Determination of Uranium in Plutonium-238 Metal and Oxide by Differential Pulse Polarography N. C. Fawcett' Los Alamos Scientific Laboratory, University of California, Los Alamos. N.M. 87545
A differential pulse polarographic method was developed for the determination of total uranlum In 238Pu metal and oxides. A supporting electrolyte of 0.5 M ascorbic acid in 0.15 N H2SOr was found satisfactory for the determination of 500 ppm or more of uranium in 10 mg or less of plutonium. A relative standard deviation of 0.27 to 4.3% was obtained in the analysis of samples ranging in uranium content from 0.65 to 2.79%. The limit of detection was 0.18 pg mi-'. Peak current was a linear function of uranium concentration up to at least 100 pg ml-'. Amounts of neptunium equal to the uranium content were tolerated. The possible interference of a number of other cations and anlons was investigated.
T h e highly a-active isotope 238Pu is used as a source of thermal energy for power in space and for human heart pacers ( I ) . Contamination of 238Pu with uranium is unavoidable because 238Pu LY decays t o 234Uat the rate of 22 ppni per day. Also, implements made from uranium metal are sometimes used in the preparation of 238Pu ceramics. It is desirable, therefore, that suitable methods be available for the determination of both 234Uand total uranium in substances containing 2:38Pu. Ideally, the methods chosen should be free from interference by 237Npfrom which 238Pu is made, 241Am which grows into 238Puat t h e rate of 1.7 ppm per day, and 232Thwhich is a persistent impurity. At this laboratory, both spectrophotometry of the arsenazo complex and x-ray fluorescence have been used t o determine total uranium in 238Pu, but these methods require separation from other actinides, and the x-ray method requires a somewhat tedious concentration of the separated uranium. Determination of uranium in 238Pu samples by differential pulse polarography (DPP) in sulfuric-ascorbic acid electrolyte offers several advantages. For example, the high intrinsic sensitivity and signal-to-noise ratio of DPP makes possible a direct determination of uranium in less than 10 mg of 238Puwithout prior concentration of uranium in all b u t t h e most recently purified samples. No separation from plutonium is required because it is reduced by ascorbic acid t o the (111)state from which it cannot be electrolytically reduced in aqueous solution. Th(1V) and Am(II1) d o not interfere at any level likely t o be encountered, and relative freedom from Np(1V) interference is also obtained. Sulfate complexes Np(IV), shifting its reduction wave 100 mV negative from that of U(V1). T h e high resolution of DPP make this shift adequate for most levels of N p encountered. Also, a lower residual current in the region of the uranium wave is obtained in sulfate as compared t o chloride medium. A t least two other differential pulse procedures for uranium have appeared in the literature: One for uranium in sea water in which the uranium is determined in tartrate chloride medium ( 2 ) ,the other for uranium as the arsenazo complex ( 3 ) . Present address, Department of Chemistry, Southwest Texas State University, San Marcos, Texas 78666.
EXPERIMENTAL Apparatus. Data were obtained with a Princeton Applied Research (PAR) Model 174 Polarographic Analyzer equipped with an X-Y recorder. The performance of this instrument has been evaluated ( 4 ) . A conventional dropping mercury electrode (DME) was used. The nominal bore of the capillary was 0.05 mm. The height of the mercury column was adjusted to give a natural drop time of -4 sec. Drop times were controlled with a PAR Model 174/70 Drop Timer. The cell consisted of a 50-ml beaker sawed-off crosswise just below the pouring spout so that the beaker would fit snugly against a Plexiglas lid. The following items were inserted through holes in the cell lid: a gas inlet tube for passing argon over the surface of the sample solution; a gas dispersion tube for de-aeration of the sample solution; a fiber junction salt bridge in which was inserted a commercial SCE; a porous Vycor tube which contained a platinum spiral counter electrode; and the DME. The Vycor tube and the salt bridge were filled with 1 N HzS04. A two-way stopcock allowed water-saturated, high-purity argon to be directed either over the surface