obvious choice for use in separating plutonium from the other alloy conA lanthanum-plutonium stituents. weight ratio of a t least one was necessary to ensure quantitative precipitation. I n the presence of 50 mg. of uranium and in 2iU hydrochloric acid coprecipitation of 10 mg. of plutonium with 10 mg. of lanthanum was quantitative, but 11% of the plutonium remained in the supernate when 5 mg. of lanthanum was used. Nitric acid solutions of both zirconium nitrate (6) and aluminum nitrate were tested as media for dissolving the lanthanum fluoride-plutonium fluoride precipitate and converting the plutonium to the quadrivalent nitrate complex. At zirconiumfluoride molar ratios greater than 1 the absorbance curve obtained with samples which had been carried through the precipitation procedure was identical to that obtained with a direct standard (Figure 1). Aluminum was also tested a t the same compiexant-fluoride ratio, but proved to be less effective. Absorbance measurements a t 475 mp were 30% low. When the aluminum concentration was increased by a factor of 10, from 0.1 to 1.OM absorbance measurements were 4% low. Although there is no significant sensitivity advantage in using zirconium rather than aluminum, small variations in the fluoride content of the precipitates result in slightly erratic absorbance-concentration plots when aluminum is used. When IO mg. of plutonium is co-
precipitated with 10 mg. of lanthanum fluoride in the presence of 50 mg. of uranium, about 1 mg. of uranium is also precipitated. This amount of uranium does not interfere in the subsequent determination, since uranium becomes a significant interference only when the uranium concentration is half the plutonium concentration. When 8.2 mg. of plutonium was determined in the presence of 22.1 mg. of uranium, the result was 670 low. Of the group VI11 elemental constituents of the alloys, none interfered when the lanthanum fluoride precipitation was used to separate them from plutonium. Each was tested a t a weight ratio to plutonium of 1.5 (10 or more times the amount to be encountered in the analysis of the alloys) and only rhodium was not completely separated. Under these conditions the plutonium absorbance was 7% high. Absorbance of Plutonium(1V) Nitrate. The absorbance is independent of nitric acid concentration in the range 2.3 to 3 . 5 M . At 1.6iW and Q.8M the absorbances were down 5 and lo%, respectively. The effect of higher nitric acid concentrations was not investigated. RESULTS
The reliability of the method was ascertained through the analysis, along with the regular samples, of a synthetic solution prepared from standard uranium, plutonium, and fission element solutions. The nominal composition
was 75YG uranium, 20% plutonium, 2.5% molybdenum, 2% ruthenium, 0.3% rhodium, and 0.201, palladium. By the solvent extraction separation, the coefficients of variation obtained in eight determinations of uranium and plutonium were 0.5 and 0.8%, respectively. h negative bias of 0.5% for the plutonium analysis is probably attributable t o incomplete reduction to the trivalent state prior t o the uranium extraction. Using the lanthanum fluoride separation when only plutonium analyses were required, the coefficient of variation was 0.7% with no apparent bias. ACKNOWLEDGMENT
The authors express their sincere appreciation to R. k1. Clarke for his numerous careful analyses. LITERATURE CITED
( 1 ) Crouthamel, C. E., Johnaon, C. E., ARAL.CHEY.24, 1780 (1952). (2) Flikkema, D. S., Larsen, R. P.,
Schablaske, R. V., U. S. At. Energy Comm., Rept. ANL-5641 (Kovernber lQS6). ~ _
_ .
(3) La;sen, R. P., ANAL. CHEM.31, 545 (19.59\. \----,-
(4) Metz, C. J., Ibid., 29,1748 (1957). (5) Seaborg, G. T., Katz, J. J., “The
Actinide Elements,” McGrsw-Hill, New York, 1954. RECEIVED for review December 14, 1959. Accepted August 11, 1960. Work performed under the auspices of the U. S. Atomic Energy Commission, Contract W-3 I-109-eng-38.
on Exchange Separation Radioeleme W. 1. BLAEDEL and EUGENE D. OLSENI Chemistry Department, University o f Wisconsin, Madison, Wis.
R. 6.. BUCHANAN Argonne National Laboratory, Lemonf, 111.
lp An ion exchange separation scheme Is described by which tracer amounts of over 35 metallic radioelements may b e separated into six groups. fn the rocedure, the sample is pretreated to put several elements into particular oxidation states and to complex some elements into anionic form, then adsorbed onto a column of Dowex 56W cation-exchangerl and finally eluted with a series of complexing eluents, each of controlled pH and ionic strength. The elements in each group are obtained in 15 Po 3 0 ml. OS solutions ~on~aining only ammonium
$66
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ANALYTICAL CHEMISTRY
salts, organic acids, or hydrochloric acid. With this procedure, each of 36 eiements falls predominantly into its own group, with less than 1% falling into any other group. Four elemenis (Pd, Au, Hg, and Ag) cannot b e tolerated in the scheme. In designing the procedure, less importance was given bo high yields than to clean separations; nevertheless, the yields of 3 1 elements are over 90%, and three more are recovered with yields above 80%. The conditions necessary fer these separations were studied in detail.
