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). '
L I T E R A T U R E 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. T h e 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
1' "
./
Table I. Retention Percentages of Solvent-Extracted Au on Srafion N M R R
80
Halide Complex
60 m m 0
IC
k-++++
O 0
L
d 20
'
40
\
'
1
60
'
'
-
"
80
100
'
120
ELUATE VOLUME (ml)
Solvent
Retention, ?4
HAuClda EtOAc 98.0 HAuC1, MIBK 96.3 HAuCI, i- Pr,O 97.8 HAuBrt EtOAc 97.0 HAuBr, MIBK 96.9 HAuBr, i- Pr,O 98.4 AuBr,-C EtOAc 97.1 AuBr,MIBK 98.1 AuBr,i- Pr,O 96.2 The aqueous solutions from which these species were extracted were: ( a ) 3M HC1; ( b ) 3M HBr; ( c ) 6M HC1-3M Br- (Naf, NHd+).
Flgure 1. Au elution profile from Srafion NMRR using 5 % thiourea Column length, 15 cm; flow rate, 1.4 ml/min. (A)% eluted (right-hand ordinate); ( 0 )Au concentration, mg/5 ml (left-hand ordinate)
Ion Exchange Columns. Srafion NMRR resin (0.3-1.2 mm) was equilibrated with 0.1M HC1 for 24 hours prior to use, and was transferred to borosilicate glass columns (18 X 1.4-cm i.d. with a 150-ml reservoir) fitted with Teflon stopcocks. The resin bed height was 15 cm. Counting Apparatus. Two gamma spectrometers were used at various stages of this work. The first consisted of a 400-channel analyzer coupled to a 3- X 3-in. well-type NaI(T1) crystal. For the precise identification of contaminant radionuclides, a 4096-channe1 analyzer in conjunction with a Ge(Li) coaxial detector was used. For this system using the 6oCo 1332-keV gamma, the resolution was 1.86 keV (FWHM) and the peak-to-Compton ratio was 40:l; readout via magnetic tape was employed in this combination. Sample solutions were counted in 17- X 100-mm polypropylene tubes. Procedures. All data in this presentation were obtained radiometrically by counting the 412-keV gamma of 19sAu using the Wasson method for photopeak integration (IO). Methyl isobutyl ketone (MIBK),ethyl acetate (EtOAc),and diisopropyl ether (i-PrzO), were each used to extract Au carrier and equilibrated tracer from each of the following aqueous solutions: (1)3M HCl; (2) 3M HBr; and (3) 6M HC1-3M Br- (Na+,NH*+). The last solution is similar to the matrix in our radiochemical procedures for the extraction of Au into EtOAc; the predominant Au species is a bromide complex. For each of the above nine extraction systems, the volume of the aqueous and organic phase was 100 ml. After extraction, the gold-containing organic solvents were transferred to the resin columns, and passed through the resin bed at a rate of 1.0-1.3 ml/min. Au retention was calculated by counting the activity in the effluent, and was corrected for losses in the extractions (which were never greater than 5%). The recommended procedures for the recovery of Au from Srafion NMRR resin are ignition at -1000 "C whereupon Au is recovered in the pure metallic form, or elution with a 5% solution of thiourea in 0.5%HC1 (11).This study was confined to the thiourea elution. Because we intended to use Srafion NMRR resin in direct conjunction with our solvent extraction procedures, elution experiments were restricted to the EtOAcl6M HC1-3M Br- (Na+, NH4+) extraction system described above. After extraction, the gold-containing EtOAc was transferred to the column and passed through the resin bed at the rate of 1.4 ml/min. The column was washed with 10 ml of 1:l ethanol-0.2M HC1 followed by 10 ml of 0.1M HC1. Au was immediately eluted from the resin with 100 ml of the thiourea solution. To monitor the elution profile, fractions of the eluate were collected at 5-ml intervals and were each counted for three minutes on the NaI detector.
