ACKNOWLEDGMENT
We thank D. S. Buster for assistance with the statistical calculations. LITERATURE CITED
( I j Coleman, C. I., Brown, Ti. Lc., Moore, J. G o ,Grouse, D. J., I n d . Eng. Chem. 50,
1756 (1958).
(2) Dougherty, T. F., Stover, R. J., Dougherty, J. H., Jel:, W.6. S.,Mays,
C. W., Rehfeld, C. E., Christensen, W. R., Goldthorpe, H. C., Radiation Res. 17, 625 (l9V2). (3) Hindman, J. C., Anies, D. P., “The Transuranium Elenients,” U. S. At. Energy Comm. Paper 4.2, IV-14B (1949j. (4) Holstein, V., Hoogmn, A. H. AI., Kooi, J., Health Phys. 8, 1 (1962). ( 5 ) McLane, C. IC., Dixon, J. S., Hindman, J. C. , “The Transuranium Elements,” IJ.S.At. Energy Comni., Paper 4.3 IV-14B (1949).
(6) Petrow, H. G., Sohn, Bernard, Allen,
R. J., ANAL.CHEM.30, 1301 (1961). (7) Sanders, 8. M., Leidt, d. C., Health Phys. 6 , 189 (1961). (8) Stover, B. J., Atherton, I). It., Bruenger, F. W., Buster, D., l b d , 8 , 589 (1962). (9) Weiss, H. V., Shipman, W. S , ASAL. CHEM.33,37 (1961). RECEIVED for review February 11, 1963. Accepted July 8, 1963. ork supported by the U. S. Atomic Encrgg Conmiissicin under Contract AT( I 1-1)-11‘3.
Am pe rometric Tit rat io n of PIutonium(VI) with Iro n(II) C. A. SEILS, Jr., R. J. MEYER, and R. P. LARSEN Chemical Engineering Ilivision, Argonne National laboratory, Argonne, 111.
b A method has been developed for the precise titration of plutonium(V1) with iron(ll) in which the end point is detected amperometrically with a rotating platinum electrode. The plutonium is oxidized to the sexivalent state in a sulfuric acid medium with argentic oxide, the excess oxidant destroyed by heating, and the plutonium titrated b y the addition of standcird iron(ll) sulfate from special weight bLrets. For 15-mg. samples, precisions of &0.0670 relative standard deviaticn are obtainable with no significant bicis. For 0.2-mg. samples, the relative standard deviation is 0.4% with a 0.6% negative bias.
A
R ~ ~ C E N T L Yas 1957 a review by Metz (8) of the analytical chemistry of plutonium revealed that only one precise method was available for the determination of plutonium. This method, which was h t reported by Boaz, et al. ( 1 ) was Lie potentiometric titration of plutonium JII) to (IV) with cerium(1V) sulfate. By the use of weight burets for both aliquoting and the addition of titrant a precision of *0.05% is obtainable (8). Iron, the most common contaminant of plutonium metal and its compour,ds, interferes and a correction for the iron by means of a spectrophotometric determination is necessary. Several tariations in the titration of the (111-1.V) couple, such as photometric end point detection (d), and use of potassium dichromate as the titrant (IO),have sine€ been introduced. More recently several new highprecision analytical methods for plutonium have been reported (11, 13, 14) in which the interference of iron has been either eliminated completely or significantly reduced. Scok and Peekema (11) and Shults (13) used controlled potential coulometry to titrate Plutonium(II1) to (IV). At the 20-mg. level, a precision of *0.06% is obtainable. At iron to plutonium molar ratios of one or greater (an unusual situation) iron
s
does interfere, but otherwise the method was remarkably free of interferences. In a method developed by Waterbury and Metz (14) plutonium(V1) (obtained by perchloric acid oxidation) is reacted with excess standard iron(I1) sulfate to form plutonium(1V) and iron(TI1). The excess iron(11) is then back titrated potentiometrically with standard cerium(1V). With 500-mg. samples, a precision of *0.02% can be obtained. Shults (1.2) demonstrated that the excess iron(I1) could be determined equally well by controlled potential coulometry. Chromium, manganese, and vanadium interfere. Sulfate cannot be tolerated as it inhibits the preliminary oxidation of plutonium to the sexivalent state. Plutonium(V1) can be titrated directly with iron(I1) by incorporating an amperometric end point detection of the type used by Kolthoff and May (4) in the chromium(V1)-iron(I1) titration. In both the plutonium and chromium titrations essentially no current is passed by the rotating platinum microelectrode until the end point is reached and thereafter the current is proportional to the amount of excess iron(I1) titrant. Argentic oxide, the reagent introduced by Lingane and Davis ( 7 ) for