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 to 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 microscopically and b y 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
1018
e
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 sulfate had O/Pu atom ratios of 2.015, atom ratios of the plutonia formed would 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 thermogravimetric behavior of plutonium metal and ture rather than undecomposed salt. 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 in air at 850' C., and that a temperaweighing the samples a t elevated ture higher than 1200' C. may be retemperatures. The presence of sigquired 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 decomposition in air of plutonium metal, at 850' C., produced B crystalline 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 to EXPERIMENTAL 800' C. indicated that plutonium Themobdance. The highly toxic dioxide is the final product (9). A helical quartz spring thermobalance nature of the materials investigated
and the difficulties encountered in completely containing the finely divided plutonium oxide formed by ignition of plutonium metal and eompounds imposed restrictions on the type of equipment that could be used. In addition, the requirement tbat the balance have a large weight capacity eliminated consideration of the quartz spring balances which eau operate in a completely closed system. Furthermore, ignition conditions should closely resemble those existing in usual ignitions in air in an analytical ehemistry laboratory. For these reasons, it waa decided to use a furnace open to the atmosphere but enclosed in a glove box to eliminate health hazards. Automatic operation of the balance in the glove box was highly desirable. Tbe Ainsworth Type BR balance with AU recorder (Wm. Ainsworth and Sons, Inc., Denver, Cola.) proved to be well suited for this purpose. This balance is a regnlar analyticd-type beam balance with a ZOO-gram capacity, and features automatic addition or suhtraetion of weights in 1OO-mg. increments up to a total of 4 grams. The sample temperature, measured with a platinum-platinum plus 10% rhodium thermocouple inserted into the furnace near the sample, is nlso recorded on the two-pen recorder. The recorder is calibrated a t the factory with the particular thermocouple used to ensure linear response and accurate temperature recording. The estimated instrument error in measuring temperature is about 0.4% or 5' C. a t the maximum temperature of 1250' C. The complete thermobalauce (Figure 1) consists of tbis automatic-recording balance and a Kanthal Type REH 7-60 resistance furnace and ceramic tube (The Kanthal Carp., Stamford, Conn.) enclosed in n 20-inch cube of Foamsil insulation (Pittsburgh Corning Carp., Pittsburgh, Pa.). The furnace element is a Kanthal A-1 alloy strip coiled with an inside diameter of 2.8 inches inside a magnesia ceramic tube support. Two vertical, motor-driven, 1-inch lead screws (see Figure 1) hold the furnace assembly with the bore in a vertical position and move the entire furnace assembly through a vertical travel of about 14 inches. The furnace and elevator assembly is enclosed in a large stainless steel glove box, and the balance is in a second glove box attached to the top of the furnace box and connected to it through highpass paper filters. Heat baffles in the top of the furnace box and in the bottom of the balance box, and the air flow through the furnace box, prevent heating of the balance whieb is about 24 inches above the top of the furnace. The balance rests on approximately 300 pounds of lead brick which act as a heat shield and a vibration damper. A 36inch length of platinum-rhodium alloy chain suspends a 20-ml. crucible of the snme alloy from the bottom o f the left pan of the balance. The links of the chain vary in length from '/g to 3 inches and therefore reduce the es-
Figure 1. Thermobalani:e enclose:d in glove boxes
tablishment of vibration nodes in the chain. The reproducibility of the weighings with this arrangement is about zt0.4 mg. instead of the 1 0 . 1 mg. reported for the balance without the long suspension system. A motor-driven, 50-ampere, variable transformer (Variae Type 5@A, General Radio Co., Cambridge, Mass.) controls the voltage to the furnace element, which has a resistance of 1.05 ohms and requires a maximum of 46 volts. A nearly linear temperature-time curve resulted, provided an initial potential of about 12 volts was applied to the furnace element and the voltage was increased a t a rate of 2.5 volts per hour. Under these couditions the temperature increased a t approximately 100' C. per hour t o a maximum of 1250' C. The furnace box is one in an assemblage of glove boxes which are used in sample handling and preparation. In operation, samples are introduced into the system through an air lock and transferred to the glove box adjacent to the furnace box. The balance beam is arrested, the furmace assembly is lowered, and the crucible is removed to the adjacent glove box. The sample is transferred to the crucible, which is then replaced on the balance chain, and the furnace is raised. After the pressures have equalized in the balance and furnace boxes, the sample is weighed, and the automatic temperature cycling is started. The accuracy and reproducibility of the thermobalanee were determined by comparing thermogravimetrie curves obtained for calcium carbonate and dibasic sodium phosphate (NalHP04. 12Hz0) with data obtained with other tbermohalances in this laboratory and
by Duval (6). No errors larger than
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served.
