relatively high values remaining among the fourteen final variables, but the average r in Table V is only 0.35. T h e deletion of the 1603,1154,888,870,846,793,and 781-cm-1 peaks should have little effect on the ability to distinguish oils by the x2 test, as t h e average si/sz for the 14 variables is 1,985, compared t o 1,863 for t h e original 21 variables. Weathering. The preceding discussion has been restricted t o fresh, unweathered oils with the exception of t h e 22 waste crankcase lubricants. Weathering of a n oil spill at sea causes several changes t o take place in t h e infrared spectrum of an oil. These include increases in many of t h e aromatic-related bands, the appearance of oxidation-related bands around 1700 cm-l and disappearance of the 673 and 697-cm-l bands. A major difficulty with weathered samples is the presence of water in the sample, which can often be removed by centrifugation a t 30 "C and t h e addition of MgS04 (14). T h e x 2 procedure described above is equally applicable t o weathered oil spectra, by considering weathering as a contributor to the estimated analytical variance, s,". I t has been suggested ( 1 4 ) t h a t a library of reference spectra might consist of spectra of oils which had been slightly artificially weathered, rather than fresh samples as used in this study. For actual field implementation of this procedure, t h e authors agree with t h a t suggestion.
ACKNOWLEDGMENT The authors acknowledge the helpful suggestions of Fredric Godshall of NOAA, Alan P. Bentz of the U.S. Coast Guard R & D Center, Morton Curtis of Rice University, and Chris Brown and Patricia Lynch of the University of Rhode Island.
LITERATURE CITED F. K. Kawahara, Environ. Sci. Technol., 3, 150 (1969). J. S.Mattson, Anal. Chem., 43, 1872 (1971). P. F. Lynch and C. W. Brown, Environ. Sci. Technol., 7 , 1123 (1973). M. E. Garza and J. Muth, Environ. Sci. Techno/.,8, 249 (1974). (5) 0. C. Zafiriou, Anal. Chem., 45, 952 (1973). (6) D. E. Bryan, V. P. Guinn, R. P. Hackleman, and H. R. Lukens, "Development of Nuclear Analytical Techniques for Oil Slick Identification (Phase I)", Report GA-9889 Gulf General Atomic, Inc., USAEC Contract AT(04-3)-67 (1970). (7) H. R. Lukens, D. Bryan, N. A. Hiatt, and H. L. Schlesinger, "Development of Nuclear Analytical Techniques for Oil Spill Identification (Phase IIA)", Gulf Radiation Technol., Report A10684, USAEC Contract AT(04-3)-67 (1971). (8) M. Anbar, A. C. Scott, and M. E. Scolnick. "Identification of Oil Spills and Determination of Duration of Weathering by Field Ionization Mass Spectrometry", Abstract No. 224, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 4-8, 1974, Cleveland, Ohio, 1974. (9) A. D. Thurston and R. W. Knight, Envifon. Sci. Techno/., 5, 64 (1971). (10) J. W. Frankenfeld, "Classification and Identification of Spilled Oil by Thin Layer Chromatography", Abstract No. 458, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 3-7, 1975, Cleveland, Ohio, 1975. (1 1) J. S. Mattson and C. A. Smith, "An On-Line Minicomputer System for Infrared Spectrometry", in "Computers in Chemistry and Instrumentation", Vol. 7, J. S.Mattson, H. B. Mark, Jr. and H. C. MacDonald, Jr., Ed., Marcel Dekker, New York, in press. (12) A. Savitsky and M. J. E. Golay, Anal. Chem., 38, 1627 (1964). (13) J. S.Mattson and A. C. McBride 111, Anal. Chem., 43, 1139 (1971). (14) C. W. Brown, University of Rhode Island, private communication, 1975. (15) M. J. Spencer, "Oil Identificationusing Infrared Spectrometry", M.S. Thesis, University of Miami, School of Marine and Atmospheric Science, Miami, Fla, July 1975. (16) W. C. Hamilton, "Statistics in Physical Science", Ronald Press, New York, 1964. (17) D. F. Morrison, "Multivariate Statistical Methods", McGraw-Hill,New York, 1967.
