Liquid-Liquid Extraction of Pertechnetate Ion with Methyltricaprylammonium Chloride G. B. S. SALARIA, CHARLES L. RULFS, and PHILIP J. ELVING The Universify o f Michigan, Ann Arbor, Mich.
b Liquid-liquid extraction with a chloroform solution of methyltricaprylammonium chloride results in the quantitative isolation of pertechnetate ion from aqueous media, ranging from 4M sulfuric acid 01 9M hydrochloric acid to pH 13. The specificity of the separation is indica,ted b y the report that a similar type of extraction results in a highly selective separation of perrhenate from ions of most other elements. The formation of a 1 : 1 pertechnetate-organic cation adduct appears to be requisite for the extraction at any pH; an excess of the organic reagent i:j only necessary where extraneous anions can compete with the pertechnetcite ion. The competition of chloride ion in the liquidliquid anion exchange system involved has been studied.
T
HE EXTRACTION {of technetium(VI1)
from aqueous acidic, neutral, and alkaline solution by a wide variety of organic compounds, including alcohols, ketones, ethers, es';ers, amines, and organo-phosphorus cr nitro compounds dissolved in nonpolar solvents has been reported ( I ) . It ha:; also been pointed out that a basic oxygen or nitrogen atom in the organic molecule is desirable in effecting partition and that extraction is often more efficierk from acidic than from neutral or alkal.ne aqueous phases. The extraction mechanism with amines dissolved in inert solvents has been described as an anion exchange (2). Churchward [see (S)]suggested the use of a quaternarj amine to extract selectively small aniounts of rhenium from basic solutions. His coworkers (3) found that methyltricaprylammonium chloride (MTC) is a highly selective reagent for the analytical isolation of trace amounts of rhenium(VI1). Consequently, it was cor sidered worthwhile to examine MTC as :t possible means for separating the pertechnetate ion. EXPERIMENTAL
Chemicals. A solution of ammonium pertechnetate in water ( p H 4) containing 46.75 mi;. of Tc99 per ml. was obtained from the Oak Ridge Sational Laboratory; this concentration was substantiated by coulometric measurement (4, 6). Technetium stock solution I was prepexed by diluting 10
ml. of this solution to 250 ml.; 10 ml. of solution I was diluted to 100 ml. for stock solution 11. A sample of "Aliquat 336," essentially methyltricaprylammonium chloride, was kindly supplied by the Chemical Division of General Mills, Inc., Kankakee, Ill. It has the general formula
where R is a mixture of Cs and Clo carbon chains, with the Cspredominating. This material has an average molecular weight of 442, on which basis solutions of approximate molarity in chloroform were prepared. In some experiments the chloroform solutions of MTC were acid-washed before being used; the results were essentially identical with those for unwashed extractant solutions. All other reagents were of Mallinckrodt or J. T. Baker analytical reagent grade quality. Nitrogen (oil-pumped) was used without further purification for purging polarographic solutions. Apparatus. A Fisher Elecdropode was used for polarographic measurements, which were made in a thermostatted H-cell us. a saturated calomel reference electrode, S.C.E. A Leeds & Northrup p H meter was used. Negative values of p H were not measured and only represent a formalized device for incorporating the results obtained in 1 t o 9 M acid. A thin window Geiger tube and Nuclear-Chicago scaler, Model-l51A, were used to measure the P-activity of technetium samples. Extraction Procedure. The test solution was usually prepared by diluting 5 ml. of stock solution I1 t o 50 ml. in a volumetric flask with the desired acid or base t o give a 0.189mM Tc solution. The pH of this solution was measured; an aliquot was transferred to a 50-nil. separatory funnel and vigorously shaken with an equal volume of 8% N T C solution in chloroform; after separation of the two layers, the chloroform layer was drawn off. The pH of the aqueous layer was measured; the latter was again extracted with an equal volume of the reagent and the organic layer was separated. One milliliter of the residual aqueous phase and 1 ml. of each extract were transferred to stainless steel planchets; any free acid was neutralized with ammonia; the resulting
