peak channel. Therefore, each peak region consisted of three bins. 3. Similarly, the peak regions of interest of each complex spectrum were extracted and combined into bins. (If the electronic shifts are small, no mathematical compensation is needed to align the complex and reference spectra.) EXPERIMENTAL
Apparatus. The Kansas State University TRIGA Mark I1 nuclear reactor was used to irradiate six fish samples. A 25-cc Ge(Li) semiconductor detector connected to a TMC 4096 multiparameter analyzer was used to collect gamma-ray spectra from each of the fish samples. The atomic absorption analyses were performed using a Jarrell-Ash Model 82-50 flame emission atomic absorption spectrophotometer. Reagents. No special preparation was necessary for the neutron activation analysis procedure. However, the fish samples were frozen prior to irradiation. Other portions of these six fish samples were ashed using a dry ash procedure. This dry ash was dissolved in 5 ml of 6N hydrochloric acid and diluted to 100 ml using deionized water. The diluted samples were analyzed for minerals on the Jarrell-Ash atomic absorption unit. Phosphorus was determined on the diluted sample colorimetrically using a Beckman Model 109200 Du-3 spectrophotorpeter. Procedure. All six fish samples were irradiated simultaneously for two minutes at a neutron flux of approximately 2.9 X 10” n/cm2 sec. After allowing from a 2- to 15-minute decay period, each spectrum was collected for 2 minutes. To determine the phosphorus content, each sample was allowed approximately a 2-week decay time and a bremstrahlung (from the pure p- emitter 32P)spectrum was collected for a 10-minute period. A time series of these bremstrahlung spectra was collected over a week period to confirm the presence of 3zPby half life comparison. RESULTS AND DISCUSSION
Table I presents the results of neutron activation, atomic absorption, and colorimetric analyses of three fish bone and three total fish ash samples. Atomic absorption or colorimetric results for all samples and for all elements were unavailable. Additionally, the NAA results for Mn and C1 are
presented for completeness. Other analyses results for these two elements were not available for comparison. The NAA results for Ca illustrate the importance of the weighting factor matrix to obtain the proper estimate. Using a spectrum with a large amount of Ca as the weighting factor to estimate Ca in a spectrum with a significantly smaller amount of Ca would produce inaccurate estimates. The converse apparently is not true for Ca. The results for Na, K, Mn, and C1 appear to be unaffected by the choice of weighting factors. Atomic absorption and NAA results for Ca are comparable, the maximum deviation being 31 % and the average deviation, 11 %. Other comparisons, though not so good, in general are reasonable. It should be noted that by allowing approximately a 1-day decay period, K can be estimated more accurately by NAA. The time series bremstrahlung spectra revealed a half life of approximately 14 days. The comparison of the phosphorus results is fair. This NAA procedure phosphorus is under further investigation. The time savings for computation, using the approximate weighting factor matrix, were as follows: NAA-1 = 0.32 minutes/sample NAA-3 = 0.14 minutes/sample which gives a time savings ratio, SR (NAA-l/NAA-3), of 2.286. CONCLUSIONS
The results given in Table I and the very significant time savings ratio strongly suggest the use of the approximate weighting factor, case NAA-3, for the analysis of these fish samples. RECEIVED for review December 13, 1971. Accepted March 15, 1972. Financial support of the Kansas Agricultural Experiment Station, Manhattan, is gratefully acknowledged. Contribution No. 12, Department of Nuclear Engineering and Contribution No. 778, Department of Grain Science and Industry have been assigned this work.
