- Email: [email protected]
Extractive separation and photometric determination of neptunium(IV) and plutonium(IV) from their mixtures
MICROCHEMICAL
JOURNAL
29, 243-252 (1984)
Extractive Separation and Photometric Determination of Neptunium(W) and Plutonium(W) from Their Mixtures J. P. SHUKLA
AND
M.
S. SUBRAMANIAN
Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085 Received November
28, 1979
INTRODUCTION
Radiometric techniques are routinely used for identification and estimation of neptunium as well as plutonium besides numerous chemical methods. Among these, the absorptiometric determination of these elements applicable down to ppm levels offers certain distinct advantages over the counting techniques as (i) it is devoid of dilution errors, and (ii) it is independent of the isotopic composition of the element which in turn changes from batch to batch depending upon the fuel burn-up. Furthermore, the estimation of Np and/or Pu by o-radioassay cannot be accomplished accurately in the presence of high salt concentrations or in the presence of each other as well as other a-emitters such as Am, etc. which calls for involved preliminary separations. Only few methods exist for the calorimetric determination of Np (2, 4, 5, 14) or Pu (7, 10, 17, 18); most of them lack the desired selectivity as well as specificity. Among the photometric methods that have been reported so far for their estimations, arsenazo III method surpasses all in sensitivity. Recently, its application has been extended for the individual assay of Np and Pu in process solutions. However, not only is this procedure lacking specificity for these actinides but fails in presence of oxidizing/reducing agents as well as several common contaminants such as HP+, Pd*+ , Th4+, U02*+, F- , NO 7, NO 5, etc. (7). Moreover, the estimation is possible only in an aqueous medium thereby necessitating the back extraction from the organic extracts. The selectivity and sensitivity of the estimation can be substantially enhanced by combining spectrophotometry with preliminary separation of the element and is best carried out directly in the organic phase without back extraction. The aim of the present work was to develop a rapid procedure to enable satisfactory separation of Np and Pu from commonly associated impurities by 2-thenoyltrifluoroacetone (TTA) in xylene as the extractant followed by their direct determination in the organic phase by spectrophotometry employing xylenol orange (X0) as the chromogenic reagent. Using 243 0026-265X/84 $ I .50 Copyright 0 1984 by Academic Press, Inc. All rights of reproduction in any form reserved.
244
SHUKLA
AND
SUBRAMANIAN
this technique, it is possible to assay down to 0.4 ppm of Np and/or Pu in presence of several interfering ions. Another advantage of the method is the simultaneous determination of Np and Pu in about 30 min when present together. EXPERIMENTAL
DETAILS
237Np,obtained from ORNL, was freed from its daughter nuclide 233Pa by the TTA extraction method (8). The stock solution of Np(IV) in 1 M HN03 was then prepared by reducing Np to Np(IV) employing 0.01 M ferrous sulfamate and 0.01 M hydroxylamine hydrochloride which also served as the holding reductant to keep Np in the tetravalent state. The tetravalency of Np was frequently checked by the ITA extraction method. Plutonium, which was mainly 239Pu,was purified by anion exchange from HN03 medium. The radiochemical purity of these tracers was established by alpha spectrometry using a silicon surface barrier detector. Laboratory reagent grade TTA supplied by E. Merck, Germany, was used without any purification. The xylene solution of TTA was preequilibrated with dilute perchloric acid for several hours to allow hydration of TTA (6). Ferrous sulfamate was prepared as described by Buchanan er al. (3) using LR chemicals. Gross Np and Pu content were determined by alpha radioassay of these nuclides using an cx-proportional counter and liquid scintillation counting, respectively. Total Np content was also ascertained by using an alphaliquid scintillation counting assembly in conjunction with a 4096 channel analyzer. The liquid scintillator solution was prepared by dissolving 0.7 g of 2,5diphenyloxazole, 0.03 g of 1,4-bis-2(5-phenyl oxazolyl) benzene, and 10 g of naphthalene in 100 ml of distilled dioxane. Solution of xylenol orange (0.2%, w/v) [C3iHs2N20t3S] procured from B.D.H. was prepared in methanol and used for color development. All other reagents were BDH/E. Merck “AR” grade, unless otherwise specified. Absorbance measurements were made on a Cary-14 recording spectrophotometer adapted for glove box operations with radioactive materials. Matched quartz cells of 1.0 cm (or 5.0 cm) path length were invariably used with a scanning speed of 10 A per second. Procedure. Two milliliters of 1 M HNOs containing microgram amounts of Np and Pu were transferred to a glass equilibration tube. The solution was adjusted to -0.01 M each in ferrous sulfamate and hydroxylamine hydrochloride and kept aside for a few minutes. Two milliliters of 0.2 M TTA in xylene was added and Np(IV) was extracted into the organic phase. All extractions were performed in duplicate at room temperature by mechanical shaking for 10 min of equal volumes of organic and aqueous phases. After settling, the combined TTA phase was pipetted into another tube containing 10 ml of 1 M HN03 containing -0.01 M ferrous sulfamate
