Radiochemical Analysis
Second group of papers from Symposium on Radiochemical Analysis, Division of Chemistry, 133rd ACS Meeting, San Francisco, Calif., April 1958. Papers from this symposium were published in November 1958 and others will appear later.
Radiochemical Separations of Indium D. N. SUNDERMAN,l 1. B. ACKERMANN, and W. W. MEINKE Department of Chemistry, University of Michigan, Ann Arbor, Mich.
b The radiochemical separation of indium by sulfide precipitation, solvent extraction of the bromide with diethyl ether, extraction with thenoyl trifluoroacetone and anion exchange in hydrochloric acid solution has been critically evaluated. Comparison of the efficiency of separation from 15 representative radioactive tracers showed that the sulfide precipitation lacked specificity as a decontamination step. Its primary utility is for the final preparation of counting plates or as a step preliminary to the determination of carrier yield by hydrous oxide precipitation. Ether extraction of the bromide gave decontamination factors as high as 1 O4 for many dissimilar elements when conditions were well standardized. Thenoyltrifluoroacetone extractions gave decontaminations of 100 to 1000 for most elements. The anion exchange technique, when operated far from equilibrium with a short column, gave decontamination factors of about lo3 for many elements with a procedure that requires 1 hour.
T
element indium could be of considerable interest in atomic energy work, because it occurs in the valley of the fission product distribution. It has not been emphasized, however, because it is difficult to separate cleanly from many other fission products. Quantitative methods (6, 21) for both separation and determination of indium generally involve either precipitation of the hydrous oxide from a neutral or slightly alkaline solution or precipitation of the sulfide in a neutral or slightly acid solution. Both procedures suffer from many interferences (12). hlany radiochemical separations of inHE
1 Present address, Battelle Memorial Institute, Columbus, Ohio.
40
ANALYTICAL CHEMISTRY
dium have utilized modifications of both the hydrous oxide and sulfide precipitations ( I S ) . Recently the anion exchange separation of indium has been described (5,10). Several extraction systems have been studied, including the ether extraction of indium bromide ( 1 , 7, 20, 21) and extraction of the chelate formed between indium and acetylacetone (15, 16) or 8-quinolinol ( I S ) . This report is a continuation of the critical evaluation of radiochemical procedures initiated with the alkaline earths and silver (18). Three of the most promising of the above procedures, as well as the separation of indium by chelation n ith thenoyltrifluoroacetone have been studied. A more detailed literature survey and discussion of these procedures is given by Sunderman ( 1 7 ) . APPARATUS, REAGENTS, A N D PROCEDURES
Apparatus. T h e extraction vessel is a n open-top glass cylinder, 3 cni. in diameter and 9 em. high, tapered sharply a t t h e bottom to a I-mm. bore stopcock. The electric stirrer, continuously variable speed, is a glass rod 7 mm. in diameter, flattened and twisted on end for stirring rod. The ion exchange column ( 5 ) is a 15ml. glass centrifuge cone with tube 4 mm. in inside diameter and 9 em. long, sealed to a point and tapered to 2 mm. a t tip. An automatic turntable for ion exchange work is designed to advance sample collection tubes a t a predetermined time interval. Other laboratory and measuring apparatus has been described ( I S ) . Reagents. Anion exchange iesin AG 2-X8, 200- t o 400-mesh (Bio-Rad Laboratories, Berkeley, Calif.) is stored in 6 J f hydrochloric acid until used. Buffer Solutions. Sulfate-bisulfate. ACS reagent grade sulfuric acid (0.7M) neutralized to p H 1, 2, and 3 with so-
