Simultaneous determination of alpha-emitting nuclides of radium

ANALYTICAL CHEMISTRY, VOL. 51, NO. 8, JULY 1979

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Simultaneous Determination of Alpha-Emitting Nuclides of Radium through Californium in Large Environmental and Biological Samples Claude W. Sill,* Forest D. Hindman, and Jesse I. Anderson Radiological and Environmental Sciences Laboratory, Department of Energy, Idaho Falls, Idaho 8340 1

A previous procedure for the simultaneous determination of all a-emitting radionuclides from radium through californium in a single 10% sample of soil has been modified for application to 50-9 samples of soil and large samples of water, air dusts, vegetation ash, and soft tissue ash. Even with such large samples of soil, fusions with potassium fluoride and pyrosulfate have been retained to ensure complete dissolution of all refractory compounds including oxides, silicates, and carbides. The actinides from large water samples are concentrated by precipitation with ferrous hydroxide so that insoluble particulates as well as soluble lorms will be collected, both of which can then be dissolved in the subsequent fusions to obtain complete recovery of the radionuclides present in whatever form. Protactinium, uranium, and neptunium are all precipitated with ferrous hydroxide to better than 9 9 % in contrast to recoveries less than 50% that are obtained by the more common precipitation with ferric hydroxide. The actinides from all sample types are then precipitated with barium sulfate, extracted into Aliquat-336, electrodeposited, and analyzed by a spectrometry. Overall yields are generally larger than 90% with most of the radionuclides with electrodeposition contributing the largest and most variable error. Detection limits for a 103-mincount at 28 % counting efficiency are about 0.2 fCi/g for 50 g of soil, 1 fCi/g for 10 g of vegetation ash or soft tissue ash, 1 fCi/L for 10 L of water and 1 aCi/m3 for l o 4 m3 of air.

In a previous publication (I),a procedure was described for the simultaneous determination of virtually all u-emitting radionuclides in a single 10-g sample of soil. This sample size is generally adequate for determination of plutonium-239 and isotopes of uranium and thorium, but larger samples are necessary in some cases to obtain increased sensitivity, precision a n d / o r reliability, such as in the determination of plutonium-238, americium-241, etc. in t h e environment. Larger samples are also desirable to reduce the effect of individual high specific activity particles on the homogeneity of the sample. However, use of large samples invariably leads to use of leaching procedures which cannot guarantee complete dissolution of refractory oxides, silicates, a n d carbides. Consequently, results of lower accuracy and reliability frequently occur. T h e present paper gives directions for scaling u p t h e previous procedure to handle a t least 50-g samples of soil by t h e same fusion technique previously used with smaller samples so that the ability to guarantee complete dissolution of all siliceous a n d refractory components is retained. Attempts by others t o scale u p the previous procedure have frequently resulted in substantial decreases in both accuracy and reliability due to unbalancing certain key parts of the procedure. Furthermore, much useful information has been gained from experience with the procedure since its original development t h a t should be applied even to the smaller samples. T h e procedure is also applied t o large samples of

other types of biological and environmental materials, such as air dusts, water, vegetation, and soft tissue. A rapid and complete separation of protactinium-231 by extraction into diisobutylcarbinol (DIBC) from strong solutions of aluminum nitrate is also included to avoid mutual interference with determination of plutonium-239 a n d / o r neptunium-237 that occurs when 50-g samples of soil are used. Separation of protactinium-231 is also necessary even on smaller samples if plutonium-242 is to be used as tracer because of overlapping of their respective energies. The protactinium-231 can be determined either by gross cy counting on barium sulfate or by electrodeposition and N spectrometry, making this procedure one of the better ones available for determination of this extremely toxic radionuclide. EXPERIMENTAL Instrumentation. The instrumentation used is the same as that described previously ( I ) . Reagents. All reagents are the same as those described previously ( 2 ) except for the reprecipitating solution for barium sulfate in which the 125 g of anhydrous potassium sulfate called for should be increased to 135 g. Tracers. The preparation of all radioactive tracers is the same as described previously (2) except in the case of protactinium which now can also be prepared in pure nitric acid solution ( 3 ) and evaporated directly onto stainless steel plates for standardization. Soil Procedure. Follow the published procedure ( 2 ) reasonably exactly except for the specific recommendations given below. S a m p l e Decomposztzon. LVith a few exceptions, the published procedure for 10-g samples is scaled up fivefold. Treat 50 g of soil in a 500-mL platinum dish with only 2 mL of concentrated nitric acid added at one edge of the sample instead of the excess called for to minimize the quantity of fixed nitrate in the subsequent fusion. Add 60 mL of 48% hydrofluoric acid carefully in 5- to 10-mL portions at first to prevent the sample from frothing over. Add tracers, mix thoroughly, and evaporate the solution to near dryness. When the wet muddy appearance has disappeared, but while the residue is still moist and soft, even on the bottom of the dish, stir the mixture and break up the lumps as fine as possible with a polypropylene stirring rod with a flat end. The more finely divided and uncaked the residue can be kept, the faster and more completely it will dissolve in the subsequent fusion. Mix the residue coarsely with 150 g of anhydrous potassium fluoride, cleaning the walls of the dish as well as possible, and fuse at the maximum temperature obtainable from the Fisher blast burner until a clear melt is obtained. As soon as sufficient liquid flux is available, begin cleaning the sides of the dish. Using long-handled beaker tongs, lift the dish and carefully roll the melt around the sides while heating the adjacent edge of the dish strongly in the flame until all sample above the liquid level has been dissolved. The size of the flame can be reduced somewhat for greater comfort in making this manual operation. When the melt is completely clear, remove the dish from the flame and swirl rapidly to distribute the cake as thinly and uniformly as possible over the sides and bottom as the melt solidifies. As soon as the cake has solidified, place the dish in a shallow bath of cold water about half the depth of the dish to fracture the cake and facilitate subsequent transposition.

This article not subject to U.S. Copyright. Published 1979 by the American Chemical Society

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ANALYTICAL CHEMISTRY, VOL. 51, NO. 8 , JULY 1979