(1) (2) (3) (4)
RECEIVEDfor review January 9, 1976. Accepted October 7, 1976. This research was supported by the U S . Coast Guard, Contract No. DOT-CG-81-75-1364 and DOT-CG-81-751383.
Radiochemical Determination of Lead-2 10 in Uranium Ores and Air Dusts Claude W. Sill" and Conrad P. Willis Health Services Laboratory, U S . Energy Research and Development Administration, Idaho Falls, Idaho 8340 1
An improved procedure is described by which cellulose, glass fiber, and polystyrene filters can be wet-ashed and siliceous samples dissolved completely wlthout loss of lead by either volatilization or spontaneous reduction to metal that occurs with dry ashlng and/or treatment in platinum containers. The solution of bismuth-210 used to callbrate the p counter is prepared by chemical separation from a solution of lead-210 which is much more convenient and results in a product of higher speclflc activity than one prepared by neutron activation of stable bismuth. After separatlon from the samples by conventlonal chemical procedures, the bismuth-210 Is preclpitated on barium sulfate for p countlng which Is more convenlent and gives a more uniform and reproduclble deposlt than Is obtained by most other means.
Lead-210 is one of the most toxic radionuclides known, its maximum permissible concentration in drinking water being smaller than that of all other radionuclides except radium-226 and -228. Its accurate and reliable measurement has been of continuing interest in this laboratory for many years, both in samples related directly t o the immediate public health, such as air, water, food, etc., and in uranium ores and products re302
ANALYTICAL CHEMISTRY, VOL. ,;9, NO. 2, FEBRUARY 1977
sulting from uranium milling operations. However, lead-210 has a half-life of 22 years and emits very weak p particles with a maximum energy of only 0.018 meV, making its direct determination very difficult. Most analytical methods involve measurement of either its 5.01-day bismuth-210 daughter, which emits a very energetic 1.17-meV /3, or its 138-day aemitting daughter. The activity of either daughter can then be related unambiguously t o t h a t of t h e lead-210 parent by appropriate growth and decay. relationships. In a previous publication ( I ) , a procedure was described for the determination of lead-210 in a wide variety of uranium mill products and biological materials. Because of the long times required for ingrowth of a significant fraction of the equilibrium activity of polonium-210 after purification of the lead210 parent, t h e procedure used was based on measurement of the much shorter-lived bismuth-210 daughter. Both lead and bismuth are separated from the main sample constituents by extraction from acid solution into diethylammonium diethyldithiocarbamate (DDTC). The two elements are then separated from each other by dithizone using a pH-2.7 buffer. T h e dithizone extract containing the bismuth-210 is evaporated t o dryness with nitric acid in a cupped planchet, and counted in a low-background p counter through a n absorber t o remove contaminating polonium-210 a activity.