dium hydroxide solution. Acidphthalate hydrochloric acid. Reagent grade potassium acid phthalate plus hydrochloric acid ( 0 . 2 M ) in proper proportions to make pH bet1veen 2.2 and 3.8 (3). Potassium chloride-hydrochloric acid mixtures were used for p H below 2.2 ( 3 ) . Diethyl ether, ACS analytical reagent, absolute (Mallinckrodt). Diisopropyl ether (alcohol-free, KO. 1193, Eastman Kodak). Hydrobromic acid, 48% HBr. Thioacetamide, 5% in water (A. Daigger Co., Chicago, Ill.). Thenovltrifluoroacetone TTA. 0.5M in benze"ne (Dow Chemical Co., San Francisco), Carrier and radioactive tracer solutions have been described (17, 18), except for indium. Carrier, indium metal dissolved as chloride to give 10 mg. of indium per ml. as In+3 in dilute hydrochloric acid; tracer, indium-114 as InCL in hydrochloric acid solution, obtained from Oak Ridge Xational Laboratory. All other reagents were of C.P. or analyzed reagent grade. Procedures. SULFIDEPRECIPITATION. Place 10 mg. of indium carrier in a 15-ml. centrifuge cone, add 10 mg. of carrier and tracer for contaminating element, and take steps necessary t o secure exchange. Dilute t o 5 ml., adjust pH roughly t o 1, 2, or 3, and add sulfate-bisulfate buffer t o desired pH. Stir thoroughly, heat to 90" to 95" in a water bath, and pass hydrogen sulfide gas through the solution for 1 minute. Digest for 5 minutes with occasional stirring. Centrifuge a t top speed for 5 minutes. Remove supernatant solution by suction, transfer the precipitate to a planchet, then dry, mount, and count with Geiger tube, or transfer to a glass culture tube, stopper, ar,d count in the scintillation well counter. ANION EXCHAXGE. Add the sample (n-ithout carriers) in a small volume of 6M hydrochloric acid to the top of an 8.5-cm. column of anion exchange resin and allow to seep into resin. Elute with 6M hydrochloric acid at a flow rate of 1
ml. in 3 minutes. Save the 6- through 20-ml. portion, which contains most of the indium. BROMIDE EXTRACTION. &fake Up 10 ml. of 4.5X hydrobromic acid solution containing 10 mg. of indium and tracer (no carrier) for contaminating element. Stir rapidly for 2 minutes with 30 ml. of diethyl ether in an extraction tube. Allow phases to separate; drain off the lower aqueous phase, allowing 1 drop of organic phase to pass through the stopcock. K a s h by stirring rapidly for 2 minutes with 10 ml. of 4.5M hydrobromic acid solution. Again drain off the aqueous phase. Repeat the wash step if greater decontamination is required. Stir the ether layer rapidly for 2 minutes with 10 ml. of 6 M hydrochloric acid. illlow phases to separate and draw off all but 1 drop of aqueous solution. Transfer a n aliquot of the aqueous layer to a culture tube for counting in the scintillation n-ell counter. T T A EXTRACTIO?;. Make u p in a n extraction tube a 20-ml. solution of acid phthalate buffer of p H 2.1 containing 10 mg. of indium carrier and tracer (no carrier) for contaminating element. Stir rapidly for 5 minutes with a 20-ml. solution of T T A in benzene. A l l o ~the phases to separate and take an aliquot of the organic layer for counting in the scintillation well counter. RESULTS A N D DISCUSSION
The sulfide precipitation separation was studied because so many past procedures had utilized it. The hydrous oxide separation was not studied, because it lacked specificity for general applicability and considerable information on contamination for this type of procedure had been obtained in connection with hydroxide scavenging (18). The anion exchange and the two extraction procedures include a study of techniques not used in previous evaluations. One goal of this work has been to reduce the over-all time required for a total procedure by recording typical yield and decontamination values for an optimum separation. It was felt that each step should not exceed 15 minutes. Thus while somewhat better yields might be possible with these four procedures under more time-consuming conditions, these slight improvements were not justified by the goal. The experimental yields for indium and for contamination of 15 typical tracer activities are summarized in Table T
1.
Sulfide. Indium sulfide can be precipitated in a sufficiently acid solution toprevent interference of elements t h a t are easily hydrolyzed or form sulfides in a neutral or alkaline solution. This precipitation m a y also be useful for determination of t h e yield of indium a t t h e end of a series of separation steps and for reducing t h e sample volume for counting. An attempt was made to use the ther-
Table I.