The time required for complete sample dissolution in the fluoride fusion is about 20 min if no large chunks of hard cake were allowed to remain after drying. Support the dish on a heavy nichrome triangle on a ring that is at least 1 inch larger in diameter than the bottom of the dish. Smaller rings or triangles covered with fireclay or vitreous silica intercept so much heat that the fusion is greatly prolonged or prevented, the triangles sometimes stick to the dish, arid the ring gets so hot that it deforms under the weight. Flex the sides of the dish to break the cake loose and turn the cake upside down. Using a pestle or small heavy-walled glass bottle as a hammer, break the fluoride cake into small chunks not over about one-half inch long, taking care not to hammer the chunks directly against the plat,inum dish. Set the dish in a small puddle of water in the hood to dissipate part of the heat to be liberated, and add 7 5 mL of concentrated sulfuric acid. Spray a small stream of water around the sides of the dish as necessary to slow the reaction. Add another 100 mL of concentrated sulfuric acid and repeat the moderate cooling. When the reaction slows down, place the dish on a hot plate covered with a piece of asbestos cloth and heat unti! the frothing again threatens to go over the side of the dish. Keep the transposition of the potassium fluoride going as fast as possible in a controlled way by moving the dish alternately among the hot plate, a piece of cement-asbestos or other nonconducting material, and a cold metal surface as necessary. Be prepared at all times to cool the dish rapidly in a cold water bath should it become necessary. After about 1 h, most of the potassium fluoride will have been transposed and the frothing due to evolution of hydrogen fluoride will have decreased markedly. As soon as residual frothing or boiling permits, place the dish on a small stainless steel planchet on the hare hot plate for another hour to evaporate as much water and excess sulfuric acid as possible. and to transpose the remaining small pieces of potassium fluoride. Finally, place the dish on a ring stand and begin heating the solution carefully with the burner until the solution boils smoothly and light fumes of sulfuric acid are evolved. As soon as sufficient liquid flux is available, work the melt around the sides of the dish to transpose all potassium fluoride above the liquid level. Before the solution begins to thicken, add 100 g of anhydrous sodium sulfate and continue heating at increasingly higher temperatures as the boiling point of the mixture increases until a clear pyrosulfate fusion is obtained as described previously ( I ) . During cooling, distribute the pyrosulfate cake around the sides of the dish and fracture the cake by setting the dish in a cold water bath. Color photographs of the entire dissolution process for 10-g samples have been published 14). Heat 1750 mL of water and 125 mI, of concentrated hydrochloric acid to boiling in a 4-L beaker covered with a watch glass and containing several 8-mesh silicon carbide boiling chips. Flex the sides of the dish to break the cake loose from the sides and add the cake carefully to the boiling solution. When the cake has dissolved, proceed with the hydrolysis of condensed phosphates, and precipitation of barium sulfate as described. Add 5 mL of 2570 potassium metabisulfite and an additional 250 mL of either hydrochloric acid and/or water depending on the quantity of calcium present and on whether or not determination of tervalent actinides is desired. On blanks and samples containing little iron, reduce the volume of 2570 potassium metabisulfite to 0.5 mL and add 5 mL of freshly prepared 20% ferrous ammonium sulfate hexahydrate to avoid reduction of platinum to the elemental state during reduction of plutonium and neptunium. Similarly, for 10-g samples, heat the 350 mL of water and 25 mL of hydrochloric acid ( 1 ) to boiling in the 800-mL beaker before adding the pyrosulfate cake to facilitate rapid and complete dissolution of calcium sulfate, and reduce the volume of potassium metabisulfite on samples to 1 mL, and the volumes of potassium metabisrilfite and ferrous anmionium sulfate on blanks to 2 drops and 1 mL, respectively. Barium Sulfate Recipitatiun. Although the quantity of barium sulfate used in the previous procedure with 10-g samples gives approximately 90% recovery on 50-g samples with plutonium, and presuniably with thorium and protactinium, the quantity of barium used must be increased at least threefold to obtain reasonable recovery of americium and the other tervalent actinides. To prevent decreasing both the efficiency of carrying the desired elements and the tolerance to interfering elements, the con-

centration of barium chloride and the number of separate additions must be kept the same. Consequently, the volume of barium chloride added in each increment is increased threefold. Add 15 mL of 0.45% barium chloride dihydrate to the boiling solution in about 1.5-mL increments every 3 s while stirring the solution vigorously and continuously with a stirring rod. Replace the cover glass, heat the solution back to boiling, and boil actively for 1 min. Repeat the stepwise addition and 1-min boiling with five more portions. If curium and californium are of primary importance and maximum yields are desired, increase the total number of additions to nine. Let the solution stand undisturbed for 15 min to settle and cool somewhat and filter the solution while still hot through a 47-mm GA-6 membrane filter (Gelman Instrument Co., Ann Arbor, Mich.) in a 300-mL borosilicate glass filtering chimney (Millipore Corp., Bedford, Mass.). Use only partial vacuum to avoid boiling over. Rinse the empty beaker, funnel, and precipitate with 0.5% sulfuric acid. Remove the funnel and, while the suction is still applied, add several drops of water around the edge of the paper to wash that part of the filter protected by the shoulder of the filtering chimney. With a few samples, the barium chloride solution can be added conveniently from a 15- to 25-mL graduated pipet to facilitate measurement of the individual increments added. However, when many samples are to be analyzed, the tediousness of making such large numbers of repetitive additions can be reduced significantly by using a mechanical volume dispenser (“Auto-Pet,’’ Lab Apparatus Co., Cleveland, Ohio) set to deliver 1.5 mL. Place the barium chloride solution in a polyethylene squeeze bottle and connect the bottle to the inlet of the dispenser by means of a piece of small polyethylene tubing inserted into the solution through the center hole in the cap. Connect the outlet of the dispenser to a piece of polyethylene tubing with a piece of glass tubing at the end, bent so that it can be hooked over the rim of the beaker under the watch glass. Displace the cover glass only enough to permit rapid strokes of a stirring rod back and forth through the center of the solution. In this way, boiling is not interrupted significantly and stirring is efficient enough to disperse the barium chloride throughout the solution before precipitation begins to obtain efficient carrying. After precipitation of each sample has been completed, rinse the outside of the glass tube and expel one volume of reagent to the sink to preclude any chance of cross contamination between samples. Reprecipitation o f Barium Sulfate. If the barium sulfate has a significant color, or if elimination of elemental platinum is to be ensured, add 5 mL each of concentrated nitric and hydrochloric acids to the flask containing the filter paper and barium sulfate and evaporate to near dryness to dissolve the metallic platinum before adding the sulfuric acid for reprecipitation of the barium sulfate. Use 7 or 10 mL of concentrated sulfuric acid depending on whether six or nine additions of barium chloride, respectively, were used instead of the 4 mL used with 10-g samples. Omit addition of the potassium sulfate, which decreases the solubility of barium sulfate, and heat the sulfuric acid to strong fumes. Add several drops of a 1-to-1 mixture of nitric and perchloric acids while swirling the solution vigorously between drops to remove the last of the organic matter, and then 2 drops of 7270 perchloric acid to remove the last traces of nitric acid. The larger quantity of nitrated cellulose produced from the filter paper during dissolution of the platinum with aqua regia requires considerably more attention to ensure its complete oxidation than if it had been oxidized more rapidly with nitric and perchloric or sulfuric acids. If not oxidized completely, the nitro compounds will oxidize most or all of the ferrous iron, and particularly tervalent titanium if used, in subsequent steps. If not reduced to the quadrivalent state, neither uranium nor plutonium will be precipitated with barium sulfate and severe losses will result. Continue heating the sulfuric acid until the barium sulfate has dissolved completely, which takes a few minutes with the larger quantities of barium sulfate, and until excess perchloric acid and the cerium and plutonium that will have been oxidized by the perchloric acid have been thermally decomposed to their reduced forms. Use a small flame to minimize the sulfuric acid allowed to escape. A slight turbidity of silica or anhydrous sulfates of iron, aluminum, etc. might remain. Cool the solution to room temperature and add 100 or 125 mL of the reprecipitating solution depending on whether ’i or 10 mL of sulfuric acid, respectively, was used. Do not delay addition