of or through the sample solution. The cell was immersed in a thermostated temperature bath ( f O . l "C) when data for plotting calibration curves of peak current vs. concentration were being obtained. For analysis of samples by standard addition, it was not necessary to thermostat the cell. The cell assembly was enclosed in a glovebox suitable for handling radioactive materials. Reagents. A stock solution of U(V1) sulfate in 1 N HzS04 was prepared from high purity NBS SRM960 uranium metal. Working reference solutions were prepared from this solution by dilution of weight aliquots with 0.05 N H2S04. The resulting solutions typically contained 600 kg ml-I of U(V1). A 238Pustock solution was prepared by dissolving electrorefined ( I ) plutonium metal in 6 N HCl. The 234Ucontent of this solution was computed from the day of purification using a half-life of 87.5 years for 238Pu.This result was checked against the 234U content determined by radioassay and against the total uranium content determined by x-ray fluorescence. These results showed that uranium isotopes other than 234Uwere present only in negligible amounts. Quartz distilled H2S04 was used throughout. Water and mercury were both triply distilled. All other chemicals were reagent grade and were used without further purification. The 0.5 M ascorbic acid solution was prepared in 0.05 N H2S04. This solution was stored in the reservoir of and dispensed from an automatic buret. The atmosphere inside the storage reservoir and buret was highpurity, water-saturated argon. A 1-ml Digipet was used t o dispense small quantities of solutions directly into the cell. Procedure. Plutonium samples, either oxides or metals, were dissolved in HCI according to a procedure given elsewhere ( 5 ) . Weight aliquots of the resulting solutions were taken directly into were added the sample cup of the cell. A few drops of 15 M "03 to each aliquot which was then converted to the sulfate by fuming twice in H2S04, the last time to dryness. These aliquots typically contained 5 to 40 gg of uranium and 2 to 10 mg of plutonium. Just prior to being analyzed, each aliquot was dissolved in 1 ml of 1 N H2S04, and then 9 ml of 0.5 M ascorbic acid in 0.05 N HzS04 was added. The sample cup was then placed in position against the cell lid and the solution de-aerated for 8 min. The differential pulse polarogram was recorded by scanning negatively from 0.00 V vs. the SCE. A measured volume of standard uranium solution was added, followed by additional de-aeration for 2 min. A second scan was then made. Uranium concentration in the sample was computed according t o the standard addition method as described elsewhere ( 2 ) . The residual current was determined from polarograms run on supporting electrolyte alone. Interference studies were made by adding volume aliquots of a sulfate solution of the ion in question t o a prepared sample solution, containing a known amount of uranium, and observing the effect on the measured content of uranium. ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
215
~
~~
Table I. Results of Analysis for Percent U in 238PuSamples Sample No.
% U ?Sa b y D P P
% U tsb b y x-ray
%
234UC
0.432 i 0.013 0.432 0.015 0.430 2 0.619 i 0.049 0.620 i 0.012 0.568 3 0.741 i 0.0021 0.742 i 0.12 0.745* 4 1.51 ~ 0 . 0 2 8 1.42 t 0.29 1.49 5 0.065 i 0.0028 0.059 6 0.165 i 0.000 0.165* 7 0.216 i 0.0085 0.214 8 0.218 i 0.0014 0.233 9 0.812 3 0.0021 0.800 10 0.825 i 0.010 0.843* 11 1.03 3 0.018 0.245 12 2.79 i 0.038 0.832 a s = standard deviation with n = 3 for samples 3 and 1 0 . n = 2 for all others. b n = 2. CValues with an asterisk were calculated using tv2 = 87.5 years for decay of 13'Pu into 234U,All other values were obtained by radio-assay. 1
T
0.2pA
1
J
I
Potential ( V ) vs SCE
Figure 1. Differential pulse polarogram of 2 pg ml-' of U(VI) in 0.15 N H2S04 and 0.5 M ascorbic acid solution containing 9 mg of 238Pu. Modulation amplitude = 100 mV, scan rate = 0.5 mV sec-', Drop time = 1 sec
For all polarograms, the following instrument settings were used: scan rate = 5 mV sec-', drop time = 1 sec, pulse height amplitude = 100 mV. The full scale current range was set to give a uranium peak height as near full scale as practical. The most sensitive range used was 1 p A for a full scale deflection of 200 chart divisions at 20 divisions per inch.
