Fluorometric Determination of Microquantities of Uranium in Mixtures of Uranium and Plutonium SIR: Fluorometric methods (1,s)have been used extensively for the determination of trace quantities of uranium. Techniques and instruments have been highly developed to permit routine application of these methods to a variety of materials. As a result of the recent increased interest in uranium-plutonium mixtures as potential reactor fuels, a need has developed for a rapid, reliable method for the determination of uranium in the presence of plutonium. This study was undertaken to determine whether the fluorometric method could be applied to the analysis of these mixtures without a preliminary separation of plutonium. Results of this investigation show that microquantities of uranium can be determined by the fluorometric method in uranium-plutonium mixtures containing U :Pu ratios as high as 1:20. EXPERIMENTAL
Apparatus and Reagents. Galvanek Morrison Fluorometer, manufactured by Jarrell Ash Co. Sodium fluoride-297, lithium fluoride flux. Several brands of reagent grade materials were tested before a number with suitable low blanks were found. The flux was prepared in large batches to ensure a uniform material of low blank. Each batch was thoroughly mixed by a modified P.K. blender for 8 hours a t 32 r.p.m. prior to use. Rocedure. The method was calibrated by analyzing aliquots of a standard uranium solution. A series of aliquots was taken a n d each trans-
ferred into a platinum dish. After evaporation to dryness, a pellet of mixed fluoride flux was prepared a n d added to each dish. The samples were fused using a propane burner a t approximately 950’ C. Fusion was continued for 1 minute after the flux had melted completely. The dishes were then cooled in a desiccator. Fluorescence measurements were made 30 minutes after completion of the fusion. Solutions containing varying quantities of plutonium and uranium were prepared and measured by a similar procedure.
uranium in the mixed sodium fluoride2% lithium fluoride flux in the presence of plutonium in ratios as high as 1:20. Samples can thus be analyzed successfully without pretreatment or chemical separation. Ratios greater than I :20 have not been investigated. The method has been applied to the analysis of planchets containing microgram quantities of uranium and plutonium deposited from a Knudsen effusion cell and an investigation of vapor pressures of uranium-plutonium carbides (2). ACKNOWLEDGMENT
RESULTS A N D DISCUSSION
The results of analyses of solutions containing microquantities of uranium plus various quantities of plutonium are shown in Table I. Duplicate analyses were performed on each series on three successive days. The results show no quenching of the fluorescence from microquantities of
Table I. Results of Analyses of Mixtures of Uranium and Plutonium Uranium found, Uranium pg. added, (mean G:Pu No. of ratio detns. rt3. value) 1:5 1:lO 1:20
12 12 12
0.040 0.040 0,040
0.039 0.041 0.043
The authors gratefully acknowledge the advice and assistance of the late Hyman Steinmetz in the preparation of this paper. LITERATURE CITED
( 1 ) Centanni, F. A,, Ross, A. M., Desesa, M. A,, ANAL.CHEM.28, 1651 (1956). ( 2 ) Strasser, A,, U. S. At. Energy Comm., ReDt. No. UNC-5094. SeDtemberr 1964. ( 3 ) Lf.S. At. Energy Com’m., Reptr No. TID-7015, Supplement 3, Method No. 1,219,240, January 1960.
ROBERT A. JAROSZESHI~ CHARLES C. GREGG United Nuclear Corp. Development Division White Plains, IL’. Y. THIS work was sponsored by the U. S. Atomic Energy Commission under Contract AT 30( 1)-3118. Present address, Nuclear Fuels Service, Inc., West Valley, N.Y.
Resistance Effects of Two Types of Carbon Paste Electrodes SIR: During some chronopotentiometric studies of carbon paste electrodes, we observed iR drops in the electrodes themselves which were of sufficient magnitude to affect the results. This observation is not surprising when the form of the carbon paste electrode is considered-carbon particles coated with a n organic solvent and pressed into a holder with only hand pressure-a form which suggests the presence of an appreciable amount of resistance. No specific mention of the need for concern about the resistance of the carbon paste electrode has been made previously. I n most of the reported works the electrodes were small in size. Apparently the resistances of these electrodes mere low enough so that with the currents used, the iR drops aere negligible. I n two recent papers 766
0
ANALYTICAL CHEMISTRY
(4, 5 ) large electrodes, similar in size to the electrodes used in this study, were employed and the data reported could include factors caused by unconsidered electrode resistances. Both the increased resistance of large electrodes and the higher currents used can cause observable iR drops. However, proper design of the electrode holders c a n lower the resistance sufficiently in some instances to give negligible iR drops. EXPERIMENTAL
Carbon Paste Electrode Preparation. The electrode holders originally were prepared by sealing a piece of platinum wire between 7-mm. and 12-mm. soft glass tubing. The 12mm. tubing was cut to form a cup and t h e end was polished flat. The area of the electrodes was 0.71 sq.