of the reprecipitating solution after the sulfuric acid has cooled or absorption of water vapor might cause premature precipitation of barium sulfate. Add 3 silicon carbide boiling chips, reducing agents, and finish as described. However, reduce the volumes of 25% potassium metabisulfite and 20% ferrous ammonium sulfate to 1 drop and 3 drops, respectively, on either 50- or 10-g samples. A small fraction of the platinum will precipitate with barium sulfate from the quadrivalent state, giving the precipitate a slight buff color but will not interfere subsequently in most cases. However, some elemental platinum will be reformed if too much bisulfite or prolonged boiling is used. Except for the scaleup in the quantity of sulfuric acid and reprecipitating solution, the other changes should also he applied to the barium sulfate obtained in the previous procedure employing 10-g samples. Gencral DinsolutiorL of Barium Sulfate and Extraction. To accommodate the threefold increase in quantity of barium sulfate used, both the dissolution of barium sulfate and the subsequent extraction must, be scaled up I.5-fold, except for the volume of Aliquat-336 used in the first extraction. If the tervalent actinides are to be determined in addition to the quadrivalent ones, the quantity of perchloric acid must be kept small, and the barium sulfate must be dissolved directly in aluminum nitrate. \Vet-ash the filter containing the barium sulfate in a 250-mL beaker with 3 mL of 1-to-1nitric and perchloric acids, swirling the solution gently Over the bottom of the beaker almost continuously until heavy fumes of perchloric acid are produced. Otherwise, the barium sulfate will cake, and complete dissolution will be severely retarded. Add 5 mL of concentrated nitric acid and fuse with 7 5 g of solid A1(N03)3.9H20,using the grinding technique described to break up the lumps of barium sulfate. As soon as the solution begins to boil, swirl the solution thoroughly to suspend the barium sulfate and set the beaker on a clay triangle at the edge of the hot plate for 5 min until virtually clear, allowing little or no further movement of the surface of the solution. While swirling the solution continuously, add 20 mL of 0.75 M aluminum nitrate to which 1 mL of 2570 sodium nitrite was added immediately before use. Heat the solution back just to boiling and set the beaker back on the clay triangle for another minute. Add another 25 mL of 0.75 M aluminum nitrate solution, and cool the solution to room temperature. Transfer the solution into a 250-mL separatory funnel with another 30 mL of 0.75 M aluminum nitrate, add 50 mL of 5 0 7 ~DIBC (Union Carbide Chemicals Co., New York, N.Y.) in xylene, and shake vigorously for 1 to 2 min to extract protactinium-231. Draw the lower aqueous phase into another 250-mL separatory funnel and scrub the organic phase for 1 min with 10 mL of the 2.2 M aluminum nitrate solution (not the 0.75 M sohtion), combining the scruh with the main aqueous phase in the other separatory funnel. Either discard the DIBC extract or use it for determination of protactinium-231 (31, if desired. In the latter case, the extract must be scrubbed at least once with wash solution (3)to remove most of the aluminum, the strip solution evaporated to a pyrosulfate fusion, and the protactinium reextracted into fresh DIBC from a chloride solution to eliminate nitrates and/or nitro compounds which interfere with reduction and elimination of iron ( 3 ) . Add 50 rnL of 3 0 7 ~Aliquat-336 and 8 mL of 25% sodium nitrite to the 250-mL separatory funnel containing the aluminum nitrate solution. Swirl the separatory funnel gently just enough to distribute the blue color of the nitrous acid throughout the aqueous phase. Let the solution stand for 2 min. extract vigorously for 3 min, and draw the aqueous phase into a 500-mL separatory funnel for determination of the tervalent actinides. Scrub the organic phase with one io-mL portion of 8 M nitric acid and reserve for recovery of the tervalent actinides present. Scrub the organic phase with three successive 75-mL portions of 10 M hydrochloric acid to remove thorium adequately from such large samples. Discard the hydrochloric acid scrubs because they will normally contain far too much thorium (ca. 750 pg) to be accommodated on electrodepositon and (Y spectrometry. However, if a 50-g sensitivity for thorium is desirable for application to particularly barren rocks or for other geochemical purposes, increase the number of 8 M nitric acid scrubs to four to remove barium. aluminum, tervalent actinides, and lanthanides more completely. Otherwise, they will strip in the subsequent hydrochloric acid scrubs and will interfere seriously with electrodeposition of thorium. Strip the organic phase with 50 mL of

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perchloric-oxalic acid and water and finish as described. Alternate Dissolution and Extraction. If determination of the tervalent actinides is not desired, the simpler perchloric acid dissolution method can be used. Dissolve the barium sulfate and filter by boiling with 2 mL of concentrated nitric acid and 10 mL of 72% perchloric acid in a 250-mL Erlenmeyer flask. Particularly after reprecipitation, barium sulfate is very finely divided and forms lumps of tightly-packed powder that require about 2 min of vigorous boiling after the perchloric acid has been reconcentrated to 7270 to disperse and dissolve completely. However, the quadrivalent elements thorium, zirconium, and cerium, all of which are carried in the barium sulfate, form sulfates with the sulfate ion from the dissolution of the barium sulfate which are almost quantitatively insoluble in boiling 72% perchloric acid. Consequently, barium sulfate that has been precipitated from samples containing these elements will not clear up in boiling perchloric acid but will clear up easily on subsequent dilution with water, dilute hydrochloric acid, aluminum nitrate solution, etc. When the refluxing perchloric acid reaches the top of the flask, cool for 1 min and add 75 mL of the acidic 2.2 M aluminum nitrate solution containing 1 mL of 2570 sodium nitrite added immediately before use while swirling the flask continuously. Heat the solution just to the boiling point, then cool and transfer to a 250-mL separatory funnel. Rinse the walls of the flask with another 25 mL of the 2.2 M aluminum nitrate and add to the separatory funnel. Add 50 mL of 5070 DIBC in xylene and shake vigorously for 1 to 2 rnin to extract protactinium-231. Scrub the DIBC with 10 mL of 2.2 M aluminum nitrate and combine with the main aluminum nitrate solution in another separatory funnel. Reserve the DIBC extract for the determination of protactinium-231 or discard as desired. Extract the aqueous aluminum nitrate phase with 50 mL of 30% Aliquat-336 after addition of 8 mL of 2570 sodium nitrite as described for the "General Dissolution". Because of the large quantity of perchloric acid present, the tervalent actinides cannot be extracted efficiently, and the aluminum nitrate phase should be discarded after the first Aliquat-336 extraction. Deterrninatzon of Uranium. Most soils contain enough natural uranium (ca. 2.5 pg/g) that analysis of 50-g samples will not usually be required. However, when higher sensitivity and/or precision is desirable for application to barren rocks or other geochemical purposes, the procedure can be scaled up similar to that for plutonium. To avoid decreasing yields on electrodeposition and degraded (Y spectra, the quantity of uranium present should be kept below 100 pg. Complete elimination of polonium-210 has been erratic at times, giving higher apparent yields of uranium-232 tracer and low results for uranium. Consequently. the following changes should be made to the previous procedure for 10-g samples in the section on "Determination of Uranium" ( I ) . Use a 0.625% solution of Te02 in 25% hydrochloric acid which is more stable than the solution recommended previously. Add 2 mL as the catalyst for reduction of iron by sulfite as described in the first paragraph, and 5 mL more as called for in the second paragraph before addition of titanium trichloride. Boil the solution actively for 5 min as described in the fourth paragraph. The quantities of tellurium carrier, titanium trichloride, and chromous chloride will all have t o be scaled up by at least a factor of two for application to 50-g samples. Determznation of TerLalent Actinides. Evaporate the 10-mL 8 M nitric scrub of the first Aliquat-336 extract to about 0.5 mL and dissolve in 75 mL of 2.5 M acid-deficient aluminum nitrate. Add 8 mL of 25% sodium nitrite and transfer to the 500-mL separatory funnel containing the acidic aluminum nitrate solution from the first Aliquat-336 extraction. Extract with 75-mL of 3070 Aliquat-336, scrub the organic phase three times with 25-mL portions of 10.8 M ammonium nitrate, and strip the tervalent actinides with consecutive 45-mL and 15-mL portions of 8 M nitric acid. Treat the combined strips with 3 mL of 7270 perchloric acid and 10 mL of concentrated hydrochloric acid and evaporate the solution to strong fumes of perchloric acid to oxidize ammonium salts and traces of residual organic matter from the extraction. Omit addition of sodium hydrogen sulfate or sulfuric acid to eliminate solubility problems from the light lanthanides. The separation of the lanthanides from the actinides must also be scaled up to accommodate the larger quantities present in 50