RESULTS A N D DISCUSSION As the gold-containing organic solvents passed through the columns, the effluents did not possess the yellow and orange colors that are characteristic of HAuC14 and HAuBr4; this gave some evidence that Au adsorption had occurred. The brochure accompanying this resin ( 1 1 ) stated that Srafion NMRR changes color from yellow to orange-red when Au, Hg, or the platinoids are adsorbed from 2038
0.1M HCl solutions. In our experiments, this color change on the resin was also observed, but only t o a very limited degree, which made the visual confirmation of Au adsorption somewhat difficult. Table I shows the percentages of Au retained on Srafion NMRR for the gold-containing solvent systems that were investigated. As the average retention for the nine systems was 97%, this method of isolating Au is more than adequate for determinations by radiochemical neutron activation analysis. Although the data are somewhat limited, it appears that no significant correlation exists between Au retention and any particular halide or solvent component in the solutions that were studied. Nadkarni and Morrison ( I ) previously reported the use of Srafion NMRR for the determination of Au and five of the platinum metals in geological materials by radiochemical neutron activation analysis. Because adsorption on this resin was 97-100% complete for the metals they investigated, no chemical yield determinations were necessary, no attempts were made to recover the adsorbed metals from t h e resin, and the resin was directly counted. While these procedures show considerable merit in terms of their simplicity, there are a variety of reasons why they cannot be employed in this laboratory. In our routine radiochemical procedures, many analytical separations are performed prior t o the time of our intended use of Srafion NMRR. Consequently, chemical yield determinations must be performed on all of our samples. Although we could conceivably count Au directly on the resin, some additional chemistry would have t o he performed after counting in order t o obtain chemical yields. For counting purposes, we prefer that the separated and purified radionuclides in our samples be in a liquid form, as the chemical yields for many of our elements including Au are determined by atomic absorption spectrometry. Therefore, the Au adsorbed on Srafion NMRR must be recovered before counting. Of the recommended methods for the recovery of Au from Srafion NMRR, elution with a 5% solution of thiourea in 0.5% HC1 was chosen. In early experiments, after the gold-containing EtOAc passed through the resin bed, the column was washed with several 10-ml portions of EtOAc. The immediate introduction of the thiourea solution resulted in the diffusion of EtOAc into the eluent and the plot of Au concentration vs. eluate volume showed considerable tailing. To overcome this difficulty, different wash solutions were substituted in the place of EtOAc. After the gold-containing EtOAc passed through the resin bed, the column was washed with 10 ml of 1:l ethanol-O.2M HCl, followed by 10 ml of 0.1M HC1. With this wash procedure, the diffusion of EtOAc into the eluent was minimized and Au was eluted as a relatively sharp peak.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
1
5.0
I
I
nu
/'
4!2
-8 Y
z
a u
a Y e
I -o" '
Flgure 2.
b THIOUREA 8b.m
m.01
!W.m
ifo.m
1bo.m
ii0.w
,;Y~;~,, NTihfB
ei;;
~
O
D~ , O D
h.m
im.m
*.m
-0.80
m.m
m.0~
uw.8
Gamma-ray spectra of ( a )EtOAc prior to Au adsorption and (b)the thiourea eluate
Nuclide gamma-ray energies are in keV. (E)= background photopeak. (C)= detector contamination. Counting duration, 20,000 sec each
