replicate analyses by either technique showed a standard deviation of &lo%. The detection limit, defined as twice the noise level, is 0.1 ppm I for X-ray fluorescence and 0.05 ppm I for the iodide electrode. The iodide electrode method is a simple, fast, and inexpensive method for determining iodine in fresh milk, but it may give erroneous results if the milk has been stored with formaldehyde. The X-ray fluorescence method is independent of chemical form and any preservative treatment, but does require considerably more work.
Table I. Concentration of Iodine in Milk Determined by X-Ray Fluorescence and Iodide Electrode .Milk sample
1 2 3 4 5 6 7
By X-ray fluorescence, ppm
clectr:Je, ppm
0.36 0.47 0.72 0.77 2.9 2.8
1.3
0.48 0.56 0.60 0.90 3.0
2.8 1.2
ACKNOWLEDGMENT The author thanks Ron Sanders for his assistance in this work.
R E S U L T S A N D DISCUSSION The standard curve for X-ray fluorescence of milk produced by the method of known additions was linear over the range 0.1-4.0 ppm I. Seven samples of cow's milk obtained from location sources were analyzed by both X-ray fluorescence and iodide electrode (Table I). The two techniques show very good agreement, particularly a t milk concentrations of greater than 1 ppm of iodine. This agreement adds confidence to prior observations that the iodine in fresh milk is mainly present as iodide. The precision for
LITERATURE C I T E D (1)G. K. Murthy, J. E. Gilchrist, and J. E . Campbell, J. Dairy Sci., 45, 1066 (1962). (2)E. W. Bretthauer. A. L. Mullen, and A. A. Moghissi, Health Phys., 22, 257 (1972). (3) C. W. Thomas, R. W. Perkins, and G. H. Mamada, Regulatory Guide 4.3, Nuclear Regulatory Agency, Washington, D.C., Sept. 1973.
RECEIVEDfor review June 4, 1975. Accepted July 14, 1975. This study was funded by the Nuclear Regulatory Agency.
Use of Chelating Ion-Exchange Resin in the Determination of Uranium in Ground Water by X-Ray Fluorescence Lawrence R. Hathaway and Gerard W. James Kansas Geological Survey, University of Kansas, l a wrence, Kan. 66044
The general usefulness of Chelex-100 chelating ion-exchange resin in selective extraction of various trace metal ions as a function of pH in batch extraction processes and the suitability of the pelletized resin for direct X-ray fluorescence analysis have been demonstrated by Blount et al. ( 1 ) . Studies involving the use of related chelating resins of the ketoiminocarboxylic acid type have shown high distribution coefficients for U(V1) in the pH range of 3-5 (2), suggesting that Chelex-100 might be of use in a study of uranium occurrence in natural waters. This report describes a method developed to survey uranium concentrations in alkaline earth-bicarbonate type ground waters of western Kansas. EXPERIMENTAL Apparatus and Operating Conditions. Analyses were performed using a Phillips Model 1410 vacuum spectrograph with an XRG 3000 generator Instrument parameters used are as follows: Tube, Chromium; power, 50 kV, 50 mA; crystal, P E T (temperature stabilized); detector, gas-flow proportional, 1400 V; PHA, base line 1.9 V, window 1.0 V; goniometer, 53.14' UMa, 54.14' background; collimator, fine. The UMa line was chosen as the analyte line because it has a greater peak to background ratio than the ULa line. Reagents. Chelex-100 (100-200 dry mesh) was obtained from Bio-Rad Laboratories. The resin was washed with 1N HCl; rinsed with distilled water; adjusted to pH 7 with a dilute NaOH solution; rinsed again with distilled water; and filtered under suction. The resin was then partially dried a t 45 'C and finally stored in a desiccator containing a saturated solution of NaHS04. The resin was allowed to equilibrate with this environment for 48 hours before being used.