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g of soil. However, recent reinvestigation of this separation has disclosed several other problems so that the reader is referred to the original work for their detailed description and solutions (5). Water Procedure. Place 3 L of water, 10 mL of concentrated hydrochloric acid, and several silicon carbide boiling chips in a 4-L beaker and evaporate to not lower than about 1 L. Add i nother 2 L of water and repeat the evaporation until a total of 10 L has been evaporated to a final volume of 2 L. If smaller samples are taken, decrease the quantity of acid added so that not more than 5 mL of concentrated hydrochloric acid is present in 2-L volume at time of precipitation with ammonium hydroxide. If more acid is present, neutralize the excess with sodium hydroxide to avoid buffering the solution with too much ammonium ion. Add 10 mL of 5% ferric chloride hexahydrate (ca. 100 mg iron) in 5% hydrochloric acid, whatever tracers are desired, and 10 mL of 25% potassium metabisulfite. Cover the beaker with a watch glass and boil the solution for 10 min to reduce the iron to the ferrous state, to hydrolyze any condensed phosphates that might be present, and to volatilize most of the sulfur dioxide which will otherwise cause precipitation of calcium sulfite when the solution is subsequently made alkaline. Without removing the cover glass or disturbing the smooth evolution of water vapor from the boiling solution in any way, add a small stream of concentrated ammonium hydroxide from a polyethylene squeeze bottle with the delivery tube inserted under the cover glass through the pouring lip of the beaker. The boiling solution stirs the solution more efficiently than is obtained with a stirring rod, and the smooth evolution of steam from the covered beaker effectively prevents reoxidation of iron by contact with air. Add ammonium hydroxide until a light, permanent turbidity is proc’uced and then add an excess of 10 mL. The precipitate is generally a light green color, presumably of ferrous hydroxide, which becomes almost white if phosphate is present. Boil the solution for 10 min to complete precipitation and flocculation of the insoluble hydroxides. Add a small additional squirt of ammonium hydroxide (ca. 1 mL) and set the beaker in a cold water bath for 10 min. Remove the beaker from the water bath and place in a vibration-free place to settle undisturbed for 30 min. Carefully decant and discard as much of the supernatant liquid as possible without disturbing the precipitate. With care, all but about 150 to 200 mL can be decanted with a loss of precipitate of about 3 to 5%. Swirl the beaker to resuspend the precipitate in the remaining solution and transfer the suspension to as many 90-mL roundbottomed centrifuge tubes as are required. Centrifuge a t 2000 rpm for 5 min. Discard the supernates. Rinse the 4-L beaker with 10 to 25 mL of nitric acid, and divide the rinses equally among the centrifuge tubes. Place the tubes in a wire rack on the hot plate and heat carefully with swirling to prevent bumping until the precipitate has dissolved. Continue heating until all occluded chloride has been oxidized as indicated by the disappearance of all odor and evolution of colored gases of chlorine and/or nitrogen oxides. Otherwise, severe dissolution of the platinum dish will occur in the next step. Transfer the solutions to a 50-mL platinum dish and rinse the centrifuge tubes each with two 2-mL portions of nitric acid, 1mL of hydrofluoric acid added dropwise around the sides to dissolve silica, and two 2-mL portions of water. Evaporate the solution until the residue is just moist with acid. Do not permit to dry and bake or significant corrosion of the platinum dish will occur and complete dissolution of the residue will be greatly retarded. Add 0.5 g of silica gel (Fisher Scientific Co., Pittsburgh, Pa., S-157, 28-200 mesh) and 6 g of anhydrous potassium fluoride, mix coarsely with a stirring rod, and fuse over a blast burner. If the water sample contains a significant quantity of siliceous sediments, reduce the quantity of silica gel somewhat to compensate. Cool, add 7 mL of concentrated sulfuric acid and transpose to a pyrosulfate fusion using 4 g of anhydrous sodium sulfate as described previously ( I , 6). Cool the dish without swirling to keep the melt in the bottom of the dish to facilitate its removal and flex the sides gently until the cake breaks loose. Transfer the contents to a 250-mL beaker containing 65 mL of water and 5 mL of concentrated hydrochloric acid at the boiling point. Rinse the platinum dish with some of the hot solution as necessary and then with 10 mL of water to obtain complete recovery of the cake. Add 1mL of 25% potassium