in mixtures of only a few activities are identified easily by simple decay and energy measurements (5), or by the more retined techniques of scintillation spectrometry (7). However, for samples containing many elements, direct identification is rarely possible, and must usually be preceded by a chemical separation. For such complex mixADIOELEMENTS
1 Present address, Chemistry Department, Franklin and Marshall College, Lancaster, Pa.
tures, a widely accepted method 10 t o add an inactive carrier for each soughtfor radioelement, and then to process the sample chemically to recover the added carrier uncontaminated by other elements (8, 12). The majority of these standard schemes employ precipitation to effect the separation. This method beconies onerous or time-consuming when inany activities are present in the sample, or when the nature of t h e activities is unknown. Examples of such cases are the determination of impurities by activation analysis (11, 16) and the determination of the composition of fission products (17). Many workers have used ion exchange resins to separate individual fission product elements. Cohn, Parker, and Tompkins (6) developed a system for separating macro amounts of some of the major fission products in high purity, but they were also concerned with high yields, and the procedure required 27 hours. Crouch and Cook (9) combined precipitation and ion exchange to separate 20 of the fission product elements, but this procedure was on the semimicro level, and time-consuming. Other workers ( I d , 82) separated up to 20 elements, but they had to recycle their fractions several times to obtain good separations. All these procedures are concerned principally with high yields of the individual elements, and are not general enough or fast enough to be useful for identifications of groups of elements. More comprehensive ion exchange schemes were proposed by Hicks, Gilbert, Stevenson, and Hutchin ( I S ) , and by Kraus and h'elson (18), both using anion exchange resins and hydrochloric acid eluents. Investigation in our laboratory showed that these schemes suffer from slow elution of some elements, and irreversible retention of other elements on the resin. Ideally, for a separation scheme to be most useful for identifications, the following requirements should be satisfied: 1, wide applicability, with the behavior of as many elements as possible being clearly defined; 2, clean separations, the principal portion of each element being in one place, with little cross contamination of or by elements from other groups; 3. rapid separations; 4, easy further processing of separated fractions, if necessary; 5, high yield of the separated elements. Since the complete satisfaction of some of these requirements excludes the satisfaction of others, i t was decided to satisfy them roughly in the order of importance given. The sequential ion exchange group separation scheme described in this paper was designed to separate rapidly a majority of the metallic radioelements, in tracer amounts, into smaller groups of elements. Such a separation into
Table I.
Group I I1
Fraction Main Wash Main Wash Buffer wash
I11
Main Wash
IV
Main Wash
V
Main Wash
VI
Main Wash
Elution Procedure Eluent, Composition, IXrections 2% citric acid (to pH 3.0 with T\"a), 20 ml. 30 ml. of pH 3 citrate Two 6-ml. portions of water 1% IiEDTA (to pH 3.0 with "a), (0.036M),25 nil. 30 ml. more of pH 3 HEDTA Three 5-ml. portions of water 0.08M formic acid (to pH 4.0 with "a), 20 ml. Water, 5 ml. a t 2.3 to 2.4 ml./mmute (All flow rates a t 2.3 to 2.4 ml./minute) 1% citric acid (to pH 5.0 with "a), 15 ml. Stop flow for hour. Resume flow with 5 ml. more 30 ml. more of pH 5.0 citrate Two 5-ml. portions of water 1%EDTA (to pH 8.5 with NHs), (0.031M), 30 ml. 25 ml. more of pH 8.6 EDTA Water, 5 ml. 2.5% EDTA (to pH 9.0 with "a), 15 ml. 15 ml. more of pH 9.0 EDTA Three 5-ml. portions of water 3M HCl, 15 ml. 15 ml. more of 3M HCI 5M HCl, 100 ml.