the oxidation of chromium, manganese, and cerium, has also been found to be particularly satisfactory for oxidizing plutonium to the sexivalent state. The precision and accuracy of the method are comparable to those obtained in other titrations. During the preparation of this paper for publication, it came to the authors’ attention that Helbig (3) had also titrated plutonium(V1) amperometrically with iron(I1). I n principle the titration which Helbig reports is nearly identical to the one reported herein, but the titrations are quite different in practice. Helbig carried out his titrations a t the 0.1- and 1-fig. levels with relative standard deviations of k 6
and =k3’%, respectively; titrations carried out in our laboratory were a t the 0.2- and 15-mg. level with standard deviations of 2~0.4and 0.06%, respectively. h thorough and comprehensive review of this phase as well as the other phases of plutonium analytical chemistry c m be found in the recently published treatise by hletz and Waterbury (9). EXPERIMENTAL
Apparatus. The titration assembly used in this work consisted of a n E. 1%. Sargent and Co. “Ampot” with the microammeter replaced by the more sensitive Ealing Corp. “Scalamp” (KO.29-222), a 600-r.p.m. synrronous rotator for the platinum microelectrode, and a low resistance mercurymercurous sulfate reference electrode. Saturated potassium sulfate solution was the electrolyte used in the reference electrode and salt bridge. Iliffrision of the electrolyte from thc bridge into the solution being titrated was prevented by a closure of unfused vyror (Corning So. 7930). Operation of the assembly is 1)ropcr if a plot of current us. weight of 0.01 meq. per gram of iron(I1) (corrected for volume changes) is linear in the ranre 0 to 50 kta. When not in use the platinum microelectrode is stored in 1.Y sulfuric acid. Before use it is operated for 2 minutes a t +1.2 volts us. the reference electrode. Without this treatment, the residual current in the first titration may be himh. ?he weight buret used in this analysis is shown in Figure 1. Its particular uses and advantages are covered in the “lliicussion” section. Reagents. The plutonium, iron(lI), and chromium(V1) solutions used in this a n 11) sis are prepared and aliquoted by w i g h t rather than volume. Concentrations are evpressed in milliequivalents per gram of solution. Plutonium chloride, 0.05 meq. per gram, is prepared by dissolving National 13ureau of Standards (NBS) high purity plutonium metal in 3N hydrochloric acid. VOL 35, NO. 11, OCTOBER 1963
1673
Ferrous ammonium sulfate, 0.05 meq. per gram, in 1N sulfuric acid is standardized daily by amperometricall y titrating against standard 0.05 meq. per gram potasqium dichromate (NBS). The spread between duplicates should be 0005% or less. Lower concentrations are prepared by dilution with 1 N sulfuric acid. Silver(I1) oxide is obtained from Handy and Harmon, 82 Fulton Street, S e w York 38, N. Y. Procedure. Dissolve or dilute the samples to a concentration of 5 t o 10 mg. of plutonium per gram of solution. Transfer 20 t o 25 ml. of this solution t o one of the special weight burets. TTeighing t h e buret before a n d after to t h e nearest 0.1 mg., transfer a n aliquot containing from 10 t o 20 mg. of plutonium t o a 50-ml. beaker. Add approximate Iy 0.5 ml. of 18N sulfuric acid and fume without spattering to dryness. Cool the sample to room temperature and dissolve the plutonium sulfate in about 10 ml. of 0.5N sulfuric acid. Vhile swirling the solution, slowly add 25-mg. portions of silver(I1) oxide until a n excess is present. The presence of excess oxidant is determined by a characteristic blackish-brown color and the presence of a black precipitate of unknown composition (possibly Ag,OS). After 10 to 15 minutes, dilute the solution to about 25 ml. with 1N sulfuric acid and warm on a hot plate to 85’ C. t o destroy the excess oxidant. (Both the dark color and the black precipitate disappear during this step.) After the solution has cooled to 40” C. or less, insert the platinum microelectrode and salt bridge arm of the reference electrode in the solution, start the rotator, and add 10 ml. of 18N sulfuric acid. With the microammeter shorted (to protect it from the charging current) apply a potential of +0.6 volt os. the reference electrode. After 1 to 2 minutes