Determination of Trace Quantities of Aluminum and Chromium in Uranium by Gas Phase Chromatography Claude Genty, Colette Houin, Pierre Malherbe, and Roger Schott Commissariat a I’Energie Atomique, Centre d’Eludes de Bruyeres-le-Chatel, Boite Postale 61, 92 Montrouge, France
The method described here combines the techniques of solvent extraction and gas phase chromatography, and enables the limit of determination of aluminum and chromium in uranium to be reduced to 0.1 ppm. Aluminum and chromium present in uranyl nitrate solutions are extracted with benzene as the trifluoroacetylacetonates; the chromatographic analysis i s carried out with an electron capture detector. The calibration curve is plotted for quantities of metal smaller than 10k4 p g . The method proposed i s compared with a conventional method for concentrations of more than 5 ppm, and for lower concentrations, it is shown that the method remains valid by adding known amounts of metal to purified uranium. The effect of extraneous elements is studied and the possibilities for extending the method are discussed.
USEOF GAS CHROMATOGRAPHY for the determination of aluminum and chromium by formation of volatile chelate compounds has undergone considerable development during the last few years. The review of Moshier and Sieviers ( I ) on the studies made with acetylacetone and volatile compounds, especially fluorine derivatives, was followed by many other publications (2-8). Use of Lovelock’s electron capture detector improved the sensitivity of the method and Ross, Sievers, and Wheeler extended the field of application of the method t o the determination of small concentrations ( I , 9, IO), but n o work had yet been done o n the determination of traces of impurities in a matrix. The problems involved in the accurate determination of trace quantities of aluminum and chromium in uranium (less than 5 ppm) are difficult to solve. The outstanding sensitivity of the electron capture detector (11) toward certain compounds, especially halogenated molecules, has enabled the determination threshold of the presently used methods to be lowered appreciably. Since the method was t o be applied t o uranium in metallic form and in nitric solution, a n extraction by a solvent compatible with the system of detection had to be used. On the basis of previous studies made on extraction of aluminum and chromium by p diketones (2, 7 , 1 2 , 13),the benzene (1) R. W. Moshier and R. E. Sievers, “Gas Chromatography of Metal Chelates,” Pergamon Press, Oxford, 1965, and references
cited therein. (2) G. P. Morie, Ph.D. thesis, Ohio State University, Columbus, Ohio, 1966. (3) G. P. Morie and T. R. Sweet, Aiinl. Chim. Acta, 34, 314 (1966). (4) G. P. Morie and T. R. Sweet, ANAL.CHEM., 37, 1552 (1965). ( 5 ) R. W. Moshier and J. E. Schwarberg, Talanta, 13, 445 (1966). (6) R. E. Sievers, J. W. Connolly, and W. D. Ross, J . Gas Chromatogr., 241 (1967). (7) J. R. Stokely, Ph. D. thesis, Clernson University, Clemson, S. C., 1966. (8) T. Michino, Y . Ishihara, K. Saito, and T. Nakazawa, Bunseki Kagaku, 15, 181 (1966); C.A.,65, 6277g (1966). (9) W. D. Ross and R. E. Sievers, Talnnta. 15, 87 (1968). (10) W. D. Ross and R. E. Sievers, ANAL.CHEM., 41, 1109 (1969). (11) D. K. Albert, ibid.,36, 2034 (1964). (12) W. G. Scribner. W. J. Treat, J. D. Weis, and R. W. Moshier, ibid., 37, 1136 (1965). (13) J. Stary, “The Solvent Extraction of Metal Chelates,” Pergamon Press, Oxford, 1964, and references cited therein.
