Determination of Uranyl and Thorium(lV) by Gas Chromatography of Volatile Mixed-Ligand Complexes R. F. Sieck, J. J. Richard, Kay Iversen, and C. V. Banks' Institute f o r Atomic Research and Department of Chemistry, Iowa S t a t e University, A m e s , Iowa A gas chromatographic method is reported for the separation and determination of UOzz+and Th(lV) as mixed-ligand complexes. Several new volatile UOzZ+ and Th(lV) complexes are reported. By using the p-diketone hexafluoroacetylacetone, H(HFA), and the neutral donor di-n-butylsulfoxide, (DBSO), to form the mixed-ligand complexes, separation and quantitative determination of UOz2+and Th(lV) was possible in the range 1 mg metal/ml to 120 mg metal/ml with a relative error of less than 10%. Detection limits were 0.4 mg/ml for thorium and 0.6 mg/ml for uranium. The use of solvent extraction procedures to obtain the mixed-ligand complexes provides a method which is quick and highly selective.
UTILIZATION OF THE GAS CHROMATOGRAPH for determination of metal halides and metal acetylacetonates has become widespread in recent years. Considerable attention has been given to the determination of uranium as the volatile fluoride UF6 (1-6). The corrosive nature of volatile fluorides such as UF6has required special column materials and modification of gas chromatographic detectors (2-6). Several attempts have been made to determine thorium(1V) as a metal acetylacetonate by gas chromatography (7-9). Yamakawa, Tanikawa, and Arakawa (9) report a gas chromatographic peak for thorium acetylacetonate, Th(AA)4. Tanikawa, Hirano, and Arakawa (8) report a gas chromatographic peak for thorium trifluoroacetylacetonate, Th(TFA)a, while Schwarberg and Moshier (7) were not able to obtain a chromatogram for Th(TFA)4. Berg and Acosta (IO) have reported volatile and stable acetylacetonates for Th(IV), U(IV), and U0z2+. They also reported volatile chelates of thorium using hexafluoroacetylacetone to give Th(HFA)4 and 2,2,6,6-tetramethylheptane-3,5-dione to give Th(THD)4. Th(TFA)4 was found by these authors to be volatile and stable, while UOZ(TFA)Zwas reported to be thermally unstable. Swain and Karraker (11) studied the U(1V) and Th(1V) Deceased, Feb. 26, 1971. (1) J. F. Ellis and C . W. Forrest, J . Inorg. Nucl. Chem., 10, 86 (1959). (2) J. F. Ellis, C . W. Forrest, and P. L. Allen, Anal. Chim. Acta, 22, 27 (1969). (3) A. G . Hamlin, G. Iveson, and T. R. Phillips, ANAL.CHEM., 35, 2037 (1963). (4) R. S . Juvet, Jr., and R. L. Fisher, ibid., 38,1861 (1966). ( 5 ) H. Shinohara, N. Asakura, and S . Tsujimura, J. Nucl. Sci. Technol, 3, 373 (1966). (6) S . Tsujimura, G. Fujisawa, and H. Ito, ibid., 5 , 127 (1968). (7) J. E. Schwarberg, R. W. Moshier, and J. H. Walsh, Talanta, 11, 1213 (1964). (8) K. Tanikawa, K. Hirano, and K. Arakawa, Chem. Pharm. Bull., 15, 915 (1967). (9) K. Yamakawa, K. Tanikawa, and K. Arakawa, ibid., 11, 1405 (1963). (10) E. W. Berg and J. J. C . Acosta, Anal. Chim. Acta, 40, 101 (1968). (11) H. A. Swain and D. G. Karraker, Inorg. Chem., 9, 1766 (1970).