The previous procedure suffers from three significant weaknesses. First, use of platinum dishes t o obtain rapid and complete dissolution of siliceous materials b y fusion with potassium fluoride results in substantial loss of lead t o the dish and subsequent contamination of succeeding samples. Consequently, another method was required t o obtain complete sample decomposition not requiring use of platinum or other metallic containers. Second, standardization of t h e p counter requires a solution of bismuth-210. Because of its short halflife, this nuclide is not commercially available and was prepared b y irradiation of natural bismuth with thermal neutrons. Nuclear reactors or other irradiation facilities capable of producing thermal neutrons at sufficiently high flux are not generally available to most workers interested in measurement of lead-210. Third, t h e technique used to prepare the final bismuth-210 fraction for /3 counting is n o t entirely satisfactory for several reasons. It was necessary t o wet-ash t h e dithizone extracts with nitric acid and hydrogen peroxide only t o avoid introducing extraneous materials that are corrosive t o stainless steel or would absorb t h e /3 radiation and produce uncontrollable changes in counting efficiency. Under these conditions, complete oxidation of t h e organic material is almost impossible to achieve in a reasonable length of time. Even mild heating over a small flame introduces considerable risk of volatilization losses. Also, transfer of the activity to a counting planchet quantitatively in a suitably small volume of liquid is a difficult and tedious chore. The deposit obtained on evaporation of the solution in a cupped planchet for /3 counting is very uneven, causing uncontrollable changes in the counting efficiency. Recently, this laboratory was required t o determine t h e equilibrium relationships existing among uranium-238 and its principal long-lived daughters in uranium ores and air dusts at various stages during processing in t h e mills. The precision and accuracy required in an equilibrium study were such that the deficiencies in t h e determination of the lead-210 mentioned above had t o be corrected. T h e primary purpose of t h e present communication is to present t h e solutions worked out to t h e three problems noted above. T h e reader is referred t o t h e original publication ( I ) for all other details of t h e procedure. It has been demonstrated that small quantities of large polyvalent elements can be carried quantitatively on a sufficiently small quantity of barium sulfate t o permit direct a counting without significant loss of counting efficiency (2, 3 ) . When t h e 11 mg of barium potassium sulfate resulting from 5 mg of barium is deposited uniformly over an area of 13.4 cm2, or about 0.8 mg/cm2, the counting rate is only about 5% less than from sources prepared b y direct evaporation of carrierfree activity ( 3 ) .Because both lead and bismuth are known to be carried efficiently in barium sulfate in t h e absence of hydrochloric acid ( 2 ) ,the same method should be equally effective in preparing the bismuth-210 fraction for (? counting its 1.17-meV /3 particles. Consequently, t h e organic extracts can be wet-ashed completely and easily in glassware using nitric, perchloric, andlor sulfuric acids as necessary without concern about corrosion of stainless steel. Either lead or bismuth or both can then b e precipitated quantitatively on an exactly reproducible quantity of barium sulfate, eliminating all extraneous materials. T h e barium sulfate can then be transferred easily a n d quantitatively to a filter, giving a completely uniform a n d reproducible deposit for /3 counting.
EXPERIMENTAL Instrumentation. A Widebeta I1 counter (Beckman Instruments, Inc., Fullerton, Calif.) with a 2%-in.diameter detector, a 7.1 mg/cm2 absorber to remove polonium-210 a particles and an automatic sample
changer was used for all p measurements. The background is generally about 3 cpm and the counting efficiency for bismuth-210 p’s through the absorber is about 38%. A conventional 3-in. by 3-in. thalliumactivated sodium iodide well counter was used for y counting lead-212 and bismuth-207 used in tracer studies. Reagents. All reagents are prepared exactly as described previously (1,3).
