Solvent Extraction Method for the Radiochemical Determination of Chromium WILLIAM J. MAECK, MAXINE E. KUSSY, and JAMES E. REIN Atomic Energy Division, Philhps Petroleum Co., ldaho Falls, ldaho
b
analysis. The desired procedure should be free of critical manipulations, and be applicable either to reactor cooling mater samples or to high level fission product samples. Based on these criteria a procedure was developed involving a preliminary hydrolytic separation of chromium (111), osidation to chromium(VI), and extraction from hydrochloric acid of the tetrabutylammonium chromium(VI) ion-association complex into methyl isobutyl ketone. Chromate is stripped from the organic phase with base, facilitating a direct spectrophotometric yield determination.
A procedure for the separation of radioactive chromium from fission and stainless steel corrosion products i s based on extraction of the chromium (VI) tetrabutylammonium ion-association complex into methyl isobutyl ketone. The yield is determined spectrophotometrically and the counting is done in a well-type Nal(T1) scintillation crystal. Precision based on replicate analysis of a chromium-51 solution i s 1 % coefficient relative standard deviation.
S
steel is commonly used in the construction of nuclear reactors as vie11 as for the cladding of reactor fuel elements. Because corrosion is a serious problem in reactor operation, reactor coolant waters are routinely analyzed for various corrosion products. A commonly encountered neutron-induced stainless steel corrosion product activity is 27-day chromium-51. For ease of manipulation and adaptation to routine operation, solvent extraction procedures are preferred in this laboratory. Pijck (4) and Morrison and Freiser (3) have described many solvent extraction procedures for chromium, the majority of which are applicable to nonradioactive materials. A disadvantage of several of the extractants described is the necessity for critical p H control, an undesirable feature in routine TAIKLESS
EXPERIMENTAL
Apparatus and Reagents. X 3 X 3 inch SaI(T1) well-type scintillation crystal coupled t o a multichannel pulse height analyzer was used for all counting. Analytical grade reagents were used throughout without purification. T h e tetrabutylammonium hydroxide was obtained from Southwestern Analytical Chemicals, Austin, Tex. Fission product mixtures Jyere prepared by irradiating highly enriched uranium in the Materials Testing Reactor. Chromium-51 tracer also was prepared in the -?daterials Testing Reactor. The chromium carrier solution was prepared by dissolving 14.135 grams of
100 Z
/
4! I-
3 80 U + X W
I-
60
z w 0
/
I
P
40
a 20
n b
I
0.01
1
/ I 1 1 1 1 1 1
I
I
I l I I t I I
0.1 1.0 MOLE RATIO, TBUT/Cr(IZI)
I
I i l l l l l l
10.0
'
" ' 1 "
100
Figure 1. Extraction o f chromium from 1M HCI as a function of the mole ratio of tetrabutylammonium ion (TBUT to chromium(V1)
1602
ANALYTICAL CHEMISTRY
potassium dichromate in w t e r and diluting to 1 liter (equivalent to 5 mg. of chromium per ml.). A calibration for the yield determination was prepared by removing a series of known aliquots of the carrier solution, diluting to volume with 231 sodium hydroxide-331 ammonium chloride, and measuring the absorbance a t 370 mp, as described in the procedure. Procedure. Pipet a sample aliquot into a suitable vessel containing 5 mg. of chromium carrier; acidify with nitric acid, and add 0.25 nil. of 1 M sodium bisulfite. Add 2 drops of phenolphthalein, followed by ammonium hydroxide t o just before the transition point. (Excess base can cause dissolution of chromium hydroxide.) Heat to coagulate the precipitate, centrifuge, and decant t h e supernatant liquid. Wash t'he precipitate with water, centrifuge, and again decant. Dissolve the precipitate in 1 ml. of concentrated perchloric acid and 0.5 ml. of concentrated sulfuric acid. Fume until the deep red dichromate color just starts to form; do not heat t o dryness, because chromium(V1) is reduced. Cool, add 15 ml. of 1-11 hydrochloric acid, and transfer to a 125-m!. separa-. tory funnel. Add 5 mi. of 0.5~11 tetra.butylammonium hydroxide and 25 ml. of methyl isobutyl ket'one, and extract for 2 minutes. Discard t h e aqueous phase. Scrub the organic phase for 3 minutes with 15 nil. of 1Jf hydrochloric acid, again discarding t h e aqueous phase. Add 20 mi. of 2.11 sodium hydroxide-3-11 ammonium chloride and agitate until the organic phase is colorless. Drain the aqueous phase into a 50-ml. centrifuge tube, add 10 mg. of iron as iron scavenger solution, heat, and centrifuge. Filter into a 25ml. volumetric flask and dilute t o volume with the stripping reagent. Transfer a 0.250-ml. aliquot' int,o a 10nil. volumetric flask and dilute to volume with the stripping reagent. Transfer to a 1-em. cell and measure the absorbance a t 370 mp. Rinse the remaining portion of the sample in the 25-ni1. volumetric flask into a suitable container for counting in a n.ell-type scintillation crystal. Gamma-scan and determine the mea under the 3'30-k.e.1;. photo peak. RESULTS A N D DISCUSSION
The multivalent properties of chromium and the amphoterism of chroniium(V1) provide convenient means.