chelates of 2,2,6,6-tetramethylheptane-3,5-dione, H(THD), and 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyloctane-4,6-dione, H(F0D). Although they found U(THD)4, Th(THD)*, U(FOD)4, and Th(FOD)4chelates to be volatile, no significant difference in volatility was observed between the U(1V) and Th(1V) chelates of H(THD) or H(F0D). This paper will describe the determinations of U0z2+ and Th(1V) by the gas chromatography of volatile mixed-ligand complexes. The utilization of mixed-ligand complexes for the gas chromatographic determination of the rare earths has been reported by Butts and Banks (12). Application of mixed-ligand complexes for the determination of U022f and Th(1V) has allowed isolation of these species from aqueous solution by solvent extraction and subsequent separation and quantitative analysis by gas chromatography. EXPERIMENTAL
Instrumentation. A Hewlett-Packard Model 5756B equipped with a thermal conductivity detector was used. A Hewlett-Packard Model 7128A strip chart recorder was used for development work while a 1-mV Bristol recorder equipped with a Disc Integrator, Model 202, was used for the quantitative studies. Gas chromatographic conditions were as follows: Inlet temperature, 250 "C; thermal conductivity detector temperature, 350 "C ; injection port temperature, 260 "C; detector bridge current, 150 mA. Helium was used as the carrier gas with a flow rate of 48 ml per minute for all columns used. Stainless steel tubing was used for the column tubing and the injection port liner. The thermogravimetric data were obtained using a DuPont Model 950 Thermogravimetric Analyzer. Thermograms were obtained using a programmed heating rate (5 "C min.), and were observed in an atmosphere of nitrogen gas with a measured flow rate of 100 ml/min. Reagents. The ligand 1,1,1,2,2,6,6,7,7,7-decafluoro-3,5heptanedione, was prepared by a Claisen condensation of ethyl pentafluoropropionate with 3,3,4,4,4-pentafluoro-2butanone according to the procedure of Springer, Meek, and Sievers (13). The ligand was obtained as a clear liquid with the fraction distilling between 93-96 "C being collected. Anal. Calcd for H(FHD), C~HZFIOOZ: C, 27.27; H, 0.66. 'Found: C, 27.33; H, 0.64. Trifluoroacetylacetone, H(TFA), and hexafluoroacetylacetone, H(HFA), were obtained from Peninsular Chem. Research. The H(TFA) was used as received. The H(HFA) was redistilled at 66 "C before use. Tri-n-butylphosphate, TBP, was obtained from Fisher Scientific Company and purified by the procedure of Irving and Edgington (14). Tri-n-butylphosphine oxide, TBPO, was obtained from Carlisle Chemical and used after recrystallization. Di-nbutylsulfoxide, DBSO, was obtained from Aldrich Chemical, Inc., and used after redistilling at 0.9 mm and 92 "C. Reagent grade thorium nitrate from Fisher Scientific Company,
(12) W. C . Butts, and C . V. Banks, ANAL.CHEM., 42, 133 (1970). (13) C . S . Springer, Jr., D. W. Meek, and R. Sievers, Znorg. Chem., 6 , 1105 (1967). (14) H. Irving and D. N. Edgington, J. Inorg. Nucl. Chem., 10, 306 (1959). ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
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Compound UOz(HFA)** DBSO UOz(FHD)z.DBSO UOz(TFA)z.DBSO Th(HFA)4* DBSO Th(FHD)a.DBSO Th(TFA)4* DBSO
Table I. Analytical Results for UOZz+ and Th(1V) Complexes Metal % Carbon MP, “C Calcd Found Calcd Found 82 31.90 31.45 25.54 25.27 58 25.80 26.51 25.25 25.05 107 36.57 36.78 29.28 29.34 82 18.97 19.11 27.51 27.16 28 14.30 13.93 26.64 26.68 oil 23.05 22.53 33,40 33.51
was used as the source of thorium. Analytical reagent-grade uranyl acetate from Mallinckrodt Chemical Works was used as the source of UOzZ+. Preparation of Solid Complexes. URANYLCOMPLEXES. One gram (2.5 mmoles) of uranyl acetate dihydrate dissolved in 50 ml of water was shaken for 5 to 10 min with 100 ml of hexane solution containing 5 mmoles of the diketone and 2.5 mmoles (0.4 gram) of DBSO. The hexane solution was separated and evaporated. When about three fourths of the hexane had evaporated, the precipitate was filtered, washed with a little hexane and placed in a vacuum desiccator to dry. THORIUMCOMPLEXES. These complexes were obtained using the same procedure as described for the uranyl complexes, except that the mole ratio for Th :b-diketone :DBSO was 1 :4:1. Analytical results for the solid complexes are shown in Table I. Solvent Extraction Procedure. An aqueous solution of ThC14 was obtained by passing a solution of thorium obtained from Th(NO&.4H20 through a Dowex 1-X8 ion exchange column in the chloride form. Th(1V) was standardized by titration with EDTA using Xylenol orange indicator (15). The solution obtained by dissolving U02(C2Ha02)2 in water was standardized by titration with EDTA using PAN indicator (16). About 0.04M stock solutions were prepared for both metals, and appropriate dilutions were made to achieve other desired concentrations. The pH of the stock solution was about 3 for Th(1V) and about 4.5 for UOp*+. Solvent extractions were performed in glass-stoppered, 15-ml centrifuge tubes. A Burrell wrist-action shaker was used to achieve extraction equilibrium. Shaking times of 30 min were allowed to establish equilibrium. Samples were prepared by contacting 