NEPTUNIUM
AND
PLUTONIUM
245
and hydroxylamine hydrochloride and the organic phase was scrubbed twice for 5 min to remove any Pu(IV) extracted along with Np(IV). After centrifuging, OS-ml portion of the TTA extract was pipetted to a lo-ml flask. To this was added 2 ml absolute alcohol, 0.5 ml glacial acetic acid, 2 ml X0 solution, and finally made up the volume with absolute alcohol. The absorbance at 535 nm was measured against a similar Np-free blank containing 0.5 ml of the extractant. The amount of Np extracted into the organic phase was computed from the calibration curve. For determining Pu, solid NaNO* (-0.03 M) was added to oxidize the unextracted plutonium to the tetravalent state. The solution was slightly warmed and after about 15 min plutonium(IV) was extracted twice into 2 ml of 0.2 M TTA. After settling, the organic layer was removed into another tube having 5 ml of 1 M HNOs containing -0.03 M NaN02 and was scrubbed twice for 5 min to remove small quantities of other contaminants extracted alongwith. After centrifuging, the organic layer was separated and 0.5-ml portion pipetted into a lo-ml flask. The color was developed as described earlier for Np estimation. In cases when pure Np or Pu alone is present, scrubbing of the organic extracts can be avoided to save time. RESULTS AND DISCUSSION Spectral Properties
The visible absorption spectrum of a solution of intense red colored Np(IV) or Pu(IV) complex extracted from 1 M HNO, with TTA in xylene gave an absorption maximum at around 535-540 nm measured against a reagent blank. Typical spectra for Pu(IV) complex including that of X0 against alcohol as blank are shown in Fig. 1. The reagent had an absorption maximum at around 435 nm and a continuous decrease thereafter. As neither of these actinide ions nor X0 showed any significant absorption at 535-540 nm, this wavelength was chosen for further analytical study. The calibration curve for determining Np(IV) is shown in Fig. 2 which shows that Beer’s law is valid up to about 2.4 ppm Np. Similar curve for Pu(IV) is also given in Fig. 2 with a validity of Beer’s law up to 3.5 ppm Pu. Values of molar absorptivity, computed from these calibration curves on the basis of Np or Pu content as well as the optimum concentration range for their estimations, evaluated by Ringbom’s method (1) are presented in Table 1. The sensitivity index of the method, as defined by Sandell (Z2), is 0.004 kg Np/cm* at 535 nm and 0.0045 t.r,gPu/ cm* at 540 nm for log ZdZ, = 0.001 and thus practical sensitivity based on an absorbance of 0.01 will be 10 times more than the respective values. Extraction
with TTA
The extractability of Np(IV) and Pu(IV) with TTA when present alone was examined at different HNOs concentrations ranging from 0.5-8 M
246
SHUKLA
AND SUBRAMANIAN
0.6
0.6
W ki d E
0.4
_ /I
ii u
/
(A) /- \ \ \ \ \ \ \ \
0.2
0.0 400
450
550 500 WAVELENGTH
600 ,nm
650
700
FIG. 1. Absorption spectra of xylenol orange (curve A) and its complexwithplutonium(rv) (curve B).
HN03. From the data obtained it was seen that the distribution ratios CD), for NpQV) and Pu(IV) passed through a maximum at around 1 A4 HN03 aq. medium (Table 2). Subsequent extraction studies were, therefore, performed at this acidity. The extraction of these actinides from -1 M HN03 using varying concentrations of TTA in xylene was also studied. Variation of TTA concentration from 0.2 to 0.5 M had little effect on their extractions. Although a single equilibration yielded more than 98% of these actinides from dilute nitric acid solutions using 0.2 M TTA, two successive extractions were invariably carried for quantitative recovery. Effect of Acetic Acid and Xylenol
Orange Concentration
Experiments undertaken to assess the effect of glacial acetic acid and X0 concentration indicated that 0.2-l ml of the former and 1.5-2.5 ml of 0.2% X0 per 10 ml of the final volume were adequate for complete color
NEPTUNIUM
,” z a
0.6-
i
0.4-
247
AND PLUTONIUM
,” 2 0.2
-
0.01 0.5
I.0
I.5
2.0
2.5
3.0
3.5
4.0
4.5
Pu(lV).yg/ml I
1
I
I
1
I
I
I
I
1
0
0.4
0.8
I.2
1.6
2.0
2.4
2.8
3.2
3.6
Np(lV),Jg/ml
FIG. 2. Calibration curve for Np(IV) and Pu(IV) (Beer’s law plot).
development. Larger amounts of both these reagents, however, tend to decrease the absorbance to some extent. In practice, therefore, 0.5 ml of glacial acetic acid and 2 ml of 0.2% X0 were always employed for absorptiometric determinations, Effect of HF Hydrofluoric acid-nitric acid mixtures are frequently employed for the dissolution of plutonium oxide. F- ions are consequently encountered in TABLE 1 SPECTRALDETAILSOFTHE COLOREDNp(IV) Spectral parameters Xmax, nm Range for adherence of Beer’s law, ppm Molar absorptivity, mol-i cm-i Optimal concentration range, ppm Sandell’s sensitivity index, pg/ml Coefficient of variation, %
AND
Neptunium(IV)
Pu(IV)
EXTRACTS
Plutonium(IV)
535
540
0.4-2.4
0.4-3.5
59103 f 304
52857 2 373
0.5-2.1
0.5-3.2
0.004 2.12”
0.0045 1.9gb
B For 2.4 pg Np ml-i (No. of determinations = 8). b For 2.6 pg Pu ml-’ (No. of determinations = 6).
