CouIometric Deter mina t io n of Ura nium(IV) by Oxidation at Controlled Potentials C. M. BOYD and OSCAR MENIS' Analytical Chemistry Division, Oak Ridge Nafional laboratory, Oak Ridge, Tenn.
b A controlled-potential coulometric method is described for the titration of uranium(1V). At potentials more positive than +0.5 volt vs. the Ag/AgCI electrode, uranium(lV) is oxidized at a platinum electrode. The oxidation is sufficiently rapid and complete at a potential of +1.4 volts for use in the titration of uranium, although the uranium(lV/VI) couple is not reversible. The method has been applied to the determination of uranium(lV) in samples of UOt and in Tho*-UO, mixtures dissolved in ti3P04. From 1 to 10 mg. of uranium(1V) in 1M solutions of H,P04, H2SO4, HCIOd, or HNO, can be determined by this method with a coefficient of variation of 0.370.
I
THE ANALYSIS of solutions of uranium salts, various oxides of uranium, or thorium oxide mixed with oxides of uranium, it is sometimes necessary to determine the concentration of uranium present as uranium(1V) and ~8 uranium(V1). Several methods based on the oxidation of uranium(1V) to uranium(V1) can be used to determine the quadrivalent uranium. The uranium(1V) can be titrated with dichromate (6) or permanganate (8) or by utilizing coulometrically generated oxidizing agents such as bromine (9) or cerium(1V) ( 4 ) . Alternatively, the uranium(1V) may be determined indirectly by analyzing the sample for uranium (VI) before and after oxidizing the uranium(1V) by suitable treatment, such as fuming it with perchloric acid, and calculating the uranium(1V) by difference. The method of controlled-potential coulometric analysis has been applied to the oxidation of iron(I1) (9), arsenic (111) (9, IO), thallium(1) (S),and plutonium(II1) ( I d ) with 100% current efficiency a t a platinum anode. With oxidations which involve reversible electrode couples, the potential of the working electrode determines the ratio of the amounts of oxidized and reduced forms that can exist in contact with the electrode, and current is forced to flow until such R ratio is reached. The comN
1 Present address, Nuclear Materials and Equipment Corp., Apollo, Pa.
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ANALYTICAL CHEMISTRY
pleteness of the reaction can therefore be controlled by the electrode potential selected (18). The uranium(IV/VI) couple is not reversible; consequently, a working electrode potential which results in easentially complete anodic oxidation of uranium(1V) cannot be predetermined by the Nernst equation. No application has been reported of a direct coulometric oxidation technique for the determinat,ion of uranium(1V). I t was observed experimentally that uraniwn(1V) is slowly oxidized to uranium(V1) at 4-0.5 volt us. the Ag/AgCl reference electrode. To be of analytical value, however, a more positive potential is required. This report contains the results of a study of the anodic oxidation of uranium(1V) in 3M phosphoric acid a t a platinum anode with emphasis on establishing optimum conditions for the coulometric determination of uranium(1V). Interferences commonly associated with the a m p l e types of interest were evaluated and the precision of the method was ascertained. EXPERIMENTAL
Apparatus. The coulometer used was the ORNL electronic controlledpotential, coulometric titrator, Model Q-2005, which has been described in detail elsewhere (6). A 25-ml. glass titration cell similar to the one described by Shults and Thomason (IS)is used. Solutions are added to the cell through a side-arm opening and may be drained through a stopcock a t the bottom. A platinum gauze anode rather than a mercury anode is used; this working rlectrodc, which is 4 by 4 cm. with an estimated arra of 25 sq. cm., is fitted in the bottom of the cell. The Ag/AgCl/KCl (saturated) reference electrodr and the isolated P t / l M II3P0, electrodes arc contained in Vycor tubes closed a t the bottom with porous-glass bulbs (Corning Glass Works, No. 7930). The cell is closed a t the top with a Teflon cap which also supports the electrodes, the nitrogen inlet tube, and the stirrer shaft. The stirrer is a Plrxiglas block cemented to the end of a 3-mm. glass rod and is driven hy an 1800-r.p.m. Bodine motor (Bodine Electric Co.). Reagents. The uranium stock solution used for testing the method contained 4.74 mg. of uranium per ml. dissolved in 1M sulfuric acid. The