1
1
IO /'
z 70
0
3-DAY COOLED FISSION PRODUCTS,
a
: 60 a I-
2 104
I-
1I-:50 z
8 40 a W LL
30 20
lo
o
1 1 NaOH
w
0
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W IL:
tl
,SEPARATED Cr FRACTION
"
~
HCI
-
I-
2
3 I
MOLAR1TY Figure 2.
IO
Extraction of chromium as a function of acidity
M o l e r a t i o of tetrabutylammonium ion t o chromium(VI). equol organic and aqueous volumes
for separating chromium from fission and stainless steel corrosion products. The initial reduction and hydrolytic precipitation of chroniium(II1) provide exchange and concentrate the chromium from water samples. They also give decontamination from nonhydrolyzable ions. Oxidation of chromium(II1) t o chromium(V1) with divalent silver prior to extraction was investigated. The oxidation was rapid; however, because of the formation of silver chromate and silver chloride in subsequent steps, the reagent \vas unsatisfactory Sesivalent chromium is conveniently obtained by fuming chromium(II1) with a mixture of sulfuric and perchloric acids. d f t e r the initial formation of the dichromate color, fuming should be stopped. Chromium(V1) is reduced if the qample is fumed to dryness. The extraction is rapid (Table I) and is belie1 ed to proceed through the formation of a monovalent anionquaternary cation [HCr,O,-, R J + ] ion-association complev in which the anion may be H C r 0 4 - or the dimer HCi-20,- id, 6 ) . A niole ratio of 1 to 1 gives 9594 extraction (Figure 1). A mole ratio of 20 is used in the procedure to override the high perchlorate concentration (perchlolate forms a stable tetrabutylammonium complex). Although chromium(V1) is evtracted directly into methyl isobutyl ketone from 1Jf or greater hydrochloric acid ( I ) , the quaternary ammonium salt is added to ensure extraction over a wide acidity range (Figure 2).
CHANNEL NO.(-IOKE(Elu!/ CHANNEL)
20 t o 1
Figure 3. Decontamination fission product mixture
from
3-day
cooled
No chromium-51 present
fl IO
I
I
20
SEPARATED C i FRACTION
\
I
!
,
j
40 60 80 100 120 CHANNEL NO. (10.5 K,EY/CHANNEL)
Figure 4. Decontamination from 3-month cooled fission product mixture containing spike o f chromium-5 1 VOL. 34, NO. 12, NOVEMBER 1962
1603
Table I.
Extraction of Chromium as a Function of Time”
Extraction, % 89.2 99.4 99.9 60 120 99.7 240 99.5 a Under conditions ae outlined in Procedure. Time, Seconds 15 30
The divalent CrO,-2 ion does not form an ion-association complex with the quaternary ammonium ion. Thus contact with caustic completely strips the chromium from the organic phase. Chromium could not be completely removed from methyl isobutyl ketone with the strip solution in the absence of the quaternary ammonium ion. This may be due to some type of solvation mechanism. Scavenging the strip phase with ferric hydroxide separates hydrolyzable ions that are coextracted with the chromium. Oxidization of the Cr(II1) in basic
solution appeared worthwhile and was investigated. A sample of aged fission products was treated according t o the first steps in the recommended procedure, followed by dissolution of the Cr(OH)a Mith 1 5 M NaOH. Sodium hypochlorite was added and after the oxidation was completed, an iron scavenging was performed. After filtering and acidifying, the Cr(Y1) n a s extracted according to the procedure. A ?-ray spectrum of the strip phase showed copious quantities of rutheniuni activity as well as some zirconiuniniobium activity in the final product. Apparently much of the ruthenium x i s in an extractable form and follon ed t!ie chrorniurn. Hence, in the recommended procedure, ruthenium is removed in the perchloric acid oxidation step. The yield is conveniently determined on a n aliquot of the strip phase by a spectrophotometric determination of chromate a t 370 mp. Decontamination by this procedure from 3-day and 3-month cooled fission product mixtures is shown in Figures 3 and 4, respectively. With irradiated stainless steel 300-type alloys, no foreign
activity was found in the final chromium fraction. Six aliquots of chromium-51 activity were carried through the recommended procedure and the area under the 320k.e.17. photopeak m-as measured. The relative standard deviation was less than 1%. The average yield is 85% and the time of analysis is approvimately 1 hour. llTERATURE CITED
(1) Dean, J. A., Beverlj-, lf. L., Ax41,. CHEY.30, 977 (1958). 1 2 ) Maeck, W.J., Booman, G. L., Kussv, 11. E., Rein, J. E., Ibid., 33, li7,5 i 1961).