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X Ray Diffraction Technique. Samples for x-ray powder diffraction analysis were ground and packed in borosilicate glass enpillaries ca.0.2 mm. in diameter with wall thickness of ea. 0.01 rnm., and the capillaries were sealed, all in a dried helium atmosphere. Diffraction photographs were obtained with Philips 114.6-mm. diameter powder cameras, using copper radiation [X(CuKaJ = 1.54050 A., X(CuKaz) = 1.54434 A.], Eastman Kodak Type AA film, Straumanis film mounting, and ea. 24-hour esposures. Values of length of unit-cell edge ao for the face-centered-isometric lattice of plutonium dioxide were determined from interplanar spacings of high-angle Ra, and K a 2 lines, after correcting for film shrinkage, by graphical extrapolation using the Nelson-Riley function. Reagents. The plutonium. metal uscd for the thermogravimetry analyses and as a starting material in preparing the plutonium compounds came from two large samples of highpurity metal which were specially prepared by Johnson (IO) and Mullins, Leary, and Bjorklund (13) of this laboratory. Sample A, which consisted of 0.2- t.o 0.3-gram pieces approximating parallelopipeds in ahape, eontained a total concentration of known impurities of 510 p.p.m. (Al, 30; Si, 30; Cu, 70; C, 30; 0 2 , 40; Ni, 75; Cr, 15; Mn, 20; and Fe, ZOO). Sample B consisted of coarse lathe turnings from & plutonium metal button having a total of 200 p.p.m. of known impurities (AI,5; C, 10; Os, 135; F, 2; and Fe, 50). The concentrations VOL. 33, NO. 8, JULY 1961
1019
of mct:d irnpuritics were dctermined by spectrographic (16) and bpectrophotometric methods. Carbon and oxygen were determined by the capillary trap method and the inert gas fusion method, respectively, described by 8miley (16, 17, 18). Most of the therniogravimetry determinations were made on Sample B material and corrections were applied for the calculated weights of the impurity oxides formed. In the investigation of the effect of the shape of the sample on its thermogravimetry behavior, sample A wm used and similar corrections were made for the impurities, The atomic weight of these plutonium samples was 239.06. All other chemicals used in the preparation of compounds were of analytical reagent grade and the water waa double distilled. Preparation of Compounds. All the compounds were prepared from accurately weighed, 0.5- to 5.6-gram ortions of the plutonium metal. e a c h metal sample was dissolved in 6M hydrochloric acid in a covered beaker, and these solutions were used in the preparations. PLUTONIUM( IV) NITRATE. Following addition of 5 ml. of 15M nitric acid, the solution was evaporated to near dryness on a steam bath. The addition of nitric acid and the evaporation step were repeated twice in the beaker and twice more after the sample had been transferred to the platinum crucible from the thermobalance. The final residue was heated for 15 hours on a steam bath. Following the ignitions of the first four samples of plutonium nitrate, which produced oxides that apparent1 were deficient in oxy en, two additions, samples of the sa% were prepared as follows: Five milliliters of nitric acid were added and the solution was evaporated to dryness in a covered beaker heated by an infrared lamp and on a hot plate. This step was repeated twice, then the salt was quantitatively transferred to the platinum crucible on the thermobalance, and an additional evaporation to dryness under the heat lamp and on the hot plate was carried out. The residue was dissolved in water and the solution evaporated on a steam bath. Water was added again and the sample was dried on the steam bath for about 15 hours. During the evaporations, the platinum crucible was contained in a beaker covered by a ribbed watch glass. Slight spattering occurred during all of the evaporations, except the last one on the steam bath. The beakers and covers were carefully washed, following each evaporation, and the washings were added to the sample. PLUTONIUM(IV) SULFATE. One miKliter of 15M nitric acid and a calculated excess of 18M sulfuric acid were added, and the solution was evaporated t o drynrss under an infrared lamp. Five milliliters of 0.1M sulfuric acid were added and the evaporation was repeated. This last step was repeated twice more and then the residue was transferred to the thermobalance crucible, using 0.1M sulfuric acid as a wash liquid. The solution was evap-
9
1020
ANALYTICAL CHEMISTRY
orated to complete dryness on a hot plate and under an infrared lamp. The residue was dissolved in water, and the solution was again evaporated to complete dr ness to expel any excess sulfuric a c i l The final residue was dissolved in water and heated on n steam bath for 15 hours. PLUTONIUM(IV) OXALATE. ' Following addition of 1 ml. of 15M nitric acid, the plutonium solution was evaporated on a steam bath. The residue was dissolved in 50 ml. of 0.5M hydrochloric acid, and a calculated 5% excess of oxalic acid dissolved in 50 ml. of 0.5M hydrochloric acid was added. The solution was warmed and allowed to stand 2 days. The precipitate, following separation by centrifuging, was washed with water, transferred to the thermobalance crucible using water as a wash liquid, and dried at 90' C. The combined supernatant liquid and washings were radiochemically anal zed, and a correction was applied for t e loss of plutonium. In a typical preparation, 5.8 mg. from an original 2.8956-gram plutonium sample were found in the supernatant liquid and washings.