groups might then allow eaaier identification of the individual elements in each group, when such identification could not be made on the original unseparated sample. This paper is concerned only with the separation into groups. The further separation of large groups, and the identification of individual elements within groups, are subjects for future work. EXPERIMENTAL
Apparatus. The ion exchange column consisted of a glass tube 17 em. long by 12 mm. in inside diameter, closed at the lower end b y a coarse sintered-glass disk upon which the resin bed rested, and fitted at the t o p with a bowl 9 cm. in diameter, t o hold eluents. h side a r m funnel, attached t o the column and connected to the counting cell inlet, system through a three-way stopcock, provided a means for calibration or check on contamination in the counting cell. A resin volume of 10.8 ml. (H-form) was used to give a bed height of about 10.5 cm. The effluent solution was continuously monitored for radioactivity with an arrangement similar to that of Ketelle and Boyd (16). An Amperex 100 CB counting tube, a Kuclear Chicago Model 1620 count rate meter, and a Varian Model G-11 recorder mere used. A flowmeter was inserted beyond the counting cell. Both Teflon (tetrafluoroethylene resin) and Lucite (acrylic resin) counting cells were used. The Teflon cell, whose construction and properties have been described in detail (a), was used for a majority of the tracers. However, the Lucite cell had to be used for a few elements, such as Ir, Os, Pd, and others, that were found to contaminate Teflon. The Lucite cell was constructed similarly to the Teflon cell, except that a Mylar
(polyester film) window was used. The Mylar window was bonded to the Lucite with Du Pont No. 4695 adhesive. Reagents. The resin was Baker Analyzed Dowex 5OW-X8, 100200 mesh. T h e radioelements were high specific activity isotopes from Oak Ridge National Laboratory. The H E D T A IN-hydroxyethyl(ethy1enedinitri1o)triacetic acid] and E D T A [as the diammonium salt of (ethylenedinitri1o)tetraacetic acid] were obtained from the Geigy Chemical Corp., New York, N. Y. AI1 other chemicals were reagent grade. The resin was cleaned in 1-pound lots. After shaking batchwise with several portions of 5M HC1 to remove most of the iron, the resin was washed in a column with the following solutions: water, 1 liter; 2.5M "8, 2 liters; water, 1 liter; 5 M HC1, 2 liters. The cycle was repeated twice. A final wash was made with 1 liter of 0.1M HC1, and the resin was stored under this solution. The resulting resin was spectroscopically free of metal impurities. The resin was then treated batchwise w+ith chlorine. A 50- to 100-ml. portion of resin mas mixed with 200 ml. of 0.1M HC1 in a narrow-necked bottle, and chlorine was passpd through with almost continuous swirling for 15 minutes. The resin was washed by decantation three times with distilled water, and stored under 0.1M HCl. Storage up to 3 months does not adversely affect the quality of the resin. After packing the column with 10.8 ml. of the chlorinated resin, i t was treated with 100 ml. of 2.5BI NH&l to convert it to the salt form, and equilibrated with 35 ml. of 2% citric acid (O.O94M), adjusted to p H 3.0 with NH3. Just prior to putting the sample on the column, the resin was equilibrated by washing with 5 ml. of a 2% citrate solution which had been treated the same as the sample batch. VOL. 32, NO. 13, DECEMBER 1960
e
7867
Experimental Approach. The following approach was used in developing the scheme. A tentative ion exchange scheme was devised. It was then tested b y running one representative tracer element through a t a time, and modified repeatedly until conditions suitable for a majority of the elements were obtained. T h e following procedure is the final recommended scheme.
9l-
F 2 w
ii5 LL
w 4
3
Sample Pretreatment. The sample solution is made about 2% in citric acid, and the p H is adjusted to 3.0 u-ith NH3. The solution is then immersed in a near-boiling (95’ to 97’ C.) water bath, and chlorine is slomly passed into t h e sample solution for 15 minutes, through a glass tube 2 mm. in inside diameter a t 60 bubbles per minute or less. The n-ater bath level should be maintained within ‘12 inch of the top of the sample container, or the ch!orine concentration may be increased appreciably from the cooler condensate on the container walls. After chlorination, the solution is cooled rapidly under a m t e r tap to room temperature, and carefully readjusted to p H 3.0 with dilute NE3. Elution Procedure. The sample is put on t h e column with a transfer pipet a t a low rate of 1.8 mi. per minute, or lees. The elution is then carried out according t o the schedule given in Table I, with a flow rate of 3.3 t o 3.5 ml. per minute except \Tc-hereothern-ise noted, All volumes refer t o solutions added; in collecting fractions, allowance should be made for the column-free volume of solution that is in the resin bed a t the time of addition. This volume is about 4.3 nil. for the 10.8 ml. of resin used. Each group is divided into two fractions, a main fraction and a wash portion. Since most of the eluted bands are sharp, the predominant portion of any element comes off in the first few column volumes of eluent (3 to 6 column volumes, or 15 to 30 mL)* Before adding the next eluent, a wash
Table
I!.
6
3
PROCEDURE
Conditions
r
0
40
80
120
Figure 1. scheme A.
Effluent pH
5-ml. sample (pH 3 citrate)
B. 50-ml. (pH 3 citrate) 10 ml. water
C.
D. 60-ml. (pH 3 HEDTA) 15 ml. water 20-ml. (pH 4 formate) 5 ml. water
E.