Contamination of Indium Separations“
(% carried)
Tracer
Sulfide (10 blg. In,, 10 Mg. Contaminant Carrier ildded)
Extraction,
~~i~~ Exchange ( S o Carrier
Added) 25
0.13
Cs’34 I I1114 b I131 1~192
RulO~,Rh106 Sbl24
13 0.6
2.0 91.8 f 2 . 5 4.3 4.9 96 96
0.11 95.5 f 1 . 2 1.1
3.7 22 3.0 0 3 0 4
100 1.6 8.7
0 1 6 2
31
(10 l l g . In, S o
Contaminant Carrier Added) Bromide TTA 0.055 0 01 0.009
0.012
n n1.i
93.0 f 2 . 0 6 . 7 , 6.8* 0 017 0.015 2 . 2 . 0.25* 23: 7* 48, 10* 0 006
0.08
0 018
1.0 0.3 0.5 0.6 0.05 89.4
1
0.35 0.7
0.1 0.1 8 0.5 3.1 49.5
Average of duplicate runs except for indium. Average of quadruplicate runs. Errors are “standard deviations.” * Values for procedures with two 4.5M hydrobromic acid washes. mal decomposition of thioacetamide to increase the specificity of this separation. Procedures using from 2% (4) to saturation or 13% (11) thioacetamide in aqueous solution have been reported; a 570 solution was used for this work. Precipitation was made a t 90” to 95’ C. on a water bath by adding 2 ml. of the reagent solution to the sample buffered a t p H 3.0, and digesting for 15 minutes. The precipitation n a s very slow and occurred a t different times in samples treated in an identical manner; the indium yield for four samples varied from 21 to 60%. Because a much longer digestion time would be required to obtain a yield with sufficient reproducibility to be of value in radiochemical work, the experiments 1%-iththioacetamide were discontinued. The use of buffer solutions for the precipitation of indium sulfide from acid solutions has been thoroughly studied by Jeffrey and Swift (8). For quantitative precipitation of indium sulfide with hydrogen sulfide they recommended a sulfate-bisulfate buffer of p H 1.6 to 1.8 with a total sulfate concentration of 0.35M. For radiochemical work both indium and sulfide concentrations must be greater than for quantitative analyses and the procedure must be completed in a short time with little chance for digestion. The yields of indium a t p H 1, 2, and 3 were determined for these radiochemical conditions. Similarly, the contamination of cobalt, representative of metals that precipitate in a solution slightly more basic than indium, was determined. Values for indium yield and cobalt contamination a t p H 1, 2, and 3 were 91.8 f 2.5 and 1.7; 98.8 + 1.8 and 4.1; and 98.1 =t 1.1 and 6.2, respectively. (Errors are “standard deviations” of quadruplicate runs.) The precipitate which forms at pH 1 is red-orange and
much more dense and crystalline than the yellow-orange variety formed at p H 2 and 3. It begins to form after about 0.3 minute of bubbling hydrogen sulfide through the solution, whereas the yellon*-orange variety forms immediately. K h e n centrifuged, it occupies only one third the volume of the yelloworange precipitate and tends to adhere to the sides of the tube, contributing to the lower yield. It is more difficult to dissolve in acid than the sulfide formed a t the higher pH. Precipitation at p H 1 was chosen for the contamination studies because the dense crystalline precipitate a t this p H showed better decontamination properties for cobalt. Bottger ( 2 ) has discussed this form of indium sulfide. I n Table I separation is found or to be expected for many metals such as antimony, silver, and tin that precipitate as sulfides in solution more acid than p H 1. The variable high contamination by cerium is unusual and the reason for it is not obvious. It may be the result of the precipitation of Ce2(S04)3, 2NazS04.2H20 from the buffered solution. In dilute acid it is possible to precipitate cerium completely in this manner by addition of sodium sulfate to a solution of a cerous salt (19). Ruthenium is slow to precipitate, b u t finally forms the brown-black sulfide resulting in almost quantitative precipitation. The case of iridium is rather interesting, in that the solution changes from brown to colorless with addition of hydrogen sulfide but little sulfide precipitates. Further passage of hydrogen sulfide through the solution would probably result in carrying down a larger portion of the iridium as the iridium(II1) sulfide. Selenium is reduced to the elemental state with the hydrogen sulfide, and then carried with the indium. StronVOL. 3 1 , NO. I , JANUARY 1959
0
41