concentration of uranium was determined by a volumetric, zinc-amalgam, potassium dichromate method (If). Aliquots of this stock solution were reduced by coulometric titration at a potential of -0.3 volt in a titration cell similar to the one described above with the exception that a mercury-pool electrode replaced the platinum gauze electrode. These solutions containing uranium(1V) were diluted with 3M phosphoric acid to obtain test solutions of the desired concentrations. Uranium (IV) is not completely stable toward oxidation by air; therefore, immediately before use of the stock solution an aliquot was first anal zed for uranium (VI). The uranium(h) concentration was subsequently calculated from the known total uranium concentration and the uranium(V1) found by couloinetric reduction (15). An alternative procedure was to transfer with a pipet aliquots of the uranium(1V) solution from the reduction cell directly into the oxidation cell. The phosphoric acid solutions were prepared from reagent grade, 85% orthophosphoric acid. The reducing substances present in the acid were eliminated by heating the acid and adding a slight excess of potassium permanganate. The excess permanganate was then reduced with the minimum necessary hydrogen peroxide. All other chemicals used were of reagent grade. Procedure. Prepare the titration cell for a titration by adding 5 to 10 ml. of 3 M phosphoric acid and deaerating with nitrogen. Pretitrate the stirred electrolyte and condition the platinum electrode a t a potential equal to or more positive than that to be used during the titration of the sample. Continue this pretitration for several minutes until the current, as indicated by the ammeter of the coulometer, has dropped to a constant value of the order of 0.1 ma. Note the value of this background current at the potential to be used during the titration of the sample. Stop the pretitration, add the sample containing the uranium(1V) to the pretitrated electrolyte through the sidearm, and oxidize the uranium(1V) to uranium(V1) a t thc desired controlled potential. The end point of the titration is indicated by the return of the ammeter needle to the background current value reached during the pretitration. From the voltage accumulated on the readout capacitor of the coulometer during the titration of the sample, calculate the
coulombs of electricity uwd. hhkc a barkground correction to this amount by subtracting the coulombs due to thc barkground, which is the product of the background current in amperes and the time of sample titration in seconds. Calculatr the amount of uranium(1V) froni this corrected number of coulombs. RESULTS AND DISCUSSION
Selection of Potential for Oxidation. The rates of oxidation of uranium(1V) at different electrode potentials and the magnitudes of the background corrections are shown in Table I. The formal electrode potential for the uranium(IV/VI) couple in 3.3311.1 phosphoric acid is reported to be 4-0.305 volt us. Ag/AgCl/KCl (saturated) electrode (1). From this value the oxidation of uranium(1V) to uranium(V1) might be expected to be 99.9% complete a t a potential of 4-0.39 volt us. this electrode. Because this couple is not reversible, a potential of at least +0.5 volt is required to initiate the oxidation of uranium(1V) while the rate and completeness of oxidation becomes practical only at potentials more positive than +1.2 volts. The time required for the titration of urltnium(1V) decreases as the titration potential is mo.de more positive. An upper limit to the usable positive potential is set by the oxidation of water which becomes appreciable a t +1.5 volts. A potential of +1.4 volh is the most practical since the time required for a titration is of the order of 15 minutes and the background s little correction is less than 3% with i as 1 mg. of uranium(1V). At high positive potentials oxidation of the phtinum electrode occurs, resulting in the formation of a film of platinous or platinic oxides (7). To prevent this reaction from giving high results for the uranium(1V) titration, the platinum electrode must be treated by a pretitration a t these highly positive potentials. After several minutes of pretitration a t +1.5 volts, the formation of oxide film on the electrode reduces the background current to a reasonable level. Accuracy and Precision. Thc accuracy and precision of the controlledpotential oxidation of uranium(1V) are shown by the data in Table 11. Various aliquots of standard uranium(1V) solution prepared by coulometric reduction were titrated in clifferent acidic media. The amount of uranium(1V) found was 100.3y0 of that taken with a coefficient of variation of 0.3%. The titration can be performed in phosphoric, sulfuric, perchloric, or nitric acids. The nitric acid medium is also made 0.01M in sulfamic acid to suppress any possible nitrite or nitrous acid interference. Most of the work
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I. Controlled-Potential Oxidation of Uranium(lV) at Various Potentials [Medium, 31\1 HsPO4; anode, Pt gauze; U(IV), 11.27 and 1.13 mg.] Potential, Volt V8. Titration Background Ag/AgCl Minutes Coulombs Ma. Coulombs U(IV), Mg.
Table
$1.2 1.3 1.4 1.5 1.2 1.3 1.4 1.5
40 30 15 10 20 16 10 6
9.14 9.15 9.18 9.33 0.92 0.93 0.96 1.09
reported herein was performed utilizing 3M phosphoric acid because this medium is best suited for samples which also contain large amounts of thorium. Table II. Precision of ControlledPotential Oxidation of Uranium(lV)
[Oxidation potential: +1.4 volts v8. Ag/AgCl/KCl(saturated)] Medium Taken Found Per Cent 3MHaPO4
1MH2SOc 1MHClOi 1MHNOi
7.90 4.74 3.16 1.58 7.90 7.90 5.81
7.94 4.75 3.17 1.59 7.92 7.94 5.79
Coeff.of var.
