trolysis is being governed by the rate of mass transfer to the electrode, which takes place solely by linear diffusion at this electrode, i o ~ 1 / 2 / Cshould o be constant over a fairly wide transition time range-e.g., 10 to 60 seconds. This is especially important in the analysis of multicomponent mixtures, where components giving rise t o later transitions nil1 be more affected by nonlinearity of diffusion and convection. Furthermore meaningful diffusion coefficients and transition time constants can only be calculated under these conditions. The optimum range of transition tinws for analytical application is 10 t o 60 (or more) seconds. The interference of electrode double layer capacitance, oxidation. and roughness takes place a t short transition times, and measurenicnts in this region are, in general, less accurate and more difficult to make. The use of long transition times also enables meawrement of smaller concrntrations of elcctroactive substances. The ratio of Qc to Q e , the electricity consumed in the electrochemical reaction, is ( 4 ):
loner the concentration that can be determined. For a one-electron reaction, with D of 1.0 x sq. em. per second and a transition time of 100 seconds, the minimum concentration that can be measured is 7.4 X 10-521f. Using present techniques, it is unlikely that concentrations much below 10-5LU can be determined chronopotentiometrically (6). Although this study was primarily concerned with solid electrodes, some of the results should be applicable to mercury electrodes as well. In general. mercury pool electrodes will be useful a t shorter transition times, since (CJav is generally smaller, and oxide film formation and roughness factors are absent. A mercury pool is usually shielded by the walls extending above it, so that diffusion is usually linear. Hon evpr since diffusion always takes place in a downward direction to a mercury pool, interference from natural convection will occur for many electrode reactions. Under these conditions short transition times would be most desirable. ACKNOWLEDGMENT
Assuming (CJav. is 20 pf. per sq. em. and AE is 0.1 volt, for QE to be smaller than lY0 of Q,,Co > 2.34 X 'nD1r2~1'2 moles per litw, so that, for a given electrode reaction, the longer the tranqition time employed (thc smaller the current density), the
The author is grateful to Clarence Williams for the construction of the shielded electrodes. LITERATURE CITED
( 1 ) Anson, F. C., Lingane, J. J., J . ilm.
Chem. SOC. 79, 1015 (1957).
(2) Bard, A. J., Anal. Cham. Acta 21, 365 (1959). ( 3 ) Davis, D. G., Ganchoff, J., J . Electroanal. Chem. 1 , 248 (1959/60). (4) Delahay, P., "New Instrumental Methods in Electrochemistry," Chap. 8, Interscience, S e w York, 1954. (5) Gerischer, H., Delahay, Paul, Anal. Chzm. Acta 18, 12 (1958). (6) Gerischer, H., Krause, RI., 2. physik. Chern. (Frankfurt)10,264 (1957). ( 7 ) Gierst, L., thesis, University of Brussels, 1952. (8) Ibl, N., Proc. 8th Meeting, Intern. Comm. Electrochem. Thermodynam. and Kinet. pp. 174-89, Madrid, 1956. (9) Laitinen, H. A., Ferguson, IT. S., AXAL.CHEM.2 9 , 4 (1957). (10) Laitinen, H. A., Gaur, H. C., Anal. Chim. Acta 18, 1 (1958). ( 1 1 ) Laitinen, H. A., Kolthoff, I. AI., J . Am. Chem. Soc. 61,3344 (1939). 112) Lineane. J. J.. AXAL.CHEJI.26, 1021
science, New Yorl (14) Ibid., pp. 626-33. (15) Riamantov, Gleb, Delahay, Paul, J . Am. Chem. Soc. 76,5323 (1954). (16) Nicholson, M. 31.; Karchmer, J. H., ANAL.CHEM.27, 1095 (1955). (17) Popat, P. V., Hackerman, N. J., J . Phys. Chem. 62, 1198 (1958). (18) Reilley, C. N., Everett, G. W., .Johns. R. H.. ANAL. CHEM.27, 483 (1955). (19) Voorhies, J. D., Furman, 3 . H., Ibid., 31,381 (1959). (20) Voorhies, J. D.. Parson, J. S., Ibid., ' 3'1, 516 (19i9). RECEIVEDfor reviex July 29, 1960. Accepted October 13, 1960. Work supported by the University of Texas R e search Institute.