The Au elution profile, including residual EtOAc and the washings, is depicted in Figure 1. For convenience, the integral elution curve is superimposed. Although tailing was still appreciable, 96% of the Au was recovered using 100 ml of the thiourea solution. The tailing can be reduced by using a shorter column or by decreasing the flow rate. Prolonged contact of Au solutions with the resin may result in the reduction of Au ions to the metal. Therefore, we strongly recommend that Au be eluted immediately after washing. The ultimate test; of the versatility of Srafion NMRR was the determination of its selectivity in terms of the exclusion of radioactive base metals, notably 59Fe.T h e weight ratio of Fe/Au in stony meteorites is on the order of lo6, although the activity ratio of 59Fe/198Au for our irradiation procedures is only about 100 owing to the lower isotopic abundance, smaller thermal neutron cross-section, and longer half-life of the 5$Fe target and 59Fe radionuclide. In our radiochemical procedures, even though most Fe is separated from Au by an ammonia precipitation of Fe(OH)3 followed by the extraction of Au into EtOAc, a minute
amount of 59Fe can often mask or distort the shape of the 412-keV photopeak of lg8Au. To examine the resin's exclusion of Fe and other metals, a special experiment was performed. Tracers of several radionuclides abundant in our samples were mixed with equilibrated Au tracer and carrier. This solution was diluted to 100 ml with 6M HC1-3M Br- (Na+, NH*+). Au was extracted into an equal volume of EtOAc. The organic layer was washed with 25 ml of 6M HC1. A 5-ml aliquot of the EtOAc with withdrawn and counted on the Ge(Li) detector; the remainder of the gold-containing EtOAc was transferred to the Srafion NMRR column and passed through the resin bed a t the rate of 1.4 ml/min. The column was washed with 10 ml of 1:l ethanol-0.2M HC1, and, as an added precaution, three 25-ml portions of 0.1M HC1, the first containing -100 mg Fe as FeC13. Au was immediately eluted with 100 ml of the thiourea solution. After elution was completed, a 5-ml aliquot of the eluate was withdrawn and counted on the Ge(Li) detector. The gamma-ray spectra of the EtOAc and thiourea aliquots are shown in Figures 2a and 2 b , respectively. The
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
2039
Table 11. Lower Limits of Discrimination Factors for Various Radionuclides Nuclide
Factor
Kuclide
152E~ 300 "sc "Cr 3000 6OCO lZ4Sb 400 59Fe j4Mn 500 65Zn a Corrected for detector contamination.
Factor
5000 60OOu
8000 200
calibration is approximately 0.5 keV per channel. A computer program by Baedecker (12) was used to locate, identify, and integrate the areas of the photopeaks. Aside from lgsAu and lg9Au,the only photopeaks appearing in Figure 2b are those routinely observed in the background of our counter, including a trace of 6oCo contamination. In this experiment, Au recovery was 94%. The selectivity of Srafion NMRR could be evaluated from the selectivity coefficients for Au and the other radionuclides that were investigated. However, the precise measurement of the selectivity coefficients was beyond the scope of this investigation. Instead, data are presented for a rough numerical measure of the combined adsorption-elution selectivity based on the counting data from Figure 2. These selectivity data take the form of, for lack of better words, "discrimination factors". They are defined as ( A i ) = / (Ai)b, where (Ai)a and ( A L ) bare the areas of the ith contaminant photopeak in the EtOAc and thiourea spectra (Figure 2), respectively. However, because contaminant photopeaks (excluding background) were not observed in the thiourea spectrum, the values for (Ai)b are purely statistical, and their upper limits are taken as 2v% where N is the number of counts summed over the channels that were used to calculate (Ai)=.Therefore, the lower limits of the discrimination factors are shown in Table 11. For these nuclides emitting more than one detectable gamma ray, e.g. 59Fe,the higher value is tabulated. All data were corrected for counting duration and aliquot removal. As a result of these investigations, we now routinely employ the following procedures for the determination of Au. Samples of stony meteorites or lunar materials weighing 50-500 mg are sealed in high-purity quartz vials. Duplicate flux monitors are prepared by evaporating weighed solutions onto cleaned aluminum foils. Samples and flux monitors are then irradiated by a thermal neutron fluence of about 5 X l O I 9 cm-2. Weighed samples are transferred to zirconium crucibles containing the inert carriers, and the mixture is fused with Na202; the fusion cakes are quenched with water and acidified with concentrated HBr. The solutions are adjusted to pH 8-9 with