248
SHUKLA
AND
SUBRAMANIAN
TABLE EFFECT OF NITRIC
ACID
2
CONCENTRATION ON EXTRACTION WITH TTA
Distribution [HWI M 0.5 1.0 1.5 2.0 4.0 8.0
205 587 141 36 0.32
0.5 M TTA 541 >5000 322 62 1.1 co.01
AND Pu(IV)
ratio, D
Neptunium(IV) 0.2 M TTA
OF Np(IV)
Plutonium(IV) 0.2 M TTA 142 373 281 26 0.72
0.5 M TTA 1157 >5000 351 57 0.86 co.01
the spent-fuel reprocessing solutions causing an incomplete extraction of Pu or Np into TTA. To prevent this interference, fluoride ions were masked with aluminum nitrate. Ft ions up to 2 kg/ml could be tolerated without using the masking agent. Higher amounts of the order of 1 mg/ml of NaF could, however, be tolerated by masking with about 10 mg Al(NO&. Effect of Process Contaminants Uranium and zirconium are the most common metallic contaminants of Np and Pu during their radiochemical processing besides other longlived fission products and cladding materials. According to Saloman et al. (II), an aliquot of the solution, resulting from dissolving uranium fuel with 16,000 MWDIT burn up, containing few milligrams of U (up to 10 mg) will be associated with a few micrograms of Np and a few micrograms of Zr (up to 25 kg) whereas quantities of Pu are much larger relative of 237Np. Moore and Hudgens (9) have demonstrated that several impurities can be separated from plutonium by the TTA extraction method. Advantageously, this serves here as a first step toward preliminary separation. It has been reported that zirconium can be eliminated during separation or alternatively be masked (2, 13) by using a strong complexing agent like EDTA or H3P04. About 10 kg/ml of Zr did not cause any interference in the final determination of Np or Pu. Here both EDTA and H3P04 were found to be unsuitable due to their adverse effects on analysis. However, no interference was observed in the final assay of a few micrograms of Np in presence of about 100 mg U, 300 kg Pu, and a few micrograms of Zr which are the usual concentrations of these elements expected to be associated with neptunium solutions (11). Also, the separation procedure was tried in the absence of Np with aforesaid amounts of these interferents individually and with a mixture of all three. The absorbance of the scrubbed organic extracts, was found to be negligible (CO.01) thereby
NEPTUNIUM
249
AND PLUTONIUM
TABLE 3 DETERMINATIONOF NEPTUNIUM(W) IN THE PRESENCEOF DIVERSE IONS [Np(IV) CONCENTRATION: 2.4 ppm, AQUEOUS ACIDITY: 1 M HNO,]
Ion Blank Cation A13+ Ce4+ co*+ WcLI2+ Fez+ Fe3+ La3+ Mn2+ Ni2+ Pu4+ Sn2+ Th4+ uo2+ Zn2+ Zr4+ Anion ClFNO5 NO? PO]SO;Acetate Oxalate
Added as
Al(N03)3 * 9H20 Ce(S04)2 . 4H20 CO(NO~)~. 6H20 CrK(SO4)2 . 12HzO (CH3C00)2 * Cu * Hz0 (NH4)2S04. FeS04 * 6Hz0 Fe(N03)3 * 9H20 LaW03h MnS04 * Hz0 NiS04 * 7Hz0 PW03)4
SnC12. 2H20 Th(N03)4 * 4Hz0 UOz(NO3)* ’ 6H*O ZnS04. 7Hz0 ZrOC12. 8H2O KC1 NaF NaN02 H3P04
Na2S04 CH3COONa NaG04
Amount added km) Nil 30 0.5 30 1
1; (0.05)~ (O.OS)C Id 5 5
20 (0.3)C 30 (0.4)‘ 100 45 (0.01)’ 30 (0.02)C I’ 35 35 2 35 5 (0.04)C
Absorbance“
Maximum absorbance” M-d
0.61b
535
0.61 Interference 0.62 0.60 0.62 0.61 0.62 0.60 0.61 0.60 0.60 0.61 0.62 0.61 0.62 0.60 0.62
535 535 535 535 535 535 535 535 535 535 535 535 535 535 535 535
0.61 0.60 0.61 0.62 0.61 Interference 0.62 0.60 0.60
535 535 535 535 535 535 535 535
u Average of 3 determinations of the same sample. b The average absorbance of five neptunium(W) samples, each containing a final Np concentration of 2.4 ppm was 0.61 with a standard deviation of 0.007. c Figures in parentheses are the maximum amounts tolerated without any interference. d 10 mg citric acid added before TTA extraction. c 10 mg AI(N03)3 added before TTA extraction.
indicating that U, Pu, and Zr up to amounts specified do not interfere in the estimation of Np. Effect of Other Diverse Ions To examine the effect of some other diverse ions which often accompany Np as well as Pu, a systematic investigation of their interferences
250
SHUKLA
AND SUBRAMANIAN TABLE 4
ANALYSIS
Taken, pg Np4+ + pu4+ 0.85 1.39 2.06 2.41
+ + + +
1.00 1.66 2.60 3.01
OF
Np
AND Pu FROM THEIR MIXTURES
Pound,n p,g Np4+ + Pu4+ 0.87 1.42 2.03 2.36
+ + + +
1.02 1.63 2.55 3.07
Error, % NP
Pu
2.35 2.16 1.46 2.07
2.00 1.81 1.92 1.99
a Average of 3 determinations of the same sample.