RESULTS AND DISCUSSION The rate of disproportionation of U(V) has been shown to depend on hydrogen ion concentration. In acid solutions below about 0.2 N, the disproportionation is relatively slow and two polarographic waves, one for the reduction of U(V1) to U(V) and the other for reduction of U(V) to U(1V) are observed (6). In the present case, a sulfuric acid normality of 0.15 was selected, and a peak for the reduction of U(V1) to U(V) was observed a t -0.17 V vs. the SCE. This corresponds to E112 of -0.22 V for a pulse height amplitude of 100 mV (4). A typical pulse polarogram is shown in Figure 1 for uranium a t a concentration of 2 pg ml-' in the presence of 9 mg of 238Pu. The peak to the right of the uranium peak is apparently due to the oxidation product of ascorbic acid. Two observations support this conclusion. First, the peak increases in proportion to the amount of plutonium in the sample and, second, a small peak a t the same potential is observed in supporting electrolyte alone, and this peak increases upon exposure of the ascorbic solution to air. The half-width ( 4 ) for the uranium peak was 145 mV. This is too large to give complete separation of the ascorbic and uranium peaks a t their base although resolution is sufficient not to cause interference with the uranium peak height provided the concentration of plutonium in the sample solution is 1 mg ml-I or less. Naturally, for a given plutonium concentration, the size of the ascorbic peak decreases relative to the size of the uranium peak as the uranium concentration is increased. T o test whether there was a difference in residual current when plutonium was present, residual current scans were obtained on the supporting electrolyte alone and on 239Pu solutions. National Bureau of Standards 239Pu was chosen for this test because it was known to contain less than 50 ppm uranium. The residual current a t the uranium peak potential was not affected by the presence of up to 1 mg ml-I of 239Pu within the limits of experimental error. A mean value for the residual current of 0.51 p A was ob216
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
Table 11. Neptunium (V) Interference0 CX
CXJCU
IACl - [LI
0.39 0.10 -0.16 1.94 0.50 -0.18 3.82 1.oo -0.07 7.43 2.02 +0.05 14.0 4.04 +0.20 23.4 7.43 +0.83 aCx = Concentration of Np ( p g ml-l). Cx/C, = Ratio of neptunium to uranium concentration. A C = Effect of neptun. ium on the experimentally determined uranium concentration. L = 2saC,where sac is the standard deviation of A C. tained. The value of the residual current for subsequent work was obtained from polarograms of the supporting electrolyte alone. A least squares fit of peak current vs. uranium concentration ( n = 9) resulted in the following equation:
i = 0.509C - 0.006
(1)
where i is the peak current in p A and C is concentration in pg/ml. The standard deviation of the slope and intercept were f0.002 pA ml pg-l and f0.016 FA. The standard deviation computed from the deviations about the regression line was f0.031 FA, which yielded a detection limit of 0.18 pg ml-1 a t the 95% confidence level ( 7 ) . Uranium concentrations above 20 pg ml-l were investigated in a separate experiment. The response was linear up to 100 pg m1-I. Concentrations greater than this were not investigated. The linear relationship of the peak current to the uranium concentration made it possible to determine uranium by standard addition. For work in a glovebox, the standard addition method is especially convenient as compared to the standard curve method where conditions such as drop size and temperature must be carefully reproduced from day to day, and check points must be obtained to ascertain that the position of the standard curve has not shifted. Results obtained by standard addition on two sets of three plutonium standards a t a uranium concentration of 8 pg ml-' resulted in a pooled standard deviation of f0.07 p g ml-l. The percentages of total U determined by D P P analysis by standard addition on 12 different 23sPu samples are shown in column 2 of Table I. These samples were either electrorefined metals or oxides containing 80 wt % or more of total plutonium. Of the total plutonium, 80 to 90% was 238Pu, the remainder being mostly 239Pu. As a check on
Table 111. Cation Interferences Concentration of other h t i o n s added ~
X
Cu(I1) Cr(II1) V(V) La(II1) MOW)
CX
CXICU
1.o
5.1 50. .30 10. 49. 50.
