Volumetric Assay Method for Plutonium Using Spectrophotometric End Point Detection C. E. CALDWELL, L. F. GRILL, R.
G. KURTZ, F. J. MINER,
and N. E. MOODY
Rocky Flats Division, The Dow Chemical Co., Denver, Colo.
b A titrimetric method for the assay of plutonium metal permits use of large samples and involves a spectrophotometric determination of the end point. The standard deviation as measured by assaying a number of aliquants of the same metal sample is 10.07a/,. 'The same precision was' obtained on .a series of separate metal samples that were assayed, then resubmitted. Only iron was present in the samples investigated in sufficient concentration to interfere. The iron reaction was stoichiometric, so a correction was applied after determining the iron in a separate aliquot of the plutonium,
A
Using a potentiometric method for determining the end point is probably the most widely used procedure for the assay of plutonium ( I , 6). The microvolumetric titration with which the authors are most familiar involved the potentiometric titration of plutonium(II1) to plutonium(1V). The plutonium was initially reduced to the trivalent state with a n excess of titanous chloride. followed by the oxidation of the excess titanous ion and then the oxidation of plutonium(I1.I) to plutonium(1V) with standard ceric sulfate. Rlicrotitration equipment was used. With this equipment, a n error of =t1 pl. was possible a t both the titanous and the plutonium end points, which is equivalent to 5~0.12% plutonium nhen applied to the 20-mg. sample used. I n the authors' experience, the potentiometric end points were not sharp enough to eliminate differences in operator interpretation of the exact end point. These breaks a t the end point nere not the same mith iron, arsenious oxide, or sodium oxalate, n-hich mere used t o standardize the ceric sulfate, as they were with plutonium that was dissolved in hydrochloric acid or 115th plutonium in nitric acid dolutions. I n addition, the indicator electrode was frequently poisoned so that replacenlent was necessary. Because of these difficulties, a n investigation was undertaken to find a procedure to replace the microvolumetric titration. The new procedure would have to eliminate two difficulties inherent in JlICROVOLU?rlETRIC TITRATIOK
346
ANALYTICAL CHEMISTRY
the microvolumetric titration procedure-the error associated with the use of small samples and the difficulty in determining the end point potentiometrically. In addition, the new procedure had to be simple and rapid. Gravimetric ( 7 ) , radiochemical ( 6 ) , titrimetric ( I , I O , I6), coulometric (3, 13), and spectrophotometric (9) methods have been described but do not meet the need for a simple rapid assay procedure having the required precision. Previous titrimetric procedures have, for the most part, used plutonium samples of 20 mg. or less (1, 15). B y increasing the sample size, an increase in precision could he expected. Such an increase in sample size IT as one of the essential elements in the precise titrimetric plutonium procedures developed by Waterbury and Metz (15),and Pietri and Baglio ( I O ) . Although in theory i t should be possible to obtain more satisfactory precison by modifying one of the microvolumetric procedures to use larger samples, the difficulties associated with the use of a potentiometric end point indicated that another approach was advisable. Therefore, a new titrimetric procedure was developed n hich permits the use of large samples and involves a spectrophotometrically determined end point. After investigating a number of redox indicators that have been used with ceric sulfate, ferroin ]vas selected for the plutonium titration. Koch used i t for the ceric titration of plutonium ( d ) , but indicated that the color change a as difficult to detect visually even with a 2.5mg. plutonium sample. With the much larger sample size in this procedure, i t was all but impossible to detect the end point visually. Plutonium(II1) is blue under daylight fluorescent light, while plutonium(1V) is brown. When present in sufficient quantities, the color of plutonium(1V) masks the red to light blue color change of the ferroin. However, this ferroin color change is detectable using a spectrophotometer. APPARATUS AND REAGENTS
Glove Boxes. Because of the toxic nature of plutonium, extreme care must be exercised (6). All work with solid plutonium was done in a glove box.
