solution to a p H of about 8 or above will cause it to retain its composition and its precipitating pon-er over a period of several weeks. KO additional effect on the stability was noticed on storing the solutions a,t elevated p H values in either clear soft glass or clear borosilicate glass containers. BY measuring the Change in absorbance (ultraviolet) when potassium
tetrapheiiylboron is precipitated n-ith a n excess of sodium tetraphenylboron and the precipitate is filtered a m y , the author has been able to establish a method for the quantitatiw estimation of potassium ion.
(2) Gloss, G. H., Olson, B., Ibid., 43, 69 I954 !. (33 \Titfig, G , , A1ngew, Chem. 62A, 2;31 (1950). (4) K i t t i g , G., Kiecher, G., Ruckert, .1., Raff, P., Ann. 563, 110, 126 (1949). (5) Kittig, G., Raff, P., Ibid., 573, 191
r
(1951).
LITERATURE CITED
( I ) Gloss, G. H., Chemist-Snalyst 42,5C-5 (1953).
RECEIVED for review March 28, 1956. -kccepted August 24, 1956.
Separation and Determination of Radiocerium
by Liquid-Liquid Extraction GILBERT W. SMITH' and FLETCHER L. MOORE Analytical Chemistry Division, Oak Ridge National laboratory, Oak Ridge, Tenn.
Because of the need for a rapid and effective method for determining radiocerium in fission products, which would give good decontamination from transuranium elements, the liquid-liquid extraction of cerium(lV) with 2-thenoyltrifluoroacetone was studied. A method for both tracer and macro levels was developed for the separation of cerium from many other elements, based on extraction into 0.5M 2thenoyltrifluoroacetone-xylene from a 1 N sulfuric acid solution containing potassium dichromate and sodium bromate. Yields are approximately 80%, with an average deviation of about +2yo. The procedure offers a fast, safe, and simple method for radiocerium with or without carrier and for the purification of radioactive or inactive cerium.
D
radiocerium b y the method of Hume, Ballou, and Glendenin (1) is tedious, requires about 3 hours, and is not readily adaptable to remote control when high levels of radioactivity are present. The more recent hexone extraction method (3) is considerably faster and applicable to rrmote control. However, special precautions are required to avoid the hexone-nitric acid hazard, and thorium, uranium, and neptunium must be renioved prior to extraction of the cerium. -4liquid-liquid extraction method for the purification and/or determination of radiocerium may be used with or without cerium carrier. I n principle the cerium(1V) ion forms a stable chelate complex with 2-thenoyltrifluoroETERMINATION Of
Present address, Curtiss-Wright Corp., Research Division, Quehanna, Pa.
448
ANALYTICAL CHEMISTRY
(1). Dissolve 31 grams of ceriuni(II1) nitrate hexahydrate in distilled I\ :iter and dilute to 1 liter. T o standardize, pipet 5-ml, aliquots of the solution into 50-ml. centrifuge tubes. T o each add 1 ml. of 6N nitric acid and 15 ml. of lvater. Heat just to boiling and add 15 ml. of Cea+4 4HTx $ CeTdx 4Ha+ saturated oxalic acid rrith stirring. Cool in an ice bath for 10 minutes with occawhere HT is the enol form of TTA and sional stirring. CeT4 is the cerium(1V) chelate. SubWash a fine, sintered-glass crucible by scripts A and X refer to the aqueous passing through 5 ml. of distilled water, and xylene phases, respectively. three 5-ml. portions of 95% ethyl alcoThe mechanism of the reaction inhol, and three 5-ml. portions of anvolves hydrogen replacement and cohydrous ether. P u t the crucible in a ordinate bonding. Both the P-thenoylvacuum desiccator without desiccant and apply vacuum for 2 minutes. Flush trifluoroacetone and the cerium chelate out the ether vapors by releasing the have negligible solubility in the aqueous vacuum, then pump out again. Release, acid solutions but are soluble in xylene. evacuate for 2 minutes, flush as before, release, and then evacuate for 2 minutes. PROCEDURES Release the vacuum and weigh the crucible. Cerium-144 Tracer. Pipet a suitFilter the oxalate precipitate through able aliquot, free of fluoride, chloride, the crucible. Wash and dry the crucible and phosphate ions, into a n extraction and contents in exactly the same way as vessel (preferably a 125-ml. separatory the crucible was treated. Weigh t h e funnel) and adjust t h e solution t o precipitate as Ce2(C204)3.!OHzO. approximately IN in