F. G.
of 15 to 35 ml. is used and the washings are rejected. Rejection of the washings greatly increases the purities of the main fractions, while losses remain tolerable (less than 1 t o 5% for most elements). The low sample flow rate aids in obtaining sharp elution peaks, and significantly improves the retention of sodium. With a sample flow rate of 1.5 nil. per minute, about 10% more of eluent milliequivalents are tolerable before sodium breakthrough than are tolerable a t a sample flow rate of 3.3 ml. per minute. The faster flow rates used for normal elution gave satisfactorily fast and complete separation, and TTere not studied further. The !ow Group I11 flow rates and the interruption were necessary to prevent excessive tailing of the lanthanides and yttrium, as shown in Table 11, which compares the elution of cerium under different conditions. Other Group I11 elements behaved similarly, but the tailing was most marked for cerium. The most complete elution of cerium
Elution of:Cerium under Various Conditions 1 2 3 4 5
Run Number Mode ofoelution Temp., C. Flax-rate (mL/ min.) MI. pH 5 citrate t o stop Length of stop
Continuous Flow 25 25 90 3.5
160
2.6
2.4
6
Interrupted Flow 25 25 25 2.3 LO
2.8 35
3.3 loa
(min.) 30 30 15 % eluted in first 20 ml. of pH 5 citrate 72.3 73.3 83.5 94.5 74.0 88.1 % eluted in next 30 ml. of pH 5 citrate 17.8 20.0 11.1 2.7 21.5 3.9 70 left on resin after 50 ml. citrate 9.9 5.7 5.4 2.gb 4.5 8.0 a Followed by 25 ml. more of H 5 citrate and another 15-minute stop. * Although this is a rather higkpercentage, it is not eluted rapidly enough to contaminate any of the following groups significantly. 5M HG1 removes the remaining cerium quantitatively. Inspection of Table I11 shows that other lanthanides behave similarly.
e
ANALYTICAL CHEMISTRY
200
240
EFFLUENT VOLUME, during
280
320
360
ML.
group
separation
50-ml. (pH 5 citrate) 10 ml. water 55.ml. (pH 6.5 EDTA) 5 ml. water 35-ml. (pH 9 EDTA) M. 15 ml. water N. 3 M HCI
H. 1. 1. K. L.
is given by the combined effects of lowering the flow rate and stopping the flow for hour after 10 ml. of citrate has passed (Run 4). The delay is more than compensated by higher yield and better separation. In addition, the disadvantage of the delay is not so great as it may seem, since the column mag be left completely unattended during the stop. Elevated temperature, commonly used t o speed removal and to reduce tailicg, improves the yield of cerium in the main fraction, but does not satisfactorily reduce taiIing (Run 3). The improvement in yield does not warranb the inconvenience of working a t elevated temperature. Formate solution a t p H 4 serves as a noncomplexing buffer in bridging the gap between the p H 3 HEDTA eluent of Group 11 and the p H 5 citrate eluent of Group 111. Without the formate, an inordinately large volume of p H 5 citrate would be required to neutralize the significant amount of acid that remains on the column after treatment with p H 3 HEDT.4. After p H 4 is reached, practically no acid remains on the column, and p H equiIibration to subsequent eluents is rapid. Figure 1 shows the effluent pH’s during the entire scheme. Data for Figure 1 were obtained by dipping a Beckman 39183 probe combination electrode into a 1-ml. cup formed at one end of a glass capillary Utube, 2 mm. in inside diameter, the other end of which was attached to the ion exchange column described earlier. Readings were made a t 2-ml. intervals .n.ith a Beckman Model G p H meter. The effluent solution spilled over into a graduated cylinder, permitting volume measurements to be made at any time. The side arm inlet attached to the column permitted frequent calibration of the meter, and also permitted the p H of the influent solution to be checked against the effluent pH. The curve was reproducible within 0.05 p H unit.
IO
' 0
IO 20 EFFLUENT
30
40
VOLUME,
ML.
50
Figure 2. Elution of Group elements with pH 3 citrate
I
Minimal water washes, used a t 6 different points in the scheme, prevent troublesome mixing of successive eluent solutions. The 10-ml. washes which follow the citrate of Groups I and 111 empty the column of the Group I and I11 eluents, thereby preventing needless contamination of Groups I1 and IV. The 15-ml. r a s h which follows Group I1 removes all the HEDTA before the p H 4 buffer is used. Khen the HEDTA is not remol-ed with water, the p H elevation with formate results in mowment and spreading of the rare earth bands. Fifteen milliliters are required since small amounts of HEDTA were retained on the resin. The 15-ml. wash following pH 9 EDTA ensures complete removal of EDTA, which otherwise might precipitate with the 3211 HC1 of Group VI. RESULTS
Quality of Separations.