100.5 100.2 100.3 100.6 100.3 100.6 99.7 100.3 10.3
The titration of uranium(1V) in a hydrochloric acid medium is not practical because of the high background currents resulting from the oxidation of the platinum anode to chloroplatinate. However, chloride in amounts equivalent to the uranium(1V) can be tolerated. Interferences. The ions which interfere with the anodic oxidation of uranium(1V) can be classified into three general groups. Chloride, bromide, and cyanide form strong complexes with platinum and thereby lower its oxidation potential, causing the background current to become abnormally high. The second group includes ions such as iodide, arsenic(III), vanadium(IV), molybdenum(III), cerium(III), manganese(II), mercury(I),and
0.00
0.00 0.10 0.30 0.00 0.00 0.04 0.50
0.00 0.00 0.09 0.18 0.00 0.00 0.02 0.18
11.26 11.27 11.18 11.28 1.13 1.15 1.15 1.12
ruthenium(III), which are oxidized at the same potentials M uranium(1V) and therefore interfere. Copper(1) and titanium(II1) are oxidized a t potentials less positive than uranium(1V). Iron(I1) is oxidized similarly if in a phosphoric acid medium. The elimination of this type of interference by a pretitration, however, is not recommended because the oxide film on the platinum electrode is partially removed during this step. The subsequent titrstion of uranium(1V) is then accompanied by an undesirable high background current. A correction for this type of interference can be calculated, however, from the results of a titration of a separate aliquot of the sample at +0.45 volt. These results can be used to correct for such interferences when they are present in amounts equivalent to the uranium(1V) without causing an error which exceeds the error of the method. Milligram amounts of CObalt(I1) and chromium(II1) can be tolerated because they are not oxidized a t +1.4 volts. Application. This coulometric titration method for uranium(1V) WBB applied to the determination of uranium(1V) in samples of uranium dioxide and in thorium oxide-uranium oxide mixtures. These samples were dissolved by refluxing, under an inert atmosphere of argon, in 7 M phosphoric acid containing a few drops of 1 to 10 hydrofluoric acid. Dissolution is thus accomplished without altering the oxidation state of the uranium. As an example of the precision obtainable, a uranium dioxide material contained 86.7 f 0.7% uranium(1V) from the
Table 111.
Uranium(lV) Content of Thorium Oxide-Uranium Oxide Samples [Medium, 3M HSPO4; oxidation and reduction potentials, 4-1.4 and -0.3 volt vs. Ag/AgCl/KCl (satd.), respectively] Cathodic Reduction, Mg. Uranium(IV), Mg. Total U U(W U(IV) Anodic oxidation Difference
0.93 3.18 6.56 2.66 10.77
0.24 2.28 6.11 0.98 8.71
0.69 0.90 1.45 1.68 2.06
0.73 0.93 1.42 1.69 2.12
+0.04 +0.03 -0.03 +O.Ol $0.06
VOL. 33, NO. 8, JULY 1961
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results of nine determinations on three different portions of the sample. The results of uranium(1V) determinations on thorium oxide-uranium oside mixtures are given in Table 111. l h e results by direct coulometric oxidation are compared to the amounts of uranium(1V) indicated by coulometric reduction of uranium(V1) in the original sample solutions and in portions of thc solutions which had bcen oxidized chemically. The total uranium content of the snmplcs ranged from 2 t o 30 mg. per grain. The amount of uranium(1V) actually titrlited was only 1 to 2 mg. and the agreement between the results obtained by the two methods was 5% or better.
LITERATURE CITED
(1) Baes, C. F., Jr., J. Phys. Chen. 60 805 (1956). (2) Carson, W. N., ANAL. CHEX. 25, 466 (1953). (3) Foieyj W. T., Pottie, R. F., Ibid., 28, 1101 (1056). ( 4 ) Furrnan, K . €I., Bricker, C. E., Dilts, It. V., Ibid., 25, 482 (1053). (6) Krllcv, M. T., Jonea, H. C., Fisher, 1). J., fbid., 31, 488 (1959). (6) Kolthoff, 1. M., Lingane, J. J., J. Am. Chem. SOC.55, 1871 (1933). (7) Laitinen, 11. A., Enke, C. G., J. Eledrochem. SOC.107, 773 (1960). (8) Lundell, G . E. F., Knowles, H. B., J. Am. Chem. SOC.47, 2637 (1925). (9) MacNevin, W. M. Baker, B. B., ANAL.CBEM.24,986 (1952).
(10) MacNevin, W. M., Martin, G , L., J . Am. Chem. SOC.71 , 204 (1949). (11) Rodden, C. J. "Anal ical Chemistry of the Manhattan goject," pp. 68-9, McGraw-Hill, New York, 1950. (12) Scott, F. A., Peekema, R. M., U. 8. At. Energy Comm., Rept. HW-58491 (1958). (13) Shulte, W. D., ThomaRon, P. F., ANAL.CHEM.31, 492 (1959).