reconnect the microammeter and record the current as soon as i t is steady. This may take as long as 5 minutes but the time is usually much shorter. The value should be 0.2 pa. or less. (Persistently higher “residual currents” indicate the presence of interfering ion species which are being oxidized at the platinum electrode. Hydrogen peroxide formed by alpha particle radiolysis has been shown to give this effect.) Weigh the weight buret containing the standardized iron(I1) solution and slowly add increments to the sample solution until a steady current of about 2 pa. is obtained. Because of the slowness of the reaction, c.f. “Discussion,” the current after the addition af each increment increases rapidly to 5 to 10 pa. and then decreases in 1 or 2 minutes to less than 2 pa. (This is particularly true at the start of the titration.) Reweigh the buret and record the weight and current reading. Add more iron(I1) solution until the current reaches approximately 20 pa. Again weigh the buret and record the weight and current reading. The end point in the titration is the intersection of the horizontal residual current line and the iron(I1) diffusion current line. The residual current line which has a 1674
ANALYTICAL CHEMISTRY
I OUNCE BOTTLE
FROM 2 OUNCE BOTTLE
Figure 1.
Weight buret
Material-Polyethylene
slope of zero is fixed by the initial current measurement] c.f. “Discussion” ; the iron(I1) diffusion current is fixed by the second and third current-weight measurements. The end point can be determined either by calculation or graphically. I n the latter case, the plotted data has the form of a reversed letter
“L.”
DISCUSSION
Plutonium (VI-Iron (11) Reaction. The rate of plutonium(V1)-iron(I1) reaction is markedly slower than t h e chromium(V1) - iron(I1) reaction. This is demonstrated by comparing the observed current-time relationships prior t o end points in t h e respective titrations. I n the chromium(V1)-iron(I1) amperometric titration (4) the current may rise momentarily after the addition of a 100-mg. increment of the iron(I1) titrant but returns to the initial value within the response time of the microammeter. Thus several points may be taken prior to the end point to help fix the residual current line. However, only the determination of the initial residual current is necessary to fix the residual current line since its slope is zero. As has been noted above in the “Procedure,” the current in the plutonium(V1)-iron (11) amperometric titration rises to about 10 pa. after the addition of the first 100 mg. of titrant and then drops off exponentially. The current reaches 2 pa. in 1 or 2 minutes and the initial residual current value of 0.1 to 0.2 pa. in 30 minutes or more. As the titration progresses, the current rise becomes smaller and the subsequent current fall becomes more rapid, but the fall is never rapid enough to warrant other measurements of the residual current. For this reason, only the initial current reading is used to establish the residual current line. The assumption that the slope of this line is also zero, as in the chromium(V1) titration, appears to be warranted by the
precision and accuracy of the present results. The increase in current response as the titration progresses suggests that the reaction products have a catalytic effect on the reaction rate. This is borne out by the pronounced effect that iron(1IT) ions have on the rate of t h r plutonium(V1)-iron(I1) reaction. The addition of 0.2 meq. of iron(III), the amount formed in titrating 24 mg. of plutonium, increases the current response at the start of a titration by about a factor of 5. (This is true of both the magnitude of the current rise on the addition of titrant and the rate of current decrease thereafter.) This effect is used to advantage in the titration of smaller amounts of plutonium where the reactant concentrations may he lower by a factor of 100. By adding a milliequivalent of iron(II1) prior t o the start of the titration, the titration can be carried out without undue delay. Oxidation. The oxidation of plutonium to the sevivalent state prior to its titration with bivalent iron requires a n oxidant which can bf readily destroyed once the ovidation has been completed. Waterbury and N e t z (14) and Shults (12) found fuming perchloric acid to be a satisfactory oxidant when the reaction