G. H., Freiser, Henry, “Solvent Extraction in -4nalytical Chemistry,” p. 201, Wiley, New York, 1957. i 4) Pijck, J., U. S. 9 t . Energy Comm. Rept. NAS-NS-3007 (1960). ( 5 ) Tong, J. Y., King, E. L., J . .4m.Chem. SOC.75, 6180 (1953).
( 3 ) -Morrison,
RECEIVEDfor review May 2 5 , 1962. Accepted August 20, 1962. K o r k done under ‘Contract AT(10-1)-205 t o Idaho Operations Office, U S Atomic Energy Commission.
Plutonium Sulfate Tetrahydrate, CI Proposed Primary Analytical Standard for Plutonium CHARLES E. PlETRl
U. S.
Afomic Energy Commission, New Brunswick, N. J ,
b Stoichiometric plutonium sulfate tetrahydrate has been prepared b y controlled crystallization from sulfuric acid solution. The formula of the tetrahydrate was found b y chemical and thermogravimetric analysis to b e Pu(S0&.4Hz0. The compound i s stable, showing no significant change in plutonium content or weight for at least 28 months. Changes in relative humidity below 75y0 had little effect on the material. The plutonium sulfate tetrahydrate prepared meets many of the requirements for a primary standard.
P
LUTONIUJI SULFATE TETRAHYDRATE,
P u ( S O ~ )4H20, ~ is proposed as a primary analytical standard for plutonium. Since the discovery of plutonium in 1940, many compounds have been investigated and some have been used as interim standards. For example, high purity plutoniuni metal (6) (99.95+y0 pure) is in general use although it oxidizes readily in air, is difficult to cut if in the massive form, and hard to clean if in the form of turnings. It must be used immediately
1604
ANALYTICAL CHEMISTRY
after removing the oxide layer or stored in sealed ampoules in an inert atmosphere to retard oxidation. Plutonium dioxide (4) can be prepared in a highly purified form, but its stoichiometry depends on the temperature of ignition and starting material. The more stoichiometric, high-fired, nonhygroscopic oxide is inert chemically : it is decomposed with difficulty with hot nitric acid-hydrofluoric acid mi.;tures, or by sodium bisulfate fusion, but more readily by ammonium bifluoride fusion and by the sealed quartz ampoule technique. Other plutonium compounds such as plutonium nitrate pentahydrate ( S ) , hydrated plutonium trifluoride ( 7 ) , plutonium peroxide (91, plutonium hydride (Z), plutonium trichloride ( I O ) , plutonium tetraiodate (S). and many organic types (6) h a r e not proved satisfactory. The plutonium sulfate tetrahydrate prepared in the present investig izt 1011 ’ meets many of the requirements for a primary standard. The compound ib stoichiometric in composition. It can be prepared in a high state of purity with relative ease and can be analyzetl
without difficulty. It is stable in air during weighings, maintains its composition in storage for periods of at least 28 months, and is relatively unaffected by relative humidities from 17 to 75%. The material has a high equivalent weight, readily dissolves in acids ( m 90 grams Pu per liter in 1M H3S04) ( I ) , and, because of its relatively large crystals, can be handled with ease under gloved box conditions. EXPERIMENTAL
High - purity plutonium metal (99.9676) obtained as pins from Hanford Atomic Products Operation, Richland, Kash., was used to prepare plutonium sulfate tetrahydrate. The atomic weight of the metai as reported by the supplier vias 239.11. Sitric and sulfuric acids were distilled from reagent-grade material and diluted with distilled water wherever necessary. Initial tests using methanol to crystallize plutonium sulfate tetrahydrate from sulfuric acid solution (I) gave low vields, produced large volumes of supernatant, and required long crystallization times. Accordingly, crystallization by controlled evaporation of