tium is carried as a result of the insolubility of the sulfate and coprecipitation with the sulfide of indium. Sulfide precipitation thus has very limited applicability for separating indium. It is, however, a high yield method (even better at p H 3) for reducing the indium to a form for counting and for the determination by weighing of the yield of the radiochemical separation. [For this latter step the sulfide is often dissolved and the hydroxide precipitated and ignited to the oxide (I,!?).] Anion Exchange. T h e two most frequently used sources of information on anion exchange were t h e papers of Kraus (9, 10) and Hicks and coworkers ( 5 ) . Kraus made a thorough study based on equilibrium conditions in columns, but his methods ivere not applicable here because he used a l o x flow rate. It was felt that a suitable radiochemical method should be completed within 1 hour. This required use of flow rates approximating those used by Hicks. The column contained 8 to 9 cm. of resin and had sufficient capacity for general radiochemical work. The flow rate was about 1 ml. in 3 minutes. but was greatly affected by the height of the resin and the method of packing the column. 1-ery reproducible flow rates (within 1%) were obtained when the column was filled by gravity with the resin slurried in &I4 hydrochloric acid. A linear variation of flow rate ith column height was found in the range from 4 to 9 em. of rrsin. The method of adding sample to the column is important. The volume of sample should be kept as small as possible. Although from 0.05 to 0.5 ml. of solution was used for these experiments, some procedures will require much more than this. The sample was added directly to the top of the resin and not allowed to contaminate the reservoir above the resin. It was then allowed to seep into the resin bed until the solution level reached the top of the resin. KO air was allo7ved Ivithin the resin bed. If the sample is “pushed” into the resin bed by air or otherwise forced into the resin, it nil1 spread over the entire column and the separation will become poorer. This is particularly important with the small column used in this work. The eluent n-as then allowed to fall dropnise on the top of the resin bed. Tyith no more than 1 nil. in the reservoir immediately above the resin. This prevented contamination of a large volume of eluent and subsequent spreading of the elution band. These manipulatory factors w r e of great importance with the rapid flow rate and low distribution coefficients used in this work. Indium was shown by Kraus to have a distribution coefficient which increases to a maximum of 10 a t about 3M hydrorhloric acid and then slowly decreases as 42
ANALYTICAL CHEMISTRY
7
6
5
> ;4 0
a W
E 3
a J W [L
2
I
0 EFFLUENT (MILLILITERS)
Figure 1.
Ion exchange elution curves
the acid concentration approaches 12M. This behavior is sufficient to allow a separation from many elements, if care is taken in the manipulations. The concentration of acid was taken at 6M because of the large number of elements n-hich were strongly adsorbed at this concentration, while the distribution coefficient of indium n a s still sufficiently high to effect a separation from the slightly adsorbed elements. Complete elution curves (carrier-free) (17) were run both for indium and for some contaminating elements. For these elements the activity of each fraction in the 6- to 20-ml. portion of eluent was summed to give the yield and contamination values reported in Table I. For the remaining elements the 6- to 20-nil. portion was collected and measured for activity without a complete elution curve. There had been considerable doubt that this type of anion exchange procedure could be made reproducible, Amounts in the 6- to 20-ml. fraction of the elution curve for indium (Figure l ) , however, mere highly reproducible, as shonn in the table. The indium yield could be increased by taking a larger portion of the eluent, but in most cases this would result in increased contamination. The contaminants fall into three groups: those so slightly adsorbed as to be eluted almost completely before the indium began to appear; those whose elution curves overlap to a large extent that of indium; and those which had not yet appeared to any significant degree a t the 20-ml. point in the elution. Elements of the first group include cerium, cesium, chromium. selenium, strontium, iridium, and ruthenium; all but the last two contaminate to less than 1%. A minimum Contamination for this procedure appears to be reached a t al)out O.lyo,and is the result of the