RECEIVED for review December 27 1960. Acceptod A ril 3, 1961. Pittaburgli Conference on Rnalytical Chemistry and Apt i e d Spectroscopy, Pithburgh, Pa., ebruar -March, 1961. Work carried out under ontract No. W-7405-eng-20 at Oak Rid e National Laboratory, operated by Lfnion Carbide Nuclear Company, a division of Union Carbide Corp. for the Atomic Energy Commission.
6
Thermogravimetric Behavior of Plutonium Metal, Nitrate, Sulfate, and Oxalate GLENN R. WATERBURY, ROBERT M. DOUGLASS, and CHARLES F. METZ 10s Alarnos Scientific laboratory, Universify of California, Los Alamos, N. M.
b A thermobalance for analysis of plutonium samples is described. Thermogravimetry curves for plutonium metal and its nitrate, sulfate, and oxalate were determined in air at temperatures between 20' and 1250' C., and the oxides produced were examined microscopicallyand by x-ray powder diffraction. No conclusive evidence of formation of suboxides of plutonium was observed. The final dioxides from all samples except the nitrate contained excess oxygen after 4 hours a t 1250' C., indicating that a higher temperature is required to form the stoichiometric dioxide b y ignition of these materials in air. The ignition products of the nitrate were slightly deficient in oxygen, having an O/Pu atom ratio of 1.989. The crystal structure of all of the products was face-centered isometric, fluorite type, with a, = 5.395 f 0,001 A. Only the sulfate formed a stable intermediate, anhydrous plutonium sulfate, which might b e used in the gravimetric determination of plutonium. gravimetric determination of plutonium by ignition to the dioxide has been used to a limited extent in measurements of the specific activity of plutonium (1, 6, 9, 11, 19). An investigation of the stoichiometry of plutonium dioxide ( 4 ) has shown that it may have a variable composition over the range between PuO,.o and Pu02.1. I n this work, samples of plutonium HE
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ANALMICAL CHEMISTRY
metal or compounds, containing bewith a capacity of 200 mg. (sample tween 200 and 500 mg. of plutonium, plus boat) and a sensitivity of about were ignited in air and weighed on a 10 pg. was used in this work. Because semimicrobalance. The final weights, of the small capacity of the balance and which are reproducible within k0.0275, the blank correction of 5 to 10% of the showed that the oxides formed at total weight change at the highest 870' C. from the metal, nitrate, and temperature, small differences in O/Pu atom ratios of the plutonia formed would sulfate had O/Pu atom ratios of 2.015, 2.046, and 2.089, respectively. Each not be discernible. In view of the temof these oxides lost weight at higher peratures reported to be nccessary in temperatures and at 1180' C. attained forming the stoichiometric dioxide, a constant weight corresponding to the the maximum temperature reached with stoichiometric dioxide, P U O ~ . o.oo8. ~ ~ this thermobalance is too low to ensure The loss in weight between 850' and complete removal of excess oxygen from 1200' C. was attributed to the presence the oxide. The purposes of the present of excess osygen a t the lower temperswork are to determine the thermograviture rather than undecomposed salt. metric behavior of plutonium metal and All of thc samples after ignition showed some of its compounds with a thermobalance having a large weight capacity and the fluorite structure (a, = 5.396 A.) with no other phases. The x-ray powder a high maximum temperature, and to diffraction pattern for the oxides determine the cryst:J lattice dimensions formed at 870' C. were less sharp than of the oxides produced by ignition of for the higher temperature oxides. various starting materinls. Possible More recent work (7) also showed that errors introduced by changes in coma nonstoichiometric oxide, P U O ~ . ~ ~position , which might occur as the is formed by ignition of plutonium oxide samples are cooled are minimized by weighing the samples a t elevated in air at 850' C., and that a temperatemperatures. The presence of sigture higher than 1200' C. may be required to remove excess oxygen from the nificant concentrations of extraneous dioxide. Hcating plutonia in argon phases in the oxides should be shown for several days a t 1600' C., followed by the x-ray diffraction studies. Data by a 4-hour heating in carbon monoside are presented for the ignition or thermal at 850' C., produced B crystalline decomposition in air of plutonium metal, plutonia corresponding in weight to the nitrate, sulfate, and oxalate. This stoichiometric dioxide. investigation is being extended to other Thermogravimetry of some Plutoplutonium compounds. nium compounds at temperatures up t o EXPERIMENTAL 800' C. indicated that plutonium Themobdance. The highly toxic dioxide is the final product (9). A nature of the materials investigated helical quartz spring thermobalance