The in-
Figure 3. Elution of Group II elements with p H 3 HEDTA
tegral elution curves of Figures 2 to 7 were obtained from t h e continuous record of effluent activity obtained when samples containing 10 to 100 pc. of single tracers were subjected to the procedure outlined above. Study of these curves led to the selection of the main fraction and wash volumes of the procedure. Some idea of the quality of separations for individual elements may be obtained from these curves, which are discussed in the following paragraphs. GROUP I. Figure 2 shows integral elution curves for typical Group I elements, removed with p H 3.0 citrate. The curves for Fe and Bi are typical of most of the elements. Curves for Cr, Hf, Zr, Nb, W,Ru, and Sb look like that of Fe, while those of &lo, Ir, Tc, and Os look much like the Bi curve. Scandium and Ga appear to have high resin affinities, and are eluted slowly. Thallium and In are the only important contaminants in Group I, their behavior being shown by the lower curves. The data shown on Figure 2 permit selection of the main fraction and wash volumes. For 90% yields of most of the elements, 15 ml. would be adequate, but an extra 5 ml. improves the yields of Sc and Ga. More than 20 ml. would improve the yields even more, but the slight gains for the majority of the elements would be a t the sacrifice of considerable dilution, which could become troublesome in further processing or counting of the main fraction. The 30-ml. wash volume is sufficient to reduce the tailing of Group I elements (except Pd and Ga) into Group I1 below 1%. Further reduction of the contamination of Group I1 would require larger wash volumes with a sacrifice of speed. The last 10% of Ga, for example, is eluted too slowly with p H 3 citrate to justify continued washing. The contaminants, T1 and In, have little bearing on the selection of volumes in this case, although the I n contamination of the main fraction and losses in the wash [believed to be caused by hydrolysis of In(II1) J grow steadily worse with increased volumes. GROUP 11. HEDTA at p H 3.0 generally elutes Group I1 elements sharply. The integral elution curve for Cu, shown in Figure 3, is typical of most elements. However, Cd is eluted more slowly, and 25 ml. was therefore selected for the main fraction instead of the 15 to 20 ml. that would be adequate for the other elements. The 30-ml. wash fraction removes the last few per cent of In, and achieves more quantitative removal of Cd. GROUP 111. The lanthanides and Y behave similarly, and are eluted with p H 5.0 citrate. Perhaps the greatest differences in behavior among the rare earths studied are those between E u and Ce, shown in Figure 4. Europium
IO 20 EFFLUENT
' 0
30
SO
40
VOLUME,
ML.
Figure 4. Elution of Group 111 elements with pH 5 citrate ELI, Ce: flow rate 2.4 ml./min., l/Z-hour stop a t 10 ml. Ce [no stop):flow rate 2.6 ml./min.
forms the more stable citrate complex, and elutes somewhat faster than Ce ($4). Figure 4 also illustrates the marked improvement brought about by interrupting the flow for '/z hour after 10 ml. has been passed. Yields of all the Group I11 elements are 95% or more in a 20-ml. main fraction, and the 30-ml. mash fraction removes another 1 to 370 at a low rate. Although a total of 50 ml. of p H 5 citrate is not enough to remove the lanthanides and Y quantitatively, the small amounts remaining on the resin do not significantly contaminate later groups, and can be removed rapidly with a 5M HC1 wash a t the end. GROUPIV. Figure 5 shows the elution curves for Ca and Sr, typical of
' 0
5
15 25 35 45 E F F L U E N T VOLUME, M L .
!
5
Figure 5. Elution-of Group IV elements with pH 6.5-EDTA VOL. 32, NO. 13, DECEMBER 1960
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Figure 6. Elution of ments with pH 9 EDTA
Group V ele-
Group IV elements that are eluted with p H 6.5 EDTA. I n addition, the gradual breakthrough of Na after 35 ml. have passed is shown in the lower right hand corner. The first 30 ml. can be collected without danger of N a contamination, while obtaining yields above 8Q% for Ca and Sr. A 25-ml. wash fraction removes Ca and Sr quantitatively, while keeping losses of Ka below 1%. GROUPV. Figure 6 contrasts the relatively rapid elutioii of Ba and T1 through complexation by pK 9.0 EDTA nTith the somewhat slower displacement of Na. (Note that over 32% of the T1 was distributed in previous groups, as will be discussed later.) The other alkalies, K, Rb, and Cs, do not appear in the effluent until after 20 ml. are passed, so that a E-mL main fraction will remain uncontaminated by them, while containing more than 97% yields of the Group V elements. An extra 15-ml. wash elutes the Group V elements quantitatively, without displacing and losing significant amounts of the Group VI elements. Losses of Group VI elements become appreciable, however, if washing is continued beyond a total volume of 30 nil. GROUPVI. The remaining alkalies are rapidly displaced by 3M HC1. The R and 61s elution curves of Figure 7 show that 15 ml. is sufficient to remove these alkalies quantitatively. A 15ml. wash with 3N’ HCI, followed by 5 M HCl, cleans all residual activity from the column. (Exceptions with certain elements are noted in Table 111.) Distribution of Elements in the Group Separation Scheme. Table 111 lists the distributions of elements in all fractions of the scheme, for all of t h e elements t h a t were studied. These distributions were calculated 8