Test Solutions. All test solutions were prepared from a common ground-water sample. Spiked solutions were prepared using a solution of uranyl nitrate. Solutions used in the studies on extraction time, pH dependence, and recovery in multiple extractions were spiked to 50 ppb (pg/l.) above the natural uranium level of the ground water. Preacidification of the water samples consisted of treatment with 3 ml concentrated HC1 per l i t g of water 1 2 hours prior to addition of resin. Samples used for the standard additions-calibration curve were triplicate sets of unspiked ground water, and ground water spiked to 20,60, and 100 ppb above the natural uranium level. Procedures. A 0.3-ml glass scoop was used to add two level scoopfulls (0.6 ml equals -60 mg dry weight) of prepared Chelex100 resin to 1-liter samples of water contained in 1-liter Erlenmyer flasks. The pH of the solutions was then adjusted to the desired value, with the aid of a pH meter, using a dilute NaOH solution. All samples, except those in the extraction time study, were stirred for 3 hours on a magnetic stirring table. All flasks were sealed during the extraction period in order to minimize contamination and COz uptake in the higher pH solutions. After stirring the solutions for the proper length of time, the resin samples were collected on 0.45-p filter membranes and then dried for 1 hour a t 45 'C. Filtrates from solutions used in the multiple extraction studies were treated as new samples and run through the above procedure two more times. The dried resin samples were each mixed with about 500 mg of Somar Mix (a binder), and then spread uniformly over the surface of 1.25-inch planchets which were half-filled with boric acid as a backing agent. The samples were then pressed to 10,000 psi. The resulting pellets were quite stable when stored in a chamber with silica gel. Pellets were counted for 100-second intervals a t both the analyte and background lines.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
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Total Uranium Content
Table I. Ionic Composition of Ground Water Base Used in Study Cations, ppm
0
20
40
80
60
100
120280
I
1
- 240
Anions, ppm
- 200 al
-0
Ca2+ Mg2'
Na'
K' Sr2+
44 19 28 5.3 0.9
HC03-
202 47 16 1.5 14
so42c 1FNos-
- 1602 C - 120 2 v + -
- 80 - 40 -
RESULTS AND DISCUSSION Individual ionic concentrations of major ions for the ground water used in the experimental work are given in Table I. Effects of pH variation on the extraction of uranium from preacidified and non-preacidified spiked samples are presented in Table 11. The major function of preacidification is to minimize the concentration and resultant effects of species related to dissolved C02, i.e., HC03- and C032-. The variation noted a t pH values of 6-9 between preacidified and non-preacidified samples probably reflects the stabilizing influence of C032- ions upon uranium solutions noted by Thatcher and Barker (3) and Garrels (4). Decreases in the extraction efficiency at higher pH values in both sets of solutions may also result from other factors such as a decrease in distribution coefficient with increasing pH, as shown for ketoiminocarboxylic acid chelating resins (2), and an increased efficiency in the extraction of major cations such as Ca2+ at higher pH values (Table 11). An operational pH value of 4 was selected for subsequent work in view of the foregoing considerations. Results listed in Table I11 suggest that more than 909b of the extractable uranium from a 1-liter spiked sample is recovered in one 3-hour extraction at pH 4. Using these extraction conditions, the natural uranium content of the ground water was determined by a least squares fit of the standard additions data, as shown in Figure 1. This same curve can also be used to determine uranium contents