mixtures were dried under an infrared lamp and counted. The distribution ratio in the present study was defined as D=- activity in the organic layer activity in the aqueous layer Close temperature control does not appear to be critical during the extractions, which were conducted a t ambient room temperatures (about 24' f 2 O C.). Polarographic Procedure. Ten milliliters of the pertechnetate solution in 2.V HC1 was shaken with a n equal volume of M T C solution in chloroform. The aqueous layer was separated from the organic layer and, after adjustment to p H 2.0, diluted t o 25 ml. in a volumetric flask. About 15 ml. was transferred t o the polarographic cell, deoxygenated by nitrogen purging for 10 t o 15 minutes, and then polarographed over the desired potential range. From the height of the technetium(VII)/(III) wave, Eliz a t -0.18 volt (4, 5 ) , the amount of technetium left unextracted in the aqueous layer was determined. RESULTS AND DISCUSSION
Radiometric assay for (Tcg9) pertechnetate ion indicates that over 99% of the technetium can be extracted by a two-stage process using equal volumes of (0.15M) solution of MTC in chloroform throughout the pH range of -0.95 to 13.05 (Table I). Chloroform alone is essentially incapable of extracting pertechnetate ion; the resulting per cent extracted of 0.02 to 0.09 can be considered as being due to the solubility of water in chloroform. The use of 8% (0.15M) extractant and the specification of the pH before extraction was consistent with prior work on this system (2). Actually, much lower concentrations of reagent (in general, comparable with the pertechnetate concentration) are satisfactory. In extraction from unbuffered media, the pH after equilibration has naturally more significance. However, since the efficiency of the extraction is not significantly pH-dependent for the system under study, the latter point is not too important. The large changes of pH noted for the 0.15M reagent (Table 11) are not obtained in the case of the less than 1mM reagent concentration actually needed in the present study. VOL. 35, NO. 8, JULY 1963
983
100
Table 1. Liquid-Liquid Extraction of Tc(VII) with Chloroform Solutions of Methyltricaprylammonium Chloride
Solution compositiono &SO4
(4&?f)
His04
(2-11)
&SO4
(1~11)
HISO,
Initial pHb
(0.5JI)
HzSO4
1.o
His04
2.02
HzSOi HC1
4.10 6.40 (9144)
HC1
(6M)
HC1
(41T1)
HCl
(2111)
HC1
0.85
HC1
2.0
HCl HCl HC1
3.40 5.50 7.0
KOH KOH KOH
9.05 11.0 13.1
197 I1 99 I99 I1 97 I98 I198 I99 I1 96 I97 I1 94 I95 I1 95 188 I92 I98 I1 97 I98 I1 99 I98 I1 99 I98 I1 99 I9i I1 98 I93 I1 9s I92 I93 I92 I1 90 I95 I95 198
LL
Dd 31 91 67 34 47 51 76 25 32 15 19 18 7.2 11 39 36 42 175 44 91 44 91 32 55 13 43 12 14 11 9.1 19 58
(I
Tcgg.
Distribution ratio, D, is defined as (activity in organic layer)/(activity in aqueous layer). These data were calculated from % E values expressed to four significant figures. d
Table 11. Change in pH on Extraction of Aqueous Tc(VII) Solution with CHCla Solution of Methyltricaprylammonium Chloride" pH before pH after
extraction
extraction
2 .oo 3.40 4 10 5.50 6.40 9.50 11.o 13.1
2.15 3.20 3.60 3.60 3.65 3.70 4.90 13 .O
Aqueous solution originally 0.189mII.f in Tc(VI1); chloroform solution 0.15M
in MTC. 984
ANALYTICAL CHEMISTRY
75
c
Original aqueous solution was 0.189mM in pertechnetate and contained acid or base indicated; chloroform extractant solution was 0.15M in MTC. * Liquid-liquid extraction with 0.15 MTC solution in CHCls involves large change in pH of pertechnetate solution after equilibration for pH range of 6 to 11 (Table 11). Roman numbers refer to successive extractions of aqueous layer. Equal volumes of aqueous and organic phase were used in each extraction in this table. Pertechnetate content of phase used in calculation is based on radiometric assay
for
IC
0
Tc extracted,c %
0
z 50 P t-u
a a:
kW
25
s I
I
1
IO
IO
1
01
REAGENT CONCENTRATION, m y
Figure 1. Liquid-liquid extraction of Tc(VI1) from several aqueous media with various concentrations of MTC in chloroform Volumes of aqueous and organic phases a r e equal