The plutonium solution was titrated in open hoods in a laboratory equipped t o handle plutonium safely. Spectrophotometer. The spectrophotometer was a modification of the instrument designed by Rost (12). The sensing unit was redesigned and consisted of the light source, a 510-mp interference filter x i t h a half band width of 10 mp, a cell compartment for a 250-ml. electrolytic beaker, and a cadmium sulfide cell. The output of the cadmium sulfide cell drives a 100-pa. meter whose calibration is proportional to transmittance. The 510-mp interference filter was selected because an absorbance peak in the ferroin spectra occurs a t that wavelength The cell compartment in the sensing unit mas not enclosed because the cadmium sulfide cell was unaffected by either the general lighting in the room or by the hood lights. Buret. A bottom filling, automatic zeroing 50-ml. buret calibrated in 0.1ml. units was used. To facilitate the addition of very small portions of ceric sulfate near the end point, the tip from a 3-ml. volumetric pipet was attached t o the buret tip with a short piece of Tygon tubing. The buret, with this tip, has a flow rate of 50 ml. in 2.5 minutes. There is a tendency for a drop of ceric sulfate to adhere to the self-zeroing tip of the buret. For greatest accuracy, this drop should be eliminated or a t least be consistent in size. By turning the stopcock off rapidly, the drop could be eliminated. All titrations were done in a temperature-controlled area, which eliminated correcting the titrant volume for variation caused by temperature change or jacketing the buret and circulating constant temperature water around it. Cerium(1V) Solution (0.0521). Five pounds of ceric bisulfate, Ce(HSOd4, was dissolved in a solution of 2.2 liters of concentrated H&04 diluted t o 40 liters with distilled water and thoroughly mixed by bubbling air through it for 3 days, After standing for a t least a month, the solution mas filtered through a fine porosity, sintered-glass filter funnel before using. The ceric solution was standardized using primary standard Asz03. When available, high purity p!utonium metal was used to standardize the ceric solution. This was preferable because the conditions are the same for standardizing as for titrating the samples.
Recently, a new plutonium standard material, csZPucl6, became available (8). As with the high purity plutonium metal, it was preferred over As203 for standardization of the ceric solution. Since plutonium in cS2Puc16 is present in the quadrivalent oxidation state, it must be reduced prior t o titration. A Jones or lead reductor is convenient for this (8).
Table I.
Sample ?;umber 1 2
Oxidation State of Plutonium Indicated b y Assay Values
Through Reductor Number Pu, mean yo aliquants normalized 100.01 4 16
100.00
Kot through R e d u c t o r Kumber Pu, mean yo aliquants normalized 4 99.98 14 100.00
RECOMMENDED PROCEDURE
Clean the metal as received t o remove the oxide. This can usually be done best by filing to a bright metal surface. If the metal mas stored under oil, mash with methylene chloride before filing. After cleaning, cut the required number of samples to approximately 500 mg. and weigh to the nearest 0.1 mg. Transfer the metal to a 250-ml. electrolytic beaker and dissolve in 5 ml. of 6.V HCI. Cover the beaker during dissolution. After the metal is dissolved completely, add 61V HzS04 to a final volume of 100 ml. -4fter adding 250 pl. of ferroin indicator (O.O23J1), transfer the beaker to the spectrophotometer, and titrate the solution. Add the ceric sulfate rapidly until near the end point, then in very small increments. At the end point there is a total break of approximately 60% full scale on the meter. This break occurs in several increments, the number of which depends on the size of the titrant increments. The end point is leached when further addition of ceric sulfate causes no appreciable deflection of the mc%er. 4 n example of the break obtained a t the end point is shonn in Figure 1. To prevent oxidat ion, titrate metal samples soon after they are dissolved. If this is not possibl(3, or if the sample is not initially available 15ith the plutonium all in the trivalent state, reduce the solution before titration using a Jones or lead reductor. To obtain the most accurate results, the ceric sulfate solution must be standardized by the same person n-ho titrates the samples. Run reagent blanks daily. They amount usually to only 0.10 ml. of the ceric sulfate solution. When Fell prepared reductors, either lead or Jones, have been used (8), they have not increased this blank,
Absorption spectra of solutions right after dissolution indicated the presence of only the trivalent state. To verify the absence of higher oxidation states, experiments were set up in nhich some samples were titrated immediately after dissolution while others were run through a lead reductor before titration. Results are shown in Table I. Statistically, there was no difference between the mean result on those samples that had been titrated immediately after dissolution and those that had been reduced before titration. To determine the maximum time of standing permissible between dissolution and titration, a series of metal aliquants was dissolved and then titrated, nith progressively longer elapsed times between the dissolution and the titration. Table I1 shows the results. These data indicate that if the metal sample is titrated within 5 minutes after dissolution, no error is introduced by prior oxidation of plutonium. But, if there is n longer elapsed time, the sample will have to be reduced before titration. Plutonium, initially present in an oxidation state higher than the trivalent, can be determined if it is first reduced to the trivalent state. For
EXPERIMENTAL RESULTS AND DISCUSSION
Oxidation State of Plutonium. The method, a s developed, requires t h a t all plutonium be in the trivalent state before titration n i t h ceric sulfate. If i t could be shown t h a t upon dissolution of a metal sample all of the plutonium is present as plutonium (111),no preliminary reduction would be necessary. B u t if not, some preliminary reductant would have t o be used. When metal samples are dissolved in hydrochloric acid, the blue color of the resulting solution indicates that initially the plutonium is in the trivalent state.