Counter Calibration. Standardize a radiochemically pure solution of bismuth-210 by liquid scintillation counting, and precipitate aliquots on barium sulfate for counting under the exact conditions to be employed in the determination. The bismuth-210 can be prepared more conveniently and in much higher specific activity by chemical separation from a solution of lead-210 than by neutron activation of stable bismuth as previously described. However, the polonium-210 daughter must also be removed completely to prevent its interference in the standardization by liquid scintillation counting. Because the polonium-210 grows back into the bismuth-210 at about 0.5% per day, preparation of the tracer should be planned to permit the calibration of the counter t o be made within a few hours. Evaporate approximately 1 X IO5 dpm of lead-210 with 2 g of anhydrous sodium sulfate and 2 ml of concentrated sulfuric acid in a 250-ml Erlenmeyer flask to a pyrosulfate fusion to ensure complete dissolution of any hydrolyzed or refractory polonium compounds that might be present. Cool the flask, add 25 ml of water and 2 ml of 48% hydrobromic acid to lower the oxidation potential of the Bi3+-Biohalf cell and to provide for the subsequent volatilization of tin. Heat the solution to boiling and add 1 ml of a 0.625% solution of tellurium dioxide in 25% hydrochloric acid. Add 4 drops of a freshly prepared 25% solution of stannous chloride dihydrate in 25% hydrochloric acid dropwise while swirling the flask continuously. Boil the solution vigorously for 5 min to flocculate the metallic tellurium. Allow the solution to stop boiling and add 1more drop of the stannous chloride solution to be sure that precipitation is complete. Place the flask in a bath of cold running water and rinse the sides immediately with a little water to prevent as much of the tellurium as possible from drying on the sides. Filter the cooled solution through a DM-450 filter (Gelman Instrument Co., Ann Arbor, Mich.) in a glass filtering chimney into a clean 250-ml Erlenmeyer flask and dispose of the filter containing the polonium in an appropriate manner. The flask in which the tellurium was precipitated should be cleaned by treatment with hot nitric or sulfuric acid t o remove the traces of metallic tellurium invariably still sticking to the sides to avoid contamination with polonium-210 the next time the flask is used. Add 0.5 ml of concentrated sulfuric acid, 3 ml of additional 48% hydrobromic acid, and a couple of drops of 30% hydrogen peroxide to the filtrate as necessary to produce a permanent orange color of free bromine to oxidize stannous tin, and evaporate to a pyrosulfate fusion t o volatilize the quadrivalent tin to prevent subsequent problems from hydrolysis. A decontamination factor of about 500 is obtained for polonium in a single precipitation if the solution is filtered on a membrane filter as described rather than centrifuged. If higher decontamination factors are required, dissolve the cake in hydrobromic acid and water and repeat the precipitation of tellurium with stannous chloride before finishing as described. Dissolve the cake and separate lead and bismuth from each other using dithizone and the pH 2.7 buffer as described beginning with the third paragraph under “Tap and River Waters” in the previous procedure ( I ) . To the 150-ml beaker containing the combined bismuth dithizone extracts after removal of the lead, add 1 ml each of nitric and perchloric acids. Evaporate the solution carefully until all organic matter has been oxidized to give a colorless solution and the perchloric acid just fumes dry. Add 2 ml of concentrated nitric acid and boil for 1 min to redissolve any hydrolyzed or reduced bismuth compounds. Add more nitric acid if necessary to give 1.5 ml and dilute to 50 ml. Evaporate 1 ml of the 0.5 M nitric acid solution of the tracer to dryness on a stainless steel plate and count in an a counter or a spectrometer to demonstrate the absence of significant quantities of polonium-210. Place another aliquot into an appropriate quantity of liquid scintillation cocktail and count in a liquid scintillation counter for 20 min at 100%counting efficiency to standardize the tracer solution. With as little delay as possible, evaporate two other 5-ml aliquots in 250-ml Erlenmeyer flasks with 3 g of anhydrous potassium sulfate and 2 ml of concentrated sulfuric acid to a pyrosulfate fusion. Precipitate the bismuth on barium sulfate, and count in the p counter through a 7 mg/cm2 absorber for 50 min as described below. Calculate the counting efficiencyof the p counter by comparison of the counting rate obtained in the @counterwith the disintegration rate obtained from the liquid scintillation counter. For the most accurate work, re-evaporate the combined filtrate, wash, and alcohol transfer liquid back to a pyrosulfate fusion in the same Erlenmeyer flask in which
ANALYTICAL CHEMISTRY, VOL. 49, NO. 2, FEBRUARY 1977