Coulometric Generation and Back-Titration of Intermediate Reagents at Controlled PotentiaI Application to the Determination of Plutonium W. D. SHULTS Analytical Chemistry Division, Oak Ridge National laboratory, Oak Ridge, Tenn.
b Coulometric reagents for secondary processes may b e electrolytically generated and back-titrated at controlled electrode potential rather than controlled electrolysis current. Some of the advantages of this technique and the factors to b e considered are mentioned, and the technique is illustrated by application to the indirect determination of plutonium(V1) with iron(11).
T
development in recent years of completely electronic instruments ( I , 5 ) has aroused interest in controlledpotential coulometric titrimetry as a practical and routine method of analysis. HE
These instruments are simple to operate, require minimum operator attention, eliminate the need for chemical or mechanical coulometers, and yet exhibit a very high degree of precision and accuracy. With instruments such as these, intermediate reagents for secondary coulometric titrations can be conveniently generated at controlled electrode potential rather than a t controlled electrolysis current. il number of advantages are to be gained by this technique in addition to those already known for controlledpotential and controlled-current coulometric titrimetry. Some degree of selectivity is available when this tech-
nique is used and, in fact, the successive generation of different intermediates becomes practical. Perhaps more important is the fact that a n intermediate reagent can be prepared in excess and, if its couple is reversible, the excess back-titrated electrolytically; this technique is particularly useful for slow reactions or for reactions that must be driven to completion. No indicator electrode system is required, because the magnitude of the electrolysis current itself indicates the degree of completion of the electrolysis. The effect of interfering substances can sonietimrs be eliminated by this technique. The application of the controlled-potential VOL. 33, NO. 1 , JANUARY 1961
15
generation technique to the determination of plutonium(V1) with electrolytically generated iron(I1) is described herein. Carson, Vanderwater, and Gile ( 2 ) studied the determination of plutonium as plutonyl ion with ferrous ion that was generated electrolytically a t constant current. Their procedure was very satisfactory in the microgram range for which it was developed, but it did require rather lengthy and complicated chemical treatment of the sample before titration. Later, Scott and Peekema (7, 8) and Shults (10) studied the determination of milligram quantities of plutonium by controlled-potential coulometric titration. The procedure used by these workers requires little or no chemical pretreatment of the sample and is very precise. It is free of a number of interferences; unfortunately, iron does interfere if its concentration is not low relative to that of the plutonium. It seemed feasible to determine milligram quantities of plutonium in the presence of large amounts of iron and uranium by reaction of plutonium(V1) with an electrolytically generated excess of iron (11), followed by electrolytic back-titration of the excess. This method is analogous to the most precise method known for the determination of plutonium-the weightvolumetric method of Waterbury and Metz (12). Generation of the ferrous excess a t controlled potential precludes the formation of any uranium(1V) by electrolytic reduction of uranyl ion a t the working electrode. As has been shown by previous coulometric studies (9, 11), that reduction is undesirable because the reaction between plutonyl ions and uranium(1V) is sluggish and because the irreversibility of that couple makes impossible the backelectrolysis of uranium(1V).
reported with this oxidant by Waterbury and hletz ( I d ) . The oxidation must, however, be made under carefully controlled conditions to minimize the production of oxidation products which may interfere in any subsequent reductimetric reaction. The conditions given below were found to be satisfactory for the present application.