cm. When the resistance problem was recognized, the cup was filled to within approximately 2 mm. of the top with Epon 828 resin (Shell Oil Co.) with the end of the wire projecting just beyond the resin. The top of the resin was painted with several coats of “silver printed-circuit paint” (GC Electronics Co.). This paint adhered well to the resin and made good electrical contact with the end of the wire. In addition to lowering the resistance while still keeping a large electrode area, this type of construction also produced the desirable condition that all points of the electrode surface were approximately equidistant from the external circuit connector, assuring that the potential would be the same at all parts of the surface. The usual construction of carbon paste electrode holders with only a wire sticking into the back of the paste does not achieve this
state. 4 n alternate method to prepare this more desirable electrode holder configuration would be to weld a wire to the back side of a platinum disk and to place this disk a t the bottom of a well machined into the end of a rod made of Teflon (Du Pont). Two types of carbon were used for the preparation of the pastes. One variety was G P 38 electric furnace graphite powder (National Carbon Co.) , which has been used by several workers for carbon paste electrodes (1, 5 ) . The powder particle size is listed as that size which passes through a 200-mesh screen (0.0029 inch) (8). The ratio of powder to Nujol mineral oil was 1 gram to 0.3 ml. This ratio gave a fairly dry paste, a property which helped to keep the resistance low. The other variety used was Graphon carbon black (Cabot Corp.). This black is prepared by heat-treating Spheron 6 channel carbon black above 2700" C. and it exists in the form of small polyhedral particles with an approximate diameter of 250 A. The surface of the particles consists chiefly of the basal layer planes of carbon, with the individual crystallites composing the particles having diameters of about 65 A. (!I). As received, the Graphon consisted of large, loosely combined aggregates of the smaller particles. To obtain a satisfactory paste, grinding in acetone with a mortar and pestle and drying in a vacuum desiccator was necessary before Graphon was mixed with Nujol. The ratio of Graphon to Yujol was 1 gram to 0.6-ml. This ratio gave a paste of about the same consistency as the G P 38 graphite powder paste. However, a smoother surface was obtained with the Graphon paste. Apparatus. The usual chronopotentiometric arrangement was employed. The current, which was measured with a 1% calibrated meter, was supplied to the electrochemical cell by a transistor constant current supply (6). The potential between the indicator electrode and a Leeds and Northrup saturated calomel reference electrode was followed with a differential amplifier whose output was fed to a Brown 10-mv. recorder with a 1-second pen response speed. The amplifier and recorder were calibrated with a Rubicon Model 2746 potentiometer. A Simpson LLIodel 260 volt-ohm-milliammeter was used to measure the electrode resistances. All measurements were made with the chronopotentiometric cell placed in a water bath held a t 25.0 0 . l 0 c. Chemicals. All chemicals were reagent grade. Solutions were prepared from doubly distilled water, the second distillation being from a n alkaline permanganate solution. Before use, the solutions were deaerated with purified nitrogen.
*
RESULTS AND DISCUSSION
I n our initial chronopotentiometric experiments with the ferrocyanideferricyanide system in 0.5M KCI, the curves usually appeared to have the shape of reversible waves, but were
Table I.
Reduction of Ferricyanide"
Transition Current, time, sec. pa.
Area. sq.
