Determination of Uranium in Solution by X-Ray Absorption T. W. BARTLETT Carbide and Carbon Chemicals Co., K-25 Plant, Oak Ridge, Tenn. The determination of uranium in solution by x-ray absorption was investigated to determine its applioahility to rapid, routine uranium determinations on a variety of solutions. With a new type of absorption cell having a solution depth of 19 mm., straight-line calibration curves indicated a sensitivity of 14 scale divisions per gram of uranium per liter. The absorption by 1% solutions of possible contaminating elements increased raoidlv with inereasine atomic
In the absence of contaminating elements, the preoision, expressed as the 95% symmetric eonfidenoe interval, was t0.05 gram per liter in the eonaentration range up to 10 gram per liter. The lower limit of detection was approximately 0.1 gram per liter. Moderate concentrations of substanoes of low atomic number, such as sodium, ammonium, and fluoride ions, &,I .^* "e ;."-" I . . . * ^L.%-:^"l ^F the ura tities 01
'
content of such solutions was checked bv by t~a. ceric sulfate titration procedure (4)until it wm established that t t k the uranium content of the salt was in agreement with the formula, UO*NOs.6H,0. The solutions descrihed in Table I1 were prepared from fresh, reagent-grade chemicals. Operation of the Instrument. The electronic circuit wm &I-
attenuator idjusted to approximate balance durhne the warm-up period for the x-ray unit.
the right and a second cell contjining water on the left' (attenuator) side. PRECISION, SENSFTIYITY, AND RANGE
The data in Table I are the results of replicate cdibrations with the Same solutions. For nine successive working days, readings were taken on aliquots of the nine uranium solutions, on water, and on an aluminum block. The widest 95% symmetric confidence interval (2) was +OX3 scale division, which corresponds to *0.05 gramof uraniumper liter.
Figure 1. Absorption Cells and Cell Holder
Frequent recalibration was found necessary because of changes in response of the instrument. Comparison of readings obtained over a. period of B month indicated a gradual drift of the readings, believed due to fatigue of the photocell. Occasionally, a more abrupt change in readings occurred, the reason for which is unknown. For example, a. tenth set of readings, not included in Table I, was obtained with the solutions described above, but these readings were omitted from the sbtistical analysis because all were approximately 1 scale division higher than had been obtained in the previous nineresdings.
705
*
706
ANALYTICAL CHEMISTRY or less in order to avoid estensive calibration and because of unsteadiness of the balance-indicating needle a t high amplification.
Table 1. Precision with Uranium Solutions 0
1
Mean 13 7 29 5 Precisionu 0 46 0 32
Concentration of Uranium, Grams per 2 3 4 5 6 7 Scale Readings 1 2 . 0 28.4 11.4 28.3 12.2 28.6 12.2 28 5 1 1 . 9 28.4 12.2 28.6 1 1 . 8 28 6 12 0 28 6 11.8 28 2 42 4 56 7 70 9 87 8 11 9 28 5 0 27 0 44 0 43 0 50 0 60 0 35
Liter 8 41.3 41.2 41.2 41.2 41.4 40.6 41 6 41 4 41 2 41 2 0 63
9
Check Reading on 100-Mil AlBlock
10
5 5 . 8 70 1 5 5 . 6 70 2 56.0 7 0 . 8 56.0 7 0 . 5 56.0 70.2 56.4 70.2 56 0 7 0 . 3 55 8 7 0 . 3 55 6 7 0 . 2 55 9 70 3 0 57 0 50
85 3 84.9 85.2 85.0 85.4 85.5 85.5 85.4 85,s 85 3 0 63
INTERFEREZCES
Uranium Foundb, Grams per Liter 1 . 0 5 1 . 9 5 2 . 9 3 3 . 9 4 5.12 5 . 9 0 7 . 0 6 7 . 9 5 8 . 9 9 9 . 9 9 Readings on solutions containing 0 t o 5 grams of uranium per liter mere made with 51-mil aluminuni block on right side. a 95% symmetric confidence interval about mean. b Calculated from above means by use of calibration equations -0.05
Table 11. Interferences Stomic Substance Kumber Fluoride 9 8odium 11 Chloride 17 Calcium 20 Iron 26 Copper 29 Bromide 35 Strontium 38 Molybdenum 42 Silver 47 Iodide 53 Barium 56 Tungsten 74 Lead 82 Thorium 90 Ammonium .. .4cetate Sitrate .. Sulfate ,. Phosphate ..
..