was attempted. Since interferents are common, only Np(IV) was extracted in the presence of each impurity ion followed by its photometric assay with X0 as per recommended procedure. Table 3 shows the absorbance values of 2.4 ppm of Np(IV) in presence of various external ions. Also, the effect of these interferents on absorption maximum (A max) was studied. An increase or decrease of ~0.02 unit in absorbance was considered to be an interference. Cations such as A13+, Cr3+, Co2+, Cu2+, La3+, Mn2+, Ni2+, Sn2+, and Zn2+ as well as common anions, viz., Cl-, NO;, NOy, sulfate, and acetate did not interfere with the final determination even at very high concentrations. Fe2+, Fe3‘, Th4+, Zfl+, and oxalate could be tolerated in moderate amounts. Higher amounts of Fe3+ caused no interference in the presence of 5-10 mg/ml citric acid which served as the masking agent. Strong interferences in the absorption measurements were obtained only in the presence of Ce4+ and phosphoric acid. Extraction
Time and Stability
of Color
It was observed that 95% extraction of Np or Pu from the aqueous phase occurred after shaking for 3 mitt, whereas beyond 5 min, essentially quantitative extraction of microgram amounts of these actinides was accomplished. All extractions were carried out for 10 min. The stability of the colored extracts was tested by measuring their absorbance at different time intervals. The absorbance was found to remain constant for at least 48 hr. The color formation was instantaneous, but the colored extracts were kept for about 10 min to attain equilibrium. Accuracy
and Precision
Table 4 summarizes some results obtained by applying this method to the analysis of varying amounts of Np(IV) and Pu(IV) contained in a mixture. It is possible to extract quantitatively and assay spectrophotometrically neptunium and plutonium in 30 min. If a 5-cm cell is used, as little as 0.4 ppm of both Np(IV) and Pu(IV) can be estimated with a
NEPTUNIUM
AND PLUTONIUM
251
precision better than 2% and a standard deviation of 0.007 and 0.006 ppm (n = 6), respectively. ACKNOWLEDGMENT The authors express their gratitude to Dr. M. V. Ramaniah, Director, Radiological Group, for his keen interest in this work.
SUMMARY A rapid and sensitive method for the photometric determination of trace amounts of neptunium and plutonium from their mixtures is described. Np(IV) is selectively extracted from about 1 M HNOs medium with TTA in xylene retaining Pu in the nonextractable trivalent state in the aq. phase with ferrous sulfamate. Plutonium in the aqueous phase is subsequently oxidized with NaNOz to the highly extractable tetravalent state and extracted with TTA. Np(IV) as well as Pu(IV) thus extracted are finally estimated in the organic phase itself spectrophotometrically employing xylenol orange as the chromogenic reagent. Their molar absorptivities are in the 5 x lo4 range. Beer’s law is valid up to 2.4 ppm Np and 3.5 ppm Pu. The color of the solutions is stable for at least 48 hr. The method tolerates large excess of several common contaminants encountered during spent fuel reprocessing. Cerium(IV) and phosphoric acid, however, interfere with the final estimation.
REFERENCES 1. Ayres, G. H., Evaluation of Accuracy in Photometric Analysis. Anal. Chem. 21, 652-
657 (1949). 2. Bryan, R. C., and Waterbury, G. R., Spectrophotometric Determination of Neptunium, LA-4061, pp. l-8 (1968). 3. Buchanan, R. F., Hughes, J. P., Hines, J. J., and Bloomquist, C. A. A., The determination of nitrogen, Americium, Neptunium and Uranium in P.P.M. quantities in pure plutonium. Talanta 6, 173-177 (1960). 4. Budesinsky, B., In “Chelates in Analytical Chemistry” (M. A. Flaschka, and A. J. Barnard, eds.), Vol. 1, pp. 15-47. Dekker, New York, 1972. 5. Bumey, G. A., Dukes, E. K., and Groh, H. J., “Analytical Chemistry of Neptunium,” Chap. 4, Progress in Nuclear Energy Series, IX, Vol. 6, p. 181. 1966. 6. Day, R. A., Jr., and Stoughton, R. W., Chemistry of Thorium in Aqueous Solutions, I. Some organic and inorganic complexes. J. Am. Chem. Sot. 72, 5662-5666 (1950). 7. Milyukova, M. S., Gusav, N. I., Santyurin, I. G., and Sklyarenko, I. S., “Analytical Chemistry of Plutonium.” Israel Program for Scientific Trans. Jerusalem, p. 130. 1967. 8. Moore, F. L., Separation and Determination of Neptunium by Liquid-Liquid Extraction. Anal. Chem. 29, 941-944 (1957). 9. Moore, F. L., and Hudgens, J. E., Jr., Separation and Determination of Plutonium by Liquid-Liquid Extraction. Anal. Chem. 29, 1767-1770 (1957). 10. Peter, J., Fehlman, D., and Aerne, E., A Spectrophotometric method for the determination of plutonium in fuel solutions. J. Radioanal. Chem. 11, 85-90 (1972). Il. Saloman, L., Lopez-Manchero, E., Lopez De Manterola, J., and Leynen, G., Etude Sur La Recuperation Du Neptunium Dans Le Cycle Du Combustible Et Sur Sa Purification. ETR-233, pp. l-34. 1968. 12. Sandell, E. B. “Calorimetric Determination of Traces of Metals,” 3rd ed., p. 83. Interscience, New York, 1959. 13. Savvin, S. B., Use of Reagents of the Arsenazo-Thoron Group in Analytical Chemistry. Russ. Chem. Rev. 32, 93-107 (1963).
252
SHUKLA
AND SUBRAMANIAN
14. Savvin, S. B., Determination of Thorium, Uranium, Protactinium, Neptunium, Hafnium and Scandium. Tulanta 11, l-6 (1964). 15. Shukla, J. P., and Subramanian, M. S., Extractive Photometric Determination of Plutonium(IV) with Aliquot-336 and Xylenol Orange. J. Radioanal. Chem. 47, 29-36 (1978). 16. Vasudeva Rao, P. R., and Patil, S. K., A spectrophotometric method for the determination of neptunium and plutonium in process solutions. J. Radioanal. Chem. 42, 399-410 (1978). 17. Wolf, I?, and Reinherdt, J., Die Spektrophotometrische Bestimmung Von Plutonium in HDEHP-Dioxan-Losungen mit Arsenazo III. Radio&em. Acta 11, 128-130 (1969). 18. Yamamoto, T., Spectrophotometric determination of plutonium with Chlorophosphonazo III. Mikroch. Actu (With) 871-877 (1974).