extraction of metal trifluoroacetylacetonates was chosen as a solution t o the problem. EXPERIMENTAL
Apparatus. The Model 1200 Varian Aerograph Chromatograph furnished by Aerograph-France was fitted with an electron capture detector with a direct current power supply. The column (borosilicate glass, length 3 ft, internal diameter 0.08 inch) was filled with 60-80 mesh silanized glass beads coated with 0 . 2 z D.C.-710. The flow rate of nitrogen used as carrier and scavenger gas was 30 ml/min. The temperatures used for column, detector, and injector were, respectively, for aluminum alone: 110, 180, and 135 “C; for chromium alone: 125, 180, and 135 “C; for simultaneous determination of aluminum and chromium: 115, 180, and 135 “C. The detector voltage was set to 20 volts. Ostaflon containers (polytrifluoromonochloroethylene) were used. Reagents. Suprapur Merck acids were used. The demineralized water used was distilled shortly before use in a quartz apparatus. The 1 mg/ml aluminum o r chromium I11 stock solution were diluted just before use. A 0.1M solution of trifluoroacetylacetone (K and K, Koch Light, Columbia Organic Chemicals Co.) in benzene was used. The trifluoroacetylacetone was purified by distillation under atmospheric pressure and collection of the fraction which passes between 106 and 107 “C. It was then stored in a polythene container and kept in a refrigerator (14). The bulyer solution was prepared with two volumes of 2 M ammonium acetate solution (Merck Suprapur) and 1 volume of M acetic acid. This buffer solution was purified by washing for 1 hour with 1 volume of distilled trifluoroacetylacetone. The 15% aqueous nitric acid solution was purified by passing down a cellulose column (length: 0.4 f t ; i.d.: 0.8 inch). The aluminum trifluoroacetylacetonate was prepared by addition of 5 grams of sodium acetate per 100 ml of solution to an aqueous solution of aluminum nitrate-9 hydrate (50 g/l.) followed by an extraction with a 0.1M trifluoroacetylacetone solution in ethanol. The precipitate was collected on sintered glass, ovendried and recrystallized (15). The melting point of aluminum trifluoroacetylacetonate is 122 “C. The chromium trifluoroacetylacetonate was prepared by addition of 90 ml of buffer solution and 10 ml of trifluoroacetylacetone t o 30 grams of chromium trichloride. After shaking for 4 hours, the precipitate was collected on sintered glass, ovendried, and recrystallized. The melting point of chromium trifluoroacetylacetonate is 151 “C. Procedure. The following procedure was used for aluminum and chromium determination. The sample (maximum 1 gram of uranium in 5 ml of 10-lN nitric acid) was introduced in a n ostaflon decanter. Fifteen ml of the purified buffer solution were added, followed by 5 ml of the distilled trifluoroacetylacetone solution. The sample was shaken for 30 minutes, and the aqueous phase was then decanted. Addition of 5 ml of trifluoroacetylacetone, shaking for 30 (14) R. E. Sievers, Aerosapce Research Laboratories, WrightPatterson Air Force Base, Ohio, private communication, 1968. (15) E. W. Berg and J. T. Truemper, J . Phys. Chem., 64,487 (1960).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 2, FEBRUARY 1971
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Figure la. Chromatogram of a mixture of AI (tfa)3 and Cr(tfah AI: 1.75 X x 10-5~
pg;
Cr: 3.5 Figure 16. Chromatogram obtained with a dirty detector
minutes and decanting the aqueous phase had t o be done three times. The aqueous phase was discarded and the organic phases were collected. The organic phase was washed for 15 minutes with 20 ml of 10-2M ammonia and the aqueous phase was discarded. The organic phase was made up to 25 mi with benzene. A 2- t o 3-pl sample was taken by means of a syringe and injected into the chromatograph. If the chromium determination was not needed, the following procedure was used : The sample was introduced in an ostaflon decanter; 15 ml of buffer solution followed by 5 ml of trifluoroacetylacetone solution were added. The sample was shaken for 1 hour and the aqueous phase was then discarded. The organic phase was washed for 15 minutes with 5 ml of 10-2Mammonia and the aqueous phase was discarded before making up the organic phase t o 25 ml with benzene. A 2- to 3-pl sample was taken with a syringe and injected into the chromatograph. In both procedures a blank test of the reagents and three points on the calibration curves had to be done at the same time. RESULTS
Chromatogram. Figure l a shows the chromatogram obtained with samples of 1.75 X 10-5 pg of aluminum and 3.5 X 10-5 pg of chromium. The form of the peaks is good, the retention times are short, and the sensitivities excellent in spite of the small quantities injected. The calibration curves are linear in the region involved, i.e., from 10-6 to 8 X 1O-j pg of aluminum and from 2 X 10-6 to 7 X lO-5pg of chromium. As pointed out by Ross and Sievers (9), these curves show that the good stability of the chromatograph and of the detector on one hand, and the reproducibility of the 2- to 3-pl injections of solution on the other, enable a direct measurement without a n internal standard. Quantities of metal injected are too small for identification by conventional methods; however it was shown with radio236