Determination of Trace Quantities of Uranium in Biological Materials by Neutron Activation Analysis Using a Rapid Radiochemical Separation Donald A. Becker and Philip D. LaFleur Activation Analysis Section, Analytical Chemistry Division, National Bureau of Standards, Washington, D.C. 20234 WHENTHE NEED AROSE to analyze a new biological Standard Reference Material for trace quantities of uranium, the technique of neutron activation analysis was employed. A search of the literature revealed a large number of nuclear techniques including direct a-counting ( I , 2), delayed neutron counting
(3), fission track counting (41, and nuclear activation techniques utilizing the 239Npdaughter of 239U (5,6), several different fission products from the fission of 23sU(7-9), and a
(1) E. E. Campbell, B. M. Head, and M. F. Milligan, U S . At. Energy Comm. Rept., LA-1920, June 1955. (2) E. B. Kurtz, Jr., and R. Y. Anderson, ibid., AECU-3177, March 1956.
i7) G . Buzzelli, ANAL.C H E M . ,1405(1965). ~~, (8) N. Ikeda, K. Kimura, N. Hasebe, and H. Shoji, Radiochim. Acta, 12,72 (1969). (9) A. A. Smales, Analyst (London),77,778 (1952).
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(3) s. *miel, ANAL. CHEM.,34, 1683 (1962), 14’) B. s. CarDenter and C. H. Cheek, ibid., 42,121 (1970). (5) D. N. Edgington, Int. J . Appl. Radiat. Isotopes, 18,11(1967). f6) M. Picer and P. Strohal.Anal. Chim. Acta, 40,131 (1968).
number of procedures using the nuclear reaction 2*W(n,y)239U(10-12). Although this last method using 239Uappeared to be the most direct, rapid, and potentially the most accurate, the short half-life (23.5 min) and low energy gamma ray (75 keV) meant that a rapid and specific chemical separation of the uranium was imperative. Of the procedures in the literature using **9U, several describe the use of multiple solvent extractions (up to eight consecutive extractions) and precipitations with chemical yields of 50-60%. Another procedure was stated to be quantitative only for low concentrations of sulfuric acid (0.1-1.5N). Since the analysis was to be for trace levels of uranium, the dissolution method of choice for biological materials was wet-ashing with mixed nitric-perchloric acids (13). The product of this dissolution technique, concentrated perchloric acid solution, was not directly usable with any of the published separation systems available. Therefore, an investigation into the development of a more appropriate chemical separation was begun. In the search for such a chemical separation, it quickly became apparent that of the separation techniques available, a good solvent extraction separation would best fill the requirements outlined above. A number of extraction systems were considered. However, the extensive literature on the solvent extraction mechanism of uranium(V1) using the reagent hydrogen bis(2-ethylhexy1)phosphate (HDEHP) (14-23) indicated this reagent should be applicable to the extraction from strong acid solutions. Previous work in this laboratory had established the usefulness of this reagent for the extraction of many other elements over widely varying acid concentrations (23). In the procedure reported here, the applicability of this reagent for the determination of trace amounts of uranium in to 10-9 gram is a wide variety of materials at levels of demonstrated. The materials analyzed include several different biological materials. The total time required for the analysis of triplicate samples ranged from 30-60 minutes, excluding the calculation of results. The evaluation of potential interferences for this chemical separation was greatly facilitated by the previous publication from this laboratory which has evaluated the HDEHP extraction characteristics of fifty-three other elements from hydrochloric, nitric, and perchloric acids (23). EXPERIMENTAL
The hydrogen bis(2-ethylhexy1)phosphate (HDEHP), 93-96 % pure, was obtained from the Union Carbide Corporation, and was not purified further. Reagents.
(10) M. Sankar Das, Unicersity of Michigan Rept., TID-18304 (1962). (11) D. Decat, B. Van Zanten, and G. Leliaert, ANAL.CHEM.,35, 845 (1963). (12) T. Nozaki, M. Ichikawa, T. Sasuga, and M. Inaridi, J . Radioanal. Chem., 6, 33 (1970). (13) T. T. Gorsuch, Analyst (London),84,135 (1959). (14) C. F. Baes, Jr., R. A. Zingaro, and C. F. Coleman, J . Phys. Chem., 62,129 (1958). (15) C. F. Baes, Jr .,J. Inorg. Nucl. Chem., 24,707 (1962). (16) T. Sato, ibid., 24,699 (1962). (17) T. S. Urbanski and S. Minc, Nukleonika, 7,703 (1962). (18) D. F. Peppard, G. W. Mason, I. Hucher, and F. A. J. A. Brandao, J . Inorg. Nucl. Chem., 24, 1387 (1962). (19) T. S. Urbanski, Nukleonika, 8,649 (1963). (20) M. Asano, Y.Okajima, and T. Nishi, Tech. Rept. Eng. Res. Inst. Kyoro Unic., 13, 107 (1963). (21) G. W. Mason, S. Lewey, and D. F. Peppard, J . Inorg. Nucl. Chem., 26,2271 (1964). (22) D. F. Peppard, Ann. Reo. Nucl. Sei., 1971,365. (23) I. H. Qureshi, L. T. McClendon, and P. D. LaFleur, “Proceedings, Modern Trends in Activation Analysis,” Nar. Bur. Srand. (U.S.) Spec. Pub/. 312, Vol I, 1969 p 666.