nitric acid, 0.1M in STANDARD METHOD.Pipet a suitable potassium dichromate, and 0.1M in sulaliquot of sample, free of fluoride, chlofuric acid (or to ]Ar in sulfuric acid and ride, and phosphate ions, into a 50-nil. 0.1M in potassium dichromate). Mix Lusteroid centrifuge tube and adjust the the reagents and allow to stand for 5 aqueous solution to a concentration of minutes at room temperature. Add an 1N in sulfuric acid, 0.1M in potassium equal volume of O.5M 2-thenoyltridichromate, and 0.2M in sodium broAuoroacetone-xylene and mix the phases mate, rvith 0.8 mg. per ml. of cerium carthoroughly for 10 minutes with a glassrier. After sn-irling to mix the reagents, paddle stirrer driven by a high-speed place the solutions in an ice bath for 5 motor. After the phases have separated, minutes. Add an equal volume of 0.5-11 draw off the aqueous phase and discard. 2-thenoyltrifluoroacetone-xyleneto the Wash the organic phase by mixing with aqueous solution and extract for 10 an equal volume of 0.1M sulfuric acidminutes with the paddle stirrer. Rinse 0.lM potassium dichromate for 3 minthe stirrer m-ith acetone after each exutes. Discard the aqueous phase and traction. then with distilled water. strip the organic phase by mixing with Centrifuge the solutions for 0.5 minute an equal volume of ION nitric acid for 1 in a clinical centrifuge and remove the minute. Discard the organic phase and aqueous phase with a transfer pipet or use an aliquot of the aqueous strip phase micropipet attached by rubber tubing for the radioactivity measurement. to a vacuum trap. B y using mild sucCerium-144 w i t h Carrier Solution. PREPARATIOS OF CARRIERSOLUTION tion and squeezing the tubing until the
acetone (TTd) in I S nitric acid. I n its simplest form, assuming no other ceriuni(1V) complexing in the aqueous phase, the over-all reaction may be written as:
+
+
pipet is at the bottom of the tuhe, the aqueous phase can be q-ithdran-n n-it11 a negligible loss of the organic phase. Wash t h e pipet after each use by dipping the tip into acetone and applying suction. Then wash down the sides of the centrifuge tube wit)h several milliliters of distilled water and centrifuge again for 0.5 minute. Again remove the aqueous layer by suction, taking care to remove a minimum of the organic phase. Carefully decant the organic phase into a clean Lusteroid centrifuge tube, add to it an equal volume of lAJrsulfuric acid, and mix the phases thoroughly for 3 minutes. Centrifuge for 0.5 minute and remove the aqueous wash solution by suction. =1ga,in wash the sides of the tube with several milliliters of distilled water and centrifuge for 0.5 minute. K i t h dran- the aqueous solution and carefully decant the organic phase into a clean Lusteroid tube as described aboi-e. Add an equal volume of 10.1: nitric acid, and, after thoroughly agitating the phases for 1 minute, centrifuge for 0.5 minute and dran- off the organic phase. Rinse the sides of the centrifuge tube with several milliliters of xylene, centrifuge for 0.5 minute, and remove the xylene by suct,ion. If desired, an aliquot of the aqueous layer may now lie decanted for counting, as in the above procedure. Add concenbrated ammonium hydroxide to precipitate cerium(111) hydroxide and centrifuge for 2 minutes. Discard the supernatant solution and wish the precipitate m l l n-ith 10 ml. of clistilled water, then centrifuge again for 2 minutes. Again decant the supernatant liquid and dissolve the cerium(II1) hydroxide in 1 ml. of 6N Itydrochloric acid. Dilute t o approximately 20 ml., heat the solution to boiling, and add 15 ml. of saturated oxalic acid. Then place the solution in an ice bath for 10 minutes. Filter t'he precipitate through a tared filter paper in a Hirsch funnel and wash as for the preparation of the cerium carrier solution. Keigh as Ce2(C204)3 .10HzO. Prepare for either beta or gamma counting, which is performed 3 hours after the final chemical separation to allow praseodjmium-144 to grow in to saturat'ion. EXPERIMENTAL
Variables in Tracer Method. To study the effect of time. t h e procedure given for tracer cerium was used, except t h a t the original aqueous phase \vas adjusted t o 1S nitric acid-0.1M potassiuni dichromate, a n d t h e extraction time n a s varied. T h e following results show t h a t substantial equilibrium m s reached in 10 minutes: Cerium-144 Extracted, LIinntes