E
Procedure. To correct the therrnogravimetry results for the combined effects of factors such as convection currents in the furnace, loss of platinum a t high temperatures, and warming of the balance, a thermogravimetric curve for the empty crucible was determined before and after each sample was analyzed. The timetemperature cycles for these blanks were identical to those for the samples. In determining these blanks, the crucible waa removed from the balance, cleaned, dried, and replaced. The crucible was tared by adding weights to the right pan of the balance, and the temperature cycle was started. When the complete curve was recorded, the furnace was allowed to cool, and the crucible was reweighed and removed from the furnace. After the sample was loaded into the crucible, it was replaced in the thermobalance and the sample weight read from the balance and recorder. (If the sample weight was greater than 4 grams, additional weights were added manually.) The automatic temperature cycle was started. Upon completion of the cycle, the weight of the ignition residue was determined at room temperature, the crucible was removed and cleaned, and a second blank was determined. The curve for the sample was then corrected for the blanks. The weights determined at room temperature showed any weight changes, such as an increase due to oxygen pickup, that occurred during the cooling of the sample. Six samples each of plutonium metal, plutonium(IV) nitrate, and plutonium (IV) sulfate, and four samplrs of plutonium(1V) osalate were examined. Several sample sizes were used to ensure that the correct curve was obtained for each material. Past experience with other materials showed that incomplete
reaction ocivsiori..'iy rcsultd if the sample size w:,j too large or the rate of temperature inciwse was too rapid. Better resolution WAS obtained using small samples and s h ~ xrates of ternpPrature increase, but 11 42ht.s with the> larger samples were more accurate In addition to varying t h e sample size, two shapes of the plutonium rnetnl samples, as described above, were used. In a few casm the change in wviglit o r the sample was recorded during long periods a t room temperature and at thc maximum furnace temperature. Plutonium dioxide ignited to 1250' C is not readily soluble. Fused nmmonium bifluoride was found to be effective in removing this refractory material from the crucible after the bulk of the oxide had been mechanically transferred. The crucible waa then washed in concentrated nitric acid and iinsllp in water. Samples of the oxides from each type of sample were analyzed by x-ray powder diffraction and by spectrographic methods for impurities. No significant pickup of contamination waa observed during the ignitions, except in rare cases when particles of the furnace-insulation materials fell into the crucible as the furnace was lowered. This contamination was obvious from a visible examination of the ignition product. RESULTS
Blank Determinations. The thermogravimetry curves for the blank determinations were consistent in showing an apparent gradual decrease in weight of the crucible with increasing temperature. A typical curve is shown in Figure 2. When the maximum temperature was reached, this weight loss was between 3.0 and 3.5 mg. in the various determinations. The weight loss continued as the temperature was held a t 1250' C., although the conditions, such as convection currents in the furnace, remained essentially constant. This continued weight change, which was evidence of a true loss, evidently of platinum, wm also shown by the final weights of the crucible a t room temperature. A consistent net loss of about 1.9 mg. was observed. Plutonium Metal. Samples of plutonium metal weighing between 0.5982 and 9.4566 grams were analyzed. The results for a small and a large sample of turnings (sample B) and a large sample of chunks (sample A) are shown in Figure 3, and data for all the determinations are presented in Table I. For ease in comparison, the per cent changes in weight (weight change X 100/plutonium weight) are plotted. These curves, which were all obtained under similar operating conditions, illustrate the effects of sample shape and size. As might be exyected, the samples having a large
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Sample Wt., G.