ACID 0 HF
Figure 1. Uranium extraction from various acids
The uranium standard used was a solution of NBS Standard Reference Material (SRM) No. 950a Uranium Oxide (U308),of 99.94% purity (24). All other chemicals met ACS specifications of reagent grade purity. Radioactivity Measurements. In aii cases, the radioactivity measured was the 75 keV gamma ray from 239U. For the extraction efficiencies a 7.5 cm x 7.5 cm (3 in. x 3 in.) NaI(Tl) scintillation detector with 1024 channel pulse height analyzer (PHA) was used. For the analyses, a large volume Ge(Li) semiconductor detector and 4096 channel PHA was used for increased photopeak resolution. Extraction Efficiencies. The extraction efficiencies for nitric, perchloric, hydrochloric, and hydrofluoric acids were studied systematically. Extractions were made with 0.75M HDEHP in petroleum ether. All extractions were made using 35-1111 screw-cap polycarbonate centrifuge tubes. Ten milliliters of HDEHP solution was used to extract the uranium from 10 ml of the appropriate acid solution. Approximately 100 pg of uranium carrier was present in all the solutions tested. After addition of the freshly irradiated Z39U solution and the reagent, the tube was tightly capped, vigorously shaken for 60 seconds, and centrifuged for 120 seconds to facilitate phase separation. The extraction efficiencies were measured radiochemically on 2-ml aliquots of both organic and aqueous layers. Decay corrections were made where necessary. The results obtained are shown in Figure 1. The extraction efficiencies obtained indicated essentially complete extraction of uranium(V1) from all concentrations of perchloric and nitric acids, from hydrochloric acid up to 4M, and from hydrofluoric acid up to 0.5M. (Because of the presence of radioactivities from *W fission products remaining in the aqueous phase, extraction efficiencies could not be evaluated above 98.5%.) Uranium was not extracted ( lo7 for sodium and >lo6 for copper, confirming the NO EXTRACTION statement in reference 23. The decontamination factor of 1 X l o 3 for mercury indicates an extraction of approximately 3 % for the mercury, since the extraction and the wash are actually equivalent to two extraction steps. This value also agrees well with that indicated in reference 23. The above results confirm the usefulness of reference 23 for the evaluation of the HDEHP extraction of uranium from a wide variety of materials. Irradiation Procedure. All samples were weighed and encapsulated in cleaned polyethylene snap-cap vials for irradiation. The biological materials had been lyophilized and stored in a desiccator prior to encapsulation. The uranium standards were packaged and irradiated separately from the samples in order to prevent the possibility of cross contamination. Copper foil flux monitors were attached to each sample and standard for flux normalization. 1510
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5 minutes in the NBS Reactor pneumatic tube facilities. The two facilities used were RT-4, which has a thermal neutron flux of 1.3 X 1013n.cm-2 sec-1 with a copper-cadmium ratio [Cu(Cd)] of 536 [Au(Cd) = 871, and RT-3, which has a
thermal neutron flux of 5 x 1013n.cm-2 sec-' and a Cu(Cd) ratio of 65 [Au(Cd) = 10.31 (25). Immediately after irradiation, the samples were processed radiochemically. Dissolution. All samples were dissolved in the presence of approximately 100 pg of uranium carrier. The biological materials (200-450 mg each) were wet ashed with mixed nitric and perchloric acids, observing the normal precautions necessary with perchloric acid (26). To speed the decomposition, 1-2 mg of vanadium were present as catalyst and 1-2 mg of chromium were present to indicate the complete destruction of organic material. The green-orange color change of Cr (111) -+ Cr(V1) occurred in 6-10 minutes. After cooling, the sample solution was diluted to 20 ml with 8M HN03 for extraction. This final solution was approximately 8 M in HN03and l M i n HC104. The aluminum samples (100 mg to 250 mg each) were dissolved in and extracted from 4 M HC1. Ten to 100 mg of the zirconium metal were dissolved in 1-2 ml of 7 M HF, then diluted to 20 ml with 8 M "OB. The uranium was then extracted from this solution, which