2.00-ml portions of aqueous solutions of Th(1V) or U02*+ with 2.00-ml portions of benzene which contained b-diketone in an amount 5 times the total metal concentration and neutral donor in an amount 2 times the total metal concentration. After equilibrium was established, the samples were centrifuged to separate the phases, the gas chromatographic samples were withdrawn by syringe directly from the organic phase of the extraction solutions. The species indicated from isolation of the solid chelates for these mixed-ligand systems are Th(L)4Y and UOZ(L)~Y where L = b-diketone and Y = neutral donor. It is, of course, possible that other species exist in solution. However, work with similar mixed-ligand systems has indicated the same stoichiometry under solvent extraction conditions for Th(IV) and UOZ2+mixed-ligand complexes (17,18). Preparation of Calibration Curves. Standard solutions of Th(HFA)4.DBSO and U02(HFA)2.DBSO were made in the gram range of about 1 X lowagram metal/ml to 120 x gram metal/ml metal/ml. Solutions in the range 1 X (15) A. I. Vogel, “Quantitative Inorganic Analysis,” 3rd ed., John Wiley and Sons, Inc., New York, N. Y . , 1961, p 442. (16) L. Lassner, and R. Scharf, 2. Anal. Chem., 164, 398 (1958). (17) T. V. Healy, J . Znorg. Nucl. Chem., 19, 314 (1961). (18) H. Irving and D. N. Edgington, ibid., 15, 158 (1960). 914
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
% Hydrogen Calcd Found 2.37 2.60 1.92 2.14 3.55 4.17 1.81 1.89 1.37 2.20 3.40 4.29
to 10 X gram metal/ml were prepared by solvent extraction while solutions of higher concentration were prepared by weighing out the solid complex followed by dissolution in known volumes of benzene. Twenty-microliter samples of the solutions of the complexes in benzene were injected into the gas chromatograph. Two or three injections were made to equilibrate the column to each particular sample, then three to five samples were run, and the average area under the G C peak of the complex per run was determined by the Disc Integrator. Calibration curves were drawn plotting integrator counts us. grams metal X 103/ml. Figure 1 shows the calibration curves obtained for Th(HFA)4.DBSO and U0z(HFA)2.DBS0. The calibration curves were checked periodically over a period of two weeks and did not vary significantly. Samples gram metal/ml and 10 X grammetal/ml in the 5 X range were chromatographed using solutions obtained both from solvent extraction and dissolution of the solid complex. The calibration points were identical for samples prepared by these different methods. In addition, the calibration curves did not vary when different gas chromatographic columns were used. It was not found practical to use the solvent extraction procedure for metal concentrations greater than about 25 X gram metal/ml. At high concentrations of complex, the density of the benzene solution became similar enough to that of the aqueous solution so that phase separation became very difficult if not impossible. Hence, for application of the solvent extraction procedure, it was found best to use 1 to 25 grams metal X 10-3/ml range along with an expanded calibration curve as shown in Figure 2. Gas Chromatographic Columns. Columns A and B were 1/4-inch stainless steel and 16 inches long. Column A was packed with Chromosorb W (100/120 mesh) coated with 10% SE 30. Column B, which gave the best separations, was packed with Chromosorb W (100/120 mesh) coated with 17.8% QF-1. The columns were conditioned at 245 “C overnight before use. RESULTS AND DISCUSSION Thermogravimetric Studies. Thermogravimetric data for several mixed-ligand complexes of UOz2f and Th(1V) are shown in Figure 3. Thermograms A , U02(TFA)2.DBSO, and D, Th(TFA)4.DBSO, indicate that after partial volatilization, thermal decomposition occurs for the mixed-ligand complexes of UOz2+and Th(1V) when the b-diketone trifluoroacetylacetone (HTFA) is used. Thermograms B, Th(FHD),. DBSO, and E, U02(FHD)2.DBSO, C, U02(HFA)2.DBSO, and G, Th(HFA)(. DBSO, and F, U02(HFA)2.TBP, indicate nearly complete volatilization with little or no residue for these different mixed-ligand complexes using the P-diketones HHFA and HFHD. Inspection of the indicated temperature at the mid-point of weight loss suggests some volatility differences for the mixed-ligand complexes of U022+ and Th(1V). Thermograms B and E show a small volatility difference for the
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mixed-ligand complexes of U0z2+ and Th(1V) using the @-diketoneH(FHD) and the neutral donor DBSO. A more pronounced difference is observed for the H(HFA)-DBSO system in thermograms C and G. These data suggest the possibility of separation of the mixed-ligand complexes of UOZ2+ and Th(IV), and this separation has been accomplished utilizing gas chromatography. It has been argued (17-19) that the synergic effects observed in solvent extraction using mixed-ligand complexes are due in large part to the displacement of water from the coordination sphere of the metal chelate by the organic base or neutral donor, to produce an anhydrous and more hydrophobic com-
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(19) J. W. Mitchell, Ph.D. thesis, Iowa State University, Ames, Iowa. 1970.