waq carried out under closely controlled conditions. The authors of the present work, however, were unable to obtain reproducible results with this reagent. This difficulty is probably due to a cooling effect created by the unusually strong draft in the fuming facility. When the oxidation mas carried out in uncovered beakers, results mere invariably low; when the oxidation was carried out in covered beakers, the results were often as much as 1%, higher than the theoretical value. Since high rcsults were always associated with small initial cathodic currents, it is presumed that the high results were caused by the decomposition of the perchloric acid and the subsequent titration of these decomposition products. Argentic oxide which was demonstrated by Lingane and Davis ( 7 ) to be particularly useful for the oxidation of chromium, cerium, and manganese has been found to be an equally satisfactory oxidant for plutonium. At room temperature and in 0.5N sulfuric acid, the plutonium is oxidized rapidly t o the sexivalent state. Hrating the solution to about 85” C. is sufficient treatment to destroy the elcess oxidant. Incomplete oxidation of the plutonium can occur at higher sulfuric acid concentrations even if a large excess of argentic oxide is added. I n 3M sulfuric acid, for example, the oxidant is rapidly destroyed by reaction with mater before the plutonium is oxidized. T o ensure a satisfactorily low acidity the samples are fumed to dryness with sulfuric acid
and then dissolved in 0.5N acid. Allowing the dry samp1.s to stand for a number of days before dissolving and titrating them did not affect the results. However, prolonged baking of the sample after the last of the sulfuric acid had been removed made it difficult t o redissolve the plutonium. Buret. Syringe-type microburets were used in t h e initial stages of this investigation t o deliver t h e small volumes (2 t o 4 ml.) of titi ant. However, as t h e method mas being refined, i t became apparent t h a t t h e precision of t h e analysis was being; limited by t h e reproducibility of the buret deliveries a n d readings. A weight buret of tEe type shown in Figure 1 has proved to be particularly convenient in this and other amperometric titrations where the increments of titrant are finite-it., no drop splitting is required. The buret is essentially a 30-ml. polyethylene wash bottle with the delivery tip d r a m down t o a diameter of about 0.03 inch. Because of the nonwetting characteristics of polyethylene and the small diameter of the delivery tube, thi’ unused titrant will return in its entiri?ty to the bottle if finger pressure is carefully applied and relieved. Drop size is about 15 pl. The buret is handled within a polyethylene secondary container and transferred from the secondary to the balance pan and back with forceps. This avoids weight changes and contamination which might occur as the result of direct contact between the buret and the analyst’s gloved hand, Aliquoting of the plutonium solutior s to be analyzed is done with the samv equipment and technique. Weight loss of the buret, presumably the resull of diffusion of water vapor through the sides of the polyethylene bottle, amounts to 0.5 mg. per 24 hours. Interferences. Such elements as vanadium, manganese, chromium, and cerium, which, like plutonium, are oxidized by argentic oxide and reduced by ferrous sulfate, will interfere in this analysis. There are, iowever, a number of proved separation methodq which will Rith comparative ease selectively remove plutonium from all these interfering elements. I n strong hydrochloric acid plutonium(1V) can either be extracted with 3001, tributyl phosphate-inert diluent (6) or adsorbed 011 Dowes-1 anion ex:hange resin (9). Back extraction or elution is effected with dilute hydrochloric acid plus a reducing agent. Plutonium(1V) is also adsorbed onto Doweu-1 from strong nitric acid (6). Elution in this case is effected with 0.3N h,?drochloric acid0.01M hydrofluoric E cid. Results of our studies confirm the findings of Waterbury and Meta (14) t h a t the precision and accuracy of results obtained with plutonium samples that are
Table I.
0
Results of Amperometric Titration of 99.98% Plutonium Metal
Plutonium No. of taken, mg. detn. 10-20 10 0.10-0.30 13 For a single determination. Average of all determinations.