JOURNAL
29, 243-252 (1984)
Extractive Separation and Photometric Determination of Neptunium(W) and Plutonium(W) from Their Mixtures J. P. SHUKLA
AND
M.
S. SUBRAMANIAN
Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085 Received November
28, 1979
INTRODUCTION
Radiometric techniques are routinely used for identification and estimation of neptunium as well as plutonium besides numerous chemical methods. Among these, the absorptiometric determination of these elements applicable down to ppm levels offers certain distinct advantages over the counting techniques as (i) it is devoid of dilution errors, and (ii) it is independent of the isotopic composition of the element which in turn changes from batch to batch depending upon the fuel burn-up. Furthermore, the estimation of Np and/or Pu by o-radioassay cannot be accomplished accurately in the presence of high salt concentrations or in the presence of each other as well as other a-emitters such as Am, etc. which calls for involved preliminary separations. Only few methods exist for the calorimetric determination of Np (2, 4, 5, 14) or Pu (7, 10, 17, 18); most of them lack the desired selectivity as well as specificity. Among the photometric methods that have been reported so far for their estimations, arsenazo III method surpasses all in sensitivity. Recently, its application has been extended for the individual assay of Np and Pu in process solutions. However, not only is this procedure lacking specificity for these actinides but fails in presence of oxidizing/reducing agents as well as several common contaminants such as HP+, Pd*+ , Th4+, U02*+, F- , NO 7, NO 5, etc. (7). Moreover, the estimation is possible only in an aqueous medium thereby necessitating the back extraction from the organic extracts. The selectivity and sensitivity of the estimation can be substantially enhanced by combining spectrophotometry with preliminary separation of the element and is best carried out directly in the organic phase without back extraction. The aim of the present work was to develop a rapid procedure to enable satisfactory separation of Np and Pu from commonly associated impurities by 2-thenoyltrifluoroacetone (TTA) in xylene as the extractant followed by their direct determination in the organic phase by spectrophotometry employing xylenol orange (X0) as the chromogenic reagent. Using 243 0026-265X/84 $ I .50 Copyright 0 1984 by Academic Press, Inc. All rights of reproduction in any form reserved.
244
SHUKLA
AND
SUBRAMANIAN
this technique, it is possible to assay down to 0.4 ppm of Np and/or Pu in presence of several interfering ions. Another advantage of the method is the simultaneous determination of Np and Pu in about 30 min when present together. EXPERIMENTAL
DETAILS
237Np,obtained from ORNL, was freed from its daughter nuclide 233Pa by the TTA extraction method (8). The stock solution of Np(IV) in 1 M HN03 was then prepared by reducing Np to Np(IV) employing 0.01 M ferrous sulfamate and 0.01 M hydroxylamine hydrochloride which also served as the holding reductant to keep Np in the tetravalent state. The tetravalency of Np was frequently checked by the ITA extraction method. Plutonium, which was mainly 239Pu,was purified by anion exchange from HN03 medium. The radiochemical purity of these tracers was established by alpha spectrometry using a silicon surface barrier detector. Laboratory reagent grade TTA supplied by E. Merck, Germany, was used without any purification. The xylene solution of TTA was preequilibrated with dilute perchloric acid for several hours to allow hydration of TTA (6). Ferrous sulfamate was prepared as described by Buchanan er al. (3) using LR chemicals. Gross Np and Pu content were determined by alpha radioassay of these nuclides using an cx-proportional counter and liquid scintillation counting, respectively. Total Np content was also ascertained by using an alphaliquid scintillation counting assembly in conjunction with a 4096 channel analyzer. The liquid scintillator solution was prepared by dissolving 0.7 g of 2,5diphenyloxazole, 0.03 g of 1,4-bis-2(5-phenyl oxazolyl) benzene, and 10 g of naphthalene in 100 ml of distilled dioxane. Solution of xylenol orange (0.2%, w/v) [C3iHs2N20t3S] procured from B.D.H. was prepared in methanol and used for color development. All other reagents were BDH/E. Merck “AR” grade, unless otherwise specified. Absorbance measurements were made on a Cary-14 recording spectrophotometer adapted for glove box operations with radioactive materials. Matched quartz cells of 1.0 cm (or 5.0 cm) path length were invariably used with a scanning speed of 10 A per second. Procedure. Two milliliters of 1 M HNOs containing microgram amounts of Np and Pu were transferred to a glass equilibration tube. The solution was adjusted to -0.01 M each in ferrous sulfamate and hydroxylamine hydrochloride and kept aside for a few minutes. Two milliliters of 0.2 M TTA in xylene was added and Np(IV) was extracted into the organic phase. All extractions were performed in duplicate at room temperature by mechanical shaking for 10 min of equal volumes of organic and aqueous phases. After settling, the combined TTA phase was pipetted into another tube containing 10 ml of 1 M HN03 containing -0.01 M ferrous sulfamate