Citrate Carbonate a
b
. ,
Compound Used .4mmonium fluoride Sodium acetate .4mmonium chloride Calcium acetate Ferric nitrate Cupric acetate Sodium bromide Strontium nitrate Ammonium molybdate Silver nitrate .4mmonium iodide Barium acetate Tungstic acid Lead acetate Thorium nitrate .4mmonium hydroxide Acetic acid Nitric acid .4mmonium sulfate JIonobasic hmmonium phosphate Citric acid Sodium bicarbonate
Forniula
-4pparent L-ranium Content of 1% Solution of Substance, Grams /Liter 0 04 0 3 0 7 1 5 2 7 3 8 5 0”
5 6 5 3 3 7 8
8 1 0 2 4 0
8
9 6 0 01
- 0 003b 0 05 0 3 NHiHzPO4 CaHi(0H) (COOH)a.HzO NaHCOa
0 2 0 02
The x-ray absorption of 22 elements and ions was measured and the extent of interference determined as apparent concentration of uranium by reading from a calibration curve. The conipounds used to prepare the solutions are listed in Table 11, and the apparent uranium content of 1 % (w./v.) solutions of the elements and ions is presented both in Table I1 and in Figure 3. Kater was the only solvent used, although the pH a a s increaded by adding a small amount of amnionia in the cases of ammonium molybdate and tungstic a c i d . T h e a b s o r p t i o n by elements other than the one being investigated \T-as neglected except for sodium compounds, when the absorption b y sodium m s subtracted from the ab~orptionby the compound. The apparent uranium content increased rapidly with atomic number up to 42 (molybdenum) arid above 53 (iodine). The absorption by silver, atomic number 47, was between that by molybdenum and iodine. The location o f the mininiium in the curve of Figure 3 intlicatce that, the “effpctive” wave length
0“
Corrected for absorption due t o sodium. Scale readings decreased v i t h increasing acetic acid content.
Kith the 30-ml. fixed-volume absorption cells, a change in uranium concentration of 1 gram per liter resulted in a change of approximately 14 scale divisions. This sensitivity is shown graphically by the slope of the calibration lines in Figure 2. The values plotted in Figure 2 are the means in Table I, but the lines were drawn by means of the following equations:
K
=
14.33U
R = 14.24U
+ 14.44 (51-mil aluminum block on right) - 72.05 (no aluminum block)
m-here R is the scale reading and U is the concentration of uranium in grams per liter. These equations were calculated, by the method of least squares, as being the straight-line equations which best fit the calibration data. In addition to the deviation of scale readings obtained on the same solution, there is error due to deviation of readings between solutions. This is illustrated by the h a 1 row of data in Table I, grams of uranium per liter found. The maximum error from this second source, 0.12, and the greatest sum of the two deviations, 0.15, occurred with the solution containing 5 grams of uranium per liter. The values were calculated from the mean and the highest of the readings on the solution, respectively. The range of uranium concentration which may be used in the small absorption cells has been considered to be 0.1 to 90 grams per liter. The lower limit is the concentration which may readily be distinguished from water, while the upper limit is imposed by the limit of amplification. It is preferable to dilute solutions containing high concentrations of uranium to 20 grame per liter
‘ O t
I
0 0
2
Figure 2.
I
i
/ I
I
I
4 6 8 10 URANIUM, G R A M S / L I T E R
1 I2
Uranium Calibration Curves
V O L U M E 2 3 , N O . 5, M A Y 1 9 5 1
707 70
14
60
M A S S ABSORPTION
c 12
.: t
*
I-
z 2
50 W
10
E
0
k W
I-’
z
4o
c”e z
8
E
0 V
E 2 6 a a
30
c
4
20
a 2
10
v)
a
3
W
a a a a
0
$
0
10
20
30
50
40 ATOMIC
Figure 3.