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0.5982 2.9427 3.9227 9.4566 8.0916" 3.4474
Chunks Chunks
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Thermogravimetry curve, blank determina-
surface area (turnings) burned rapidly and completely a t lower temperatures than the chunk samples. For the 0.5982-gram sample of plutonium turnings, the slow increase in weight occurring between 145' and 270' C. indicates either formation of unstable lower oxides of plutonium or, more probably, surface oxidation. At 230' C. an empirical formula, of Pu00.00 is indicated by the sample weight. This slow initial weight increase waa less pronounced for the 9.4566-gram sample of turnings. Sufficient heat may be generated in the initial oxidation of the large sample to raise the local temperature and increase the rate of oxidation, thus elimininating or greatly shortening the initial oxidation time. The chunk samples showed the slow increase i n , weight but at higher temperatures, and the rapid burning also waa in a higher temperature range. The decrease in oxidation rate of the 8.0916-gram sample a t about 480' C. suggests the possibility of a mixed oxide, having an empirical formula PudOr, which was then oxidized to the dioxide. Lower oxides of plutonium form upon ignition in atmospheres having lower oxygen content than air, and the mixed oxide may result from depletion of oxygen near the burned metal or, more probably, limited access to air caused by the arrangement of the oxide on the metal. Because formation of this mixed oxide waa not shown by the other chunk sample, it is probably related to physical factors rather than to chemical stability of the mixed oxide. The investigation of the possible forma-
Form
"
Pu TURNINGS,
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b Figure 3. Thermogravimetry curves for oxidation of plutonium metal in
14
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TEMPERATURE
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tion of the mixed oxide at a lower heating rate waa not undertaken because the compound, if formed, is obviously unstable and has no significant effect on the composition of the final oxide. The samples of plutonium turnings combined with slightly higher relative amounts of oxygen than did the chunk samples and at lower temperatures. All the curves are similar in showing a slow loss in weight as the temperature is increased and held at 1260' C. In no determination WM the stoichiometric dioxide formed even on prolonged heat-
ing at 1260' C. The plutonia in these ignitions waaa nonhygroscopic, olivedrab powder. Under a polarizing microscope, it was seen to consist of irregular, highly angular, splintery fragments up to 60 by 90 microns, with some dense, granular aggregates. The material was transparent to translucent, greenish yellow to brown in color by transmitted white light, and optically isotropic with refractive index n = 2.40 for sodium light. No other phases were
485 495 420 400 670 555
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detected. The x-ray powder diffraction patterns obtained from a11 of the samples were indistinguishable from one another, all lines being accounted for with no missing or extraneous lines, on the basis of a facecentered-isometric, fluorite-type phase with length of unit-cell edge a0 = 6.396 f 0.001 A. Spectrographic analysis of the products of the ignitions did not show significant increasea in the concentrations of the impurities. AE might be expected, there waa a slight pickup of aluminum, dicon, and magnesium from the furnace, but the h l oonoentratione of thew elementa did not materially change the correotiona applied to the weights. Plutonium Nitrate. The thermogravimetry curves for the six samples, which contained between 0.4782 and 5.6425 grams of plutonium, were all similar to the representative curve shown in Figure 4. The initial compositions of the salts varied between Pu(NOJ4.1.32H~0 and Pu(NO&.
Table 1. Thermogravimetry Data for Plutonium Metal Temp., 'C. O/Pu Atom Ratio For ra id For max. oxid: wt. 230'C. 480" C. Max. wt. 1250'C.
270-470 255-495 200-420 180-400 3W50 270-555
--
2.01s 2.015 2.0175 2.0176 2.014, 2.014
1250' C. (4hr.) 2.015 2.012 2.016 2.0170 2.013, 2.013
1250" C. ( 1 1 hr.) 2.01,
Slight evidence for unstable oxide of this composition. Slieht change in slope of curve appeared at 155" C. This sample showed a weight increase of only 0.4 mg. during 20 hours in the furnace at room temperature. Slight evidence for an unstable oxide of compoeition Pu40s.