NEPTUNIUM
AND
PLUTONIUM
245
and hydroxylamine hydrochloride and the organic phase was scrubbed twice for 5 min to remove any Pu(IV) extracted along with Np(IV). After centrifuging, OS-ml portion of the TTA extract was pipetted to a lo-ml flask. To this was added 2 ml absolute alcohol, 0.5 ml glacial acetic acid, 2 ml X0 solution, and finally made up the volume with absolute alcohol. The absorbance at 535 nm was measured against a similar Np-free blank containing 0.5 ml of the extractant. The amount of Np extracted into the organic phase was computed from the calibration curve. For determining Pu, solid NaNO* (-0.03 M) was added to oxidize the unextracted plutonium to the tetravalent state. The solution was slightly warmed and after about 15 min plutonium(IV) was extracted twice into 2 ml of 0.2 M TTA. After settling, the organic layer was removed into another tube having 5 ml of 1 M HNOs containing -0.03 M NaN02 and was scrubbed twice for 5 min to remove small quantities of other contaminants extracted alongwith. After centrifuging, the organic layer was separated and 0.5-ml portion pipetted into a lo-ml flask. The color was developed as described earlier for Np estimation. In cases when pure Np or Pu alone is present, scrubbing of the organic extracts can be avoided to save time. RESULTS AND DISCUSSION Spectral Properties
The visible absorption spectrum of a solution of intense red colored Np(IV) or Pu(IV) complex extracted from 1 M HNO, with TTA in xylene gave an absorption maximum at around 535-540 nm measured against a reagent blank. Typical spectra for Pu(IV) complex including that of X0 against alcohol as blank are shown in Fig. 1. The reagent had an absorption maximum at around 435 nm and a continuous decrease thereafter. As neither of these actinide ions nor X0 showed any significant absorption at 535-540 nm, this wavelength was chosen for further analytical study. The calibration curve for determining Np(IV) is shown in Fig. 2 which shows that Beer’s law is valid up to about 2.4 ppm Np. Similar curve for Pu(IV) is also given in Fig. 2 with a validity of Beer’s law up to 3.5 ppm Pu. Values of molar absorptivity, computed from these calibration curves on the basis of Np or Pu content as well as the optimum concentration range for their estimations, evaluated by Ringbom’s method (1) are presented in Table 1. The sensitivity index of the method, as defined by Sandell (Z2), is 0.004 kg Np/cm* at 535 nm and 0.0045 t.r,gPu/ cm* at 540 nm for log ZdZ, = 0.001 and thus practical sensitivity based on an absorbance of 0.01 will be 10 times more than the respective values. Extraction
with TTA
The extractability of Np(IV) and Pu(IV) with TTA when present alone was examined at different HNOs concentrations ranging from 0.5-8 M
246
SHUKLA
AND SUBRAMANIAN
0.6
0.6
W ki d E
0.4
_ /I
ii u
/
(A) /- \ \ \ \ \ \ \ \
0.2
0.0 400
450
550 500 WAVELENGTH
600 ,nm
650
700
FIG. 1. Absorption spectra of xylenol orange (curve A) and its complexwithplutonium(rv) (curve B).
HN03. From the data obtained it was seen that the distribution ratios CD), for NpQV) and Pu(IV) passed through a maximum at around 1 A4 HN03 aq. medium (Table 2). Subsequent extraction studies were, therefore, performed at this acidity. The extraction of these actinides from -1 M HN03 using varying concentrations of TTA in xylene was also studied. Variation of TTA concentration from 0.2 to 0.5 M had little effect on their extractions. Although a single equilibration yielded more than 98% of these actinides from dilute nitric acid solutions using 0.2 M TTA, two successive extractions were invariably carried for quantitative recovery. Effect of Acetic Acid and Xylenol
Orange Concentration
Experiments undertaken to assess the effect of glacial acetic acid and X0 concentration indicated that 0.2-l ml of the former and 1.5-2.5 ml of 0.2% X0 per 10 ml of the final volume were adequate for complete color
NEPTUNIUM
,” z a
0.6-
i
0.4-
247
AND PLUTONIUM
,” 2 0.2
-
0.01 0.5
I.0
I.5
2.0
2.5
3.0
3.5
4.0
4.5
Pu(lV).yg/ml I
1
I
I
1
I
I
I
I
1
0
0.4
0.8
I.2
1.6
2.0
2.4
2.8
3.2
3.6
Np(lV),Jg/ml
FIG. 2. Calibration curve for Np(IV) and Pu(IV) (Beer’s law plot).
development. Larger amounts of both these reagents, however, tend to decrease the absorbance to some extent. In practice, therefore, 0.5 ml of glacial acetic acid and 2 ml of 0.2% X0 were always employed for absorptiometric determinations, Effect of HF Hydrofluoric acid-nitric acid mixtures are frequently employed for the dissolution of plutonium oxide. F- ions are consequently encountered in TABLE 1 SPECTRALDETAILSOFTHE COLOREDNp(IV) Spectral parameters Xmax, nm Range for adherence of Beer’s law, ppm Molar absorptivity, mol-i cm-i Optimal concentration range, ppm Sandell’s sensitivity index, pg/ml Coefficient of variation, %
AND
Neptunium(IV)
Pu(IV)
EXTRACTS
Plutonium(IV)
535
540
0.4-2.4
0.4-3.5
59103 f 304
52857 2 373
0.5-2.1
0.5-3.2
0.004 2.12”
0.0045 1.9gb
B For 2.4 pg Np ml-i (No. of determinations = 8). b For 2.6 pg Pu ml-’ (No. of determinations = 6).