In another set of experiments, the concentration of technetium was kept fixed and the concentration of MTC mas varied to find the average number of extractant molecules, n, associated with the extracted metallic species. The results are shown in Figure 1, where the extraction data for pertechnetate ion at pH -0.G (HzSOa),6.4, and 13.05 (KOH) are consistent with n = 1. I n all of these media, 90 to 9801, extraction obtains with but one MTC per Tc04- ion present. The fact that the extraction of pertechnetate ion from 2M HC1 with MTC (Figure 1) requires a tenfold excess of reagent for nearly complete extraction must be due to a less favorable competition between pertechnetate ion and chloride ion for the cation of the reagent. To ascertain that MTC in the presence of 2M HCl does not act as a reducing agent, the aqueous extract after extracting pertechnetate from 2M HCl was diluted to a known volume, the p H adjusted to 2, and the solution scanned polarographically. The most positive polarographic wave (4, 5 ) corresponding to the 4-electron reduction of Tc(VI1) to (111) appeared; its height due to unextracted pertechnetate was in agreement with the results of the radiometric determination (Table 111), confirming the validity of the radiometric measurements and the nonreducing action of the reagent. I n connection with the further study of the chloride effect in this system, some additional polarographic checks were made after extraction from 0.5M acid. Evidently, (a) a 1:1 compound exists, (b) both the reagent and the MTCpertechnetate adduct distribute predominantly into the chloroform layer, (c) HTc04does not extract significantly, and (d) reduction of Tc(VI1) is not important. Considering only the major aspects of the chloride competition in the heterogeneous system as a whole,
Table 111. Comparison of Radiochemical and Polarographic Measurements of Residual Tc(VII1) in Aqueous Layer after Extraction"
Reagent concn.. mM
70
'
0.384 0.76i 1.92 0.192 0.384 0.768 5
Aqueous
Extraction measured by Polar@-activity ography 40.2 67.5 85.5 51.2 70 . 2 86.7
39.5
70.9 85.2 52.2 70.1 86.9
solution
was
originally
0.189mM in Tc(VII), and 2Min HC1 (Nos. 1 to 3 ) and 0.5M in HC1 (Nos. 4 to 6 ) .
Defining D as,
then,
or (5)
K i t h low concentrations of RSTC, of course, appropriate correction must be made on the "free" MTC concentration for the portion bound to extracted pertechnetate. Values of K. calculated from thc present data for 2111 TIC1 and from 0.1 to 8-11 N T C concentrations range from (7.2 to 7.8) X 103. For 0.5N HCl and MTC concentrations of 0.08- to 0.8mJ1,
the calculated values of K , range from (4.8 to 6.1) X lo3. Consequently, a K , of (6.5 f 1) X lo3is indicated, and the conception of the mechanism involved appears to bl: essentially valid. A more refined treatment is not warranted until a reagent of 1 0 0 ~ opurity is available. Attempts were also made to use MTC as an extractant for ‘Tc(II1) or Tc(1V). Only 2.40/, of the technetium could be extracted from the green colored solution of Tc(II1) which was obtained by coulometric reduction of a Tc(VJ.1) solution, 0.1M in HC1 and 0.5M in KCl, a t -0.03 volt us. S.C.E. Since a polarographic run on the Tc(II1) solution used showel the presence of about 3% of Tc(VII), i t may be concluded that i t is not possible to extract Tc(II1) with the qua1,ernai-y ammonium salt. No significant extraction was found for Tc(1V) ohtained by the reduction of pertechnetate with ascorbic acid. The prior work on perrhenate with MTC (3) indicates the general utility
and great selectivity of this extraction in isolating Re-and therefore also Tcfrom most other elements. Chromium, cobalt, rhenium, and vanadium are among the few elements which might coextract with technetium. Unlike technetium, however, the distribution in some of these systems is strongly pHdependent; hence, resolution from technetium may still be feasible by control of pH in the aqueous phase. Perrhenate can be recovered from the quaternary ammonium salt extract by stripping the latter with 2.5M perchloric acid in three stages (3); over 90% is recovered in the first stripping solution. An attempt was made to separate pertechnetate and perrhenate from 2M HC1 solutions with MTC extraction, which involved measuring the amount of Tc in both aqueous and organic layers by the radiometric method and the unextracted Re in the aqueous layer polarographically. Rhenium follows technetium so closely that their separation from each other is not possible from the indicated medium. This separation
might be feasible, however, using sulfuric or perchloric acid media. ACKNOWLEDGMENT
One of the authors (G.B.S.S.) thanks the National Academy of Sciences (U. S. A.) for an appointment supported by the International Cooperation Administration under the Visiting Research Scientist Program. LITERATURE CITED
(1) Boyd, G. E., Larson, Q.V., J . Phys. Chern. 64, 988 (1960).