active chromium (jlCr, TI/?= 28 d) that the chromium trifluoroacetylacetonate is quantitatively eluted. The calibration curves need to be checked daily for they will fluctuate as the detector becomes dirty. The detector should be cleaned once a week. Otherwise the dirtiness is such that the aluminum peak becomes increasingly anomalous as a negative peak follows the main peak (Figure 16); this defect can be reduced by cleaning in a n ultrasonic device. A calibration problem arises for chromium trifluoroacetylacetonate. In effect this compound is made up of two isomers, cis and trans, but the ratio of one to the other varies according to the experimental procedure involved. Under our working conditions the two isomers are eluted at the same time, and a 10-ft column is necessary to separate them. Even this would not solve the problem, because it would be necessary to prepare them separately in the pure state in order to be able to measure their response coefficient, and this is not possible at present. In order to counteract this difficulty it is essential, when preparing the pure chromium trifluoroacetylacetonate standard, t o use the same extraction conditions as for the analysis itself. The fact that there are two isomers is then irrelevent, since their ratio will be the same in the standard sample and in the sample analyzed. Extraction Conditions. I n order to determine the best experimental conditions, the influence of the following factors was studied : p H of the aqueous solution, extraction time, and trifluoroacetylacetone concentration in the organic phase.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 2, FEBRUARY 1971
For chromium the extraction conditions were checked by using radioactive chromium. pH OF THE AQUEOUSSOLUTION AND EXTRACTION TIME. The pH is adjusted uith ammonia and the value measured after extraction; it has been shown that the extraction of aluminum is quantitative for pH values greater than 5 and for an extraction time of one hour. For the following tests a buffer solution at pH 5.8 + 0.2 was used. For chromium it has been checked that at this pH value, four extractions of half an hour each are necessary t o get a yield of 100%. TRIFLUOROACETYLACETONE CONCENTRATION. Since the quantities of metal involved are very small, only minute concentrations of trifluoroacetylacetone (of the order of 10-5M) would suffice, theoretically, for quantitative extraction. The tests showed that a very large excess of trifluoroacetylacetone (10-lM) was required to make the extraction quantitative. This excess could be distributed as follows: a n undissociated fraction in the organic phase; an undissociated fraction in the aqueous phase, this fraction probably being negligible; a fraction combined with elements of the aqueous phase which cannot be extracted under the experimental conditions used ; and a fraction combined in the organic phase as trifluoroacetylacetonates of metals other than aluminum and chromium. The presence of an excess of trifluoroacetylacetone in the aqueous phase causes a disturbance in the detector (9). An attempt was made, after extraction of aluminum and chromium, to remove the excess of trifluoroacetylacetone by washing the organic phase with lo-" and 10-2N ammonia solutions. The study was carried out by infrared spectrophotometry, using the 9.02-pm absorption band of trifluoroacetylacetone. For a 10-2Nconcentration of ammonia a 15-minute washing time is enough to remove completely all excess of trifluoroacetylacetone in the organic phase, without any aluminum or chromium loss. However after washing the organic phase with a 10-1N ammonia solution, it was found that, for 2.5 pg of aluminum in the organic phase initially, 0.8 pg was present in the aqueous phase and 1.7 pg in the organic phase. For all later tests, therefore, the 15-minute washing with a 10-2N ammonia solution was chosen, and the detector disturbance mentioned above disappeared. QUALITY OF REAGENTS AND BLANK TESTS
Although the combination of solvent extraction and gas phase chromatography lowers the determination limit obtainable by conventional methods, manipulation is more difficult and extreme care must be taken in purifying the reagents and in avoiding pollution of the solution to be analyzed. By the purification of reagents and the systematic use of polythene or ostaflon, it was possible to improve the quality of the chromatograms and to lower the blank test value to about 0.08 pg of aluminum and 0.05 pg of chromium for 25 ml of aqueous solution. The different reagents contribute to the blank test value in the following proportions for aluminum: 40 % for nitric acid, arn40% for buffer, and 20% for trifluoroacetylacetone monia. For chromium: 30 % for nitric acid, 60 % for buffer, and 10 % for trifluoroacetylacetone ammonia.
+
+
For aluminum: (4.2 i 0.5) X 10-3pg/ml and 6.2 X Extreme values 3.4 X Taken4.0 X 8 experiments (8.4. 0.5) x 10-3~g/mi Extreme values 7.6 X and 9.2 X Taken 8.0 X loW3 10 experiments
For chromium : (2.1 =t0.1) x 10-2pg/ml Extremevalues2.3 X 10-2and 1.9 X lo-* Taken 2.0 X 10-2 7 experiments (7.6 i 0.9) x 1 0 - 3 ~ g / m i Extreme values and 6 X Taken 8.0 X 10 experiments (3.4 + 0.7) x 10-3~g/mi and 4.8 X Extremevalues (2.4 X Taken 4.0 X 7 experiments APPLICATION TO URANIUM
At the pH of the aqueous phase used for the extraction, uranium trifluoroacetylacetonate is partially extracted, but in an acetic buffer solution, the complexing capacity of the acetate ions is sufficiently high for uranium trifluoroacetylacetonate extraction to be negligible. The uranium present in the organic phase was determined by fluorimetry and its concentration is of the order of 0.2 to 1 pg/ml. Comparative Tests with Uranium Samples. Samples were used to compare the chromatographic method and the spectrophotometric methods used in our laboratory (spectrophotometry of aluminum oxinate and chromium diphenylcarbazide). Each sample is dissolved in nitric acid and the solution divided into two parts, one for chromatographic analysis and the other for the spectrophotometric one. The results are shown in Tables Ia and Ib. Tests Carried Out with Purified Uranium. The spectrophotometric methods are not suitable for aluminum or
Table Ia. Comparison of Results Obtained by Chromatography and Spectrophotometry Aluminum in uranium. ppm _ ~ _ _ . _ _ _ Samples Chromatography Spectrophotometry A 13 14 A
12.5
15
A
10 11
14 15
A A A
B
B C D
REPRODUCIBILITY TESTS
Reproducibility tests involving the whole manipulation operation were carried out. The results reveal that the 95 % confidence limits for the average values are :
loF3
E F G
H
_ _ ~ ~
12
11 3.6 3.6 13 34 45 97 158 205