248
SHUKLA
AND
SUBRAMANIAN
TABLE EFFECT OF NITRIC
ACID
2
CONCENTRATION ON EXTRACTION WITH TTA
Distribution [HWI M 0.5 1.0 1.5 2.0 4.0 8.0
205 587 141 36 0.32
0.5 M TTA 541 >5000 322 62 1.1 co.01
AND Pu(IV)
ratio, D
Neptunium(IV) 0.2 M TTA
OF Np(IV)
Plutonium(IV) 0.2 M TTA 142 373 281 26 0.72
0.5 M TTA 1157 >5000 351 57 0.86 co.01
the spent-fuel reprocessing solutions causing an incomplete extraction of Pu or Np into TTA. To prevent this interference, fluoride ions were masked with aluminum nitrate. Ft ions up to 2 kg/ml could be tolerated without using the masking agent. Higher amounts of the order of 1 mg/ml of NaF could, however, be tolerated by masking with about 10 mg Al(NO&. Effect of Process Contaminants Uranium and zirconium are the most common metallic contaminants of Np and Pu during their radiochemical processing besides other longlived fission products and cladding materials. According to Saloman et al. (II), an aliquot of the solution, resulting from dissolving uranium fuel with 16,000 MWDIT burn up, containing few milligrams of U (up to 10 mg) will be associated with a few micrograms of Np and a few micrograms of Zr (up to 25 kg) whereas quantities of Pu are much larger relative of 237Np. Moore and Hudgens (9) have demonstrated that several impurities can be separated from plutonium by the TTA extraction method. Advantageously, this serves here as a first step toward preliminary separation. It has been reported that zirconium can be eliminated during separation or alternatively be masked (2, 13) by using a strong complexing agent like EDTA or H3P04. About 10 kg/ml of Zr did not cause any interference in the final determination of Np or Pu. Here both EDTA and H3P04 were found to be unsuitable due to their adverse effects on analysis. However, no interference was observed in the final assay of a few micrograms of Np in presence of about 100 mg U, 300 kg Pu, and a few micrograms of Zr which are the usual concentrations of these elements expected to be associated with neptunium solutions (11). Also, the separation procedure was tried in the absence of Np with aforesaid amounts of these interferents individually and with a mixture of all three. The absorbance of the scrubbed organic extracts, was found to be negligible (CO.01) thereby
NEPTUNIUM
249
AND PLUTONIUM
TABLE 3 DETERMINATIONOF NEPTUNIUM(W) IN THE PRESENCEOF DIVERSE IONS [Np(IV) CONCENTRATION: 2.4 ppm, AQUEOUS ACIDITY: 1 M HNO,]
Ion Blank Cation A13+ Ce4+ co*+ WcLI2+ Fez+ Fe3+ La3+ Mn2+ Ni2+ Pu4+ Sn2+ Th4+ uo2+ Zn2+ Zr4+ Anion ClFNO5 NO? PO]SO;Acetate Oxalate
Added as
Al(N03)3 * 9H20 Ce(S04)2 . 4H20 CO(NO~)~. 6H20 CrK(SO4)2 . 12HzO (CH3C00)2 * Cu * Hz0 (NH4)2S04. FeS04 * 6Hz0 Fe(N03)3 * 9H20 LaW03h MnS04 * Hz0 NiS04 * 7Hz0 PW03)4
SnC12. 2H20 Th(N03)4 * 4Hz0 UOz(NO3)* ’ 6H*O ZnS04. 7Hz0 ZrOC12. 8H2O KC1 NaF NaN02 H3P04
Na2S04 CH3COONa NaG04
Amount added km) Nil 30 0.5 30 1
1; (0.05)~ (O.OS)C Id 5 5
20 (0.3)C 30 (0.4)‘ 100 45 (0.01)’ 30 (0.02)C I’ 35 35 2 35 5 (0.04)C
Absorbance“
Maximum absorbance” M-d
0.61b
535
0.61 Interference 0.62 0.60 0.62 0.61 0.62 0.60 0.61 0.60 0.60 0.61 0.62 0.61 0.62 0.60 0.62
535 535 535 535 535 535 535 535 535 535 535 535 535 535 535 535
0.61 0.60 0.61 0.62 0.61 Interference 0.62 0.60 0.60
535 535 535 535 535 535 535 535
u Average of 3 determinations of the same sample. b The average absorbance of five neptunium(W) samples, each containing a final Np concentration of 2.4 ppm was 0.61 with a standard deviation of 0.007. c Figures in parentheses are the maximum amounts tolerated without any interference. d 10 mg citric acid added before TTA extraction. c 10 mg AI(N03)3 added before TTA extraction.
indicating that U, Pu, and Zr up to amounts specified do not interfere in the estimation of Np. Effect of Other Diverse Ions To examine the effect of some other diverse ions which often accompany Np as well as Pu, a systematic investigation of their interferences
250
SHUKLA
AND SUBRAMANIAN TABLE 4
ANALYSIS
Taken, pg Np4+ + pu4+ 0.85 1.39 2.06 2.41
+ + + +
1.00 1.66 2.60 3.01
OF
Np
AND Pu FROM THEIR MIXTURES
Pound,n p,g Np4+ + Pu4+ 0.87 1.42 2.03 2.36
+ + + +
1.02 1.63 2.55 3.07
Error, % NP
Pu
2.35 2.16 1.46 2.07
2.00 1.81 1.92 1.99
a Average of 3 determinations of the same sample.
