotplate and allow the reaction to proceed until addition of heat is again required to speed up the dissolution. Do not heat any more than is necessarv to maintain a moderate rate of dissolution of the cake to prevent unnecessary loss of hydrogen chloride which will lower the efficiency of the subsequent extraction. Stir the solution occasionally and use the glass rod to break up the larger lumps of cake. When sample dissolution is complete and a clear solution is obtained, place the beaker in a bath of cold running water and cool to room temperature. Transfer the cool solution to a 250-mL separatory funnel and rinse the beaker with an additional IO mL nf concentrated hydrochloric acid. Extract with 50 mIJ of DIBC and finish as described above for Uranium Ores and Mill Tailings. With smaller samples, use 1.7 mL of 72% perchloric acid per gram of hone ash hut not less than 5 mL for dissolution of the original bone ash, and 5 mL of concentrated hydrochloric acid per gram of bone ash with a minimum of about 25 mL for dissolution of the perchloric acid cake. The remainder of the procedure is as described. If higher sensitivity is desired and adequate sample is available, dissolve 100 g of hone ash in 170 mL of 72% perchloric acid in a I-L beaker, dissolve the perchloric acid cake in 500 mL of concentrated hydrochloric acid and make two successive extractions in a 1-L separatory funnel each with 50 mL of DIRC. allowing a t least 10 min each time for adequate phase separation. Combine the two organic extracts in a 250-mL separatory funnel and wash with three successive 50-mL portions of protactinium wash solution to obtain adequate elimination of extracted calcium.
RESULTS AND DISCUSSION All procedures were checked on several different types of actual ores and soils in the presence of tracer. Each fraction was kept separate t o determine the recovery and distribution actually obtained under the recommended conditions. Because of t h e hydrolytic nature of protactinium, every piece of glassware was cleaned thoroughly after use with either 25 m L of 1-to-1hydrofluoric acid a n d / o r a pyrosulfate fusion, and the resulting solution was counted along with t h e other fractions t o determine all sources of loss and t o study contamination of glassware. Even with as many as 19 fractions, material balances of 10070 were usually achieved within the statistics of counting, generally a relative standard d e v i a h n of *0.370. Approximately 3 X lo5 y cpm of protactinium-233 was used per run, and each fraction was counted for 5 min in a 3 inch X 3 inch thallium-activated sodium iodide well crystal in a volume of 50 m L in polystyrene vials t o give a standard deviation of about 0.087~on the major fraction. The results were compared against a standard prepared and counted under identical conditions t o determine t h e percentage recovery. During the early part of this investigation, 40 m L of water and 50 m L of concentrated hydrochloric acid were used t o avoid precipitation of sodium chloride from t h e pyrosulfate fusion when the hydrochloric acid concentration is raised for t h e extraction of protactinium. Also, the early emphasis was on precipitation of protactinium with barium sulfate. T h e distribution of protactinium obtained is shown in test 1 of Table I. Because the major fraction was carried on through the procedure, the values enclosed in parentheses were obtained by difference of the minor fractions from 100c70. Several important conclusions should be noted. Over 96% of t h e protactinium is recovered in the final barium sulfate from a single extraction and a single strip. Generally, repeating treatments and recovering wash solutions are not worth t h e effort. T h e quantity of protactinium remaining unprecipitated by barium sulfate is about 0.0270,similar t o that found previously for other u emitters ( 2 2 ) . The total loss of protactinium due to hydrolytic deposition on t h e walls of
ANALYTICAL CHEMISTRY, VOL. 50, NO. 11, SEPTEMBER 1978
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Table I. Distribution of Protactinium in the General Procedure with and without Niobium
fraction first wash of Te precipitate second wash of Te precipitate Te precipitate (Po fraction) flask from original fusion centrifuge tube from separation of Te first DIBC extract second DIBC extract aqueous phase after two extractions first wash of first DIBC extract second wash of first DIBC extract first strip of first DIBC extract second strip of first DIBC extract first DIBC extract after two strips separatory funnel after extraction aqueous supernate after separation of BaSO, wash of BaSO, final BaSO, after washing (Pa fraction) flask from precipitation of BaSO, centrifuge tube from precipitation of BaSO, material balance
Recovery of 233Pa,%
3
3
4
0.44 0.04 0.27 0.14 0.04 (97.97) 0.43 0.08 0.04 0.02 (96.83) 0.72 0.36 0.59 0.02 0.02 96.80 0.07 0.16 100.24
-
1.31 0.03 0.03 < 0.01 < 0.01 (98.22) 0.11 0.02
Test‘
(93.25) 3.35 2.67 0.03
-
1.53 1.05 0.73
-
-
5 1.13
< 0.01