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the original barium sulfate precipitation was carried out. Repeat the precipitation of barium sulfate, count in the p counter and add the result to that from the main fraction to correct for small losses (about 0.5%) occurring in the procedure. Propagate the uncertainties in all random variables to the final counting efficiency. Dissolution of Ores and Mill Tailings. Because of the easy reducibility and volatility of most lead compounds at elevated temperatures, samples being analyzed for this element must not be heated strongly as in dry ashing, or treated in metallic containers, including platinum. Severe losses and contamination will result. Organic material including filter papers must be wet-ashed, hydrofluoric acid must be used extensively to dissolve silica, and a pyrosulfate fusion is desirable to guarantee complete dissolution of refractory compounds, particularly of ter- and quadrivalent elements. Teflon FEP melts at 205 OC. and will not stand temperatures high enough for rapid charring of some organic compounds such as polystyrene filter materials, but is excellent with respect to chemical stability and freedom from sorption effects. Teflon TFE withstands considerably higher temperatures but exhibits undesirable memory effects if used repeatedly, and is too expensive to be discarded after a single use. Because isotopic tracers for lead-210 and many other radionuclides are not available, sample decomposition must be complete, routinely reproducible, and free from spattering, incomplete recovery, or other physical losses. Place a 1-gsample into a 100-mlbeaker made of Teflon FEP plastic and add 1 ml each of concentrated nitric and perchloric acids, 1 ml of lead-bismuth carrier ( I ) , and 10 ml of 48% hydrofluoric acid. Place the beaker on a hot plate with a continuously variable heat control (such as the Model PC-35, Corning Glass Works, Corning, N.Y.), and gradually increase the temperature until the bottom of the Teflon FEP beaker just barely becomes tacky after several minutes in direct contact with the plate, then back the control off slightly. A conventional hotplate can also be employed if it is covered with 2 or 3 layers of asbestos paper. Heat the solution until fumes of perchloric acid are evolved but do not allow to dry and bake. Move the beaker frequently to prevent its sticking to the plate or covering. Also, do not leave the beaker unattended on the hot plate for prolonged periods or the temperature will continue to rise and the bottom of the beaker will melt. Add 5 ml of concentrated sulfuric acid slowly while swirling the beaker continuously. Continue heating the solution in the uncovered beaker until light fumes of perchloric and/or sulfuric acid have been evolved for at least 10 min to remove as much hydrofluoric acid as possible, to oxidize most of the organic matter, and to make the solution so that it no longer wets the sides of the beaker. When the solution rolls around the bottom of the beaker like a pool of mercury, the solution c w be poured out quantitatively without rinsing. Collect any isolated droplets by carefully rolling the solution around the sides and pour the solution into a 250-ml Erlenmeyer flask containing 5 g of anhydrous sodium sulfate. Without delay, evaporate the solution over a blast burner to a pyrosulfate fusion to remove the last traces of hydrofluoric acid resulting from transposition of nonvolatile fluorides as quickly as possible to minimize attack on the glass and reintroduction of insoluble silica into the sample. Any turbidity in the pyrosulfate melt is a good visual indication of the quantity of original sample remaining undissolved and/or silica reintroduced from the glass flask, generally the former. Cool the flask, add 25 ml of 3 M hydrochloric acid (1:3) and heat gently until the cake has dissolved. Add an additional 15 ml water, 1 ml of 25% potassium metabisulfite and boil the solution for 2 to 3 min to dissolve lead sulfate or bismuth phosphate, and to hydrolyze any condensed phosphates that might have been produced during the fusion from orthophosphates present in the sample. The cake must be dissolved in premixed 3 M hydrochloric acid to avoid trapping lead or bismuth in the barium sulfate lattice before their complexation by chloride ion could occur. If the solution is virtually clear, cool and transfer to a 250-ml separatory funnel and rinse the flask with an additional 45 ml of water to give about 80 ml of 0.9 M hydrochloric acid. If a significant quantity of insoluble material is present, centrifuge the solution in a 100-mlLusteroid centrifuge tube, decant the supernate into the separatory funnel and transfer the insoluble matter back to the Teflon beaker with 1ml of concentrated sulfuric acid and 5 ml of hydrofluoric acid. Evaporate to fumes of sulfuric acid, dissolve in 20 ml of water containing 1ml of hydrochloric acid, and combine with the main filtrate in the separatory funnel. If a white precipitate is present that dissolves in the hot concentrated sulfuric acid and reprecipitates on addition of dilute hydrochloric acid, barium sulfate is indicated. Small quantities can be removed from hydrochloric acid solution by recentrifugation before combining the supernate with the main filtrate without significant loss of lead or bismuth. 304