for ground water samples with similar major ion compositions. The standard deviations shown as bars in the figure reflect three countings of each of the triplicate samples of each concentration set (mean and standard deviations being 37.1 f 2.5 counts/sec unspiked, 79.8 f 2.8 countslsec 20ppb spike, 162.7 f 5.8 counts/sec 60-ppb spike, and 261.1 & 4.6 countshec 100-ppb spike). A response of 2.2 counts/sec per ppb uranium is obtained from the curve. For a rootmean-square standard deviation of f4.2 countslsec, the ratio of variances for instrument operation (counting and sample relocation in the spectrograph) to sample preparation (extraction and pelletization) is about 1:2. Instrument operation variation amounts to about 2.5 countdsec as determined from replicate measurements on individual pellets. Variations in sample preparation account for the remaining deviation noted for the different concentration
i t / ,
20
I
I
0
20
,
I
I
1
40
I
60 Level of Uranium Spike (ppb)
1
80
I
I
1
100
0
Flgure 1. Standard additions-calibration curve for uranium in ground
water sets, with variations in pelletization probably being the most important contributing factor. Thus, improvement of fit in the calibration curve can probably be achieved through greater care in pellet preparation. The presence of excessive amounts of iron (Fe K a1,2 52.62O-2nd order) and calcium (Ca K a1,2 45.20°) in the sample being analyzed raises the background for the uranium analysis. The iron content of the alkaline earth-bicarbonate type waters to which this method has been applied is low (range 0-158 ppb, mean 17 ppb for 40 samples), and appears to present little problem since the backgrounds for pelletized extracts of these .water samples are relatively constant (range 19-28 counts/sec, mean 23 counts/sec). Calcium concentrations in these same waters have a range of 33-163 ppm, with a mean value of 67 ppm. Trends between high calcium andlor iron values and increased backgrounds were not evident. Comparable extraction efficiencies for uranium and pellet backgrounds of -24 counts/sec have been obtained from a standard additions analysis of an alkaline earth-sulfate type water containing about ten times the dissolved solids and calcium concentrations of the ground water used in this study. Thus, calcium interference appears to be minimal using the pH 4 extraction conditions suggested above. Field treatment of samples collected for regional studies should include filtration, if necessary, and addition of 2-3 ml of concentrated HC1 to prevent loss of uranium. The value of 16 ppb for the ground water sample reported here and a regional average value of 14 ppb (range 6-23 ppb) based upon samples from 35 different sites are in general agreement with earlier values of 11 and 14 ppb, as determined by the sodium fluoride method, for ground water samples from the same aquifer and regional area (5). The detection limit for uranium in a single count determination for one pellet using the calibration curve for the ground water studied here is defined as that concentration giving a net signal equal to three times the standard devia-
Table 11. Effect of pH Variations upon Extraction of Uranium a n d Calciuma PH 4
5
Uranium, preacidified 148 152 (net counts/ s ec ) Uranium, non -preacidified 161 155 (net counts/sec) Calcium: preacidified 9,750 13,500 (gross counts/sec) a Samples spiked to 50 ppb uranium level above natural content. 2036
2
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
6
7
9
147
97
16
114
47
9
19,550
20,600
24,600
Table 111. Effect of Extraction Time a n d Number of Extractions upon Uranium Recoverya Time, hr 1
Net counts/sec
142
3
6
151
149
Extractions
Net counts/sec ( 3 - h r extractions)
1S t
2 nd
3rd
156
9
4
a Samples spiked to 50 ppb uranium level above natural content. Extractions a t pH 4.
