Separation of Boric Acid from Uranyl Nitrate by Anion Exchange Chromatography R. A. A. Muzzarelli Department o,f Chemistry, Faculty of Sciences, Unioersity of Sherbrooke, Sherbrooke, P. Q.,Canada
THE SEPARATION OF BORON from uranium has been studied extensively in order to lower the detection limits or to obtain better precision of measurement (1). However, such a separation is a delicate a i d difficult operation. Currently the most common method is the distillation of methyl borate from solutions obtained by dissolving uranium in nitric acid or in bromine ( 2 ) . The distillations do not give reproducible results past a concentlation of about 0.3 ppm of boron in uranium (3). The difficulties associated with the distillation separation have been indicated ( 4 ) , the most important of which is the introduction of boron with the large amount of calcium chloride or beryllium powder required by the distillation methods. Attempts to overcome some of the difficulties primarily involve the preliminary concentration of boron by ion exchange chromatography in aqueous solvents (4,5); however, the chromatographic methods described were limited as to the amount of sample and because of precautions made necessary when evaporating the boric acid solution. The cation exchange separation was elaborated and improved, offering the possibility of separating 0.1 pg of boron from 2 grams of uranium and determining with a 12 relative standard deviation (6). Uranyl nitrate is very soluble in ethyl ether, and boric acid has a solubility of 7.8 mg in 100 ml of ethyl ether. The chromatographic behaqior of these compounds on ion exchange resins in ethyl ether is not well known: uranyl nitrate has been studied in etiers (7, 8) but no data are available concerning boric acid. The present study ias been conducted to establish the chromatographic behavior of boric acid on anion exchange resin in ethyl ether, and to find the experimental conditions for a rapid quantitative: separation of boric acid from bigger samples of uranyl nitrz te, with no hazard of losses of boric acid by evaporation. EXPERIMENTAL
Reagents. Reagent-g:rade chemicals and solvents were used for the preparation of the following solutions: for flame spectrophotometric determinations, amounts of uranyl nitrate hexahydrate va-ying between 1 and 30 grams were
(1) C . J. Rodden, editor, Analysis of essential nuclear reactor materials, USAEC Division of technical information, 1964. 32, 146 (1960). (2) A. R. Eberle and M. W. Lerner, ANAL.CHEM., (3) M. Freegarde and J. Cartwright, Analyst, 87, 214 (1962). (4) A. R. Eberle, M. W. Lerner, and H. Kraner, USAEC Report NBL-143, April 1958. (5) G. A. Barnett and G. W. C. Milner, UKAEA Report AEREC/R-2307, Dec. 1957. (6) A. W. Wenzel and C . E. Pietri, ANAL.CHEM., 36, 2083 (1964). (7) S. Urubay, J. Korkisch, and G. E. Janauer, Talanta, 10, 673 ( 1963). (8) V. M. Vdowenko, A. A. Lipovskii, and M. G. Kuzina, Radiokhimiya, 3, (3), 365 (19C~l).
dissolved in 300 ml of ethyl ether containing variable amounts of boric acid up to 20 mg; for emission spectrographic determinations, amounts of uranyl nitrate hexahydrate varying between 0.1 and 1 gram were dissolved in 100 ml of ethyl ether containing 3.10-6 gram of boric acid; standard solutions for flame spectrophotometry were prepared by dissolving different amounts of boric acid in aqueous hydrochloric acid 5N, to cover the ranges 0-50 and 0-500 mg of boron per liter, and the corresponding calibration curves were drawn. Resin and Columns. Dowex-1 X 8, 100-200 mesh, (chloride form) was stirred several times in water, in methanol, and finally in ethyl ether and was used to prepare a resin bed in ethyl ether 1 cm in diameter and 10 cm high. A smaller column, 0.8 cm in diameter with a bed 4 cm high, was used for separation of smaller quantities to be determined by emission spectrography. Instruments. Boron was measured with a Beckman flame spectrophotometer Model DU-2400 with atomizerburner 4020. The spectrophotometer was set at 519.5 nm to give two calibration curves in the ranges 0-50 and 0-500 mg/liter of boron. Flame height, mirror position, and fuel pressure were adjusted in the usual way (9-11) to obtain maximum intensity of the emitted light. The determinations made by emission spectrography were routine analyses based on the carbon porous cup technique (12) and were done at the Oak Ridge National Laboratory. The preliminary detection of uranium in effluents and columns was done by spot test with ferrocyanide, and the determination of traces of uranium was done by gammaray spectrometry using a 128-channel ND-110 spectrometer coupled to a NaI (Tl) crystal (13). Procedure. A sample of uranium metal is weighed and dissolved in concentrated nitric acid in a silica apparatus. Uranyl nitrate is crystallized and dissolved in ethyl ether to obtain a concentration of about 10 grams of uranyl nitrate in 100 ml of ethyl ether: the solution is immediately poured into the chromatographic tube and passed at a flux of 5 ml per minute. A sample of 30 grams of uranyl nitrate can be passed in 1 hour. The column is washed with 100 ml of pure ethyl ether, and then boric acid is eluted by careful introduction of 24 ml of 5NHCl. The determination of boric acid is made on this solution, and the recovery of pure uranyl salt is effected by adding some water to the ether solution and blowing off the ether. In order to establish the procedure, two solutions were prepared, one containing only boric acid in ethyl ether, the second containing the same amount of boric acid and gram amounts of uranyl nitrate. Two identical columns were prepared and the chromatographies carried out simultaneously under identical conditions. Experiments were repeated to cover the quantity ranges indicated. The effluent con-