73
2
82.1 96.8 98.2 97.9
7
10 30
It is suiprising that the dichromate ion will oxidize ceiium(II1) to ceiium(IV) to the extent shown above, because the standard oxidation potential (EO)of the cerous-ceric couple is higher (more negative b y the Latimer convention) than that of the dichromatechromic couple. Such a phenomenon has also been observed b y Pitzer (6). The oxidation may take place because the relatively small number of ceiium atoms involved n-ith cerium-144 tracer, together with their removal from the aqueous phase, may cause the reaction to go in the direction Ce(II1) Ce(1T'). Effectively, the ccious-ceiic couple potential is probably Ion ered relative t o the dichromate-chromic potential. K i t h earlier-free ceriuni-l4& tiacer, sodium bromate could not be used in the system as an ovidant because a very slight scum of uridetermined nature, containing >907, of tracer, usuall>- appeared a t the interface. I t was established that this tiacer cerium behavior vas due to the copiesence of sodium bromate and xylene. If it is not known whether earlier ceiium is present or not, a small amount of cerium carxier should be added (-0.5 mg. per ml.) and the macro method used. Experiments showed that variation in nitric acid acidity during dichromate oxidation had negligible effect in cerium tracer TT-ork. The procedure described above was used. and the acidity 11-as adjusted after the 5-minute ovidation period. The use of less than 1 S nitric acid is inadvisable, because of inadequate decontamination from other elements. --f
HXOI,
s
0 5 1 0
1 5
2 0
Cerium-144 Extracted, %c 98 98 96 92
5 0 4
5
Washing the organic phase containing the cerium-144 tracer with dilute nitric acid containing 0.1M potassium dichromate resulted in severe losses. A wash solution consisting of a n equal volume of 1S sulfuric acid resulted in 5 to 10% losses of cerium-144 tracer. n-hereas a wash solution of 1 5 sulfuric acid0.1M potassium dichromate gave losses of only 0.7% in 3 minutes. A wash solution consisting of a n equal volume of 0.1M potassium dichromate and 0.1M sulfuric acid gave losses of only 0.1%. The presence of a n oxidant in the n-ash phase was necessary to prevent excessive losses in carrier-free cerium tracer n-ork. If the initial aqueous solution was made 0.1M in sulfuric acid, the yield of cerium-144 tracer was increased from about 98 to 99%. The cerium-144 tracer was
readily stripped quantitatively from the organic phase into a n equal volume of 1O.Y nitric acid. Typical decontamination results ale shown in Table I. These values arc the averagr of duplicatcs.
Table 1. Decontamination from Various Elements in Cerium Tracer Method -1mount Found in
Added.
Strip Phase,
Element
C.P.11.
c-
Eu-152-154
3 . 1 X 10'
0.04
Ru-106
Zr-95-Sh-95
1-131
Pa-233 PU-230
2.8 X 5.9 X 6.2 X G G X 6.0 X
/C
10' 107 lo7 lo6 lo6
0.2
0.01* O.lh O.l' 0.5
10.1- nitric acid strip solution lvashed
for 5 minutes with an equal volume of 0.5JI TTA-xylene. Iodine removed by extraction into carbon tetrachloride after addition of 20 nig. of potassium iodide carrier.
Variables with Carrier Cerium. EFFECT O F REAGEXT COKCESTRATION, T I h r E , TEMPERATURE, hXD h I D I T t . K h e n more t h a n traces of inactive carrier cerium are present or when t h e solution may contain unknown impurities, i t is necessary to add carrier cerium (-1 mg. per nil.) and deteimine the extraction yield by oxalate precipitation ( 1 ) . As expected from the oxidation potentials involved, experiments showed that dichromate ion failed to oxidize cerium(II1) carrier appreciably. Table I1 shows the effect of varying the concentration of cerium carrier added, using the tracer procedure described above.
Table II. Effect o f Varying Concentration of Cerium Carrier on Extraction of Cerium- 144
Cerium Carrier Added, y
Cerium Extracted,
70
0
99 0
8
99 0 33.Y
80 800
8G
Sodium bromate was the most convenient reagent for the oxidation of macroquantities of cerium. As the presence of dilute sulfuric acid in the aqueous solution stabilized the ceric ion, it was decided to study the more important variables in a sulfuric acid system. I n the cerium distribution experiments each initial aqueous phase contained 8 x lo5 gamma counts per minute of cerium-144 in 12-ml. rolVOL. 29, NO. 3, M A R C H 1957
e
449
Table 111.