mechanical problems involved in washing the last portion of the sample through the column. Even though the aqueous volume of the column is about 0.1 ml., a large number of column volumes are necessary for complete change of the solution in the column. The peaks in the elution curves of iridium (Figure 1) and ruthenium occurred a t about 2 nil. The iridium concentration in the eluent drops very rapidly after the peak is reached, while the concentration of ruthenium remains high, resulting in 3.7 and 22% contamination of indium by these elements. The elements of the second group, which overlap indium during elution, are cobalt, silver. and zirconium-niobium. Zirconium and niobium must be considered together, because they were present in secular equilibrium in the tracer solution, each contributing about 50% of the gamma activity. The elution curves of both silver (Figure 1) and cobalt reach a peak of 5 ml., but the cobalt concentration in the eluent remains high to 16 ml. while the silver is reduced rapidly. Zirconium and niobium caused general contamination of the entire first 20 ml. of the eluent. A slight peak in the elution curve was observed a t 15 ml., but it was a very broad one. The elements of the third group, which have not yet been eluted to a significant degree a t the 20-nil. point, are antimony, iodine, tantalum, and tin. The highest contamination in this group is caused by tantalum, which is eluted continually but at a low rate. The elutions of iodine and antimony are similar to this but of lower degree. Tin is held very tightly to the column, resulting in the low contamination of 0.4%. Anion exchange with 6;M hydrochloric acid is of limited value for the separation of indium. Table I shows that it is considerablj- better for separation from tin and selenium and somewhat better for iodine than solvent extraction and sulfide precipitation. It cannot, however, compete with solvent extraction for separation of indium from most elements. Bromide Extraction. Hudgens and Selson ( 7 ) recommend the use of specially purified diisopropyl ether as the organic phase in the radiochemical separation of indium by solvent extraction. They extracted the indium from 4.5M hydrobromic acid and removed it from the organic phase with 6-11 hydrochloric acid. Their work was based upon the investigations of K a d a and Ishii (20, 21) ~ ~ idiethyl t h ether. For any procedure to be adaptable for routine use, it was felt that the reagents should be commercially available in sufficient purity to obtain satisfactory results. Thus an attempt was made to reproduce the work of Hudgens and Selson, using diisopropyl ether commercially available. TT4thout further purification it was impossible to obtain distri
Figure 2. Extraction of indium into thenoyltrifluoroacetone
I
G 9 5 0 M T T A CARRIER F ? E E I 0 5c TTA IO WG. C A Q R I E R ~
i
d 0 2 5 hl T T A CARRIER FREE A 3 e5 T T A 10MG. CARRIER1 0
e
bution c*ocficic~ntsgrcater than ;i with 4.5M hytlroljroniic acid aiid I O me;. of indium cwricr. This distribution cocffic h i t v a s considered iiisufficicmt, brcymsc one \\-ash of the organic phase ~ o u l dreduce t h possil~leyipld to ahout 607,. Thc us(’ of diethyl ether \vas thrn investigatctl, Ijcwiuse it is readill- :ivailable in high purity and hcmusc of t,hc considcrahly Iiighcr distribution cocfficicnts reportot1 for it by l\.ada and Ishii (20, 21) and b!. I(ock and con.orkers ( I ) . Disntlva n t : i ~ wof d icbtli>.l et 1ic.r :I re its higlicr \olatility antl highcr soluliility in the aquc~)iisphase. T h use of 10 mg. of indium in a s y s t c m cwriposed of tliethyl tthcr and 4..iJI hydrobromic~acid resulted in a riistrihut,ion cooficiciit of 65. suffirimt to give high yiclds. Thest. solvcnt clxtraction scpar a t‘Ions werr conducted n-ithout adding carrier amounts of the interft,riiig elmients. Indium wrrier was added, Iion.cvcr, prior to extraction to facilitate thi. tletcrininntion of yields and sulmqucnt manipulatioiis. The high distribution cwffiricnt oht a i n d with tlicthyl othcr uiitlcr the conditions of the proccdurc givc ykld of over 90%. It’ was found that phase equilibrium could lw rcwhctl in between 1 and 2 minutcs of rapid stirring. L o s s r s of indium arc d u e to niec.hanicd rather than rhemicnl effects. The method of separation of phascs sacrifiws a sniall amount of indiuni in each contact to gain a btttc>r decontaniination. The distribution factor is so high that w r y little loss is expcrienccd b>-a second wash n-ith 4.5.W hydrobromic acid solution.