Add sufficient ascorbic acid to reduce all iron and leave an excess of 0.5 g and extract with three successive 10-ml portions of 1%DDTC in chloroform. Combine the extracts in a 250-ml Erlenmeyer flask, add 2 ml of 50% sodium hydrogen sulfate and 2 ml of concentrated nitric acid, and continue as described beginning with the second paragraph under “Tap and River Waters” ( I ) through the dithizone extraction and strip with pH 2.7 buffer. Oxidize the dithizone extract containing the bismuth and precipitate barium sulfate as described below. Dissolution of Air Dusts. With 4-in. filters, fold the paper in half with the sample side toward the inside, tear the paper carefully into three or four strips, holding the paper over the beaker while tearing to avoid any loss of sample. Place the strips into the appropriate beaker as described below. Cellulose Fzlters With cellulose filters such as the MSA 2133, place the filter strips into a 250-ml borosilicate glass beaker. Add 3 ml of concentrated sulfuric acid, 15 ml of concentrated nitric acid, and 1 ml of lead-bismuth carrier. Cover the beaker with a watch glass and evaporate on a regular hot plate until evolution of red-brown gases of nitrogen dioxide ceases, the remaining organic matter has charred to a completely dry and immobile black mass and slight fumes of sulfuric acid are visible inside the beaker. With smaller or thinner papers, the mass will not become completely dry but char as thoroughly as possible. Cool the beaker, add 10 ml of concentrated nitric acid and 3 ml of 72% perchloric acid, replace the cover glass, and evaporate the solution to near dryness. Leave the residue just slightly moist with acid and do not allow to cake. Oxidation of residual organic matter, including the graphite generally present in MSA 2133, takes place smoothly with scarcely a vigorous reaction. Add 1ml of concentrated nitric acid and 4 ml of water to the cooled residue in the beaker, and heat to boiling with vigorous swirling as required to loosen the residue from the beaker. Transfer the suspension to a 100-ml Teflon FEP beaker, and rinse the glass beaker with a few 2- or 3-ml portions of water. Use as little water as possible to transfer the solution and insoluble residue quantitatively. Reevaporate the solution to 1or 2 ml, add 10 ml of 48% hydrofluoric acid, and re-evaporate again to near dryness on the low-temperature hot plate, taking care not to allow the bottom of the beaker to overheat. Add 5 ml of concentrated sulfuric acid and heat to fumes to remove as much hydrofluoric acid as possible. Do not swirl the solution higher than necessary on the side of the beaker to facilitate complete transfer later. When the solution no longer wets the sides of the beaker, transfer to a 250-ml Erlenmeyer flask containing 5 g of sodium sulfate and complete as described above for ores and tailings from that point. Polystyrene Fdters. With 4-in. polystyrene filters of Microsorban S, distribute the filter strips as uniformly as possible over the bottom of a 250-ml borosilicate glass beaker. Add 5 ml of concentrated sulfuric acid, 5 drops of concentrated nitric acid, and 1 ml of lead-bismuth carrier. If cellulosic materials were used to support or cover the polystyrene filter and must be included in the analysis, add an additional 2 or 3 ml of concentrated nitric acid to dissolve and oxidize the cellulose. However, it is not desirable to use the support paper normally supplied with the Microsorban because of the substantial quantities of titanium and other impurities that are present. Cover the beaker with a watch glass and heat strongly on a bare hot plate until the paper has charred to a completely dry and immobile black mass. Be sure that the paper is completely wet with the sulfuric acid and no large masses of polystyrene remain uncharred. With regular Microsorban, which is only half as thick as Microsorban S, the mixture will not go dry, but should be heated to an oily black liquid free of distinct particles of uncharred paper. Polystyrene is not attacked significantly by hot concentrated nitric acid, but is attacked rather explosively by hot 72% perchloric acid alone. Consequently, the polystyrene must be thermally charred with sulfuric acid completely before proceeding or the subsequent oxidation will be incomplete. Cool the beaker, add 10 ml of concentrated nitric acid, and repeat the evaporation to strong charring in sulfuric acid. Cool and add 10 ml each of concentrated nitric and 72% perchloric acid. Replace the cover glass and continue heating to oxidize the organic material. After most of the nitric acid has been expelled, the more resistant organic matter is oxidized rather vigorously but smoothly by the perchloric acid. Add several drops of concentiated nitric acid repeatedly throughout the fuming with perchloric acid. When the vigorous reaction has subsided and the organic matter has been oxidized completely, remove the cover glass and allow the excess perchloric acid to evaporate, leaving the residue slightly moist with sulfuric acid. Suspend the residue in 1ml of nitric acid and 4 ml of water, transfer to a 100-ml Teflon FEP beaker, and finish as described above for cellulose filters.