was approximately 8M in H N 0 3 and 0.5M in HF. The copper(1) oxide single crystals were dissolved in and extracted from 8 M H N 0 3 . Extraction Procedure. The extraction procedure was essentially the same for all samples. Exactly 10.0 ml of HDEHP solution was added to the sample solution, the extraction container (35-ml polycarbonate centrifuge tubes) shaken vigorously for 60 seconds, and the samples were centrifuged for approximately 120 seconds to facilitate phase separation. The aqueous phase was removed with a disposable pipet and discarded. The organic phase was then washed a t least once with 8 M H N 0 3 , and the entire solution, or an aliquot depending on the activity expected, removed for counting. For the biological materials (or whenever additional decontamination is desired), the uranium was stripped from the organic phase with 14M HF. Again, only a short, vigorous contact time of 60-120 seconds was required. The aqueous phase, or an aliquot, was then removed for counting. For the analysis, the uranium standard consisted of about one milliliter of 1.02 pg U/ml standard solution, which was encapsulated in a polyethylene vial for irradiation. After its return, a small aliquot (50-250 pl) was removed and diluted to the appropriate volume. Several milligrams of dissolved inactive uranium were used as carrier during this dilution to prevent losses of the small amount of radioactive uranium. For counting, the liquid volumes of both sample and dilute standard were kept constant. In order to check against the possibility of neutron selfshielding in the uranium standard, solutions of several uranium concentrations were made (1.02 ,ug/ml to 1.02 mgiml), and cross checked against each other and against a very small amount of pure uranium oxide powder (ca. 300 pg). The results indicated less than 2 % difference between the specific activity of the various uranium solutions and the oxide for both irradiation positions. The results of this check also eliminate the possibility of a bias due to loss of a portion of the dilute uranium standard by adsorption or other means. In addition, no significant enhancement was noted due to increased thermalization of the neutron flux for the solution standard. Chemical Yields. As can readily be seen from the extraction efficiencies reported above and as determined from repeated measurements, the extraction yield is essentially quan(25) P. D. LaFleur and D. A. Becker, Ed., "Activation Analysis Section: Summary of Activities, July 1969 to June 1970," Nut. Bur. Stand. (U.S.) Tech. Note, 548,6 (Dec. 1970). (26) G. F. Smith, Anal. Chim. Acta, 5,397 (1953).
titative. However, mechanical losses invariably lead to a small reduction in the chemical yield, with the final yield dependent on the number of manipulations taking place. This is especially true with a radioisotope having a short half-life, when rapidity of handling is important. For these analyses, the chemical yield for the complete analysis was determined to be 95 f 3 % for the metals (using the centrifuge technique and one wash) and 91 += 4 % for the biological materials (using the centrifuge technique, two washes, and stripping the uranium from the organic phase with 14M HF). The use of separatory funnels for the extractions increased the yields by several per cent, indicating lower mechanical losses using this procedure. However, the rapid phase separation engendered by centrifugation was felt to be more important than a slightly higher chemical yield. RESULTS AND DISCUSSION
The results of a number of analyses using this procedure are found in Table I. For several of the samples and standards, decay curves were obtained which substantiated radiochemical purity. The gamma-ray peaks were integrated, background was subtracted, and corrections were applied for flux normalization, yield, and decay. The accuracy of the procedure can be seen by the good agreement with other analytical techniques as shown in Table I. Analysis of zirconium metal, SRM No. 1210, produced good agreement with the certified uranium concentration of 1.8 ppm. In the case of SRM 1571, Orchard Leaves, the agreement among the three analytical techniques is good, and the uranium value has recently been certified.