0 10 0 IC
TTA CARRIER T-A IO MG.CARRlER
1
i
Tlie distribution factor in t,hc final extraction step with 6-11 hydrochloric acid \ w s lcss than 0.005, allowing complete rec:ovcry of the indium present in the organic phase. A hydrochloric acid solution \vas used a t this point t,o prevent contamination by macro amounts of iron oftcn found as either mounting inaterial in bombardment procedures or a contniiiinaiit in process solutions. It also r ~ l u c e coiitaniination s by antimony antl tin! because the>-are appreciably scluble in the organic phase in this extraction step. Tlicrc~arc two separation steps in the stantlard c.xtraction: ether extract,ion from hydrobromic acid and the hylrohromic arid wash. Addition of a second hydrohrornic. acid ivash n-ill increase the tlecwitaniination factor by an amount tlepcmlent upon the distrihution factor for the elenient bcing considered. For most of the elements (not starred in Tablc I) this increase n-ill be ahout a factor of 100. For cslcnicnts which arc apprc&hly cxtracted, n second wish mag prove i i w f u l , ns for antimony antl scleniuni; for iodine and tin little wuld be ga incd. Thus for most clciiicmts tl(w)ntuniination factors of lo4 arc possiblr with this procedure in about 15 niinutcs. and can hc increased to 106 by the addition of aaother wash step requiring about 5 niinlites. The method is neat and rapid, gives a high yield of thc desired constituent, and is easily adapted to routine operation. Thenoyltrifluoroacetone Extraction. This chelating agent has been used extcnsivrly in analytical and nuclear
chemistry (Id),although no study 1i:td been made with it for indium extraction. As in the bromide extraction, 10 mg. of indium carrier n-ere used throughout but the contaminants were tested carrierfree. Curves of per cent extraction us. pH after e x t r a h o n were determined for both carrier-free indium and saniples with carrier a t concentrations of 0.10, 0.25, and 0.50Ji thcnoy1trifluoro:mtone in benzene (Figure 2 ) . There is w r y lit,tle difference between t h e c u r w s for carrier and carrier-frce extractions. A pH of 2.1 a t 0.SM therioyltrifluoroncetone was selected for the contamination studics. Because hydrogen ions arc released as a rcwilt of the chelation of indiiini 17-ithtlieiio!ltrifluoroacetonc. the pH values rcferred to for this prowdure were determined after the chclation was complet,ed. For 10 nig.of irirliuni carrier the pH change was of the order of 0.2 p H unit a t this p H levcl. ?*lost elements (except zirconium and tantalum) are separated cleanly at’ this pH. pH conditions for this separation may he t,oo critical for very rapid scparation-but addition of premixed huffer solutions to approximately neut’rxl saniple solutions should p r o w wtisfactory for most cases. Conihinat,ion of sevcxral of these scparation stcps ran provitlc a procdiirc to fit many t y p c ~of mixturrs. Tuble 11 illustrates the results of conihining bromide arid tli~rioyltrifluoroacctone extractions. If tin \\-ere a major interfcrcncc in the sample, an anion cxrhange step c~oulcl11c suhstitutcd for thc ttwnoyltrifluoroucetoiic extraction. by :idding the solution from step 3 dirrctly to the colun1n. LITERATURE CITED
(1) 13ock, It., Iiusche, H., Hocnk, I 188 iI950 \ .
H. G , , Gilbert, 11. d., Stevenson, P.C., Hutrhin. \\-.H., “Qiialitntive Aiiionie Behavior of a Sumher of Netals Jvith an Ion Exchange Resin ‘I)o~1~1111~clI~ (i. 1 et a/.,“Applied Inorgitnic, .\rial, JTiley, Yew York, 705X. (5) -H&,,
(7) Hudgrris, J. E., Selson, I,. C., .Is.IL. CHEM.24, 1472 (195%):. ( 8 ) ,Jeffreys: C. E:. I’., S m t t : 1.:. H., J . -1)j~. ( h e m . Soc. 54, 3219 (1932). (0) Kraus, K. *I.! Selson, F., Int,rrnation:tl Conference on Peaceful Cses of .Itomic ICnergy, Geneva, Paper 837 (US-I) i 195.5 I . (10) K&s, I