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Figure 3. Thermograms of mixed-ligand complexes of U02+2and Th(IV)
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
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Figure 4. Gas chromatogram for mixedligand complexes of U O P and Th(1V) with H(FHD) and DBSO Column A, Temp 200 "C A . Benzene C. Th(FHD)( * DBSO B. DBSO D. UOz(FHD),.DBSO
plex. It has also been observed by Berg and Acosta (10) that many anhydrous metal complexes can be volatilized while hydrated complexes tend to decompose when volatilization is attempted. Hence, it is not surprising to observe volatilization without apparent decomposition of the anhydrous mixed-ligand complexes shown in thermograms B, C, E, F, and G. However, the data shown in thermograms A and D indicate that factors other than thermal decomposition initiated by waters of hydration can be involved in thermal degradation. It wou!d seem that some peculiarity of the mixedligand complexes using the P-diketone H(TFA) results in thermal instability for these complexes. Other mixed-ligand complexes studied in this laboratory using H(TFA) have been observed to be thermally unstable, but as yet no explanation for this behavior has been formulated. Gas Chromatographic Studies. The solvent extraction procedure was utilized to prepare all the possible mixedligand complexes of U O Z ~ +and Th(1V) with the ligands H(HFA) and H(FHD), and the neutral donors TBP, DBSO, and TBPO. Evaluation of these complexes allowed the identification and development of the most promising mixedligand systems. Good chromatograms were obtained for UOz(FHD)z.TBP, Th(FHD)4. TBP, UOZ(FHD)Z. TBPO and Th(HFA)(. TBPO. However, no separation of mixtures of the UOs2+and Th(IV) mixed-ligand complexes was achieved for complexes which utilized TBP or TBPO as the neutral donor. A chromatogram for a mixture of UO~(FHD)Z-DBSO and Th(FHD)d.DBSO is shown in Figure 4. As had been anticipated on the basis of the thermogravimetric data, it was possible to obtain partial separation of these complexes of UOZ2+and Th(1V). However, base-line separation of the U022+and Th(1V) complexes of this system was not achieved
7 Figure 5. Gas chromatogram for mixedligand complexes of UOz+2 and Th(1V) with H(HFA) and DBSO Column B, Temp 192" and 210 "C A . Benzene, HHFA B. DBSO C. Th(HFA)r.DBSO D. Temp to 210 "C E . UOz(HFA)2*DBSO
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
Table 11. Carbon and Hydrogen Results on Complexes Trapped from the Gas Chromatograph Hydrogen Carbon Calcd Found Calcd Found 2.37 2.36 24.59 UOz(HFA)z.DBSO 25.54 1.81 2.49 27.64 Th(HFA)4.DBSO 27.51 Table 111. Analysis of Unknowns Th, grams X lOs/ml Error, Error, Present Found Present Found 1.a 4.9 4.9 0.0 4.4 4.7 6.8 2.b 36.4 34.3 5.8 23.7 24.4 3.0 3." 4.b 20.8 19.3 7.2 14.1 13.4 5.0 4 Prepared by solvent extraction. b Prepared by weighing out solid complex. U, grams X lO3/ml
even though several columns and a range of gas chromatographic conditions were employed. The failure to separate these complexes is somewhat surprising in light of their similarity to the HFA-DBSO complexes which were separated easily. The chromatogram for a mixture of UOZ(HFA)Z.DBSO and Th(HFA)4.DBSO is shown in Figure 5. This chromatogram illustrates the excellent separation of U0z2+and Th(1V) obtained by using this mixed-ligand system. The column temperature for this analysis was held at 192 "C until the Th(1V) complex was eluted, then the temperature was elevated to 210 "C for elution of the U022+complex. The small rise in base-line after the temperature elevation, which is indicated between point D and peak E in Figure 5, is probably due to the effect of thermal changes resulting from the step program. As can be seen, it does not interfere with the analysis. Base-line separation for