60
70
80
90
NUMBER
Apparent Uranium Content of 1% Solutions
100
0
E
2
the x-ray photometer. The choice of separation method depends upon the nature and the amount of the contaminating elements, the amount of uranium present, and the accuracy desired. Ether extraction from nitrate solution (8) separates uranium from a large number of elements, it is applicable to a wide range of uranium c o n c e n t r a t i o n s , and an aqueous solution, well adapted to use in the x-ray photometer, may be 01)tained by evaporation of the ether over water. ACKXOWLEDGMENT
The allsorption cells described herein were designed and constructed by S . -k. Teasly. I n addition, assistance in planning and performing the work was given by R. B. Bauder, E. B. Olszeuski, J. H. Lykins, I-. G. Iiatzel, Richard Aiken, W, R. Rossmassler, and R. H. Lafferty, Jr. LITER 4TURE CITED
of the y-rag beam is approximately 0.5 A. To illustrate this, the published values of the mass absorption coefficients for 0.497 1. x-rays (7) have also been plotted against atomic number in Figure 3. The organic anions and those such as carbonate, nitrate, and fluoride, which contain only elements of very low atomic number, showed little interference. Sulfate, phosphate, and chloride were slightly more serious, Jvhile bromide, iodide, molybdate, and tungstate were very seiious interferences. I-ariation of the piiinary voltage in the range 60 to 120 volts had only FI minor effect on the scale ieadings obtained with uranyl nitrate fiolutione and, generally, with the solutions described in Table 11. However, in the cases of molybdenum, silver, and iodine, a marked decrease of scale reading occurred when the primary voltage a a s decreased to 60 or 70 volts. This effect was preaumably due to the increase of effective wave length above the K critical abfiorption wave lengths of these elements. Because interference by most elements may be materially reduced only by removal of the element, chemical separations have usually been necessary when determining uranium by means of
(1) .kboln, R. H.. and Ht.own, 11, H., 1x0. ENG.CHESI., ASLL. ED., 1, 26 (1929). (2) Brownlee, K. A , , “Industrial Experimentation,” 2nd American ed., Brooklyn, K.Y., Chemical Publishing Co., 1948. (3) Compton, A. H.. and Allison, S. K., “X-Rays in Theory and Experiment,” 2nd ed., Kew York, D. Van Nostrand Co.. 1935. (4) Furman, X. H., and Schoonover, I. C., J . Am. Chem. Soc., 53, 2561 (1931). (5) Lambert, RI. C.. Pacific Northwest Regional Meeting AM. CHEM.SOC.. Richland. Tash.. June 1950. (6) Liebhafsky, H. A,, Smith, H 3f., Tanis, H. E., and Winslow, E. R., Ihid., 19,861 (1947). (7) hfichel, P. C., and Rich, T. A , , Gen. Elec Reu., 50, 45 (February 1947). (8) Rodden, C. J., ANAL.CHEX.,21, 330 (1949). (9) ANAL. ~, Sullivan. 31. V.. and Friedman, H.. IND.ENG.CHEM.. ED.. 18, 304 (1946). (10) Vollmar, R. C., Petterson, E. E., and Petruazelli, P. A . AXAL. C H E M . . 21, 1491 (1949). RECEIVED December 14, 1950. Presented before the Division of Analytical Chemistry at the 118th RIeeting of the -~\IERIC.AX CAEJrICaL SOCIETY, Chicago, Ill. Prepared from A E C D 2765 by T. W. Bartlett, R. B. Bauder, J. H. Lykins, E. B. Olszewski. and R. H. Lafferty.Jr., and from A E C D 2766 by T. W. Bartlett.
Analysis of Uranium Solutions by X-Ray Fluorescence L. S. BIRKS
T
AYD
E. J . BROOKS, C. S. .\-am1 Research Laboratory, Washington, D . C.
HE principle of the x-ray fluorescence method of chemical analysis as developed a t the Naval Research Laboratory (4) is shown diagrammatically in Figure 1 of (5). With the presentday, high-intensity, sealed-off x-ray tubes and sensitive Geiger counter circuits, it is feasible t o locate the specimen outside the x-ray tube and evcite i t t o fluorescence with the direct x-ray heam. The fluorescent radiation characteristic of the elements in the specimen is collimated b y a n array of nickel tubings. T h e resulting parallel, polychromatic beam is analyzed by a single-crystal Geiger counter spectrometer in which each characteristic x-ray line from the specimen is diffracted at a particular angle according t o Bragg’s law. I n the past, the x-ray fluorescence method has been applied successfully t o both solid ( 9 ) and liquid (3) specimens. It has an advantage over x-ray absorption analysis in that most impurities in the specimen do not affect the results. However, any impur-
ity whose x-ray lines do interfere with those of the desired element, or whose absorption coefficient is high enough to affect the intensity, must be considered and the analysis corrected for its effect. I n some liquid samples, the background scattering is so high that the accuracy is affected, and it becomes necessary to evaporate the liquid before making analysis. SPECIMEN PREPARATIOY
When water solutions of uranium salts were examined in liquid cells, the background near the uranium La line was high because of scattering b y the hydrogen in the water. For a 1 gram per liter solution of uranium (as the nitrate), the background intensity from a tungsten-target x-ray tube operated at 50 kv. and 25 ma. was 35 counts per second; the uranium La line was only 15 counts per second above background. With a molybdenum-target tube and the same operating conditions.
