Production of Thorium-234 Tracer K. S. Bhatki Tata Institute of Fundaniental Research, Colaba, Bombay 5 B R , India
VARIOUS EQUILIBRIA and reactions of Th+4 could be studied conveniently if a proper thorium tracer was used to facilitate analyses. Of the 13 isDtopes of thorium known to date ( I ) , 234Th(UX,) seems to ke the best because it has a convenient half life of 24.1 days; it can be readily prepared in the laboratory from natural uranium-238, of which it is the first decay product; its short-lived (1.18-minute) daughter, UXz(234mPa), is a very energetic beta emitter, so that the detection of UX1 is greatly facilitated; and the tracer can readily be assayed even in liquid samples by eniploying a well-type NaI crystal spectrometer. Methods employed to separate UX1from uranium have been reviewed by Hyde ( 2 ) and Dyrssen (3). Dyrssen (3) first removed the bulk of uranium by ether extraction and then purified the tracer, UX1, remaining in the aqueous layer by cation exchange resin. Bane ( 4 ) ,using Amberlite resin TR-1, proccssed 0.15M uranyl nitrate solution t o recover UXI without prior removal of uranium. Murase, Lind, and Nelson ( 5 ) OIL the other hand, employed 0.4M uranium solution to get microcurie amounts of the tracer, utilizing Douex 50 resin. The carrier-free separation of 234Pa(tlj2 = 6.7h) after removing UX, from 100 kg of uranyl nitrate has been described by Smit t’t al. (6). Anion exchange resin IRA-400 in the chloride form was used by Berman, McKiiney, and Bednas (7) to recover UX1 from uranium. A similar procedure was employed recently by Sill (8) to process 1 pound of uranyl nitrate hexahydrate (UNH). The highly charged nature of the Th+‘ ion in solution indicates that it is very strongly adsorbed on a cation exchange resin (1,2), while U02+*is only moderately strongly held. If a solution of uranyl nitrate is percolated through a column of cation exchanger, therefxe, the UX1 is mainly adsorbed near the top of the column. Uranium can then be selectively eluted from the column by dilute acid or ammonium chloride solution and UX1 finally removed by a suitable complexing agent for thorium, such as oxalic acid or ammonium carbonate. ~
~~
(1) J. J. Katz and G. T Seaborg, “Chemistry of the Actinide Elements,” pp. 1 6 6 6 , hlethuen & Co., London, 1957. (2) E. K. Hyde. U. S. At. Energy Comm., Rept. NAS-NS-3004 (1960). (3) D. Dqrssen, Scen. Kern. Tids. 62, 153 (1950). (4) R. W. Bane, U. S. At. Energy Comm., Rept. CC-3336(Nov. 23, 1945). ( 5 ) T. Murase, E. L. Lincl, and F. Nelson, J. Chromatog. 14, 478 (1964). (6) J. van R. Smit, M. Peisach, and F. W. E. Strelow, Proceedings of International Conference on Peaceful Uses of Atomic Energy, Geneva, 1958, Vol. 20, p 62, P/1119 (1958). (7) S. S. Berman, L. E. McKinney, and M. E. Bednas, Talanta 4,153 (1960). (8) c. W. sill, ANAL.CHEhl. 36,675 (1964).
It became interesting to study the adsorption of carrier-free thorium tracer on a cation exchange resin in the presence of varying amounts of uranium, from which it is normally obtained. EXPERIMENTAL
Reagents and Chemicals. Crystallized uranyl nitrate hexahydrate more than 6 months old was used to prepare solutions in 0.1N “ 0 3 in concentrations of 0.1 to 1 mole per liter, Baker analyzed Dowex 50W-X8 (200- to 400-mesh) resin was employed in the present investigation. It was air-dried and contained 32 moisture. Glass Apparatus. Ground-glass joint centrifuge tubes with stoppers and capacity of about 10 ml were used. A glass centrifuge tube (20-ml capacity), the end of which was blown off, was fused to a glass tubing of 4-mm internal diameter and used as a column to be filled with resin. Distribution Studies. A weighed quantity of the air-dried resin (50 to 100 mg) was taken in a glass-stoppered centrifuge tube. After addition of a known volume of U N H solution, the tube was shaken horizontally on a shaking machine for about 5 hours. The solution was then filtered through a borosilicate glass filter to get rid of suspended resin particles. A known volume of the filtrate was counted immediately, employing an end-window Geiger-Muller counter. A blank solution was similarly counted. The experiments were carried out in duplicate. The distribution coefficient, D, was then calculated using the formula
where I , Fa
= =
s
=
L:
=
initial activity of solution, cpm final activity of solution, cpm weight of resin taken, grams volume of solution, ml
The data so obtained for various molarities of U N H are plotted in Figure 1. To confirm these results the adsorption of thorium- 234 (UXJ from various concentrations of uranyl nitrate on the cation exchange resin and its breakthrough by the higher molarities of U N H were studied. When a solution of uranyl nitrate (UNH) showing betagamma activity, caused by the presence of the daughters UX1 and UX2 (234mPa)in dilute nitric acid, is passed through the resin bed, both these activities are adsorbed. The effluent, then, coming out of the resin, shows no beta-gamma activity but only alpha, caused by uranium-238 = 4.51 X logy). This continues for some time but, after more U N H is passed, the first adsorbed activities of UX1 UXZ start leaking through and the effluent then shows beta-gamma activities. This leakage or breakthrough for UX1 is highly dependent upon the initial U N H concentration. Adsorption of UX1 therefore is more efficient from lower concentrations of UNH.