The copper(1) oxide-single crystal sample listed in Table I was made by NBS research scientists by oxidizing copper metal strips at high temperature and oxygen pressure. An attempt was made to introduce uranium as a dopant into this cuprous oxide at the 1 % level. Resistivity measurements showed no significant change upon addition of the uranium. Therefore, these analyses were made in an effort to verify the presence or absence of the uranium dopant. The results indicated the presence of small amounts of uranium much less than 1 %, distributed inhomogeneously. In addition, the analysis for uranium in housekeeping quality aluminum foil was attempted, primarily to determine the applicability of the separation technique. The result obtained was 0.49 ppm uranium, which agrees closely with the values obtained on similar quality aluminum foil by other scientists (11). Very good decontamination was obtained from both the aluminum and the relatively large manganese concentration in the foil using the HDEHP extraction. In conclusion, the procedure reported here is applicable to the determination of trace levels of uranium in a wide variety of sample matrices from convenient, strong acid solutions. RECEIVED for review October 29, 1971. Accepted March 14, 1972. In order to specify procedures adequately, it has been necessary to identify commercial materials and equipment in this report. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the purpose.
Spectrophotometric Determination of Sulfite with Mercuric Thiocyanate and Ferric Ion Willie L. Hinze, James Elliott, and Ray E. Humphrey' Department of Chemistry, Sam Houston State University, Huntsville, Texas 77340 SOLUBL~, SLIGHTLY DISSOCIATED mercuric thiocyanate reacts with certain anions which form even less dissociated or insoluble mercuric compounds to release thiocyanate ion. A solution of ferric ion is then added and the FeSCN2+ complex formed which absorbs in the visible region. These reactions have been made the basis of a spectrophotometric procedure for chloride which seems to be one of the most common methods for that anion (1-4). This method has also been developed into an automated procedure for chloride (5). The method is not selective for chloride as many other anions, including iodide and cyanide (6),bromide (7),
Author to whom correspondence should be addressed. (1) I. Awasaki, S. Utsumi, and T. Ozawa, Bull. Cl7em. SOC.Jup., 25, 226 (1952). (2) R. P. Marquardt, ANAL.CHEM., 43,277 (1971). (3) D. M . Zall, D. Fisher, and M. 0. Garner, ibid.,28, 1665 (1956). (4) T. M . Florence and Y . J. Farrar, Anal. Chim. Acfa, 54, 373 (197 1). (5) R . D. Britt, ANAL. CHEM., 34,1728 (1962). (6) S. Utsumi, J. Cl7em. Soc. Jup., Pure Chem. Sect., 74, 32 (1953). (7) S. Utsumi, ibid.,73, 889 (1952).
and sulfide, thiosulfate, nitrite, iodate, and bromate (8) have also been determined spectrophotometrically using these reactions. Similar methods have been reported involving the reaction of anions with insoluble metal thiocyanates to displace the thiocyanate ion which is then complexed with ferric ion. Cuprous thiocyanate has been used for the determination of cyanide (9) and thiosulfate ( I O ) while silver thiocyanate has been used for the measurement of sulfide (12) and bromide or iodide in the presence of chloride (12). We have investigated the reaction of sulfite i o n with mercuric thiocyanate as a possible method for the determination of that anion. N o reaction occurs in water alone, but thiocyanate ion is released in ethanol and methanol and in aqueous ethanol. Sulfite can be measured at rather low levels in these solvents by reaction with mercuric thiocyanate and formation of the ferric thiocyanate complex. This method may have some advantages in some instances for the spectrophotometric determination of sulfite ion or sulfur dioxide. (8) Zbid.,p 838. (9) Zbid.,74, 479 (1953). (10) Ibid.,p 526. (11) Ibid.,p 358. (12) Zbid.,p 35. ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
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