ANALYTICAL CHEMISTRY
from t h e continuous record of effluent activity obtained as described previously. Coincidence corrections were made wherever necessary. Relative errors range from about 6% for main fractions (containing more than 80% of the element eluted) up to severalfold for other fractions. The following conclusions may be drawn from Table 111: Thirty-six elements are well separated. Each of these elements goes predominantly into one group, and less than 1% falls in any other group. Two elements (Ga and Ti) are poorly separated; up to 10% of each of these elements falls into groups other than its main group. These elements must be classified as interferences in the scheme if they are major constituents in a sample. Four elements (Pd, Au, Hg, and Ag) are very poorly separated; these elements are found in several groups in the scheme in amounts over 10%. These elements must be removed from a sample before proceeding M-ith the separation of the other elements. Preliminary experiments indicated that a prereduction with Hg, based on the work of Furman and Murray ( I O ) , might remove these interferences. I n addition to the separations achieved, the yields of 31 elements are over 90%, while three more are obtained with yields above 80%. These elements may be determined ai least semiquantitatively. About 3 l / 2 hours are generally sufficient t o go through the whole scheme. This allows 45 minutes for the sample pretreatment and Z3/( hours for the elution. Poorly Separated Elements. GOLD. T h e principal difficulty with this element is believed to be hydrolysis of Au(III), which could account for t h e slow elution and leakage of activity into all of the groups. This hypothesis is supported by t h e fact t h a t water alone continues t o elute gold activity after the Group I wash. I n addition, there appears to be some reduction to the metal; even after the entire procedure and a dilute HF wash, dilute aqua regia removes an additional 2% of activity from the column. SILVER.Chloride introduced by preoxidation Tyith chlorine causes a cloudy AgCl suspension to appear. Even after centrifuging out the solid particles, the remaining silver activity leaks through in all groups of the scheme. Attempts to increase the solubility by increasing the chloride concentration to 1.OM with NH&1 failed to improve the separation. If a chlorine-free system is used, Ag first appears in the pIf 6.5 EDTA of Group IV, but tails, 3% more being eluted in p H 9 EDTA, and 5% more in 3iM HC1.
EFFLUENT
VOLUME,
3 ML,
Figure 7. Elution of Group VI elements with 3M HCI MERCURY.Irreversible contamination of both Lucite and Teflon cells caused the data to be poor. Mercury not only goes appreciably into both Groups I and 11, but about 18% remains on the resin, as ascertained by monitoring the resin through the glass column, and by a material balance on the amount of activity originally in the sample. When both the sample and Group I citrate eluent are made Q.01M in chloride with KHdCl, over 99% of the Hg remains in Group I, but the effect of this modification on the other elements was not studied. PALLADIUM. Hydrolysis of this element is believed to be the principal cause of difficulty, since extended water washing a t several points in the scheme continues to bring the element off. Latimer (19) gives 1Q-a’ as the solubility product of Pd(QHJ2. I n addition, removal of the last 2% of the activity from the column can be accomplished only with chlorinated HC1 or dilute aqua regia, indicating that some reduction to the metal mag take place. GALLIUX. This element is only very slowly removed from the resin, which indicates a high resin affinity. Hydrolysis can also be postulated since water washes continue to bring the element out. Latimer (16) gives 1 0 - 3 7 as the solubility product of Ga(OH)a. THALLIUM. If chlorine is omitted from the procedure the separation is greatly improved. Tl(1) remains quantitatively on the resin until the p H 6.5 EDTA wash fraction begins to bring out about 2% of the activity, and p H 9 EDTA of Group V rapidly removes the remaining 98%. However, when chlorine is used, as is necessary for many of the other elements, the Tl(II1) formed seems to hvdrolyee, as evidenced
d
1
c4
00
6
010
5:
ope'
W
L-:
?
08
a
rnrnNCJri
??????
$*1330QC
9 Q, 01"
?