VOL. 33, NO. 8, JULY 1961
1021
2.68H20. Upon standing at room temperature the water content of each sample increased slowly-for example, the nitrate prepared from the O.4782-gram Eample of plutonium metal increased in weight from 1.0221 grams (1.32H20) to 1.0625 grams (2.44HsO) during a 27-hour period, and the weight was still increasing. A significant loss in weight occurred at temperatures above 60' C., but there was no indication of the formation of stable hydrates or of the anhydrous salt. At temperatures between 205" and 215" C. a slight decrease in the rate of weight loss occurred, giving evidence for the formation of a slightly stable mixed compound having the empirical formula PuO(N08)z or PuOz.Pu(NO& No evidence for this mixed salt existed for the larger samples. At temperatures between 265" and 290' C. the plateau for PuOz starts, and the weights corresponded to those calculated for the stoichiometric dioxide in the temperature range between 675' and 805' C., depending upon the sample size. The loss in weight was continuous, and a t 1250' C. the calculated O/Pu atom ratio was 1.989. Continued heating a t 1250" C. did not reduce this ratio during a &hour period. The plutonia formed from one of the nitrate samples was of two colors. One part of the oxide consisted of darkbrown, vesicular, slaggy fragments, with one surface of each exhibiting yellowish green color and dull luster. On being crushed to a powder, the material was macroscopically bright greenish yellow in color. Under a polarizing microscope the powder was seen to consist of irregular, angular fragments comprised of aggregates of grains averaging about 1 micron across, transparent to translucent, greenish yellow to greenish brown in color by transmitted white light, and optically strictly isotropic with refractive index as reported above for oxide prepared from the metal. No other phases were detected in this dark-colored portion. The other part of the oxide consisted of irregular, yellowish green, vitreous fragments, with dark-brown, vesicular, slaggy patches. After being crushed to a powder this material was indistinguishable from the other part of the oxide. The dark-brown material was not formed from the other nitrate samples. The x-ray powder diffraction patterns from these products were identical to that from oxide prepared from the metal. The spectrographic analyses of the products showed an increase in the aluminum concentration slightly greater than that occurring during the ignition of the metal samples. The total correction to the oxide weight changed the O / h atom ratio by -0.0030. Plutonium Sulfate. The preparation of these samples differed from 1022
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ANAlWlCAL CHEMITRY
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Thermogravimetry curves for Plutonium nitrate, sulfate, and oxalate
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the method recommended by Pietri and Baglio (14) for preparing plutonium sulfate tetrshydrate, which is being tested as an analytical standard for plutonium. The present sulfate salts, which were prepared from 0.58- to 4.88-gram samples of plutonium, had various compositions corresponding to hydrates containing between 3.3 and 4.5 molecules of water. These variations in water content probably resulted from the different drying periods used. Each of the samples was hygroscopic below 50" C., but lost weight above 60" C., forming the tetrahydrate a t about 70' C., as shown in the representative thermogravimetry curve, Flgure 4. This salt was not highly stable and decomposed rapidly at temperatures above 85" C. An inflection in the decomposition curve at temperatures between 150" and 175' C. indicated formation of the unstable dihydrate, which decomposed rapidly below 230' C. and then more slowly as the last of the water was evolved to form the anhydrous salt a t 490' C. for the smallest sample to 535" C. for the largest. Slight evidence existed for the formation of the monohydrate at 235" C . and the tritahydrate at 415' C. The slopes of the curve in these temperature ranges showed that these hydrates are unstable. Very rapid decomposition of the anhydrous salt occurred between 605" and 810' C., at which temperature the plateau for the dioxide started abruptly. The O/Pu atom ratios calculated from the weights were 2.09 at 810' C., 2.056 a t 1ooO" C., 2.022 at 1250' C., and 2.017 after 5 hours a t 1250' C. The final weights of the six samples corresponded to O/Pu atom ratios between 2.011 and 2.017.