Electrode E l / , us. SCE, volt Slope cm. Platinum 0.226 f 0.001 0.064 21.4 f 0 . 1 169 0.29 GP 38 graphite powder paste 0 222 f 0.001 0.064 24 8 i 0 2 363 0 71 Gra hon carbon bPack paste 0.226 f 0.001 0 065 30.2 =t0 1 363 0.71 Electrolyte, 0.5M KC1; ferricyanide concentration, 10.0mM. Values given are the averages for at least three runs with precision given by range. 5
displaced on the voltage axis from the expected potentials for reversible behavior by 10 mv. to 100 mv. Chronopotentiograms with a platinum electrode indicated that the experimental set-up was functioning properly. Thus, the potential shift had to be a function of the carbon paste electrodes. Because the ferrocyanide-ferricyanide system has exhibited both reversible and irreversible behavjor a t carbon surfaces, depending upon the type of carbon and the experimental conditions ( 2 , 7 , fO), some potential shift could be caused by irreversibility. However, the reversible appearance of the curves suggested another reason for the potential shiftsiR drops within the electrodes themselves. When the resistances of the electrodes were measured, values from 30 ohms to 100 ohms were found for the G P 38 graphite powder paste electrodes. Even higher resistances were found for the Graphon paste electrodes, especially before the method of grinding with acetone was employed. Calculated iR drops agreed with the observed potential shifts of the curves. When the electrode holders were modified, the resistances dropped to approximately 1 to 2 ohms for both carbons. These resistances gave iR drops in the same order of magnitude as the error in the potential measurements. Table I gives some typical results for the ferrocyanide-ferricyanide system in 0.5M KC1 for the two carbon pastes in the modified holders along with the results for a platinum electrode. The E l l 4 potentials and the log plot slope values agree well with previous data for platinum electrodes and pyrolytic carbon film electrodes from this laboratory ( 2 ) . The variance in transition times for the two types of pastes, even though the geometrical areas are the same, is not unexpected, because the sensitivity depends upon the carbon to Nujol ratio (9). With the chronopotentiometric conditions used, essentially no difference was noted between the two types of carbon pastes. Although the use of carbon blacks for carbon paste electrodes has been mentioned previously (9), this report is believed to be the first mention
of the use of a well defined black. We believe that the use for paste electrodes of a carbon such as Graphon that has been employed in studies on the nature of carbon and whose, properties are known is more desirable than the use of an ordinary graphite powder with many unknown characteristics. Correlation of observed electrochemical phenomena with the nature of the carbon surface would be easier and would have more meaning. For example, one of the properties of a carbon which could affect its electrochemical response is the number of edge carbon atoms present compared with the number of surface atoms. Because the structure of Graphon is fairly well defined, an estimation of this ratio could be made for it. To give an estimation of this ratio for a graphite powder like G P 38 would be difficult, because the powder particles are a mixture of many sizes and shapes. I n chronopotentiometric studies of the bromide-bromine couple Davis and Everhart (4) found a difference between the El/, value for a platinum electrode and the value for a carbon paste electrode. The given current level of 0.846 ma. and a reported area of 1.25 sq. cm. (3) suggests the possibility that some of the potential shift could be caused by iR drop. Davis (3) reported that the resistances of the carbon paste electrodes were larger than the resistance of the platinum electrode, but that no actual values were recorded. To check the possibility that some of this potential shift was from iR drop we carried out the oxidation of lOmM NaBr in 1'34 H2SO4with our modified holder using the G P 38 graphite powder paste. As a check, with a platinum electrode an El,, value of $0.94 volt us. SCE was obtained. Davis and Everhart reported a value of f0.92 volt. This discrepancy with platinum might be explained from our use of a cylindrical wire electrode compared with their use of a shielded electrode. Because of the problem of bromine sorption a t the carbon paste electrode we had difficulty in obtaining consistant El/, values. However, the values averaged around f1.06 volt VOL. 37, NO. 6, MAY 1965
767
and were never greater than f1.08 volt. Davis and Everhart reported a value of + l . l O volt. If we assume that the difference between their value and ours is iR drop and if we take a value of 40-mv. for this iR drop, an electrode resistance of approximately 50 ohms is calculated with their current value of 0.846 ma. This calculated resistance value falls within the range of resistances observed by us in our unmodified electrode holder. The possibility of the inclusion of iR drop in the potential values of Davis and Everhart is thus suggested. Another possibility which cannot be discounted from the available data as a source of a t least part of the disagreement between El,, values is differences in the properties of the carbons used. However, in both cases graphitic carbons were used and their properties should be similar enough to minimize this source of disagreement. T h a t this possibility can be raised lends support to our previous statement that well defined carbons should be used for electrochemical studies involving carbon electrodes. I n ‘the anodic stripping voltammetry of gold and silver reported by Jacobs
( 5 ) , large area carbon paste electrodes were used also. With a variation of electrode areas from 0.196 sq. cm. to 1.32 sq. cm. he found that the plating rate decreased as the area of the electrodes increased. This variation of plating rate with electrode area suggests a possible resistance effect. However, from the magnitudes of the currents given, iR drops would be negligible unless the electrode resistances were high. Some other effect is probably the main cause of the variation. Although carbon paste electrodes have been used successfully for many studies, the brief observations reported here show that there are many things that need to be learned about the nature of carbon paste electrodes and about the nature of carbon electrodes in general. One still must approach results obtained with carbon electrodes with care.