00
E:
3
VQL. 32, NO. 13, DECEMBER 1960
m
1871
by a flat-topped elution peak in Group I, and continued removal of activity with water washes. Latimer (19) gives as the solubility product of T1(OH)s. Elements Obtained in Low Yield. Four other elements (Sc, Ga, Sr, and T a ) are well separated, b u t deserve special mention because of their relatively lorv yields (75 to 85%). T h e slow elution which results in t h e lorn yields can be attributed largely t o the high resin affinities competing m-ith the relatively weak anionic complexes formed (9). Tantalum is unusual in that the p H 3 citrate removes only about 85% of the element, with all other eluents in the scheme being ineffective for the remaining 15%. Dilute HF rapidly removes the remaining portion, but the reason for its strong retention is unknown. DISCUSSION
Chlorine as Oxidant. Chlorine is used in this scheme to oxidize the sample, so that as many elements as possible will be in single, well defined oxidation states. This was important only with the Group I elements, many of which behaved poorly in the absence of chlorine. A simple preoxidation improved the separation of Fe, b u t a holding oxidant was necessary t o keep t h e platinum metals, particularly Ir, in higher oxidation states. Chlorine was chosen as the oxidant because it can oxidize a large number of metals, and because resinchlorine systems are fairly stable after proper treatment (vide infra). Further, the small amounts of chloride formed from the chlorine are tolerable. Many other strong oxidants-e.g., KZCrzO?, KMnOp-do not have these advantages. MacSevin and McKay (91) also found chlorine useful in maintaining I r in the oxidized state in separations on Dowex 50. It is necessary to pretreat the resin with chlorine (4); otherwise, i t is removed at the top of the column, and is not available as a holding oxidant for the elements moving down the column. In experiments with Ir, evidence of reduction by the resin was obtained when an unchlorinated resin was used with a procedure otherwise identical to the one described. I n this case, the small amount of chlorine in the sample solution was completely removed from solution at the top of the resin bed, and the resulting chlorine-free citrate solution eluted only about 9570 of the Ir. Practically no Ir was removed by other eluents until pH 9 EDTA was used, which removed the remaining 5% as a sharp peak. I n contrast, the recommended procedure with a chlorinated resin gives detectable amounts of chlorine in the effluent through a t
1872 *
ANALYTICAL CHEMISTRY
least 30 ml. [o-tolidine spot test ( I ) ] , with improved elution of Ir, as shown in Table 111. It was postulated that two oxidation states of I r were present during the run with unchlorinated resin, the bulk of the I r being in the quadrivalent state, and the remainder (elutable with pH 9 EDTA) being in the trivalent state. The tailing into Groups I1 and 111 during the run with chlorinated resin may be due to hydrolysis of Ir(1V). On the other hand, the concentrations of chlorine in the sample solution must be kept small, as specified, to minimize undesirable side reactions with ammonium ion and with citrate. Chlorine reacts with ammonium salts to liberate nitrogen gas, proceeding through a hydrazine intermediate (19). If strongly chlorinated eluents are passed through the column in the ammonium form, slow chlorination of the ammonium ion occurs, accompanied by gas evolution. This gas evolution is particularly serious if attempts are made to chlorinate the resin in the ammonium form, so the resin is treated in the acid form. However, chlorination of ammonium ion is slow, and the chlorine in the solutions causes no detectable gas evolution when passed through the resin column in the recommended procedure. Chlorine also reacts with citrate in the hot sample solution. If chlorine is bubbled through 5% citrate (at p H 3.0 with NaOH) for 30 minutes a t 85" C., 30y0 of the citrate complexer is destroyed. Under similar conditions with 0.570 citrate, 50% is destroyed, indicating a dependence on the citrate concentration. (These semiquantitative data were obtained by amperometric titration of citrate with CuSO*, before and after chlorination.) The rate of the reaction between chlorine and citrate is also dependent on the chlorine concentration, as indicated by the magnitude of the p H change of the sample batch a t different temperatures. When chlorine is bubbled through 5% citrate (pH 3) a t 85' C. for 10 minutes, the p H falls to 1.1 to 1.3, whereas a t 95' C. the p H falls to only 2.0 to 2.2. With the conditions prescribed for the separation scheme (15 minutes of slow chlorine bubbling through the 2% citrate sample solution a t about 95' C.) the pH drops t o 2.2 to 2.4, indicating only moderate reaction. High chlorine levels are also undesirable because the larger amount of SHt required for readjustment of the p H to 3.0 increases the ionic strength of the sample solution and causes sodium ion to break through earlier. Resin chlorination causes a slight decrease in strong acid capacity, and is accompanied by an almost equal increase in the number of weak acid groups (4). However, these deleterious
effects have little practical effect 011 the separation scheme. Thus, monitoring of the effluent pH's during the entire sequential separation scheme showed no significant differences among unchlorinated resin, 15-minute chlorinated resin, and even 4-hour chlorinated resin. Also, when representative elements with single oxidation states were put through the entire scheme, no significant differences were found between unchlorinated and 15-minute chlorinated resin. Batch Precomplexation in the Sample. It was necessary to complex a number of the elements batchmise before putting the sample on the resin, to prevent many of these elements from becoming almost irreversibly adsorbed on t h e resin. Chromium particularly is very difficult to remove rapidly from the resin, if i t is once allowed to be adsorbed. The low rate of complexation of Cr is m-ell known (83). The batch-complexing conditions of 15 minutes and 95" C. were selected to put Cr quantitatively into the pH 3 citrate main fraction of Group I. Fifteen minutes at room temperature, for example, complexed only a small proportion of the Cr; the remaining CrT3 was adsorbed on the resin and was later slowly eluted into several fractions. Likewise, several other metals (Ru, Os, Ir, &Io, and Bi) are difficult