the H(HFA)-DBSO mixed-ligand complexes of UOZ2+ and Th(1V) was also achieved on column A , but not with as much resolution as was observed on column B. The mixed-ligand complexes UOZ(HFA)Z. DBSO and Th(HFA)4.DBSO were individually trapped as eluted from the gas chromatograph. Data from the carbon and hydrogen analysis of these trapped complexes are shown in Table 11. These data indicate that the mixed-ligand complexes are being eluted from the G C without appreciable decomposition. In mixedview of the stability observed for the "FA-DBSO ligand system and the separation which could be achieved for mixedthe U0z2+and Th(1V) complexes, the "FA-DBSO ligand system was the choice for the quantitative study. This system was found to have the best chromatographic characteristics of those observed in this study. Quantitative Studies. The preparation of the calibration curves shown in Figures 1 and 2 has been discussed above. It should be noted that and response for a given weight of thorium as shown in Figure 1 is greater than that for an equivalent weight of uranium. This is to be expected since the molecular weight of Th(HFA)(.DBSO is 1222.5 grams/ mole while that of U02(HFA)2.DBS0 is only 846.3 grams/ mole. Assuming the thermal conductivity of the mixed-
ligand species to be approximately the same, the thermal conductivity detector should respond more to the heavier complex. Detection limits were determined for both mixed-ligand complexes used in the quantitative study. The detection limit was taken to be that minimum amount of mixed-ligand complex necessary to give a chromatographic peak response for the complex equal to or greater than twice the background response, Detection limits were observed as follows: Th: 0.4 X 10-3 gram/ml, and U : 0.6 X gram/ml. Four unknown samples were prepared by an independent worker and analyzed on the gas chromatograph. The procedure for analysis of the unknown was the same as that used to obtain the calibration curves. The data obtained for the analysis of the unknowns are shown in Table 111. Interferences. There should be no major interferences for this method of analysis of U0z2+and Th(1V). Mitchell (19) has shown that various mixed-ligand complex systems can be utilized to isolate U 0 2 + and Th(IV) from common interferences such as Sc(III), Fe(III), Al(III), and the rare earths. After isolation by extraction, the UOzZf and Th(1V) can be back extracted and prepared as the HFA-DBSO complexes for G C analysis. CONCLUSIONS
The determination of U022+ and Th(1V) described here is a new addition to the list of inorganic species which can be determined by gas chromatography. The selectivity afforded by the utilization of solvent extraction with mixed-ligand complexes and the resolution obtained for the complexes of U02+and Th(1V) make this potentially a very useful method. Studies are in progress to extend the sensitivity of the method into the trace range by use of an electron-capture detector. ACKNOWLEDGMENT
The authors acknowledge the many helpful discussions concerning solvent extraction with J. W. Mitchell. The technical assistance of Hugh Woodruff is also appreciated.
RECEIVED November 9, 1970. Accepted March 15, 1971. Work was performed in the Ames Laboratory of the U. S. Atomic Energy Commission. Contribution No. 2893.
Correction Determination of the Number and the Nature of the End Groups of Poly(Viny1 Chloride) In this article by Marc Carrega et al. [ANAL.CHEM.,42, 1807 (1970)l on page 1809, just preceding the Acknowledgment, the following lines were inadvertently omitted from the published text. Computing the methyl content of PVC H according to the ASTM method D 2238-68 yields CH3-values 25% lower and the regression formula: A CHI % exp = (0.79 0.35) A CHI % theor. (0.004 0.030).
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