metabisulfite, cover the beaker with a watch glass, and boil the solution for 5 min to hydrolyze any condensed phosphates that might have been formed during the fusion, and to reduce plutonium and neptunium completely to the ter- or quadrivalent state. Add 5 mL of 0.45% barium chloride dihydrate from a Mohr pipet to the boiling solution a t a rate of about 0.5 mL every 3 s while stirring the solution vigorously (1). Boil the solution for 1 min and repeat the addition of barium chloride and 1-min boiling two more times. Filter the solution while still hot through a 47-mm GA-6 membrane filter in a filtering chimney. Wash the precipitate with 0.5% sulfuric acid. Reprecipitate the barium sulfate and dissolve in either perchloric acid or aluminum nitrate fusion and finish as described previously (I). If desired, the conditions for dissolution of barium sulfate and the subsequent extraction can be reduced to one-half that given to make them comparable to the quantity of barium sulfate used. If only small samples are available or necessary, the procedure can be simplified somewhat. For volumes of fresh water of 500 mL or less containing less than about 100 mg of calcium, evaporate the solution containing 0.5% hydrochloric acid nearly to dryness. Add 10 mL of concentrated nitric acid and boil to eliminate chlorides. Transfer the solution to a 50-mL platinum dish and evaporate to near dryness. Finish as described above beginning with the 6-g potassium fluoride fusion. Up to 15 mL total of concentrated hydrochloric acid can be used as necessary to dissolve calcium sulfate if only nuclides of protactinium through plutonium are to be determined. If the tervalent nuclides are desired, and the calcium sulfate cannot be dissolved with 5 mL of hydrochloric acid, use a smaller sample or make the ferrous hydroxide separations. Because of the relatively large quantities of calcium present in most waters, volumes larger than 500 mL must be treated by the full procedure given for soil or, preferably, separated with ferrous hydroxide. However, if phosphate is present, no separation from calcium will be obtained, and smaller samples will have to be used. If uranium, neptunium and/or protactinium are not to be determined, the procedure for plutonium and the tervalent actinides can be simplified somewhat. Decrease the quantity of 5% ferric chloride solution to 5 mL, eliminate the potassium metabisulfite, and add about 1 g of hydroxylamine hydrochloride after the neutralization with ammonium hydroxide. By leaving the iron carrier in the tervalent state, the more insoluble ferric hydroxide will flocculate faster and more completely, and more of the supernatant liquid can be decanted. However, under these conditions, precipitation of uranium, neptunium, and/or protactinium will be markedly incomplete. Soft Tissue Ash. Weigh 10 g of soft tissue ash into a 250-mL platinum dish, add 30 g of anhydrous potassium fluoride and 1 g of potassium nitrate, and mix thoroughly with a spatula or stirring rod. Pretreatment of the ash with nitric and/or hydrofluoric acids as recommended for soil, or with perchloric acid, is not recommended because of corrosion of the platinum dish when the solution is subsequently evaporated to dryness, apparently due to the presence of phosphates. Add 1 mL of plutonium-236 or other tracer in 10% nitric acid dropwise uniformly over the mixture. Place the dish on a hot plate for a few minutes until the tracer solution has evaporated. Heat the mixture over a high-temperature blast burner as described for soil (1) until the frothing ceases and a clear melt is obtained. If unoxidized organic matter still remains, which appears black when cold or reddish at the temperature of the fusion, additional solid potassium nitrate should be sprinkled into the melt in small quantities and the heating continued until the organic matter has been oxidized completely. A considerable quantity of organic matter can be oxidized in this way in the fusion so that excessive time should not be wasted in obtaining a pure white ash during muffling. However, potassium nitrate decomposes t o the oxide during the fusion so that no more than necessary should be used to minimize frothing and the effect of excessive alkalinity on the solubility of heavy metals in the alkaline flux. Cool the melt to room temperature and add 35 mL of concentrated sulfuric acid in increments not larger than 5 to 10 mL at a time, as the vigor of the reaction will permit, with sufficient time between increments for the frothing to subside to avoid frothing over the sides of the dish. Be prepared to set the dish in a bath of cold water should the necessity arise to prevent


frothing over. After all the sulfuric acid has been added and the reaction has subsided, heat the dish on the hot plate intermittently as frothing will allow until the cake is transposed completely and the frothing ceases. The potassium fluoride cake from 10 g of soft tissue ash transposes with sulfuric acid much more rapidly and with more frothing than with 10 g of soil so that more care and watchfulness are required during the transposition. Add 20 g of anhydrous sodium sulfate and finish as described for soil ( I ) . Because of the significant quantity of phosphate present in many soft tissues, the hydrochloric acid solution of the pyrosulfate cake must be boiled for a t least 15 min to hydrolyze the pyroor other condensed phosphates that will have been formed from the orthophosphates during dry-ashing and/or the pyrosulfate fusion, and which will interfere seriously with precipitation of heavy metals on barium sulfate (and with many other separations) if not removed by hydrolysis. If much calcium is present, calcium sulfate will frequently precipitate after the sequestering effect of the condensed phosphates is removed by hydrolysis. If the solution becomes turbid on boiling, more hydrochloric acid should be added immediately. If more calcium is present than can be kept in solution with the quantity of hydrochloric acid prescribed in the soil procedure, the solution should be diluted and an aliquot taken as described in the soil procedure ( I ) . Although the procedure described will handle 10 g of ash, the ash from cartilaginous tissues, trachea, and some lungs generally contains too much calcium to permit such large samples to be used. Vegetation Ash. Place 10 g of vegetation ash into a 250-mL platinum dish and add 5 mL of water at one side of the sample. Add 2 or 3 mL of 48% hydrofluoric acid to the water and carefully allow the powdered ash to slide into the acid solution as slowly as the vigor of the reaction requires. If the sample is added too rapidly to too strong acid, the vigorous reaction and evolution of carbon dioxide will blow the light dry powder out of the container and large losses of sample will result. U’hen effervescence becomes slow, add additional small quantities of hydrofluoric acid and repeat the careful addition of the ash until the entire sample has been contacted with acid and evolution of carbon dioxide ceases. About 10 to 15 mL of 48% hydrofluoric acid will be required. Add 1 mL of concentrated nitric acid, whatever tracers are desired, and evaporate the solution until most of the free liquid phase has evaporated. Leave the cake distinctly moist with acid to facilitate subsequent dissolution and transposition and to minimize the attack on the platinum dish by any phosphates that might be present. Add 30 g of anhydrous potassium fluoride, stir to a coarse mixture, and fuse over a blast burner. Add 35 mL of concentrated sulfuric acid, transpose to a pyrosulfate fusion using 20 g of anhydrous sodium sulfate and finish as described for soil (I). As with soft tissue ashes, some vegetation ash also contains too much calcium to permit samples as large as 10 g of ash to be used. Some vegetation ash also contains so much barium that the quantity of barium used in the precipitation should be reduced somewhat to compensate. The residue from dry-ashing fecal samples can be dissolved by a similar procedure. Air Dusts. Collect the air dust on a 4-inch filter of suitable retentivity and low pressure drop using a high-volume sampler a t 30 to 40 cfm for 1 day to give at least lo3 m3 of air. Most glass fiber filters contain large quantities of calcium and/or barium and excessively high blanks of naturally occurring radionuclides and should he avoided. However, the 1HV paper (Schleicher and Schuell, Keene, N.H.) has been found to be excellent in both respects, and has the distinct chemical advantage that it can be dissolved completely with hydrofluoric acid in a platinum dish, evaporated to dryness, and then fused with potassium fluoride. With combustible filters, fold the filter in half with the sample toward the inside. Tear the half-circle in half and then the resultant quarter-circles again in half through the right-angle corner. Stack the four wedge-shaped pieces together and place them in a 50-mL platinum dish with the broad edge at the bottom. Ignite the pointed end of the filter with a small flame from a blast burner and allow the filter to burn slowly. If necessary, turn the hood exhaust fans off to avoid blowing some of the very fragile ash from cellulose filters out off the dish. Heat the dish from the bottom with a small flame from the blast burner as necessary to complete the combustion of the filter. Polystyrene filters such BS Microsorban will require external heating on a hot plate or over