was attempted. Since interferents are common, only Np(IV) was extracted in the presence of each impurity ion followed by its photometric assay with X0 as per recommended procedure. Table 3 shows the absorbance values of 2.4 ppm of Np(IV) in presence of various external ions. Also, the effect of these interferents on absorption maximum (A max) was studied. An increase or decrease of ~0.02 unit in absorbance was considered to be an interference. Cations such as A13+, Cr3+, Co2+, Cu2+, La3+, Mn2+, Ni2+, Sn2+, and Zn2+ as well as common anions, viz., Cl-, NO;, NOy, sulfate, and acetate did not interfere with the final determination even at very high concentrations. Fe2+, Fe3‘, Th4+, Zfl+, and oxalate could be tolerated in moderate amounts. Higher amounts of Fe3+ caused no interference in the presence of 5-10 mg/ml citric acid which served as the masking agent. Strong interferences in the absorption measurements were obtained only in the presence of Ce4+ and phosphoric acid. Extraction
Time and Stability
of Color
It was observed that 95% extraction of Np or Pu from the aqueous phase occurred after shaking for 3 mitt, whereas beyond 5 min, essentially quantitative extraction of microgram amounts of these actinides was accomplished. All extractions were carried out for 10 min. The stability of the colored extracts was tested by measuring their absorbance at different time intervals. The absorbance was found to remain constant for at least 48 hr. The color formation was instantaneous, but the colored extracts were kept for about 10 min to attain equilibrium. Accuracy
and Precision
Table 4 summarizes some results obtained by applying this method to the analysis of varying amounts of Np(IV) and Pu(IV) contained in a mixture. It is possible to extract quantitatively and assay spectrophotometrically neptunium and plutonium in 30 min. If a 5-cm cell is used, as little as 0.4 ppm of both Np(IV) and Pu(IV) can be estimated with a
NEPTUNIUM
AND PLUTONIUM
251
precision better than 2% and a standard deviation of 0.007 and 0.006 ppm (n = 6), respectively. ACKNOWLEDGMENT The authors express their gratitude to Dr. M. V. Ramaniah, Director, Radiological Group, for his keen interest in this work.
SUMMARY A rapid and sensitive method for the photometric determination of trace amounts of neptunium and plutonium from their mixtures is described. Np(IV) is selectively extracted from about 1 M HNOs medium with TTA in xylene retaining Pu in the nonextractable trivalent state in the aq. phase with ferrous sulfamate. Plutonium in the aqueous phase is subsequently oxidized with NaNOz to the highly extractable tetravalent state and extracted with TTA. Np(IV) as well as Pu(IV) thus extracted are finally estimated in the organic phase itself spectrophotometrically employing xylenol orange as the chromogenic reagent. Their molar absorptivities are in the 5 x lo4 range. Beer’s law is valid up to 2.4 ppm Np and 3.5 ppm Pu. The color of the solutions is stable for at least 48 hr. The method tolerates large excess of several common contaminants encountered during spent fuel reprocessing. Cerium(IV) and phosphoric acid, however, interfere with the final estimation.
REFERENCES 1. Ayres, G. H., Evaluation of Accuracy in Photometric Analysis. Anal. Chem. 21, 652-
657 (1949). 2. Bryan, R. C., and Waterbury, G. R., Spectrophotometric Determination of Neptunium, LA-4061, pp. l-8 (1968). 3. Buchanan, R. F., Hughes, J. P., Hines, J. J., and Bloomquist, C. A. A., The determination of nitrogen, Americium, Neptunium and Uranium in P.P.M. quantities in pure plutonium. Talanta 6, 173-177 (1960). 4. Budesinsky, B., In “Chelates in Analytical Chemistry” (M. A. Flaschka, and A. J. Barnard, eds.), Vol. 1, pp. 15-47. Dekker, New York, 1972. 5. Bumey, G. A., Dukes, E. K., and Groh, H. J., “Analytical Chemistry of Neptunium,” Chap. 4, Progress in Nuclear Energy Series, IX, Vol. 6, p. 181. 1966. 6. Day, R. A., Jr., and Stoughton, R. W., Chemistry of Thorium in Aqueous Solutions, I. Some organic and inorganic complexes. J. Am. Chem. Sot. 72, 5662-5666 (1950). 7. Milyukova, M. S., Gusav, N. I., Santyurin, I. G., and Sklyarenko, I. S., “Analytical Chemistry of Plutonium.” Israel Program for Scientific Trans. Jerusalem, p. 130. 1967. 8. Moore, F. L., Separation and Determination of Neptunium by Liquid-Liquid Extraction. Anal. Chem. 29, 941-944 (1957). 9. Moore, F. L., and Hudgens, J. E., Jr., Separation and Determination of Plutonium by Liquid-Liquid Extraction. Anal. Chem. 29, 1767-1770 (1957). 10. Peter, J., Fehlman, D., and Aerne, E., A Spectrophotometric method for the determination of plutonium in fuel solutions. J. Radioanal. Chem. 11, 85-90 (1972). Il. Saloman, L., Lopez-Manchero, E., Lopez De Manterola, J., and Leynen, G., Etude Sur La Recuperation Du Neptunium Dans Le Cycle Du Combustible Et Sur Sa Purification. ETR-233, pp. l-34. 1968. 12. Sandell, E. B. “Calorimetric Determination of Traces of Metals,” 3rd ed., p. 83. Interscience, New York, 1959. 13. Savvin, S. B., Use of Reagents of the Arsenazo-Thoron Group in Analytical Chemistry. Russ. Chem. Rev. 32, 93-107 (1963).
252
SHUKLA
AND SUBRAMANIAN
14. Savvin, S. B., Determination of Thorium, Uranium, Protactinium, Neptunium, Hafnium and Scandium. Tulanta 11, l-6 (1964). 15. Shukla, J. P., and Subramanian, M. S., Extractive Photometric Determination of Plutonium(IV) with Aliquot-336 and Xylenol Orange. J. Radioanal. Chem. 47, 29-36 (1978). 16. Vasudeva Rao, P. R., and Patil, S. K., A spectrophotometric method for the determination of neptunium and plutonium in process solutions. J. Radioanal. Chem. 42, 399-410 (1978). 17. Wolf, I?, and Reinherdt, J., Die Spektrophotometrische Bestimmung Von Plutonium in HDEHP-Dioxan-Losungen mit Arsenazo III. Radio&em. Acta 11, 128-130 (1969). 18. Yamamoto, T., Spectrophotometric determination of plutonium with Chlorophosphonazo III. Mikroch. Actu (With) 871-877 (1974).