Of the elements tested, only bismuth appears to be critical. Normally, less than 100 p.p.m. bismuth is found by spectrographic analysis, and 100 p.p.m. would not interfere. The usual concentration of the other elements shown in Table is far below that which could cause measurable interference. Several of the rare earths are commonly present in amounts to loo P*P*m',but no interference with the thorium determination would be expected.
v
ACKNOWLEDGMENT
The authors thank s. H. Huston and s. E. Mueller for their help in making the precision studies. LITERATURE CITED
(1) Adler, I., Axelrod, J. M., ANAL. CHEM.27,1002 (1955). (2) Adler, I., Axelrod, J. M., Norelco Reptr. 3, 65 (1956). (3) Banks, C. V., Weiss, G. L., U. S. At. Energy Comm., Rept. ISC-873 (1957).
(4) Campbell, W. J., Carl, H. F., ANAL. CHEM.27, 1884 (1955). (5) Claisse, F., Norelco Reptr. 4, 3 (1957). (6) Cope, J. H., Ibid., 3,41(1956). (7) Peed, W. F., Wright, W. B., Jr., Rogosa, G. L., U. S. At. Energy Comm., Rept. ORNL-1419(Dec. 6,1952). (8) Pish, G., Huffman, A. A,, ANAL. CHEM.27,1875 (1955).
RECEIVED for review March 17, 1961. Accepted August 24, 1961. Presented in part at Pittsburgh Conference on h a lytical Chemistry and Applied Spectroscopy, March 3, 1959.
Radiochemical Separation of Cadmium by Amalgam Exchange J. R. DeVOE,' H. W. NASS, and W. W. MEINKE Deparfment of Chemistry, University of Michigan, Ann Arbor, Mich.
b The radiochemical separation of cadmium b y an amalgam exchange technique has been critically evaluated. The cadmium amalgam exchange step i s followed by a back-extraction with thallous ion to remove the cadmium selectively from contaminants in the mercury. Cadmium yields of 80% were obtained with less than 0.1% contamination of most typical elements. Indium, thallium, and selenium contaminate the separation to a greater extent. The procedure can be carried out in 8 minutes with no special equipment. Mineral acids below 1M do not affect the exchange, but oxidizing agents such as U t 6 and CeS4 must be given special attention. This procedure may prove particularly useful in studies of the decay schemes of short-lived cadmium fission products.
A
communication from this laboratory (1) described preliminary results obtained for a number of elements using a novel radiochemical separation technique based on amalgam exchange. The separation of the radioisotope takes place by virtue of the rapid exchange which is known to occur between an element in the form of a dilute amalgam and its ions in solution. If there are many more inactive atoms of the element in the amalgam than there are of its radioisotope in solution, the amalgam exchange will result in most of the activity being incorporated in the amalgam. In this it is somewhat similar to PREVIOUS
Present address, National Bureau of Standards, Washington, D. C.
the isotopic exchange separation developed b y Sunderman and Meinke (9).
A recent report (7) describes the successful application of this technique to determination of in reactor effluent. In the present paper we extend the preliminary survey of the method by a critical study of its application to the radiochemical separation of cadmium. Since considerable work has already been done in this laboratory on the element cadmium (Z), this study provided an excellent opportunity to compare the amalgam exchange method with several other methods which had previously been considered optimum. The separation technique uses the exchange of cadmium amalgam in the extraction step, followed by a backextraction or elution with thallous ion to remove the cadmium selectively from contaminants in the mercury. The extraction step can 'be represented by the following reaction: Cd (Hg)
+ Cd*+*e Cd* (Hg) + Cd+2
where the asterisk denotes a radioisotope of cadmium. The procedure consists merely of shaking the cadmium amalgam with an aqueous solution containing the radioisotopes of cadmium. The degree of separation and yield of cadmium which can be obtained with such a separation technique were measured with radioactive tracers. APPARATUS,
REAGENTS, A N D PROCEDURES
Apparatus. During the separation, the amalgam was agitated in a 50-ml. round-bottomed centrifuge tube b y an electric stirrer manufactured by Eastern Industries, New Haven,
Conn. The stirring rod was 7 mm. in diameter with a glass propeller on the end. Gross y-ray measurements were made with a Nuclear-Chicago scintillation well counter as described previously (2, 8, 9) and gamma spectrum measurements with a special 100-channel y-ray scintillation spectrometer (6). Reagents. Cadmium metal foil, 99.9% pure, Belmont Smelting and Refining, Brooklyn, N. Y. Chromous sulfate solution. Reduce chromic sulfate solution [-lo0 mg. of chromium sulfate (green powder), Crt( S 0 4 ) 3 . ~ HJ. ~ 0T., Baker Lot 2487, per ml.] in 0.1N sulfuric acid by stirring with 5% by weight zinc amalgam. Copper pellets, Mallinckrodt, analytical reagent grade. Mercury, Baker and Adamson, analytical reagent. Nitrogen gas, water pumped, 99.99% pure, Liquid Carbonic Co. Thallous acetate, Fisher Chemical Co., purified. Solution 75 mg. of T1+ per ml. in 0.1N nitric acid. All other chemicals used were analyzed reagent grade. All radioisotopes used as tracers have been described [Table I (Z),and Table 11 @)I. Preparation of Cadmium Amalgam. Wash cadmium metal foil in 1N nitric acid until surface is completely etched. Wash with distilled water, dry, and weigh. Add enough cadmium foil to a weighed portion of mercury stored under several milliliters of 0.1N nitric acid t o make a 2% amalgam. Agitate for a short time to amalgamate the cadmium completely. Amalgam-Exchange Procedure. Place 2 ml. of solution containing tracers of contaminating ions plus microgram amounts of inactive cadmium in a 50-ml. centrifuge tube. Radioactive cadmium and nonradioVOL 33, NO. 12, NOVEMBER 1961