lot 41.68
41.70 41.72 41:74 41:76 41:78 rnl 0.05 N Ce (M Sulfate
41.80
Figure 1 . Typical break obtained on spectrophotometer at end point Sample composition, 500 mg. Pu, 5 ml. 6N HCI, 100 ml. 1N HzS04, and 250 PI. 0.025M ferroln
Table It. Influence of Time between Dissolution and Titration on Assay Result
Elapsed Time between Dissolution and Titration, Minutes 0 5
10
20
40
60
Pu, (;To
Normalized 100.00 100.02
99.91 99.84 99.75 99.69
direct application of the recommended procedure, however, the reduced plutonium solution must not contain any excess reducing agent. The simplest way to accomplish this reduction without the introduction of interfering reductant ions is to use a metallic reductor, as stated previously. Interferences. I n the metal samples assayed by this procedure, the only titratable impurity present in concentrations high enough to interfere is iron. It was present in the 200- to 800-p.p.m. range. I n the titration, the reaction of iron is stoichiometric, and so a correction could be determined for the assay value by determining iron in a separate plutonium aliquot, either colorin~etrically (11, 16) or spectrographically ( 2 ) . For the work in this report, a colorimetric method was used (11). Other metals would be expected to interfere if present as impuritiese.g., chromium, titanium, molybdenum, tungsten, uranium, and vanadium. If preliminary reduction of the plutonium using the lead or Jones reductor is required, nitrate, in addition to the metals noted above, mould interfere (6, 14). Therefore, if the samples contained any of these impurities in concentrations sufficient to cause interference, and if the reactions of these impurities were not stoichiometric, as is that of iron, it would be necessary to remove them before the plutonium could be determined. A possible separation process for the metallic impurities would require a n anion exchange resin column. Plutonium would be retained on the resin from an 8N nitric acid sohVOL. 34, NO. 3, MARCH 1962
* 347
Table 111.
Normalities of Ceric Sulfate Solutions
Pu of known purity and AszOI used t u standards Ceric Sulfate Solution Ceric Sulfate, N No. As203 Pu 1 0.04978 0,04977 2 0.04973 0.04971 3 0.04859 0,04856 4 0.04826 0.04828
tion, while the metallic impurities would pass through the column (1) I
RELIABILITY
A measure of the precision of the method was obtained by assaying in succession 59 weighed aliquants of the same plutonium metal sample. The assays were done by two analysts. The standard deviation was =kO.O?%. To measure the precision of the method over several weeks, a number of samples were submitted and then resubmitted under different laboratory identification numbers. Duplicate assays were run on each submission, giving a total of four assays on each sample. The standard deviation of the four results on each sample was calculated and combined to give a pooled standard deviation for all of the samples submitted. On one group of data containing 31 samples, the standard deviation was k0.06’%; on another containing 29 samples, +0.07%.
When the method was adopted as the assay procedure for plutonium metal samples in this laboratory, a control program was set up based on resubmitted samples. One hundred eighty-eight samples were resubmitted and analyzed in duplicate. A standard deviation was calculated using the means of the samples and their resubmissions. This standard deviation, based on mean results, was =k0.05%, equivalent to a standard deviation of *0.07%, based on individual results. A measure of the accuracy of the method can be obtained from data on the standardization of ceric sulfate solutions. When plutonium metal of known purity has been available, i t has been used with As203 in ceric sulfate standardizations. The normalities of four ceric sulfate solutions obtained using both Asz03 and plutonium metal of known purity are shown in Table 111. Another measure of accuracy of the method can be obtained by comparing the mean plutonium assay obtained using this method on a number of samples with the mean by-difference assay obtained on these same samples. A by-difference assay is calculated by subtracting the impurity concentration in the sample from 100%. On 255 samples, the mean difference between the assay obtained using the method in this report and the by-difThis ference assay was -0.03%. difference includes errors in the individual impurity analyses as well as any errors in the plutonium assay.