(9) J. A. Dean and C. Thompson, ANAL.CHEM., 27,42 (1955). (10) P. T. Gilbert, R. C. Hawes, and A. 0. Beckman, Zbid., 22, 772 (1950). (11) T. Yoshizaki, Zbid., 35,2177 (1963). (12) C. Feldman, Zbid., 38, 1062 (1961). (13) P. Bussikre, Bull. SOC.Chirn. France, 1966, p. 1134. VOL. 39,
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Table I. Recovery of Boron Added to Uranium Samples Boron, grams Added Recovered Uranium, grams 0.0059 0.0059 0.0030 0.0031 0.0029 0.0029
0.0062 0.0059 0.0031 0.0032 0.0031 0.0032
RESULTS AND DISCUSSION
It has been observed that uranyl nitrate in ethyl ether is not retained on the anion exchange resin, and that the resin column can be washed free from uranium with 100 ml of ethyl ether. On the other hand, boric acid is collected on the resin bed from the uranyl nitrate solution in ethyl ether, and the volume of this solution as well as its uranyl nitrate concentration does not affect the retention of boric acid. Aqueous 5N hydrochloric acid can elute boric acid and only 24 ml of this solution are required to obtain a quantitative elution of boric acid from the longer column. The elution curves of the different chromatographic separations correspond to the curves represented in Figure 1: more than 80% of boric acid is eluted with the first 9 ml of 5N hydrochloric acid. Typical results are given in Table I. The elution of boric acid from the smaller columns follows the same curve, and 5 ml of HC1 were usually sufficient to obtain the quantitative elution of boric acid. The collected elution fractions were measured for radioactivity before determining boron, to be sure that no traces of uranium were present. When the washing of the column was sufficiently prolonged (generally 100 ml of ethyl ether, until no reaction with ferrocyanide occurred), no uranium could be detected by gamma-ray spectrometry in the elution fractions containing boric acid. Also, distilled water is a convenient eluting agent for boric acid, and the elution curve resembles the one for 5N HC1 presented in Figure 1. In any case, hydrochloric acid solution is preferred because eventually it might be possible that a column is not perfectly packed and some uranium is retained: in this case anionic chloride complexes of uranium are formed and they are retained on the resin and not eluted with boric acid. Hydrochloric acid 5N in methanol has also been tried, and it is useful for elution of boric acid. One would expect to obtain some advantage in flame spectrometry because the use
ANALYTICAL CHEMISTRY
ethyl
ether
HCI 5N
1,3208 1.5864 4.0160 4,0456 6.9891 15.0600
taining boric acid was collected in eight volumetric flasks (3 ml per flask) and diluted to 10 ml for flame spectrophotometry. When using the smaller columns, and determining by emission spectrometry, the effluent (5-ml total) was treated according to the cited procedure (12).
366
IO0
I
1001
ml
effluent
vs
elution
%
Figure 1. Elution curve of boric acid from 1 X 10-cm Dowex-1 column, performed with 5N hydrochloric acid Ethyl ether (100 ml) was first used to wash the column free from uranyl nitrate
of organic solvents enhances the intensity of atomic emission (9, 14). In fact, line intensities were higher by a factor of two, but the slope of the calibration curve in the range 0-50 mg of B was not the most suitable for exact readings. An application of particular importance is the separation of boron from uranium for preparative or analytical purposes after separation of traces of numerous metals by chromatography on cellulose column in ethyl ether (15). The solution of gram amounts of uranyl nitrate in ether passed through the cellulose column can be directly purified from boric acid by passing it on a column of anion exchange resin in ether. The rapidity permitted by the high-flow-rate chromatography and the use of a reduced volume of acid solution to elute boric acid, are advantages of this method. The possibility of subjecting to chromatography a 30-gram uranyl nitrate sample extends the separation method to samples of high-purity uranium and permits the determination of boron at lower levels of concentration in uranium. ACKNOWLEDGMENT
The author is indebted to Roland Reverdy who operated the flame spectrometer, and to the Analytical Chemistry Division of the Oak Ridge National Laboratory where the emission spectrographic measurements were made. RECEIVED for review October 24, 1966. Accepted December 14, 1966. Research partially conducted under Grant No. 4200-20 from the National Research Council of Canada. (14) J. Elhanan and W. D. Cooke, ANAL.CHEM., 38,1062 (1966). (15) R. A. A. Muzzarelli and L. C. Bate, Tuluizta, 12, 823 (1965).