Macroscopically the plutonia formed in these ignitions was a bright yellowish green powder. Under a polwining microscope it waa seen to consist of irregular aggregates of anhedral grains averaging slightly less than 1 micron across, with very few exceeding 2 microns across. The grains were transparent, greenish yellow in color by transmitted white light, and largely optically isotropic with refractive index as reported above. Many grains showed small optical anisotropy (low birefringence) which was presumably due to residual strain. No other phases were detected. The x-ray powder diffraction pattern from these products was identical to that from oxide prepared from the metal. The spectrochemical analyses of the products were also comparable to those obtained for the oxides prepared from the metal, and the corrections to the weights due to the small increases in impurities were not significant. Plutonium Oxalate. Because of differences in the drying periods, the four oxalate samples, which contained between 0.7982 and 3.7925 grams of plutonium, varied in initial composition between Pu(CsO&. 1.47H20 and Pu(CnO&. 4.64H20. The compound reportedly precipitated from aqueous solution is Pu(CSO4)r.lOHzO(9). All preparations were hygroscopic a t room temperature and steadily increased in weight. For example, the weight of the sample which contained 2.9740 grama of plutonium increased from 6.21338 grama to 6.2632 grams in 7 hours, corresponding to a calculated increase in water of hydration from 4.64 to 4.90H10. HOWever, the lack of any evidence of stable hydrates in the decomposition curve indicated that the increase in weight
may have been due to absorbed water. As shown in Figure 4, the samples began to lose weight slowly above 50" C. and very rapidly above 150" C. At temperatures between 290" and 330" C., the rate of the weight loss slowed markedly as the plateau for plutonium dioxide started. No indication of stable hydrated oxalate or of carbonate was shown. The O/Pu atom ratios calculated from the weights varied from 2.044 a t 330" C. to 2.014 after 4 hours a t 12,50° C. The O/Pu atom ratios calculated from the final weights for the four samples were between 2.010 and 2.014. The plutonia formed was macroscopically greenish brown in color. Under a polarizing microscope the powder was seen to consist of irregular aggregates of anhedral grains averaging about 1 micron across, but a few grains up to about 5 microns across were observed. Many of the aggregates appeared pseudomorphous after single crystals, or aggregates of crystals, of the parent oxalate. The grains were transparent to translucent, greenish yellow to grcenish brown in color by transmitted white light, and mostly optically isotropic with refractive index as reported above. Many grains exhibited low to moderate birefringence which was presumably due to residual strain. No other phases were detected. The x-ray powder diffraction pattern from these producte was identical to that from oxide prepared from the metal. The spectrochemical analyses of the products were also comparable to those for the oxides prepared from the metal, and no significant changes in the corrections t o the weights were required. DISCUSSION
The thermogravimetry curves for plutonium metal and its sulfate and oxalate show that ignition of these materials a t a temperature higher than 1250" C. is necessary to form stoichiometric plutonium dioxide in air, and that no stable lower oxides of plutonium are formed. At 1250' C. and at lower temperatures a slight excess of oxygen remains in the product plutonia. Temperatures of 600" to 900" C.,recommended for forming the dioxide, are not adequate if highly accurate results are required. The results obtained for the plutonium nitrate show the formation of an oxygen-deficient plutonia at 1250' C. The reason for the low final weight of the oxide is not known, but a possible reason may be a physical loss of plutonium resulting from decrepitation of the nitrate in the ignition. No evidence of such a loss was observed, however, It may also be possible that the method of preparation of the nitrate leaves a trace of chloride which could cause a