1713
active interferences are used for yield determinations, Chemical concentration of solution prior to amalgam exchange is given in Table I. Add 50 111. ( 4 . 6 8 gram) of cadmium amalgam containing 2% cadmium by weight (-14 mg.). Stir vigorously for 5 minutes. Remove the aqueous phase by suction and wash amalgam twice with 2-ml. portions of 0.1N nitric acid and twice with 2-ml. portions of distilled water. Transfer amalgam to a stoppered bottle containing 2 ml. of thallous acetate solution. Shake for 1 minute. Remove a 100-,ul. aliquot of aqueous layer containing separated cadmium isotopes and count. Total time for separation is 8 minutes. Cadmium Separation from Fission Products. Reduce sample of irradiated uranium with chromous sulfate solution. [In these experiments 100 mg. of uranyl nitrate were irradiated for 10 minutes a t a flux of 10'2 n cm.+ sec.-1 in the pneumatic tube facilities of the Ford Nuclear Reactor of the University of Michigan (6). ] The sample was dissolved in 1 ml. of water and transferred to a stoppered bottle. To this solution were added 1.5 ml. of chromous sulfatesolution to reduce the U+8to U+4. This was about 50% more than the stoichiometric amount of reducing agent.] Then use the amalgam-exchange procedure above. Total time for separation is 10 minutes. DISCUSSION AND RESULTS
A number of trial separations were made in which the concentration of the amalgam, the time of stirring, and the volume of the aqueous phase were varied, to maximize the separation yield for cadmium. Various selective back-extractants were also tried to remove the cadmium metal from the mercury. The resultant optimum separation procedure is that given above. The degree of separation of cadmium obtained with this procedure from a number of elements representative of the periodic table is shown in Table I. The elements have been listed in the order of their reduction potentials. The data in Table I have been subdivided to show the degree of separation for the amalgam-exchange step and for the elution step using thallous ion. Those elements above cadmium do not contaminate the exchange separation within the sensitivity of the experiments-Le., with the amount of tracer used, 106 to 108 counts per minute. On the other hand, elements below cadmium do contaminate the amalgam-exchange step, probably because of reduction of the ions by the amalgam. When an eluent such as thallous ion is used, considerable selectivity can be obtained through the preferential oxidation of cadmium without oxidizing the contaminating elements in the mercury. From Table I it can be seen that those elements between thallium 1714
ANALYTICAL CHEMISTRY
l o o , ~ l - ,
'I
lo'
,
,
l
i
l
l
Cs Bo-La CeR Zr-Nb Ta
i Cr
l
i
'
y---a.
I
R A D I O C H E M I C A L SEPARATION OF C A D M I U M E x t r a c t t o n Y l e l d 1 1 7 - I 7%) Ion Exchange Y ~ e l d 1 8 0 f 1 6 % 1 Amalgam Exchange Yleld 1 7 6 1 33%1
/
'
"9,
i
\1
1
/A \
\
l
l
l
l
Co Ru.Rh Ir
Ap
Zn
Hg
l In
l
l
l
!
TI
So
Sb
Se
I
Figure 1. Experimental contamination for three types of cadmium separations
and cadmium-i.e., indium-continue to contaminate the separation, while elements below thallium with the exception of selenium and iodine do not contaminate the elution step. It is suspected that the very small amount of I1al that separates with the amalgam adsorbs on the surface of the amalgam as the iodide and then elutes as thallous iodide. Selenium is reduced to the metal by cadmium amalgam but, instead of amalgamating,
Table
I.
Tracer* I131
Cs'*' Ba'N-La140 S~W-Y~O
Ce14cPrl44 ZP-Nbgs Zn86
Cr61 CdIlSm In11~ 1 2 0 4
Separation of Cadmium and Contaminants (Amalgam Exchange ProcedureI5
Solution for Exchange (No Carrier Added). I-, 0.2N "03 (C.F.) 0.1N HCl(20 pg.) 0 . 5 N HC1 (C.F.) H,O (c.F.) 1:5NH ' N03 (C.F.) 0.5N H2C204(C.F.) 1.ON HCl(O.8 mg.) 0.2N HC1 (0.1 p g . ) ~
(18 ua.) 0.51\r'HN6j3'(50pg ) 0 5N HNOI ~- 10.83 - - me.) 0 15% " 0 8 (7 pg.) ' 0
0.9NHC1(1.1 mg.) \
cow
Sn"3 Sb124 Ru~OB
SeT6
forms a thin film on the surface of the mercury at the concentration of selenium used in these experiments. The selenium is then removed from the surface of the amalgam during the mechanical agitation in the elution step. Interferences. The mineral acids (nitric, hydrochloric, and perchloric), in concentrations up to 3 N , do not affect the cadmium yield when the separation is carried out at room temperature. Sulfates, sulfites, sul-
-
SbO+, 0.5N "0s
(10.rg.) RuCl-;, 0 . 5 N "03, 0.7N HC1 (6 p g . ) HISe030.1N HC1 (0.12 mg.) IrC1-t 0 . 1 N HC1
( 0 . 2k . ) 0 . 5 N HNOs (0.2 mg.) Hg2oa AgllOm 0.8N HNO (20 pg.) Average of duplicate runs.
Reduction Potentiald
0.005
I
-2.92 -2.90. -2.89: -2.48; -1.53, -0.76 -0.74
% Separated Exchange Elution Total step step Separation
-2.52 -2.37 -2.47 -1.1