LITERATURE CITED
(1) Fudge, A. J., Wood, A. J., Banham, M. F., Atomic Energy Research Establ. ( G . Brit.) Rept. AERE-R 3264, April 1960 (unclassified). (2) Johnson, A. J., Vejvoda, E., ANAL. CHEM.31, 1643 (1959). (3) Kelley, M. T., Jones, H. C., Fisher, D. J., Talanta 6,185 (1960). (4) Koch, C. W., “The Transuranium Elements,” Natl. Nuclear Energy Series, Div. IV, Vol. 14-B,p. 1337, McGrawHill, New York, 1949. (5) Laitinen, H. A., “Chemical Analysis,” pp. 353-5, McGraw-Hill, New York, 1960. (6) Metz, C. F., ANAL. CHEY. 29, 1748 (1957). (7) Mills, W. JJ7., U. S. Atomic Energy Comm. Revt. HW-51822, 1957 . August (unclassified). (8) Miner, F. J., DeGrazio, R. P., Byrne, J. -~ T.. Rockv Flats Division. The Dow Chemical-eo., 1961: unpubiished data. (9) Phjllips, G., Analyst 83, 75 (1958). (10) Pietri, C. E.,. Badio, - . J. -4,Talanta ‘ 6 , 159 (1960). (11) Plock, C. E., Caldwell, C. E., Rocky Flats Division, The Dow Chemical Co., 1961, unpublished data. (12) Rost, G. A., ANAL. CHEM.33, 736 (1961). (13) Scott, F. .4., Peekema, R. M., Paper 914, Second Intern. Conf. Peaceful Uses of Atomic Energy, Geneva, Switzerland, 1958. (14) Sill, C. W.,Peterson, H. E., ANAL. CHEK 24, 1175 (1952). (15) Waterbury, G. R., Metz, C. F., Ibid., 31, 1144 (1959). (16) Waterbury, G. R., Metz, C. F., Talanta 6, 237 (1960). RECEIVEDfor review October 16, 1961. Accepted December 18, 1961. Work done under U. S. Atomic Energy Commission Contract AT(29-1)-1106.
Iron(ll) Determination in the Presence of Iron(ll1) Using 4,7-DiphenyI-l,lO-phenanthroline LEWIS J. CLARK Metallurgy Division, U . S. Naval Research laboratory, Washington 25,
b Ferrous iron in ferrous-ferric mixtures is determined colorimetrically with 4,7 diphenyl 1/10 phenanthroline. Samples of 1 to 10 mg. are prepared b y digestion in hydrochloric acid under a flow of carbon dioxide. Test solutions are stabilized with ammonium dihydrogen phosphate. The red ferrous complex with 4,7-diphenyl-1 , I 0phenanthroline is developed and simultaneously extracted with isoamyl acetate. Spectrophotometric measurement of the extract a t 530 mp permits determination of 0.2 to 30 pg. of iron (II) in IO-mI. test solutions of ferrous and ferric ions. Prepared sample solutions and extracts are oxidatively stable. Interference effects and toler-
-
348
-
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ANALYTICAL CHEMISTRY
D. C.
ance limits for diverse metallic ions are reported. The method is applicable to ferrous determinations in mixed iron oxides, ferrites, and corrosion products.
I
steel corrosion investigations the need arose for precise determinations of ferrous iron in reagents and oxidation products. The corrosion materials usually contained only microgram quantities of ferrous iron in association with relatively large amounts of ferric iron. Samples were often limited to a few milligrams in weight, consequently, standard titration procedures were not applicable. N
Attempts were made to determlne the bivalent iron of magnetite colorimetrically with 1,10-phenanthroline in aqueous solution. Results were not usable because of marked instability of the colored complex. Additives, including phosphoric acid, ammonium fluoride, ammonium tartrate, and disodium EDTA failed to stabilize the ferrous-ferric solutions. The related compound, 4,7-dimethylI,lO-phenanthroline, introduced by Brandt and Smith (W),was tested and produced a sensitive red complex with the ferrous iron in magnetite. I n aqueous solution, however, this dimethyl derivative suffered the same instability as 1,lO-phenanthroline. The possibility