+
VOL 39, NO. 3, MARCH 1967
401
Table I. Volume Distribution Coefficients for UX1from Breakthrough Volumes and Calculated from Weight Distribution Coefflcients, D, for Various Molarities of UNH Employing Dowex 50W-X8 resin D, from Mole/liter D, = Dp" breakthrough UNH D ( p = 0.5569) vol. 0.1 0.2 0.3 0.4 0.5 0.6
0.7 0.8 0.9 1.o
5250 1500 860 46 5 240 160 125 78 47 31
2924 835.4 478.9 259.0 133.7 89.1 69.61 43.44 26.17 17.27
lo4
-
-
280.1 150.2 94.32 68.78 50.16 38.41 27.34
ri LL
$ lo3 V
z a
0
Resin bed density.
-
t2
mE t-
2
In the actual study of breakthrough of UX1, a glass column (about 4 mm) was filled with Dowex 50W-X8 resin in the hydrogen form to a known height. A solution of UNH of known concentration was passed through this resin bed and the effluent collected in 2-ml portions, which were monitored. The collection of fractions was continued until the counting rate of the effluent was half that of the original UNH solution before passing through the resin. The total volume of the effluent so collected was measured. Employing fresh resin beds of known dimensions and various UNH solutions, the data were collected. From these breakthrough volumes, the volume distribution coefficient, D,, was calculated from the relation
D, =
breakthrough vol, ml resin bed vol, ml
These values of D, for various concentrations of UNH are given in Table I, with calculated values obtained from D,the weight distribution coefficients from the relation D, = Dp, where p is the bed density (kilograms of dry resin per liter of bed). It was determined in a separate experiment by measuring the volume of a settled wet column containing a known weight of the dry exchanger and found to be 0.5569. When these values were plotted on log-log paper us. the concentration of UNH, a straight line of slope -2 (not shown), similar to that of Figure 1, was obtained. Procedure. A liter of a 0.5M solution of UNH corresponding to 250 grams per liter was prepared in 0.1N nitric acid. It is observed from Table I that to collect practically all UX1 from this solution, a resin bed of more than 8 ml is required. A resin column of 2 sq cm X 5 cm (10 ml) was set up with water-washed Dowex 50W-X8 resin (200- to 400mesh) in the hydrogen form. The UNH solution was then passed through the column at a flow rate adjusted to about 1 cm per minute. After the solution had passed through, the resin bed was treated with 3 to 4M NHICl to elute uranium adsorbed on it and also Fe(III), if any. This washing with NH4C1was continued until the effluent showed no color and was neutral (pH -6.0). The column was washed with water to remove excess NH4C1and UX1 was eluted with a saturated solution of ammonium carbonate. The eluent was evaporated almost to dryness by infrared heating, treated with concentrated nitric acid to get rid of organic matter, if any, and dried. The sample was taken up in dilute nitric acid for further studies. 402
ANALYTICAL CHEMISTRY
0
I02
-
I
I
0.1
0.2
I 0.4
I
I
1
0.6 0.8 1.0
I 2.0
M U02 (NO,),
Figure 1. Distribution coefficient of UX1 (234Th)between Dowex SOW-X8 resin and aqueous medium containing various amounts of UNH
If the thorium tracer, after removal of ammonium carbonate, still contains a small amount of uranium or iron as seen from the color of the dried mass, a second purification is done on a much smaller column (-0.25 ml) employing the procedure described above. By this procedure about 40 pc of the tracer are obtained from 250 grams of UNH, sufficient for a considerable amount of tracer work. DISCUSSION AND RESULTS When a solution of UNH is passed through a cation exchange resin in the H+ form, it is converted essentially to the U02+2form and the exchange reaction can then be written as 2(U02+'),
+ Th+4*(Th+4)r + 2U02+'
where subscript r denotes the resin phase. The equilibrium constant, K, is then
and
where Dv =
counting rate for UX1 per ml of bed counting rate for UX1 per ml of solution
Taking logs and reananging, we get log D v = log K'
4-2 log (UOz+'),
- 2 log U O Z , + ~
Since (UOz+Z)7is constant, the differential, d l o g Dt/dlog (UOz+') = -2 Hence, a log-log plot 0:'either D, us. UOz+2or D us. UOz+2 concentration should be a straight h e of slope -2. That this is really so, can be seen from Figure 1. In the above calculations, the activity coefficient term in the exchange reaction was ajsumed constant and the possibility of hydrolysis or nitrate coniplexing of U(V1) and T h (IV) was ignored. However, since Th(1V) is a minor component and is more strongly complexed by nitrate than UOz+z,the slope may be more negative than - - 2 at high UNH concentrations.