1
''-8
27
___
06 C5 Ecpvi S C E , r a t
-
4
c 3
Figure 1. ( = Q) for
E c p vs. readout voltage electrolytic reduction of 5.1 19 mg. of PuIV and 1 .OO mg. of Fer11 in HClO, and HzS04
of 25.593 mg. per ml. and the atomic weight of plutonium in this solution was found to be 239.07 grams by mass spectrometric assay. The plutonium concentration of the solution was estimated to be 25.57 mg. per ml. *0.7% (average of eight determinations) by potentiometric titration using ceric sulfate as titrant (3). Coulometric titration a t controlled potential (10) indicated that the solution contained 25.58 mg. per ml. h O . l % of plutonium (average of five determinations). Solutions were prepared from iron wire and U308, in In1 HCl and 1M "03, respectively. The iron solution contained 5.00 mg. per ml. of iron; the uranium solution contained 199.1 mg. per ml. of uranium. Aliquots were taken from and dilutions were made on these solutions as needed. All other reagents were analytical reagent grade. Tank COz was used to provide an airfree atmosphere within the cell; it was fed to the cell through distilled water contained in a single gas-washing tower. DISCUSSION AND RESULTS
APPARATUS AND REAGENTS
Instrument. An ORNL Model Q-2005 automatic electronic controlled-potential coulometric titrator (4-6) was used for this study. Minor changes (6, 10) had been made in the circuitry of that instrument in order to provide the increased sensitivity necessary for titration of substances of high equivalent weight. Instructions for the calibration and operation of this type of instrument are available (4). Titration Assembly. The titration assembly is conventional, incorporating a platinum gauze working electrode, saturated calomel reference electrode, and mechanical stirring; it has been described fully (10). Reagents. Plutonium metal of 99.93% purity was obtained from the Los Alamos Scientific Laboratory. A known weight of the metal was dissolved and diluted t o volume with 6 M HC1. T h e solution was prepared to have a plutonium concentration 16
ANALYTICAL CHEMISTRY
Preparation of Plutonium(VI). Hot, concentrated perchloric acid will quantitatively oxidize plutonium to P U O ~ and + ~ excellent results hare been
Table 1.
Effect of Solution Age upon Titration Results [Three standard solutions were prepared to contain 25.59 mg. of Pu as Pu(V1) by
the method described in the text. Aliquots were analyzed periodically during 24-hour periods.] Time Lapsed slI1ce Pu Found, Mg., Diluted in Dilution, 1M 1M 1M Hr. HC10, HCl HzSOa 0 0.5 3 24
25.56 25.64 25.67 25.63
25.57 25.64 25.68 25.74
25.60 25.68 25.62 25.85
The sample, 1 ml. of concentrated HClOd, and 2 drops of concentrated HNO, are placed in a 30-ml. borosilicate glass beaker and evaporated to fumes under an infrared lamp. Use of a lamp for this initial evaporation eliminates boiling and/or spattering of the solution. The small amount of HNOI apparently reacts n.ith the more easily oxidizable components of the sample and results in less degradation of the HC1O4 and in better precision in subsequent titrations. The beaker is then transferred to a hot plate, 2 ml. more of concentrated HC1O4 is added, and the fuming is continued for 10 to 15 minutes. Ovidation of plutonium to plutonium (VI) is complete in 3 to 5 minutes; additional fuming removes nitrate from the solution. Samples have been fumed as long as 60 minutes without detectable effect upon the titration results. The solution is cooled briefly and is then quantitatively transferred to a 10-ml. volumetric flask; 1M HCl or 1174 H2S04 is used as wash solution and diluent. Normally, I-ml. aliquots are taken from the dilution for titration. Standard plutonium(V1) solutions were prepared in this manner using 1M HC104, 1M HCl, and 1M HZSO, as diluents. Aliquots from these solutions were then titrated periodically during a 24-hour period to determine what effect aging has upon the results. The results of these titrations are listed in Table I. Surprisingly, the effect is small. The data indicate that best results are to be expected when the time lapse between dilution and titration is minimized, but that the error is not large even when a diluted sample is allowed to age several hours before titration. No attempt was made to eliminate this effect by storage of the solutions in a controlled atmosphere. Choice of Titration Conditions. Either 1 M HClOb or 1M H2S04 can be used as the supporting-electrolyte solution in the present procedure. Both solutions were evaluated for this purpose, and data are given for each. Figure 1 shows the relationship between readout voltage (a measure of current consumed), &, and control potential, ECP, when 5.119 mg. of plutonium(1V) and 1.00 mg. of iron(II1) are successively reduced a t a platinum electrode in 1M HC10, and in 1-74 H2SO4. Data for this type of curve are obtained by making the titration in normal fashion, except that periodically during electrolysis the current is stopped by adjustment of the control potential, and the values of Q and E C P
a t that time are noted. Curves obtained by plotting a series of these points are especially pertinent because they are indicative of the reaction progress under actual operating conditions. The curves in Figure 1 show that the redox potentials for the iron(I1, 111) and plutonium(II1, IV) couples are separated approximately 250 mv. in 1M HC10, but that the potentials are very nearly equal in 1M &Sod. A separation of only 30 mv. was measured from similar Q us. EcP data obtained by electrolysis of the two couples separately.