to remove from the resinafter they are once adsorbed, and later contaminate several groups to the extent of 10 to 257,. With the batch conditions prescribed, all of these elements pass nearly quantitatively into the effluent of Group I. The p H 3 citrate complexer also causes some other elements-e.g., Fe, Hf-that could be satisfactorily adsorbed as cations and eluted from the resin to be anionically passed into the effluent of Group I, which is a very large group. However, citrate is the most satisfactory batch complexer of several tried. Unidentate ligands in general mere rejected because of the multiplicity of complexes formed with each element, and the resulting danger of distribution of an element among several fractions. The following conlplexers were also tried and rejected: Oxalic acid is too weak a complexer for several elements-e.g., Fe is partially complexed and partially retained by the resin. hIalonic acid at pH 3.5 behaves much like citrate, except that Hf is not a t all complexed, and Bi is only partially complexed, HEDTA a t p H 3 t o 3.5 does not complex Cr, and does complex several divalent metals-e.g., Zn, Co, and Cu-to no particular advantage since they can be easily removed from Dowex 50W after adsorption as cations. The p H of 3 chosen for the citrate batch is considered to be optimum. A
lower p H cannot be used to decrease the number of Group I elements because p H equilibration of the resin to later eluents is made more difficult. Higher p H only causes niore elements to go into the already large Group I. Control of Displacement. T h e procedure described deals with such a wide range of elements t h a t both complexation and displacement must be used to elute groups of elements. This is necessary because the alkalies, for example, are poorly complexed, while the other elements, such as Cr and Fe, are only slowly displaced. As a result, i t is necessary t o remove elements with the strongest affinity for the resin jirsl, by complexation. while the less strongly held are removed last, mainly by displacement. This order of removal is unusual, and requires some explanation. I n displacement, the order of elution of cations is univalent > divalent > trivalent, whereas by complexation, the order is roughly reversed. Thus, if an attempt is made to remove the alkalies first by displacement, upon switching to a complexing eluent the trivalent metals would have to overtake and pass the divalent metals that have already been spread along the column by the displacing eluent. This would result in very diffuse peaks and poor separations. The batch precomplexation procedure further commits the scheme to early elution by complexation. To reduce displacement in the early parts of the scheme, the ionic strengths of all eluents are kept low. It was found that 15 to 16 meq. of animonium salts can be tolerated in the scheme before Na, which is the least strongly held of the elements in the scheme, breaks through. (This number of milliequivalents of ammonium ion was found to be largely independent of the particular concentration of ammonium ion or of the sequence of washing.) Toward the end of the scheme, where only uncomplexable elements remain on the column, the ionic strengths of the eluents are raised to give rapid elution. Effect of Deviations from Recommended Procedure. T h e recommended scheme is designed for use on the tracer level, starting with a 5-ml. sample solution t h a t contains less than 1 meq. of electrolytes. If the concentration of t h e sample solution exceeds this limit, N a may begin to break through before t h e main Group V fraction. This limitation on the
sample composition may be severe, but it is important in cases where radiosodium is an important component of the sample. On the other hand, if radiosodium is not an important part of the sample, larger amounts of electrolytes may be tolerable in the original sample. The same consideration applies to the other alkali metals that are eluted by displacement, but to a lesser extent, since they are eluted less rapidly than sodium. Caution must also be used in attempts to apply the scheme to semimicro or macro amounts of the metals. Some exploratory runs with milligrani amounts of various metals (most of which were not readily available as radiotracers) were made, elutions being followed by spectroscopic analysis. Most of the metals each fell cleanly into a single group (Ti and R h into Group I; Zn into Group 11; Ho into Group 111; Rlg and M n into Group 117). But on the other hand, larger amounts of Zn and Ni (eluted in Group 11) and 20-mg. amounts of Na caused tailing of these elements into the group following the main group. Interactions were also found to occur on the milligram level that might not occur on the tracer level. Thus. milligram amounts of 57 precipitated \?,hen mixed with Sn and the rare earths. Lastly, a note of caution on the history of the sample must be given. Some metals, particularly those of the Pt group, exist in forms that are highly dependent upon the sample history, and that are slow or difficult to decompose ($0). Such forms might not be converted to the citrate complexes by the sample pretreatment described in this paper, with consequently poorer separations than those described in Table 111. Continuous Monitoring of Efiuent. Even on the tracer level, the well defined volumes t h a t have been recommended in this scheme should be regarded only as a compromise for a sample t h a t may contain many elements. When a sample contains fewer elements, as may often be the case, continuous monitoring of the effluent may allow great flexibility of procedure. If, for example, a certain main fraction eluent is void of all activities, the wash fraction can be eliminated to save time, and the next group eluent tried. In brief, the rigid specification of procedure should not be interpreted to mean that continuous monitoring of the effluent is unnecessary or undesirable.
ACKNOWLEDGMENT
Financial support in the form of a fellowship from the Minnesota Mining and ?YIanufacturing Co. and research grants from the T;. S. Atomic Energy Commission are gratefully acknordedged. The authors thank Ronald A. Razner for performing the spectroscopic analyses.
RECEIVEDfor review June 13, 1960. Accepted August 29, 1960. Taken in part from the Ph.D. thesis of Eugene D. Olsen, UniverEity of Wisconsin, 1960.
VOL. 32, NO. 13, DECEMBER I960
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