concentrated NHdOH, which precipitates the hydrous oxide of Ir, the hydroxides of Ga, Ge, and In, and partially precipitates the hydroxides of Zn and Cd; Au, Ni, and the remaining Zn and Cd are retained in solution as ammonia complexes. After centrifugation, Ni is precipitated from the ammoniacal supernate with dimethylglyoxime. The Ni precipitate is filtered and the filtrate containing Au, Zn, and Cd is acidified with HCI. Au is extracted into an equal volume of EtOAc while Zn and Cd remain in the aqueous phase. The EtOAc is washed twice with 25-ml portions of 6 M HC1, and is transferred to the Srafion NMRR resin columns, whereupon adsorption, washing, and elution are identical to the methods that lead
2040
to the presentation of Figure 2b. Au is then precipitated from the thiourea complex as the sulfide by the addition of ammonia. The precipitate is dissolved in aqua regia and diluted for counting. Chemical yields are determined by atomic absorption spectrometry. Au flux monitors are dissolved in 6M HC1 with a trace of "03 to ensure dissolution, extracted into EtOAc, and treated identically to the samples for the remainder of the procedure. It was mentioned that Ir is also determined in our suite of elements, and one might ask why we do not employ Srafion NMRR in our Ir procedures. Tracer experiments showed that the adsorption of 4 mg Ir (presumably in the +3 or +4 oxidation states) on Srafion NMRR was on the order of 98-99% complete from 0.1M HCI solutions. However, contrary to statements in the literature ( I I ) , Ir could not be recovered either by using immediate elution with 5% thiourea or by a batch extraction with the same solution. This can possibly be attributed to the greater stability of the Ir(1) chelate with the resin relative to the Ir-thiourea complex. Another possibility is that, upon adsorption, the higher oxidation states of Ir were reduced at least in part to the metal, since after prolonged attempts to remove Ir from the resin with a Soxhlet extractor using 9M HC1, the resin still maintained substantial Ir activity. Various schemes for the determination of Au by radiochemical neutron activation analysis have been recently summarized ( 1 3 ) . With chemical yields on the order of 70-80%, total processing time including counting approximately five hours for six samples and two flux monitors, and radiochemical purity for most samples similar to Figure 2b, we feel that our present procedures are better than or comparable to most of those described. The reader should have no difficulty adapting the key steps of our Au procedure to the radiochemical activation analysis of Au in other matrices where another suite of elements is to be simultaneously determined.
ACKNOWLEDGMENT The author is greatly indebted to P. A. Baedecker, W. V. Boynton, K. L. Robinson, and J. T. Wasson for their able assistance in the preparation of this manuscript, LITERATURE CITED (1) R. A. Nadkarni and G. H. Morrison, Anal. Chem., 46, 232 (1974). (2) G. Koster and G.,Schmuckler, Anal. Chim. Acta, 38, 179 (1967). (3) T. E. Green, S.L. Law, and W. J. Campbell, U S .Bur. Mines. Rep., No, 7358 (1970). (4) T. E. Green, S. L. Law, and W. J. Campbell. Anal. Chem., 42, 1749 (1970). (5) S.L. Law, Science, 174, 285 (1970). (6) H. A. Das, R. Jannsen. and J. Zonderhuis, Radiochem. Radioanal. Lett., 8, 257 (1971). (7) C. W. Blount, E. D. Leyden, T. L. Thomas, and S. M. Guill, Anal. Chem., 45, 1045 (1973). (6) P. J. Ke and R. J. Thibert. Mikrocbirn. Acta, 1973, 417. (9) J. Korkisch, "Ion Exchange in Mixed and Non-Aqueous Media", Progress in Nuclear Energy Series IX, Analytical Chemistry, Vol. 6, Pergamon Press, Oxford, 1966, pp 1-94. (10) P. A. Baedecker, Anal. Chem., 43, 405 (1971). (11) Ayalon Water Conditioning Co., Ltd.. Haifa, Israel, Product Information Bulletin. (12) P. A. Baedecker, "Advances in Obsidian Glass Studies", R . E. Taylor. Ed., Noyes Press, Park Ridge, N.J., in press. (13) F. E. Beamish, "Recent Advances in the Analytical Chemistry of the Noble Metals", Pergamon Press, Oxford, 1972, pp 150-165.
RECEIVEDfor review December 16, 1974. Accepted June 19, 1975. This study was supported in part by NASA Grant NGR-05-007-367.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