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a small flame almost from the outset to keep the plastic burning smoothly at a reasonable rate. Several 4-inch filters in succession or larger filters can be accommodated by the same technique when larger samples or composites are desired. Tests with radioactive tracers have demonstrated that no significant loss of nonvolatile elements occurs, even in the very sooty smoke from burning polystyrene, provided the light ash from cellulose papers or the liquid melt from plastics is not physically blown out of the container. Ignition has the additional advantage of volatilizing much of the polonium-210 which is perhaps the most common naturally-occurring N emitter to be encountered in air dusts, and which interferes with the determination of much smaller quantities of other N emitters having lower energies. Fuse the ash in the platinum dish with anhydrous potassium fluoride and finish as described beginning with the second paragraph under “Determination of Thorium-230 in Uranium Ores and Mill Tailings” ( 7 ) with the following exceptions. Add any tracers desired in nitric acid solution (2) to the residue in the platinum dish and evaporate to dryness before the potassium fluoride fusion. Use only 2 mL of 8 M nitric acid in each of the three nitric acid scrubs. If the tervalent actinides are to be determined, dissolve the barium sulfate in a 10-g aluminum nitrate nonahydrate fusion as described originally ( I ) instead of the perchloric acid. All other reagents called for are also scaled down by a factor of 5 . If radiochemical determination of uranium is desired on the same sample, treat the filtrate from the barium sulfate precipitation above (from Ref. 7) as described under “Determination of 234Uand 238U‘r(8) with one very important modification. Because the diethyldithiocarbamate extraction employed in the previous work ( 8 ) to remove lead-210 is not used, any polonium-210 not volatilized in the original sample treatment will still be present in the solution and will interfere seriously with the use of uranium-232 tracer if not removed. Also, 1 or 2 mg of platinum will be present in the present application from having made a pyrosulfate fusion in a platinum dish which was not permitted in the preceding work involving determination of lead. Because elemental platinum will be precipitated by the powerful reducing agents required to reduce uranium to the quadrivalent state and must be filtered off, the same operation is used for separation of polonium-210 instead of the dithiocarbamate extraction Evaporate the combined filtrate and wash from the original precipitation of barium sulfate in the 100-mL beaker ( 7 ) back to about 35 mL. Add 2 mL of 0.625% solution of tellurium dioxide in 25% hydrochloric acid to the boiling solution and proceed with addition of Safranine 0 indicator, titanium trichloride and chromous chloride as described in the first part of the second paragraph under “Determination of 2341! and 23RU”(8). Boil the solution vigorously for 5 min to obtain complete precipitation and flocculation of tellurium, platinum, and polonium. Cool the solution to room temperature and filter through a DM-450 membrane filter in a glass filtering chimney. Heat the filtrate back to boiling, add 2 or 3 more drops of chromous chloride to ensure complete reduction of uranium and the titanium holding reductant, precipitate barium sulfate containing quadrivalent uranium as described in the latter part of the same paragraph, and finish as described thereafter (8). When wet-ashing is desirable. fold the 4-inch filter in half with the sample toward the inside, tear the paper in narrow strips and insert them into a 250-mL Erlenmeyer flask. Add 3 mL of concentrated sulfuric acid, 15 mL of concentrated nitric acid, any carriers or tracers desired, and heat gently on a hot plate until evolution of red-brown gases ceases and the solution has evaporated to a charred and essentially dry and immobile mass of residual organic matter. Cool somewhat, add 5 mL of a 1-to-1 mixture of concentrated nitric and perchloric acids, and continue heating on the hot plate until all organic matter has been oxidized and excess perchloric acid has been eliminated. With the MSA 2133 all-dust paper, add 2 mL more 1-to-1nitric-perchloric acid mixture after the perchloric acid has begun to fume strongly to oxidize the carbon present (if the solution is still black). Nitric acid added to boiling concentrated perchloric acid is much more effective in oxidizing carbon than either acid alone. Add 4.6 g of anhydrous potassium sulfate and evaporate to a pyrosulfate

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fusion. Add 2 g of anhydrous sodium sulfate and continue heating until the added salt has dissolved to ensure complete dissolution subsequently in water if significant quantities of heavy elements such as iron, aluminum, chromium, etc. are present. Dissolve the cake in 35 mL of water and 3 mL of concentrated hydrochloric acid and proceed with the barium sulfate precipitation as described above.

RESULTS AND DISCUSSION Soil Procedure. If excess nitric acid is added to 50 g of soil, so much of the acid becomes fixed because of its neutralization by the basic components of the soil that extended bubble formation occurs during the subsequent potassium fluoride fusion owing to thermal decomposition of the nitrate. The continuous presence of bubbles in the liquid flux makes it difficult to tell when the sample has dissolved completely. Furthermore, in samples containing high concentrations of iron or other elements forming very insoluble hydroxides, thermal decomposition of the metallic nitrates results in rapid precipitation of oxides or hydroxides on top of the remaining sample, making sample decomposition incomplete. Obviously. any siliceous material not dissolved in the potassium fluoride fusion will not dissolve in the subsequent pyrosulfate fusion after the oxide cover has been redissolved. If hydrofluoric acid is used to neutralize the carbonates and other basic components of the soil, and only a small controlled quantity of nitric acid sufficient to oxidize the organic matter during the subsequent fusion is added, complete dissolution of a wider variety of s.mples is more readily achieved and the liquid flux appears completely clear. A small controlled quantity of nitrate is also very beneficial in the dissolution of carbides. For example 0.5 g of carborundum (silicon carbide) can be dissolved easily and completely in a flux composed of 3 g of potassium fluoride and 1 g of potassium nitrate. Limiting the quantity of nitric acid added is sufficiently beneficial that similar changes should be made in the previous procedure for 10-g samples. Use only 0.5 mL of concentrated nitric acid with excess hydrofluoric acid before evaporation to near dryness. With samples containing sulfides, evaporate 10-g samples with excess nitric acid to complete dryness, then reevaporate to near dryness with 48% hydrofluoric acid. An increase in alkalinity normally occurs during a potassium fluoride fusion due to slow hydrolysis of fluoride ion. In the presence of significant concentrations of heavy metals, precipitation of insoluble hydroxides occurs, again coating over the remaining sample making dissolution incomplete. Potassium pyrosulfate was used originally to neutralize some of the alkali but was effective only in very mild cases. More effective remedies have now been found. For example, if an iron salt containing 100 mg of iron is fused with 6 g of potassium fluoride, such as in the water procedure in which ferric hydroxide is used as a carrier, a deep red-brown turbid melt is obtained with almost black crystals sticking to the bottom and sides of the platinum dish. In contrast, a 10-g sample of soil containing a t least five times as much iron gives a completely clear and almost colorless melt with 30 g of potassium fluoride. However, if the same quantity of soil is leached with nitric acid and the acid-soluble portion is evaporated to dryness and fused with the same quantity of potassium fluoride, the same highly-colored, turbid melts that are obtained with pure iron compounds are obtained. Obviously, some nonleachable component of the soil prevents precipitation of ferric hydroxide when the entire soil sample is fused intact. It has now been demonstrated that this component is the silica itself. Addition of a small quantity of silica to the pure iron salt before fusion with potassium fluoride causes the melt to be completely clear and almost colorless. Apparently, the silica acts as a solid acid, neutralizing the excess alkali to form metasilicates. However, addition of too much silica must be avoided or the excess