The data are in good agreement with those of Murase, Lind, and Nelson (5)) who used a 10-ml bed of Dowex 50-X8 to process 2 liters of 0.4M UNH. This corresponds to a feed of 200 bed volumes. Table I shows a D, of 280 at 0.4MUNH, so that the bed volume used was adequate to collect practically all of the UXI present in the feed. It follows from the data of D and D, in Table I that any concentration of UNH in 0.1N nitric acid could be processed to get a quantitative recovery of z34Th(UXI) tracer from natural uranium, provided a proper volume of the resin bed is chosen from these data, The method of employing cation exchange resin, developed by Murase, Lind, and Nelson (5) and Smit et al. (6)and systematically discussed here is rapid, since the ammonium carbonate is readily removed, to get the tracer in carrier-free form. ACKNOWLEDGMENT The author is grateful to Frederick Nelson, Oak Ridge National Laboratory, for his suggestive comments on the discussion part of this paper. RECEIVED for review August 2, 1966. Accepted November 30, 1966.
Conditions for Optimum Sensitivity in Thermogravimetric Analysis at Atmospheric Pressure Lee Cahn and Norbert C. Peterson Cahn Instrument Company, Paramount, Calif. WEIGHTSENSITIVITY is it critical parameter in thermogravimetric analysis (TGA). Finer sensitivity permits improved precision and accuracy with a given sample size, or reduction of sample size for a given precision level. Smaller samples will give sharper resolu :ion and more accurate temperature values at a given scanning rate, or permit faster scanning for the same level of performance. Or, benefits may be taken in all of these parameters. The situation can be compared with that already shown for differential thermal analysis (DTA) (1). Balances are now available (2) with sensitivities of 0.1 pg in room air, and in vacuum at elevated programmed temperatures. Larger fluctuations in TGA in air at atmospheric pressure mu st then be explained by real physical phenomena associated with those being measured, such as aerodynamic forces on the sample, rather than as balance artifacts. Sensitivities finer than 1 pg can be attained under either of two conditions (3): in sample hangdown tubes of 9-mm inside diameter at atmospheric pressure, or in larger tubes at reduced pressures (41-mm i.d. at 150 torr). The 9-mm diameter tubes limit sample size to about 15-20 mg, and require some care in tube alinement. The reduced pressures require a vacuum pump and may be less convenient when flowing gas streams are used. There has thus been interest in operation with larger tubes at atmospheric pressure with and without flowing gas streams.
(1) C. Mazieres, ANAL.CHEM., 36,602 (1964). (2) L. Cahn and H. Schultz, "Vacuum Microbalance Techniques," VoI. 3, p. 29, Plenum Pres, New York, 1963. (3) L. Cahn and H. Schultz, ANAL.CHEM., 35, 1729 (1963).
I50
4. & 100
50 0 1
0 ID
-
MM
Figure 1. Noise, pg peak-to-peak us. inside diameter of sample tube, in air at atmospheric pressure A systematic investigation was undertaken to determine the effect on sensitivity of tube diameter at atmospheric pressure, the limiting pressure for 1 pg noise as a function of tube diameter, and the effect of flowing gas streams. EXPERIMENTAL All measurements were made using a Cahn model R G Electrobalance in a glass vacuum bottle (2-4) with chromelalumel thermocouple. A Marshall No. 1804 furnace, rated to 980" C, was programmed manually with a variable autotransformer at about 10" C/min. All runs were made from ambient to 950-80" C. Sample tubes were quartz or Vycor, of the various diameters reported. For the flowing gas runs, tubes of the Cahn Flothru ( 4 ) type were used, about 780 mm (4) Bulletin 119, Cahn Instrument Co., Paramount, Calif. VOL. 39, NO. 3, MARCH 1967
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