(2) Moore, Fletcher L., “Liquid-liquid
Extraction with High-Molecular Weight Amines,” NAS-NS 3101, Office of Technical Services, Washington, 1960. (3) Peterson, H. E., MacDuff, J. . S., Hovey, hl. W., Report 5889, [Jnited States Department of Interior, Bureau of Mines, 1961. ( 4 ) Salaria, G. B. S., Rulfs, C. L., Elving, P. J., ANAL.CHEW35, 979 (1!163). (5) Salaria, G. B. S., Rulfs, C. L., Elving, P. J., J. Chem. SOC.2479 (1963). RECEIVED for review February 8, 1963. Accepted AprilJO, 1963.
A Scheme for the Separation of Platinum, Palladium, Rhodium, and Iridium by Solvent Extraction G. H. FAYE and W. It INMAN Mineral Sciences Division, Mines Branch, Deparfmenf o f Mines and Technical Surveys, Ottawa, Canada
b A new scheme is described for the fractionation of solutions containing microgram or milligi*am quantities of platinum, palladiurn, rhodium, and iridium by solvent extraction procedures. The iodide complexes of platinum and palladium are extracted with tributyl phosphote, and thereby separated from rhodium and iridium. Modifications and extensions of existing methods are used for the separation of palladium from platinum and for the separation of iridium from rhodium. The addition of phosphoric acid improves the precision of the stannous chloride-hydrobromic acid method for the spectrophotometric determination of iridilJm. have been developed in these luhoratories for the determination 3f the platinum metals (I1, 12) have involved the use of an anion exchange procedure ( 9 ) for the separation of platinum, palladium, and rhodium in sample ~~olutions obtained subsequent to fire assay. This method is tedious, time consuming, and involves the use of large quantities of perchloric HE METHODS TH4T
and hydrochloric acids; in addition, it does not readily permit the isolation of iridium from a solution of mixed platinum metals. Therefore, an alternative method was sought that would overcome these problems. Various solvent extraction methods for fractionating certain binary mixtures and for separating pairs of the platinum group metals have been proposed (4-6, 16),but, until recently, the literature did not reveal a solvent extraction scheme that will permit the fractionation of a solution containing platinum, palladium, rhodium, and iridium by simple batch-wise operations. After the work described in this paper had been completed, Sen Gupta and Beamish (16) described a solvent extraction scheme in which micro amounts of platinum, palladium, rhodium, and iridium are Separated by a combination of the methods of Yoe and Kirkland (18),and of Tertipis and Beamish (16). The present paper describes an alternative solvent extraction procedure for the isolation of the individual platinum metals from solutions containing microgram or milligram amounts of platinum, palladium, rhodium, and iridium. Plati-
num and palladium are separated simultaneously from rhodium and iridium by extraction of their iodide complexes with tributyl phosphate. Palladium is then separated from platinum with p-nitrosodimethylaniline by the method of Yoe and Kirkland (18), and iridium is separated from rhodium with tributyl phosphate by a modification of the method of Wilson and Jacobs (17). EXPERIMENTAL
Reagents and Solutions. Equilibrated Tri-n-butyl Phosphate (TUP), This was prepared by shaking the purified grade of T B P (Fisher Scientific Co.) with an equal volume of 6 N hydrochloric acid in a separatory funnel for approximately 1 minute. Standard Platinum and Palladium Solutions. These were prepared separately by dissolving an accuratelyweighed quantity of Johnson, Matthey and Co., Inc., Specpiire platinum or palladium sponge in aqua regia, rvaporating the solution to dryness several times with hydrochloric acid, thcn dissolving the residue and diluting to a known volume with 1N hydrochloric acid. VOL 35, NO. 8, JULY 1963
985