Effect of Variables on Extraction of Macroquantities of Cerium into 0.5M 2-Thenoyltrifluoroacetone-Xylene
Cerium Extracted, %
Aqueous Phase
Oxidation Temperature 5 Slin. at room temp. (24" C.)
HS03,
s
I
5 Min. in
ice bath
60.3 66.9 90.2
74.0 79.9
1 S H2SOa-O.lJI K2Ci-20,0 . 2 X XaBROa 1.V H B 0 ~ 0 . 2 ~ XaBrO? 2I
.M
JI ..
0.6 0.6 0.2 0.2
0.1 0.05 0.25
86.9 91.8 91.6 sa.9
94.3 74.4 53.2
Minutes 5 10 15 20
Cerium Extracted, c
63 94 95 97
0 6 5 3
Two 10-minute extractions are rec-
450
ANALYTICAL CHEMISTRY
ommended for approximately quantitative recoveiy of the cerium, but for radiochemical analytical purposes nhere a yield correction is applied, one extraction is adequate. Use of a double volume of the organic phase did not increase the yields appreciably. The follon.ing data indicate that sulfate stabilization of cerium(1V) is important to effect a good yield and that moderate concentrations of nitric acid can he tolerated.
1 0.8 0.4
1 1
Table IV. Effect of Cerium Carrier on Extraction of Cerium into 0.5M 2Thenoyltrifluoroacetone-Xylene
Cerium Carrier,
cerium Extracted,
0 0 0 1 2 4
70
47 2 86.7 82 2
A solution of cerium, oiiginally contained in nitric acid, should be adjusted to approximately 1\' sulfuric acid-0.5N (or less) nitric acid before performing the extraction. At cerium concentrations greater than approximately 1 mg. per ml. in the aqueous phase, the extraction behavior was someu-hat erratic, although yields were still adequate for a n analytical radiochemical method (Table IV). The decreasing extractability of cerium with increasing concentration a t constant acidity may be an indic a t'ion of the hydrolytic polymerization of ceiium(1V) to form nonextractable species. Such a phenomenon for zirconium ( 2 , 4, 5 ) has been observed in this type of system. I n cerium carrier work the aqueous phase after the initial extraction n-as usually slightly cloudy, and a small amount of precipitate settled out within a few hours. However, the
5 6 6 03
Less than 0.1% cerium washed out of the organic phase in 1-minute distilled water wash after standing overnight, indicating that the cerium(1V) chelate is rather stable in the 0.5Y 2-thenoyltrifluoroacetone-xylene phaae. However, the standard n.ash solution selected was 1.lr sulfuric acid, because it gave more efficient decontamination from other elements. The use of oxidants in the wash phase did not appear necessary under the conditions used in cerium carrier expeiinients.
Cerium E\tractetl,
Mg./Sll.
Aqueous Phase Acidity H2S04, HPU'O,, s N
0 0 0 u9 9 >99 9 >Y9 9
Even smallei volumes of 1 O S nitric acid stripping agent may be used. The standard stripping solution selected was an equal volume of IOS nitric acid with a 1-minute agitation period. A. nitric acid concentration of u-as
also efr”cc.tive. requiring only a few seconds longer than 1OS nitric acid. The main criterion in st’ripping the cerium carrier is decolorization of the organic phase. ilctually, a 1-minute agitation n-it11 :in equal volume of 1011’ nitric acid is :I several-fold excess. Comparahle concentrations of hydrocliloi~ic~ :\cid :iiitl sulfuric acid, or dilute hytlroflrioi~ic acid and vaiious reducing agents, niay be usrd to strip the cerium. Sitric acid \\-:is sclrctttd as the standard stripping solution because occasionally it may l w clcxsiralile t o re-extract the strip pliaw to give more effective decontamination from zirconium, protar ti n u m , a lit1 i roii.
Table V. Effect of Hydrochloric Acid Concentration on Extraction of Cerium(lV) Carrier Aqiieoiic Phase
H,SO,. .\1 1 1
HC1,
Cerium Extracted,
0 05 0 5 1
10 2 5 2 1.7
s
Table VI.
Element Eu(152-154) Nb-95 Pa-233 Zr-95 Ru-106 U-233 Th-232 Np-239 Pu-239 1-131
Sb-124 Fe-59
Decontamination of Carrier Cerium from Various Elements
Amount Added, Counts/Min.
x 107(?) x 106 (Y) 1 2 x 106 ( y ) 1 I, x 1 0 7 ( ~ ) 4 0 x lOj(?) 12
2 3
3 8 X 1O6(a) 1750 (mg.)