ACKNOWLEDGMENT
Graphon carbon black was obtained through the courtesy of Walter R. Smith of the Cabot Corp. Electronic equipment was designed and constructed by Richard Mueller.
LITERATURE CITED
(1)Adams, R. N., Rev. Polarog. (Japan) 11, 71 (1963). (2) Beilby, A. L., Brooks, W., Jr., Lawrence, G. L., ANAL.CHEM.36,22(1964). (3) Davis, D. G., Louisiana State Uni-
versity at New Orleans, New Orleans, La., private communication. (4) Davis, D. G., Everhart, ST. E., ANAL.CHEM.36. 38 (1964). ( 5 ) Jacobs, E. S., ibzd.; 35, 2112 (1963). (6) Malmstadt, H. V., Enke, C. G., Toren, E. C., Jr., “Electronics for Scientists,” p. 148, W. A. Benjamin, Inc., New York, 1962. (7) Morris, J . B., Schempf, J. M., ANAL. CHEM.31, 286 (1959). (8)TNational Carbon Co., New York, Tu. Y., Catalog- Section 5-7655 (February 1960). (9) . . Olson. C.. Adams. R. N.. Anal. Chim. ’
Acta 22: 582 Cl96Oi. (10)Olson, C., Adams, R. N., Zbid., 29, 358 (1963). ( 1 1 ) Walker, P. L., Jr., Am. Scientist 50, 259 (1962). I
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ALVINL. BEILBY BRUCER. MATHER Department of Chemistry Pomona College Claremont, Calif. 91713 Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the Undergraduate Research Participation Program of the Xational Science Foundation for support of this research.
Errors in Vacuum Thermogravimetry SIR: For a research program which involves evaluating kinetics of decomposition of polymers, the author recently built a vacuum microthermobalance. Small samples were used to minimize diffusion controlled weight loss and to promote better homogeneity of temperature than can be obtain-d with larger samples. The value of vacuum is its tendency to remove decomposition gas molecules from the area of the polymer so that secondary reactions are reduced. A sketch of the thermobalance system is shown in Figure 1. I t is based on the
VACUUM SYSTEM WATER-COOLED .OlNT
SAMPLE PAN THERHOCWPLE
REFERENCE PAN CERAMIC THERKICRRLE INSULATOR bND PAN SUPPDRT
SCREEN W W R T ASS SUPPORT F W
Figure 1.
768
Cahn Model R G electrobalance. The furnace is the Hevi-Duty Type MK1012s. The temperature controller and recorder is a Minneapolis-Honeywell system composed of a programmed temperature controller-recorder with proportional control output to a magnetic amplifier and saturable reactor. Data are recorded with an Electronic Associates, Inc., Model 1110 Variplotter. The only unusual features of the system are that the thermocouple which is used to measure and control temperature was spot-welded to a pan which was placed so as to be in a zone where the temperature would be equal to that of an empty sample pan, and that a grounded Type 316 stainless steel screen cylinder liner was used to reduce electrostatic effects. Several types of errors including bouyancy effects, radiometric effects, and thermomolecular flow, can influence vacuum thermogravimetry. Some of these have been discussed in the three volumes on Vaccuum RIicrobalance Techniques (5-7) and by Duval ( 3 ) . Others nere demonstrated by Cahn and Schultz ( 1 ) . Curtiss ( 2 ) pointed out that momentum transfer effects could be important in vacuum gravimetry, according to the relationship
~ = m - 1- - dm Thermobalance system
ANALYTICAL CHEMISTRY
g
cyv
dt
where weight as read by the balance, grams m = actual weight of sample, grams g = acceleration due to gravity, cm./sq. second ff = geometric factor 21 = velocity of ejected gas, cm./second dm/dt = rate of change of actual weight, gram/second w
=
* -0.21
1
\
SAMREBELWMN
W HEATINGRATE: IO‘CIMIN.
400
5 00
600
700
TEMFfRATURE.’C
Figure 2. Effect of Teflon pyrolysis gases on empty sample pan