potassium fluoride will be inadequate to obtain complete elimination of silica. Boric acid is another solid acid that can be added directly to the hot melt without splattering to dissolve the metallic hydroxides and allow dissolution of the uncovered siliceous material to proceed to completion while the fluoride flux is still present. In fact, boric acid is somewhat more effective than silica in dissolving metallic hydroxides already precipitated because of the neutralizable hydrogens present. Like silica, boric acid is volatilized virtually completely, probably as fluoboric acid, during transposition of the potassium fluoride cake with sulfuric acid. However, too much boric acid will eventually use up so much fluoride that volatilization of silica will be incomplete, resulting in turbid pyrosulfate melts. A t least 0.5 g of boric acid has been used with fusions of about 50 mg of iron in 3 g of potassium fluoride without any detectable adverse effects. As mentioned previously ( I ) , calcium sulfate becomes distinctly less soluble in dilute hydrochloric acid the longer it is allowed to remain in the crystalline state before dissolution. Much more complete dissolution of calcium can be obtained by adding the pyrosulfate cake to the dilute hydrochloric acid after the latter has been heated to boiling as described above. Even the small quantities of strontium and barium indigenous to the soil generally dissolve temporarily. The cake generally dissolves initially to give a completely clear solution that develops a fine white turbidity on boiling for 2 or 3 min that has been shown conclusively to consist of barium and/or strontium sulfate. Apparently, barium is not present as barium sulfate in the solid pyrosulfate cake, and the barium and sulfate ions produced on dissolution require a few minutes of boiling to precipitate completely from the low concentrations involved. This reverse order of adding the solid cake to the boiling solution is particularly desirable on 50-g samples, but is so convenient and beneficial that it should be used even on 10-g samples. T h e activity of protactinium-231 naturally present in average soil from the uranium-235 chain is normally about 0.1 dpm/g. The current level of plutonium-239 from global fallout is also a t about the same level or below. Although both nuclides end up in the same fraction in the previous procedure ( I ) , no mutual interference is encountered on 10-g samples because the cy spectra are completely resolved unless degraded by the presence of absorbing impurities. However, when the sample size is increased to 50 g, increasing the activity of both nuclides fivefold, the spectra of the two nuclides overlap substantially. Although a fair correction can be made with experience, the extrapolation of each curve under the other is highly subjective and much less accurate than if the curves were completely resolved. Consequently, a simple but efficient separation of protactinium-231 by extraction into DIBC has been included in the 50-g procedure. The separation is based on the discovery that, like Aliquat-336, DIBC extracts protactinium almost as efficiently from strong nitric acid-aluminum solutions as from the more usual chloride systems ( 3 ) . Consequently, the extraction can be carried out under the identical conditions otherwise used for extraction of the other actinides into Aliquat-336 so that no additional treatment other than the extraction itself is required to accomplish the separation. No significant loss of other actinides occurs in the protactinium fraction. Under the conditions described above, recovery of protactinium-233 tracer in the final strip of the DIBC extract is well over 99%. Because over 99% recovery is also obtained in the barium sulfate separation, the overall recovery for the procedure prior to electrodeposition or precipitation on barium sulfate for gross counting is over 98%. Such a high recovery certainly makes this procedure one of the most accurate and precise as well as simple and


reliable ones available for application to large samples of highly siliceous materials. Because there are no other protactinium isotopes having a significant (Y emission with half-lives longer than a few hours, the gross (Y count on barium sulfate after the DIBC extraction can be interpreted reliably as coming exclusively from protactinium-231. T h e sensitivity is not quite as good as is obtained with (Y spectrometry because of the higher backgrounds with the gross counter, but excellent results can be obtained with far less expensive and sophisticated equipment. Also, because electrodeposition is not required, yield corrections can be eliminated without much loss of reliability, if desired. Separation of protactinium-231 from the plutonium fraction is even more necessary to prevent mutual interference when plutonium-242 is used as tracer instead of plutonium-236, in the determination of neptunium-237, or in the determination of uranium-233 and uranium-234 when uranium is carried in the plutonium fraction. Omission of the 0.5 g of potassium sulfate during dissolution of the barium sulfate in concentrated sulfuric acid prior to reprecipitation is very important, and the potassium sulfate should be omitted even with smaller samples of soil. This small quantity of potassium sulfate was added originally in a t attempt to increase the solubility of anhydrous metallic sulfates in hot concentrated sulfuric acid ( 4 ) ,particularly those of iron and aluminum which occur in soil to the extent of 5% t o 10% each. The presence of small quantities of potassium sulfate does not improve the solubility of anhydrous sulfates significantly because of the repressive effect of the large quantity of sulfuric acid on the formation of sulfate ions. On the other hand, the potassium ions present markedly decrease the solubility in the subsequent aqueous solution of the anhydrous sulfates that do precipitate. Moreover, the presence of even small quantities of potassium sulfate markedly reduces the solubility of barium sulfate in sulfuric acid and increases the opportunity for production of high-temperature forms of barium sulfate that will not subsequently dissolve in aluminum nitrate fusions. Water Procedure. In anticipation of applying the procedure to a t least IO-L samples of both surface waters and water from deep wells leading directly into the underground aquifer, and possibly to seawater, a procedure was desired that would begin with precipitation of a very insoluble flocculant compound. Such a step would serve not only as a concentrating step for ionic actinides but hopefully would also recover nonionic species as polymers and refractory particles for subsequent dissolution by fusions or other methods by which complete decomposition and conversion to the ionic state can be guaranteed. It was also desirable to avoid phosphates or unduly alkaline conditions higher than about p H 10 to avoid precipitation of large quantities of calcium and magnesium t h a t interfere with subsequent fusions or use of sulfate systems. In the initial procedure tried, 10 L of untreated tap water was evaporated to 2 L in the presence of 20 mL of concentrated hydrochloric acid. After addition of 5 mL of 5% ferric chloride solution and the individual tracer being tested, the solution was neutralized at the boiling point with concentrated ammonium hydroxide, 5 mL of excess ammonium hydroxide and 1 g of hydroxylamine hydrochloride were added, and the solution was boiled vigorously for 10 min. The individual activities remaining unprecipitated in the entire 2-L filtrate a t the final p H of about 7.8 were 0.57% with thorium-230, 0.34% with plutonium-239, 3.7% with americium-241, 0.95% with curium-244, and 0.74% with californium-252, However, the quantity remaining unprecipitated increased drastically to 12.6% with protactinium-231, 38.7% with uranium-233,

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and 45.2% with neptunium-237. Because of the much lower solubility of neptunium and uranium hydroxides from the quadrivalent than from the penta- or hexavalent states, most of the iron was reduced with excess potassium metabisulfite and a few drops of titanium trichloride and/or chromous chloride solutions were then added to reduce the elements to the quadrivalent state before neutralization with ammonium hydroxide. However, the ferrous hydroxide would not flocculate, and the solution was virtually impossible to filter a t the relatively low p H (7.8) obtained. Consequently, the quantity of hydrochloric acid added initially to the first 3 L of water was reduced to 5 m L so that only about 3 mL would remain after evaporation of the entire 10 L to 2 L. This quantity of acid will produce enough ammonium ion to give a final p H after boiling of about 9.2 with 5 mL excess ammonium hydroxide. The quantity of iron carrier was also increased to 10 mL. Under these conditions, the ferrous hydroxide precipitate flocculates well, the solution is easily filtered, and precipitation of uranium was over 99% complete. Subsequently, it was found t h a t all three nuclides were precipitated completely without using either titanium trichloride or chromous chloride. The quantities remaining unprecipitated were 0.10% for protactinium-233, 0.18% for uranium-233, and 0.25% for neptunium-237. This suggests that ferrous hydroxide is itself powerful enough to reduce all three elements to the quadrivalent state in an alkaline solution. I t is not unlikely considering the greatly reduced solubility of the hydroxides of iron in the oxidized state and the other three elements in the reduced state. However, ferrous hydroxide is much more finely divided and flocculates more slowly than ferric hydroxide, and the greater surface and time of contact before aggregation might account for part of the increased collection efficiency of the very small aggregates of the carrier-free tracer hydroxides. This effect would also be expected to be more effective in collecting small particles of refractory compounds present. The ter- and quadrivalent elements were not retested with the ferrous hydroxide procedure because there was no reason to believe that recovery would be any less efficient, particularly at the higher pH and iron concentration employed. High and reproducible yields have been confirmed in real samples, however. The final procedure recommended above has been further modified to give a slightly higher concentration of ammonium hydroxide. As should be well known, the p H obtainable with ammonium hydroxide depends on the concentration of the ammonium ion present as well as on the concentration of the excess ammonium hydroxide. Consequently, the quantity of acid neutralized with ammonium hydroxide must be controlled to obtain the proper pH. If the p H is less than about 9, the precipitate of ferrous hydroxide will not flocculate well and is very difficult to filter or centrifuge. If the p H is much above 10, calcium and magnesium hydroxides will precipitate and cause other problems later. Of course, if much phosphate is present in the sample, most of the calcium and magnesium will precipitate as phosphates a t much lower pH, and no separation is possible from these two elements. Not more than about 5 mL of concentrated hydrochloric acid can be present at the time of neutralization with ammonium hydroxide to produce the desired p H range with an excess of 10 mL. If more acid is present, or if the quantity present is unknown, the solution after evaporation to 2 L can be neutralized with 10 M sodium hydroxide to the first permanent light brown color of ferric hydroxide, and then 5 mL of concentrated hydrochloric acid is added before proceeding with the reduction and neutralization as described above. The directions given above are intended to apply to normal waters which usually contain less than a few parts per million