loss of plutonium by sublimation as the chloride during the heating cycle. Plutonium(II1) chloride sublimes readily a t 800' C. However, the repeated evaporation to dryness makes the retention of significant chloride unlikely. Any small lose of sample could readily account for the slightly low final weights. For example, a loss of 2.6 mg. of plutonium from a 3gram sample would cause an apparent decrease in the O/Pu atom ratio of the product from 2.000 to 1.985. Another possible cause for the low results may be the formation of plutonium nitride during the decomposition of the nitrate, with incomplete oxidation of the nitride during the heating cycle. The rate at which the last trace of combined nitrogen is eliminated a t 1250" C. from the interior of crystals of plutonium dioxide is unknown, but it may be low enough to cause the low results. There is evidence that the formation of an oxide with a O/Pu atom ratio less than 2.000 is unlikely: Samples of PuOz heated at 1500' C. for 1 hour in hydrogen or in contact with graphite were reported to show no decomposition to a suboxide (8). The thermogravimetry curves for the three salts show that only the sulfate forms an intermediate, anhydrous plutonium sulfate, which might be usable in the gravimetric determination of plutonium. An ignition temperature between 500" and 550' C. should be satisfactory for forming anhydrous plutonium sulfate. No stable intermediates are formed in the decomposition of the nitrate or oxalate. These results are in general agreement with data reported by Dawson and Elliot (3), who stated that anhydrous plutonium sulfate is stable at temperatures between 450" and 650" C. A gravimetric determination based on the weight of this anhydrous salt would be subject to a small error, because the plateau for this compound is not entirely flat (Figure 4). The x-ray powder diffraction photographs show that these products and material previously considered to be "standard" plutonium dioxide all have the same crystal structure and, within the limits indicated, identical unitcell dimension. These x-ray analyses are insensitive to the small differences in O/Pu atom ratios observed in the various oxides. The unit-cell dimension found in this work agrees well with that reported by other workers (.4,12,20). The results of microscopic examination of the products are in agreement with results reported by Francis and Sowden (8),who found that the preparation route had a marked influence on the physical characteristics of the oxide product. As in the present study, plutonium nitrate and sulfate gave irregular aggregates of grains averaging
about 1 micron across, whereas the oxalate yielded particles pseudomorphous after crystals or aggregates of crystals of the parent oxalate. Many of these preparations likewise exhibited optical anisotropy, indicative of lattice strain. These workers did not prepare plutonium dioxide by ignition of the metal; in the present study, this method yielded relatively coarse, splintery fragments. ACKNOWLEDGMENT
The authors express their appreciation to Joel Dahlby for his assistance in assembling the thermobalance and in preparing some of the compounds, and to Robert Phelps and John Walden for the spectrochemical analyses, LITERATURE CITED
(1) Allison G. M., At. Energy of Canada,
Ltd., dhalk River Project, unpublished work. .._~.._ . . . . 19.5'2. (2) Chikalla, T. D., U. S. At. Energy Comm., Rept. HW-63081 (1959). (3) Dawson, J. K., Elliot, It. M., U. K. At. Energy Authority, Rept. AEREI
C/R-1207 (1953). (4) Drummond, J. L., Welch, G. A., J. Chem. SOC.1957,4781. (5) Duval C., "Inorganic Thermogravimetric kalvsis." Houe_ . D._0. Elsevier. , ton, 1953. (6) Farwell, G. W., Roberts, J. E., Wahl, A. C., Phys. Rev. 94,303 (1954). (7) Ferguson, I. F., Street, R. S., D'Eye, R. W. M., U. K. At. Energy Authority, Rept. AERE-R-3344 (1960). (8) Francis, K. E., Sowden, R. G., Zbid., AERE-R-2939 (1959). (9) Hall, G. R.,' Waiter, A. J., Ibid., AERE-C/R-874 (1951). (10) Johnson, K. W. R., J. Inora. & Nuclear Chem. 9,200 (1959). (11) Moulton, G. H., U. S. At. Ener Comm., Rept. LA-172 (1944); t h r o u g U. S. Atimic Energy Comm. Nuclear Sci. Abstr. 10, No. 3500, 447 (1956). (12) Mulford. R. N. R., Ellinaer. F. 8.. . j . Phys. Chem. 62, 1460 (195'8).' (13) Mullins, L. J., Leary J. A., Bjork-
lund, C. W. U. S. At. dnergy Comm., Re t. LAM$-2441 (1960). (14) hetri, C. E., Baglio, J. A. New Brunswick Laboratory Rept. of deetm of Advisory Committee for Standari Reference Materiala and Methods of Measurement, New Brunswick, N. J.,
pp. 33-65, 1960. (15) Rein, J. E., Langhorst, A. L.,
Elliott. M. C.. Los Alamos Scientific Laboratory, Los Alamos, N. M., unpublished work, 1952. (16) Smiley, W. G., ANAL.CHEM.27,1098
(1958). (17) Smiley, W. G., Los Alamos Scien-
tific Laboratory, Los Alamos, N. M., unpublished work, 1953. (18) Smiley, W. G., U. 5. At. Energy Comm., Rept. LA-I128 (1950). (19) Westrum, E. F., Jr., "Preparatiofi and Properties of Plutonium Oxides," in Seaborg, G. T., Katz, J. J., Mannin W. M., "The Transuranium Elements!' NNES Div. IV, Vol. 14B, Part 11, pp. 936-44, McGrsw-Hill, New York,
1949. (20) Zachariasen, W. H., Acta Crysl. 2, 388 (1949). RECBIVED for review Februa 13, 1961. Accepted April 24, 1901. %ork done under auspices of the U. 5. Atomic Energy
Commission.
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