9.8. 12. 1.9 9.5 9.5 4.7
cu(I1)
Cr(II1)
Mo(V)
V(V)
Ca(I1)
Al(II1)
IACI
Zn(I1)
1.0 9.8 9.8 9.8
-0.17 -0.18
1.9 1.9
+1.7 +1.7 +3.1
.10
9.3
ILI
+1.0
9.3
1.0 1.0 1.o
-
-0.056 -0.19
.23
Ca( 11) 3.5 18. -0.19 3.5 Al( 111) 25. 125. -0.17 25. 3.5 Mn( IV) 50. 250. -0.083 T1( 11) 24. 120. -0.10 Zn(I1) 50. 260. +0.11 50. Ti(1V) 50. 280. +0.04 Th(1V) 20. 7 .O -1.4 Th(1V) 80. 28. -0.17 Pt(I1) 8.7 44. +5.5 Fe(II1) 19. 96. -0.12 a All concentrations are in p g ml-’. x stands for the impurity being tested. u stands for uranium A C = the effect o f x on the experimentally determined uranium concentration. L = 2sa,, where SA, is the standard deviation of A C. the accuracy of the method, the first four samples were also analyzed by x-ray fluorescence. These results are shown in column 3. In cases where the date on which the samples were electrorefined were known, the expected percent 234Ubased on the decay rate of 238Pu is shown in column 4 and marked with an asterisk. Otherwise, column 4 contains percent 234U determined by radioassay. All analyses for the same sample have been given as though obtained on the same day. In some cases, this required the addition of small corrections based on decay of 238Pu.All results are based on an atomic weight of 234 for uranium. T h e relative standard deviations of the radioassay results are estimated to be between 1 and 5%. The relative standard deviation of the result calculated from the decay rate of 238Puis -0.1% (8). In each case, the difference between the D P P and x-ray results is less than one standard deviation of the difference. Because total uranium is determined by both D P P and xray, results in columns 2 and 3 should be equal to or greater than the percent 234U in column 4. T h e D P P results for samples 3 , 8 , and 10 shows total uranium to be less than the 234U content; however, the difference is not significant for samples 3 and 10. For sample 8, the D P P result may or may not be significantly lower than the radioassay result, depending on what precision is attached to the radioassay result. Disregarding the highly fortuitous agreement obtained for sample 6, the relative standard deviations for D P P analysis ranged from 4.3% for sample 5 t o 0.27% for sample 3. T h e possible interference of a number of cations was studied, and the results of these tests are summarized in Tables I1 and 111. Interference was detected by computing the quantity I ACI - ILI, where AC is the indicated uranium concentration in the presence of substance n minus the indicated concentration in the absence of x , and L is a limit which was arbitrarily taken as twice the standard deviation of the concentration difference ( s A c ) . This latter quantity is given by SAC
=
(2)
where s, is the standard deviation for a single determination. The value of s, used was f 0.07 wg ml-I and was obtained by repetitious ( n = 6) determination of uranium
I
I
I
,
,
T
t
05ra
E
B
-0.1
-02
-03
-0.4
4 5
Potentiol (V)vs SCE
Flgure 2. Differential pulse polarogram of 4 Fg ml-’ U(VI) in 0.15 N H2S04and 0.5 M ascorbic acid in the absence ( a )and the presence (b)of 16 p g ml-’ Np added as Np(V) sulfate. Modulation amplitude = 100 mV, scan rate = 0.5 mV sec-’, Drop time = 1 sec
concentration by standard addition. This resulted in a value for L of 0.198 wg ml-l. A positive value for the quantity I ACI - JL)suggests significant interference a t a level of 2sA,. All concentrations are expressed as gg ml-l. As mentioned previously, 238Pu samples typically contain small amounts of 237Npas a n impurity. Hence, a separate study of neptunium interference was made. The results of this study are shown in Table 11, where cx is the concentration of neptunium added as Np(V) sulfate and C, is the uranium concentration. T h e data indicate t h a t neptunium may be present a t the same concentration as U without significant interference. Addition of a few drops of 15 M “ 0 3 to the sample prior to fuming in sulfuric acid assures that neptunium will be present primarily as Np(V). The reduction of Np(V) to Np(1V) is highly irreversible ( 9 ) ,therefore Np(V) does not interfere with the determination of uranium. Interference by neptunium is probably caused by slow chemical reduction of Np(V) to Np(1V) in the sulfuric-ascorbic acid elecANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
217
trolyte. Any Np(1V) formed may then be reversibly reduced to Np(II1) a t the electrode. When a large amount of Np(1V) relative to the amount of uranium is present, a separate peak for the reduction of Np(1V) may be observed a t about -0.3 V vs. the SCE. The effect of an amount of neptunium (added as Np(V) sulfate) sufficient to cause slight interference with the uranium peak height is shown in Figure 2. Note the broadening of the uranium peak toward a more negative potential. The possible interference of fourteen other cations was investigated and the results are summarized in Table 111. Zinc(II), Mo(V), Ti(IV), and Pt(1V) were found to interfere a t the levels tested. The effect of chromium is not clear. By itself Cr(II1) did not affect the uranium peak height; however, addition of Cr(II1) in the presence of molybdenum caused a significant increase in peak height. No interference from Th(1V) was observed. This is important because 23sPu metal is often cast into thorium oxide crucibles, and thorium is a common impurity. I t is important to note that the oxidation states given for ions listed in Tables I1 and I11 are the nominal oxidation state in which the ions were added to the electrolyte, which is a combination of a mild complexing agent and a reducing agent, Le., sulfate and ascorbic acid. Americium was not tested separately; however, all samples contained some americium. For most samples, the weight percent of americium was about one-tenth that of the uranium. No evidence of americium interference was noted as evidenced by the agreement between D P P results and the results of other methods of uranium analysis. Americium(II1) is not considered to be electroreducible in aqueous solutions, and it is likely t h a t any reasonable amount of americium could be tolerated. Several anions were checked for interference. A fivefold concentration excess of ClOa- or NOS- over uranium had no effect. Phosphate and F- interfere strongly even in small amounts and must be excluded. Chloride interferes by increasing the level of residual current in the region of the uranium peak.