Rel. std. dev., 70a 0.06 0.4
subjected to separation procedures are seldom as good as those obtained with pure materials. However, Kressin and Waterbury (5) have recently reported excellent results when using the nitric acid-Domex-1 system on larger samples (200 mg). Difficulty is, of course, encountered in the sulfuric acid fuming step when the amount of salts other than plutonium becomes large. Spattering, precipitation, and/or incomplete chloride and nitrate removal may be encountered. Since the precision and accuracy requirements become correspondingly less severe as the concentration of plutonium in alloys or processing samples decreases, the extraneous salt problem can be circumvented t o some extent by reducing the amount of plutonium t o be titrated from 10 t o 1 mg. or less and titrating with a correspondingly more dilute iron(I1) sulfate solution. Particularly careful consideration should be given t o the interference problem in samples where the plutonium concentration is low, since seemingly insignificant impurity concentrations may affect the accuracy of the results. For example, in a 1% plutonium-uranium alloy, 10 p.p.m. of manganese (equivalent weight equals 11.0) will introduce a +1% error in the plutonium (equivalent weight equals 119.5) result.
Plutonium purity, %* 99.97 99.4
Error, yo -0.01 -0.6
?‘he method was also tested on a limited number of samples which required a preliminary separation of the plutonium from interferences. Coprecipitation of plutonium with lanthanum fluoride (6) from a hydrochloric acid solution of a uranium-20ye plutoniumB%-fission element alloy resulted in plutonium recoveries of 98.6 rt 0.5%. The low recovery and lack of precision are very probably attributable to incomplete precipitation rather than difficulties in the titration. Separation from a 200-fold excess of magnesium was effected by an extraction of plutonium(1V) from hydrochloric acid into 30Ye tributyl phosphate-carbon tetrachloride solution (6). Recovery was 99.9% with a relative standard deviation of 0.2%. LITERATURE CITED
(1) Boaz, H. E., Numerof, P., Potratz, H. A., Throckmorton, W. H., U. S. At. Energy Conim, Rept. MDDC-279
(1946).
(2) Byrnes, J. T., private communication,
Rocky Flats Plant, Dow Chemical Company, 1960. (3) Helbie, W.. 2. Anal. Chem. 182. 19 ‘ (1961).’ (4) Kolthoff, I. N., May, D. R., ANAL. CHEM.18, 208 (1946). ( 5 ) Kressin, I. IC, Waterbury, G. R., I b i d . , 34, 1598 (1962). 16) Larsen, R. P.. Seils. C. A,. Ibid.. 32. 1863 (1960). ’ ( 7 ) Lingane, ’J. J., Davis, D. G., Anal. Chim. Acta 15, 201 (1956). ( 8 ) Mctz, C. F., ANAL. CHEM.29, 1748 (1987). (9) Metz, C. F., Wnterburx: G. R., “Treatise on Anal. Chem., Part 11, Tolume 9. I). 189. Interscience Publiphers. Sgw’York. 1962. (10) Piebi, C. E., haglio, J. A., TuZantu ,
RESULTS
The precision and accuracy of the method were checked by analyzing a plutonium solution which had been prepared from high purity plutonium metal. The results of two typical tests are summarized in Table I. The purity of the metal, 99.98%, wa? determined by difference after spectrochemical, carbon, oxygen, and nitrogen analyses. The combined concentration of impurities such as chromium, vanadium, and manganese which would lead to high results was negligible. The titration of the more concentrated plutonium solution was done with 0.05 meq. per gram of iron(I1) sulfate; the less concentrated solution with 0.001 meq. per gram of iron(I1) sulfate. The small negative error, O.Olye, at the 15-mg. level is not statistically significant. No explanation can be offered for the small negative bias at the 0.2-mg. level.
I
6, 159 ( I 960). (11) Scott, F. A.,
Peekema, R. M., U. S. At. Energy Comm. Rept. HW58491 (1958). (12) Shults. W. D.. ANAL. CHEM.33. Shults, W. D., U. S. At. Energy Comm. Rept. ORNL-2921 (1960). ( 1 4 ) Waterbury, G. R., Metz, C. F., ASAL. CHEX 31, 1144 (1959).
(13)
RECEIVED for review February 78, 1963. Accepted July 12, 1963. Presented a t the Fourth Conference on Analytical Chemistry in Xuclear Reactor Technology, Grttlinburg, Tenn., October 1960. Argonnc National Laboratory is operated by the University of Chicago under Contract S o . W-31-109-eng-38. This work was performed under the auspices of the U. S. Atomic Energy Commission. VOL. 35, NO. 1 1 , OCTOBER 1963
1675