version of P U O + ~to plutonium(IV), a 2-electron change: 4 H + PUOZ+' + Pu(") 2 H20
+
+
These considerations illustrate the care that must be exercised in choosing potentials for the coulometric generation and back-titration of intermediate reagents. Choice of the potential for generation of an intermediate is straightforward when the redox potential for that reagent is known. I n choosing a potential for back-electrolysis, however, consideration should be given to the species present, their valence states, and the desired course of the over-all reaction. Obviously, this technique should find greatest application for those reactions wherein the substance being 1M HC10, +0.72 $0.47 determined is not electrolyzable a t the 1M HzS04 $0.48 +O ,45 electrode during back-titration-that is, the secondary process involves an The control potentials chosen for irreversible couple or a precipitation generation of iron(I1) are only slightly reaction. The potential for backdifferent in the solutions of these two electrolysis can be chosen solely on the electrolytes and correspond to E'p. basis of the intermediate itself in that -180 mv. in each case: $0.290 volt instance. If the substance being deus. S.C.E. in HC1O4 and +0.270 volt termined does react reversibly a t the us. S.C.E. in HzS04. At these potentials, electrode, its formal redox potential iron(II1) is reduced to iron(I1) with would have to be sufficiently different 100~o current efficiency. The iron(II), as i t is generated, reacts with P U O ~ + ~from that of the intermediate to allow back-electrolysis of the excess interin the solution t o give plutonium(1V) mediate alone. In that situation, the and more ison(JI1) present technique would seldom offer any advantage over direct controlledpotential coulometric titration. I n the present procedure, 1M HzS04 is preferable to 1M HC104 as the supportr The plutonium(1V) itself is then elecing electrolyte because the complete trolytically reducible to plutonium(II1) reduction-oxidation cycle can be aca t the potentials used, so that when an complished in a smaller potential span excess of iron(I1) has been generated in H2S04. The performance of the there exists in the solution a mixture method was evaluated with each elecof iron (11) iron (111), plutonium (111), trolyte, however, and is summarized and possibly plutonium(1V). The relin Table 111. Five to eight determinations were made a t the concentration ative concentrations of these species will depend upon the amount of relevels listed and in each electrolyte duction that has taken place. An excess in order to estimate the precision and of iron(I1) is definitely required in accuracy of this method. The precision order to obtain accurate results, as the and accuracy that can be obtained with data in Table I1 indicate. These data this method are limited by the procedure were obtained by titrating aliquots of a for preparation of plutonium(V1)that is, by the operations that are instandard plutonium(V1) solution indirectly as outlined below but with difvolved and by the perchloric acid degferent excesses of iron(I1) being genradation products that are introduced. erated. I n general an excess of 10 to Use of larger samples and/or aliquots 20 peq. is generated; a larger excess would minimize these effects, but this does not adversely affect the results. was impractical. Because of the mixture of valences Table IV presents the results obtained existing when the reduction electrolysis when a known amount of plutonium is complete, the potentials for backwas determined in the presence of electrolysis are considerably different various amounts of iron and uranium. in the two electrolytes studied. They The data indicate that plutonium can correspond to Eopu 180 mv. in each be determined accurately when the iron case: +0.900 volt us. S.C.E. and/or uranium weight ratio to plutoin 1M HClO4 and +0.660 volt US. S.C.E. nium is as high as 10 to 1; above the in 1M H2S04. At these potentials ratio unreliable answers are obtained because the formation of solids interiron(I1) is oxidized to iron(II1) and feres with the fuming step. Examinaplutonium(II1) is oxidized to plutonium (IV), each with 1 0 0 ~ current o efficiency. tion of the effects of ions other than iron The net result in the generation and and uranium was unnecessary for the back-titration cycle therefore is conpresent application. That information
Table II. Results of Indirect Titration of Plutonyl Plutonium When Various Excesses of Titrant Are Generated
(Pu taken Excess
= 25.59
mg.)
Fe(r1)
Generated, req.
Pu Found, Mg.
Error,
0 5.0 10.0 10.6 19.9 40.0
24.13 25.46 25.60 25.57 25.59 25.55
-5.7 --0.5