3 7 X l o 6 (y) 6 2 x 106 ( a ) 1 n x 107 (y)
6.6 4.6
x 106 (y) x 105 ( ? )
- Found in Strip Phase,
70
After one wash
0 0004 0 07
1 16
0 0 0 0 0 0 0 OF HYDROCHLORIC ACID. Though c*hlorideion is known t o greatly accelerate the reduction of cerium(IV), several experiments viere performed in the presence of varying amounts of hydrochloric acid to determine the degree of interference. Table V indicates that even 0.05S hydrochloric acid interferes severely with the oxidat’ion of cerium(II1). Hydrochloric acid (or chloride ion) should be removed or destroyed prior to the oxidation of cerium. One convenient method would be to precipitate cerous hydroxide \Then carrier is present, wash thoroughly, dissolve the precipitate in sulfuric acid, and then use the standard method. DECONTAMINATION FROM OTHER ELEMENTS
Elements found in fission product solutions were tested for degree of separation from cerium. Each pure element i w s studied individually in the standard procedure and the analyses were perfoimed in quadruplicate. The standaid procedure was used. Appropilate ieagents were added and mived hefoi e placing the solution in the ice bath. After completion of the pioceduic,. the strip phase was analyzed foi the appropriate element. I n several instances (Table VI) the strip phase \\as washed for 5 minutes with an equal volume of 0.5-I4 2-thenoyltrifluoroacetone-xylene to achieve increased decontamination. The alkalies and alkaline eaiths n-ere
not tested because they do not form chelates in 1 N acid with O.5M 2-thenoyltrifluoroacetone-xylene. The trivalent actinides, americium and curium, behare similarly to europium(II1) in this system. Decontamination appeared to be essentially the same n-hether cerium carrier (0.8 mg. per nil.) \vas present or absent. The decontaniination results may. therefore, be regarded a s conservative if an additional oxalate precipitation is performed in the radiochemical analytical method. A P PLI CAT10 NS
The method discussed offeis a rapid purification technique for radioactive or inactive cerium, and may occasionally prove useful for the elimination of cerium interference in other work. In carrier-free cerium tracer work, the technique may be used to estimate fission product cerium in an all-extraction method. For instance, radiocerium ma!be counted directly in an aliquot of the strip solution. This technique is often adequate in process control-type work. The carrier method offers a fast, safe, and simple technique for the determination of radiocerium. Yields average approximately 80% and the precision is within about 2%. The method requires about 1 hour. Although separatory funnels or other extraction vessels may be used, 50-ml. Lusteroid centrifuge tubes were used in this work. They are inexpensive enough to discard after use, and eliminate much washing of glassware as w-ell. During the course of this work, i t was observed that approximately 700 y of cerium per ml. of 0.5M 2-thenoyltrifluoroacetone-xylene gave a very
dark reddish-broivn color. Upon dilution i t n-as possible to detect a few tenths of a microgram of cerium visually. Instrumentally, the sensitivity could be extended considerably. The color was stable for several weeks.
ACKNOWLEDGMENT
The authors gratefully acknowledge the able assistance of George R. Wilson in performing some of the thorium analyses and of Willie Bruce in counting various solutions.
LITERATURE CITED
(1) Boldridgp, IT. F., Hume, D. K., “Radiochemical Studies. Fission Products,” C. D. Coryell, N.Sugarman, eds., Kational Siiclear Energy Series, Div. IT’, vol. 9, p. 1693, NcGraw-Hill, Kew Tork, 1951. (2) Connick, R. E , hIcVey, W. H., J , Ani. C h e m SOC.71, 3182-91 (1949). (3) Glendenin, I,. E., Flynn, K. F., Buchanan, R. F., Steinberg, E. P., h A L . CHEM. 27, 59-60 (19%). (4) Huffman, E. H., Iddings, G. hI., OSborne, R. S . , Shalimaff, G. V., J . Bni. Chenz. SOC.77, 881 (1955). (5) Moore, F. L., .\SAL. CHEM.28, 9971001 (1956). (6) Pitzer, E. C., U.S. Patent 2,615,798 (Oct. 28, 1952). RECEIVED for revie-. August 8, 1956. Accepted November 1, 1956. VOL. 29, NO. 3, MARCH 1957
451