1314


of iron. If the sample contains much iron relative to the 100 mg added as carrier, e.g., rust from iron casings in stagnant wells, the iron will not dissolve in the pyrosulfate fusion resulting from use of only 6 g of potassium fluoride. If the fusion is prolonged or heated hotter in a n attempt to obtain complete dissolution, dissolution of platinum from the dish is increased markedly. Because of its small size, quadrivalent platinum is precipitated only partially with barium sulfate, particularly in the presence of chloride with which it forms a stable complex. However, if the platinum becomes reduced to the elemental state by organic compounds, sulfite, etc., it will be filtered off quantitatively with the barium sulfate producing a gray-to-black coloration. After dissolution of the barium sulfate, the elemental platinum will be gathered into the DIBC or Aliquat-336 extracts, most noticeably at the aqueous interface, some of which is subsequently removed in the strips giving a pseudo extraction effect with subsequent electrodeposition and degradation of the a spectra. After evaporation of the strip solutions to dryness, the residue will have a yellow color if much platinum came through. Elemental platinum can be eliminated from the barium sulfate easily by oxidation with aqua regia before reprecipitation of the barium sulfate from sulfuric acid as described above. However, the original attack on the platinum dish should also be reduced. When significant iron is present in the sample, compensate by reducing or eliminating the iron carrier added, and scale up the procedure, including the silica, as necessary to accommodate the larger quantities of iron present, up to the regular conditions used with 10-g samples of soil. The volume of ferrous hydroxide in the centrifuge tube relative to t h a t normally obtained with the regular 100 mg of iron carrier can be used to indicate the approximate scaleup required. The pyrosulfate fusion should not be heated at the full heat of the blast burner for more than about 1 min after the fusion is obtained. Ferrous hydroxide has been used previously by Wong (9) for carrying plutonium from seawater with improved yields but apparently for a different reason. He obtained an average plutonium recovery of 52 h 18% on 30 samples of 5 to 60 L with the ferrous hydroxide method compared to only 25 f 14% on 38 samples by the ferric hydroxide method (9). He offered a possible explanation that plutonium is lost by formation of plutonium polymers that are not recovered in the ferric hydroxide method. His recommended procedure "avoids the difficulty with plutonium(1V) polymers, by reduction of Pu(1V) to Pu(II1) with sulfite and then separating the plutonium from seawater by coprecipitation with iron(I1) hydroxide." Although the possibility of having plutonium present in the form of nonionic polymers and/or particulates is deserving of real consideration and attention, the present information suggests that the ferrous hydroxide plays a substantially more important role than simply being an unavoidable consequence of the reduction of plutonium polymers with sulfurous acid. Probably, both the increased reducing strength of ferrous hydroxide in alkaline solution and the physical characteristics

of ferrous hydroxide itself play a substantial part in obtaining increased recovery of plutonium as is shown to be true with protactinium, neptunium, and uranium. Also, the marked increase in recovery of plutonium to over 99% in the present procedure compared to that obtained by Wong is probably due more to the hot precipitation followed by 10 min of vigorous boiling in an oxygen-free environment than to the fact that precipitation was made from a volume of only 2 L. Undoubtedly, much larger volumes can be used with very little decrease in recovery. With samples so large t h a t boiling becomes decidedly inconvenient, recovery with cold precipitation can probably be improved by keeping the solution stirred overnight to prevent flocculation and settling of the ferrous hydroxide. Vegetation Ash. Vegetation ash is strongly alkaline and must be neutralized by pretreatment with acid or it will not dissolve in the subsequent potassium fluoride fusion. The large quantities of carbonates present cause a copious evolution of carbon dioxide on acidification that will cause loss of sample if care is not taken. If the neutralization is made entirely with nitric acid, the resulting high nitrate concentration lowers the melting point drastically, makes the potassium fluoride melt become alkaline much more rapidly, further lowering the solubility of the components in the melt, and causes prolonged frothing in both the potassium fluoride and subsequent pyrosulfate fusions. Consequently, most of the neutralization should be made with hydrofluoric acid and only a small controlled quantity of nitric acid added to help oxidize residual organic matter. Furthermore. without acid pretreatment, the potassium fluoride cake will not break loose from the platinum dish, nor transpose with nitric acid when a sulfate system is not desired. Except for the decomposition of the sample, the analysis and results are the same as described for soil. Also, certain vegetation such as pine trees frequently contains so much barium that a reduction in the quantity of barium added or the sample size or both is required.

LITERATURE CITED Sill, C. W.; Puphal, K. W.; Hindman, F. D. Anal. Chem. 1974, 46. 1725. Sill, C . W. Anal. Chem. 1974, 46, 1426. Sill, C. W. Anal. Chem. 1978, 50, 1559. Sill, C. W. "Problems in Sample Treatment in Trace Analysis", in Proceedings of the 7th Materials Research Symposium on Accuracy in Trace Analysis: Sampling, Sample Handling, Analysis, National Bureau of Standards, Gaithersburg, Md., 1974, Natl. Bur. Stand. ( U . S . )Spec. Pub/. 1976, No. 422, LaFleur, P.D., Ed.: U S . Government Printing Office, Vol. 1, p 463. Martin, D. B.; Pope, D. G. "Improved Separation of Tervalent Lanthanides from Actinides by Extraction Chromatography"; Radiological and Environmental Sciences Labcfatorv. of Enerov: , Deoartment . -. Idaho Falls. Idaho 83401, 1979. Sill, C. W.; Williams, R. L. Anal. Chem. 1969, 4 7 , 1624 Sill, C. W. Anal. Chem. 1977, 4 9 , 618. Sill, C. W. Health Pbys. 1977, 33, 393. Wong, K . M. Anal. Cbim. Acta 1971, 56, 355.

for review March 19,1979. Accepted April 24,1979. Use of commercial product names is for accuracy in technical reporting and does not constitute endorsement of the product by the U S . Government. RECEnED

Simultaneous determination of alpha-emitting nuclides of radium

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