ANALYTICAL CHEMISTRY, VOL. 49, NO. 2, FEBRUARY 1977
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Glass Fiber Filters. Place the filter strips into a 100-mlTeflon FEP beaker and add 1 ml each of concentrated nitric and 72% perchloric acids, 10 ml of 48% hydrofluoric acid and 1ml of lead-bismuth carrier. Evaporate the solution to near dryness on the low-temperature hot plate, taking care not to melt the bottom of the beaker. Add 5 ml of concentrated sulfuric acid and continue heating to sulfuric acid fumes to volatilize most of the hydrofluoric acid. When the solution no longer wets the sides of the beaker, transfer the solution to a 250-ml Erlenmeyer flask containing 5 g of sodium sulfate, and complete as described above for ores and tailings. Most glass fiber papers contain relatively large quantities of calcium and barium, both of which form precipitates in sulfate systems and must be provided for in the analytical procedure. If the final pyrosulfate cake does not dissolve in dilute hydrochloric acid to give a clear solution, large quantities of barium in the glass fiber should be suspected. The spectrograde filter (Gelman Instrument Co.,Ann Arbor, Mich.) contains less barium than other glass fiber papers and is recommended when glass papers must be used. Precipitation of Bismuth on Barium Sulfate for p Counting. The quantity of bismuth present must not be larger than about 1mg or the capacity of the barium sulfate to carry the bismuth quantitatively will soon be exceeded. The color of the dithizone extract from which the lead has been stripped can be used to indicate the approximate quantity present. Draw the dithizone extract containing the bismuth-210 into a 250-ml Erlenmeyer flask rather than the 150-ml beaker previously recommended. Add 3 g of anhydrous potassium sulfate (not sodium sulfate), 2 ml of concentrated sulfuric acid, 1 ml of concentrated nitric acid, and 6 drops of 72% perchloric acid to the flask, and heat gently on the edge of a hot plate covered with a piece of asbestos cloth until the chloroform has evaporated and the dithizone has been completely oxidized to give a colorless solution. Heat the flask over a Fisher blast burner until excess sulfuric acid has been expelled and a clear pyrosulfate fusion is obtained. Cool the melt to room temperature, add 0.5 m! of concentrated sulfuric acid and 25 ml of water and heat the solution to boiling. Add two 1-ml portions of 0.45% barium chloride dihydrate solution dropwise to the boiling solution at a rate not faster than about 1 drop every 2 or 3 s with 1min of boiling after each portion. Centrifuge,wash, and mount the barium sulfate as described previously ( 3 ) .Place the 47-mm DM-450 vinyl membrane filter in a suitable steel planchet, and count in a lowbackground p counter through the absorber to eliminate polonium210 a particles as described previously ( I ) . The barium sulfate can also be filtered on a 1-in. filter for counting wth smaller detectors with about a 20% decrease in counting efficiency due to the thicker deposit, which, however, must be included in the calibration. Correct the final lead-210 activity obtained for an overall chemical yield of 98 f 1%. Either the dithizone extract containing both lead and bismuth before stripping or the lead fraction from the pH 2.7 buffer strip after re-extracting into dithizone as described ( I ) can be treated similarly. However, more time will have to be allowed for any lead-212 present to decay. Also, there is frequently sufficient natural lead present, particularly in ores and mill tailings, to precipitate by itself as lead sulfate under the conditions used for precipitating barium sulfate, adding additional variable absorber to the deposit being p counted.