The method of analysis described works equally well for determination of uranium in compounds of 239Puprovided uranium is present a t about 500 ppm or higher so that the concentration of plutonium in the electrolyte does not need to exceed about 1 mg ml-l, in which case the peak due to the oxidation product of ascorbic acid may interfere. Because the concentration of uranium in many 239Pusamples is well below 500 ppm, a t least a partial separation of uranium from plutonium would often be required. However for 238Pusamples, where the uranium concentration is almost always 500 ppm or higher, the present method offers the advantages of simplicity and directness combined with relative freedom from interferences. The method is especially convenient when a coulometric determination of plutonium is to be carried out on the same sample, because aliquots of the same sample solution serve for both procedures without additional dilutions.
ACKNOWLEDGMENT The helpful interest of R. Gillette Bryan is gratefully acknowledged.
LITERATURE CITED (1) L. J. Mullins, Los Alamos Sci. Lab. Rep, LA-4940, Oct. 1972. (2) G. W. C. Milner, J. D. Wilson, G. A . Barnett, and A. A . Smales, J. Electroanal. Chem., 2, 25 (1961). (3) Y . Chapron, Commis. Enerq. At. (Fr.) Rapp., CEA-R 3299, 1967. (4) J. H. Christie, J. Osteryoung. and R. A. Osteryoung, Anal. Chem., 45, 210 ( 1973). - -, (5) J W. Dahlby, R. R Geoffrion, and G. R. Waterbury, Los Alamos Sci. La6 Reo.. LA-5776. Jan. 1975. (6) I. Hodara and I.'Balouka, Anal. Chem., 43, 1213 (1971). (7) R. K. Skogerboe and C. L. Grant, Spectrosc. Lett., 3, 215 (1970). (8) Mound Laboratory Report, MLM 1996, 1964. (9) J. C. Hindrnan, D. Cohen, and J. C Sullivan, Proc. Conf. Peaceful Uses At. Energy, 7955,7, 345 (1956) \
RECEIVEDfor review August 4, 1975. Accepted September 19, 1975. Work done under the auspices of the Division of Space Nuclear Systems of the Energy Research and Development Administration.
Digital Simulation of Differential Pulse Polarography James W. Diliard and K. W. Hanck" Department of Chemistry, North Carolina State University, Raleigh, N.C. 27607
Digital simulation is used to evaluate the effects of moduiation amplitude, number of electrons transferred, and charge transfer kinetics (a,ko) on differential pulse poiarograms. The combined use of least squares curve fitting routines and digital simulation allows heterogeneous charge transfer rate constants In the range 10-1-10-4 cm/sec to be measured using differential pulse polarography. Kinetic studies may be M. Experimental performed on solutions as dilute as data are reported for Cd2+/Cd, Ti+/Ti, Zn2+/Zn, and Eu3+/ Eu2+.
Differential pulse polarography (DPP) has proved to be a versatile and reliable trace analytical tool. Although there are several instrumental variations of D P P ( 2 - 4 ) , in its most common form, a small amplitude voltage pulse of short duration is superimposed on the slowly increasing 218
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
linear voltage ramp applied to the cell. Timing circuits ensure that the pulse is applied a t the same point in the life of each mercury drop; the output current is the difference between the current flowing just prior to the end of drop life and the current flowing just prior to the application of the voltage pulse. Unlike other electroanalytical methods, very few investigations of the theory behind D P P have been made. Parry and Osteryoung (5) used a rather simple subtractive model to evaluate the analytical current of DPP. Their model assumes Nernstian behavior and ignores the hydrodynamic peculiarities of the dropping mercury electrode. To evaluate the use of D P P in speciation studies involving complexes of trace metal ions, we have found it necessary to investigate the theoretical behavior of D P P in systems complicated by rate determining chemical and electrochemical steps (6). We have elected to apply digital simulation to D P P rath-