tion of the background signals. For the 36 pellet determinations of the standard additions set (background 21.8 f 1.2) this value is 2 ppb, as compared with a value of about 12 ppb reported for the Rhodamine method (6). Our value includes variations in count rate and sample positioning in the spectrograph. Extraction of uranium from distilled water prepared samples and detection a t the 5 ppb level (8 counts/sec net) have been achieved, but serve as only a gross check of the detection limit since the background of these pellets is lower (-15 counts/sec) than that found for ground water samples and equivalency of the extraction efficiencies for the two types of samples has not been established. A distinct advantage of the present method over either
the Rhodamine B or sodium fluoride method for uranium determination in ground waters is the durability of the pellets which makes it possible to establish sample libraries covering regional areas of interest. The resin batch extraction procedure also simplifies sample handling prior to analysis. The sodium fluoride method requires careful control of the fusion operation and samples can be subject to quenching and enhancement effects produced by the presence of various cations and anions which may necessitate additional treatment of the samples to eliminate interferences (7). The Rhodamine B method, while apparently sensitive to fewer interferences than the sodium fluoride method, requires a liquid-liquid extraction step in which care must be taken to exclude water from the organic phase to be analyzed to prevent errors due to light scattering (6). '
LITERATURE CITED (1) C. W. Blount, D. E. Leyden, T. L. Thomas, and S. M. Guill, Anal. Chem., 45, 1045 (1973). (2)M. Marhol and K. L. Cheng. Talanta, 21, 751 (1974). (3)L. L. Thatcher and F. 8.Barker, Anal. Chem., 29, 1575 (1957). (4)R . M. Garrels, "Mineral Equilibria", Harper, New York, 1960,p 186. (5) R . C. Scott and F. 8. Barker, U S . , Go/. Surv., Prof. Pap., 426, 47 (1962). (6)N. R. Andersen and D. M. Hercules, Anal. Chem., 36, 2138 (1964). (7)American Society for Testing and Materials, "Microquantities of Uranium in Water by Fluorometry". D2907 in 1970 Annual Book of ASTM Standards: Part 23,Water: Atmospheric Analysis, Philadelphia, Pa., 1970,pp 943-949.
RECEIVEDfor review April 28, 1975. Accepted July 3,1975.
Direct Adsorption of Solvent-Extracted Gold on a Chelating Ion Exchange Resin L. L. Sundberg Department of Chemistry, University of California, Los Angeles, Calif. 90024
Meteorites are of great scientific importance because of their considerable age: a billion years older than the oldest known terrestrial rocks, and 500 million years older than unaltered lunar rocks. Trace metal analyses of meteorites provide useful information about the mechanisms of their formation. In our laboratory, stony meteorites (those consisting largely of silicate minerals together with 5-20% metallic iron-nickel) and returned lunar material are analyzed for Au, Cd, Ge, Ga, In, lr, Ni, and Zn by radiochemical neutron activation analysis. One of the early steps in our radiochemical procedures is the separation of Au from Zn and Cd by the extraction of Au into ethyl acetate from an HCl solution; Zn and Cd remain in the aqueous phase. For the removal of coextracted radionuclides, Au is back-extracted with 25% NH40H and precipitated with hydroquinone. The metal is redissolved in aqua regia, the Au is extracted into di-isopropyl ether from 3M HBr, and then back-extracted into water. In an effort to simplify the above procedures for the purification of Au, our attention was directed toward a chelating anion exchange resin, Srafion NMRR (Ayalon Water Conditioning Co., Ltd., Haifa, Israel), reported t o be highly selective for Au, Hg, and the platinum metals (1-8). This resin is a styrene-divinylbenzene copolymer with chelating guanidine residues. The resin is selective for those ions in
the d8 electronic configuration which form square-planar complexes, e.g., Au(II1). In most instances, aqueous solutions are employed for the adsorption and elution of metals from ion exchange resins, although lately much research has been devoted to the use of organic solvents for these purposes (9). Since our decontamination procedures for Au could be considerably shortened if Srafion NMRR could quantitatively retain Au that was solvent-extracted into ethyl acetate (thus eliminating the need for back-extraction) and exclude the undesired coextracted radionuclides of other elements, the present work was undertaken to evaluate this possibility. In addition, Au retention on Srafion NMRR from methyl isobutyl ketone and di-isopropyl ether was studied. I believe that this is the first report of the direct adsorption of solventextracted Au on this resin. EXPERIMENTAL Reagents. All materials used in this investigation were reagent grade. Gold carrier (10 mg/ml) was prepared by dissolving Au splatters in a minimum of aqua regia and diluting with distilled water; 2 ml of this carrier was used in all determinations. Gold-198 tracer with a 64.7-hr half-life was prepared by the reaction 197Au(n,y)198Aua t the UCLA nuclear reactor. Experimental tracer activity was such that both inordinately long counting times and serious dead time losses in the analyzers were avoided, while still maintaining good counting statistics for data reduction.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
2037