Table 1. Precipitation of Lead and Bismuth with Barium Sulfate" with and without Hydrochloric Acid Element Pb
Bi
Quantity present
C.F. 1 mg C.F. 1 mg C.F. 1 mg C.F. 1 mg
Hydrochloric acid present 0 0 5 ml 5 ml 0 0
5 ml 5 ml
Element remaining in supernate, % 0.069
0.36" 88.9 86.8 0.27 0.89 99.7 99.8
Conditions: Element and tracer fused with 3 g K2S04 and 2 ml H,SO,; cake dissolved in 25 ml HzO and 0.5 ml HzS04, and Bas04 precipitated by slow dropwise addition of two 1-ml portions of 0.45% BaClz 2HzO a t the boiling point. Solution was cooled and centrifuged. Carrier-free. Solution milky due to PbS04 before addition of barium.
RESULTS AND DISCUSSION Because of the substantial changes made from the original procedure with respect t o sample decomposition, mounting t h e final fraction on barium sulfate for ,B counting, and in standardization of t h e ,B counter, the accuracy of t h e overall procedure was rechecked by analyzing a standard pitchblende sample. This material has been shown by extensive previous work to be in true secular equilibrium and of accurately known activity ( I , 4 , 5 ) .Over a period of several days, results of 6.02, 6.08, and 6.04 f 0.07 X lo3 dpm/g were obtained, all of which are in exact statistical agreement with the known value of 6.06 f 0.04 X lo3 dpm/g. An average yield of 98 f 1%was used in the calculations t o correct for small losses of 0.5 to 1%each, occurring in the sample decompositon, main chemical separations, and in the final mounting on barium sulfate. A re-evaluation of the efficiency of precipitating lead and bismuth with barium sulfate under the exact conditions being used currently, and t h e effect of hydrochloric acid and mass of element present on t h e precipitation are shown in Table I. At least 1 mg of either lead or bismuth can be present in a sulfate sysem without increasing the losses substantially, b u t
addition of 5 ml of concentrated hydrochloric acid prevents the precipitation almost completely with bismuth and reduces precipitation of lead to 10 to 15%. Obviously, chlorides or other strong complexing agents for either lead or bismuth must not be present in significant quantities. T h e small quantity of chloride added with the barium chloride (and titanium trichloride (2)) and a slight scum of barium sulfate known to remain a t the surface and not pulled down by centrifugation are responsible for t h e small quantities found in the supernatant liquid in the absence of hydrochloric acid. Similarly, hydrochloric acid eliminates virtually completely t h e approximately 92% precipitation of polonium that occurs on barium sulfate in a sulfate system (3). Reduction of Lead in Platinum. I n the original work on the present procedure ( I ) , several percent of t h e lead-212 tracer was always observed to remain with the platinum dish after a potassium fluoride fusion. Only part of the residual activity could be removed in t h e subsequent pyrosulfate fusion. These results have been confirmed and additional information obtained in t h e present investigation. About IO5 cpm of lead-212 tracer and 5-min counts in a well counter were used in each test. T h e volume and composition of each solution was adjusted to be identical to that of the standard so that the counting efficiencies would be identical. Even in the presence of 1 g of potassium nitrate to prevent reduction of lead by traces of organic matter in the alkaline fluoride fusion, recovery of the lead-212 tracer in the final solution after a 3-g potassium fluoride fusion in a 50-ml platinum dish was only 84%. The lead-212 activity on the dish could not be determined quantitatively because of the unknown counting efficiency for the direct measurement on the dish. However, the activity was substantial, giving a counting rate 59% of that of the standard but under markedly different counting conditions. Because there is no evidence of significant loss by volatilization, the 16% difference obtained between the lead-212 recovered and t h e standard probably reflects fairly accurately the contamination of t h e dish. A similar quantity of lead-212 activity was found in the platinum dish after 3 g of potassium sulfate and 3 m l of concentrated sulfuric acid were fused in a 250-ml Erlenmeyer flask with a few drops of perchloric acid to remove all organic material and the pyrosulfate melt then re-fused in a platinum dish. This result is consistent with t h e previous observation that pyrosulfate fusion is not effective in removing more than part of any lead contamination in a platinum dish. T h e most effective removal was obtained by boiling t h e dish with 1 to 1 hydrochloric acid but was still not complete. Significantly, no loss of lead-212 could be detected (
