centration including the radioequilibrium ratio of about 1 to 2. I n considering the utility of the liquid scintillation method, it is well to survey the methods already available for quantitative analysis of ZrQ6-?;b’J6mixtures. The close proximity of their y-ray energies, 0.74 and 0.76 m.e,v., respectively, make precise results difficult even with the finest y-ray spectrometric instrumentation. The precision of a time-consuming beta particle absorption measurement on mixtures of Zr96 and NbV6 is affected by self-absorption of the sample and by the necessary large counter corrections. iifter a consideration of these factors, the liquid scintillation method seems to be the most applicable to fast routine analysis of these isotopes. Standardization of Zre6 and Nbg5. Calculation of t h e absolute disintegration rate in coincidence
measurements is given by Putman (6) as :
where G y is the mean of the product of the beta and gamma detection efficiencies, while ip and zy are the means of the individual efficiencies. The ratio in brackets reduces to unity when either of the detectors is equally sensitive to all parts of the source. This is obviously true for the beta sensitivity in a liquid scintillator and equally true for the beta sensitivity of the 2a proportional counter mounted directly over a source. Results of coincidence measurements with both types of p-y counters are given in Table 111. The standard deviations of the individual measurements are also included.
LITERATURE CITED
(1) Barnothy, J., Forro, M , Reu. Sci. Instr 22,415 (1951). (2) Dunvorth, J. V., Ibid., 11, 167 (19.10). (3) Horrocks, D. L., Studier, 31. H., ANAL.CHEJI.30, 1747 (1958). ( 4 ) Kinard, F. E., Xucieonics 15, 92 f1957). ,\ - - -
(5) Putman, J. L., Atomic Energy Research Establ. Rept. I/M 26 (1953).
(6) Putman, J. L.; “Beta and Gamma Ray Spectroscopy,” K. Siegbahn, ed., Chap. 26, North Holland Publishing
co.. -19,55. -->
( 7 ) Scadden, E . hI,, Ballou, S . E., ASAL.
CHEJL25, 1602 (1953).
(8) - S t e p , J., Haasbroek, F. J., Proc. l ~ . 1, Intern. Conf. Peaceful Cses Atomic Energy, 2nd Geneva, 21, 95
(1958), (9) Strominger, D., Hollander. J. II., Seaborg, G. T., Rem. X o d e r n Phys. 30, 585 (1958). RECEIVED for review Sovember 2 i , 1959. Accepted February 23, 1960. Work performed under Contract KO. AT(45-1)1350 for the United States Atomic Energy Commission.
Determination of the Stoichiometry of Uranium Dioxide Polarographic Determination of Uranium(V1) in Uranium Dioxide HISASHI KUBOTA Analytical Chemisfry Division, Oak Ridge National laboratory, Oak Ridge, Tenn.
The deviation of the composition of uranium dioxide from the stoichiometric oxygen-uranium ratio of 2.000 is calculated from the assay for the uranium(V1) in the oxide. The sample is dissolved in hot phosphoric acid under an inert atmosphere to preserve the oxidation states of the metal, and the uranium(V1) is determined polarographically in a phosphoric-perchloric acid medium. Because this is a direct determination of the deviation from stoichiometry, it is best applied to samples which are nearly stoichiometric, and the composition of a uranium dioxide sample which has an oxygen-uranium ratio of 2.03 or less can be determined to within 0.001 oxygen atom. m P i I u h I DIOXIDEis
widely used as a nuclear fuel in both the bulk and dispersed form. A dense, crystalline material may be prepared by the reduction of uranium trioxide monohydrate in a nonoxidizing atmosphere, such as hydrogen or argon, and high-firing at 1700’ C. Bulk UOz is formed by compressing and sintering this granular product. I t s composition approaches the theoretical 1to 2 ratio of uranium to 610
ANALYTICAL CHEMISTRY
oxygen, but the exact stoichiometry is seldom attained, because more than the stoichiometric amount of oxygen is almost invariably present. This is referred to hereafter as excess oxygen. Several approaches to the determination of this composition have been reported. They include assay methods (8, S), determination of excess oxygen (6),and determination of uranium(V1) (7). The two last-named methods give a direct measure of the deviation from stoichiometry. Among the actinide elements uranium compounds exhibit the greatest deviations from the law of definite proportions. This has been attributed to the multiplicity of stable oxidation states coupled with the small differences in energy between the states. The deviations are most pronounced with compounds of uranium and oxygen, nitrogen, sulfur, selenium, and others in which the anion-uranium bonds are partially to wholly semimetallic. As a consequence, stoichiometric LO1 has a great tendency to chemisorb oxygen and hold it in the lattice as oxide ions. The electrons that bring about the reduction of the oxygen come from U(1V) atoms which are oxidized to higher states (4). Thus, determination of the uranium in
valence states higher than 4 in EOr should serve as a measure of the excess oxygen and, subsequently, as a way to defermine the esact composition of the material. With this end in view a scheme of analysis has been developed in which the U(V1) content of dissolved KO2 is determined and directly correlated n ith the excess oxygen. A similar approach has been reported by Simmler (71, who dissolved the oxide in phosphoric acid and determined U(V1) by titration with a standard titanous solution. I n the method described below, thp samples are dissolved in phosphoric acid under an argon atmosphere, and the U(V1) is determined polarographically. The uranium-phosphoric acid system has two distinct advantages for this particular determination. I t not only allows a dissolution of uranium osides ~ i t no h change in osidation states but also provides a medium in the subsequent polarographic determination of the U(V1) where a two-electron reduction of uranium can be effected, thereby providing a more sensitive procedure compared to reductions in media where one-electron reductions take place. The polarography of uranyl ions in phosphate has been under study a t this
laboratory for several years and the results are now being prepared for publication. Our work is in agreement with the published results of Chi ( I ) and is summarized very briefly in Figure 1. At low phosphate concentrations two waves are observed, which are attributed to the successive reduction of U(V1) to U(V) to UiIV). Increasing phosphoric acid concentration causes the waves to merge, and a t a sufficiently high concentration (0.5M) an irreversible reaction which corresponds to the reduction of U(T’1) t o U(1V) is the only wave seen. The half-wave potential is about -0.17 volt relative to the S.C.E. and shifts very slightly with change in phosphate concentration. The addition of perchloric or oxalic acid also helps t o bring about a single wave. Maxima effects are observed a t high U(V1) concentrations (>lo0 y per ml.), which can be suppressed with gelatin. PROCEDURE
A convenient sample weight for an oxide with 1 to 5% excess oxygen is 100 to 120 mg. Proportionately less sample is required the greater the excess oxygen. Bulk samples should be on the order of 200 mg. The sample weight is adjusted, whenever possible, to provide a solution for the polarographic determination which contains 5 t o 50 y per nil. of U(T’1). The sample is covered with 40 nil. of 85% phosphoric acid in a boiling flask calibrated a t 100 ml. and fitted with an air reflux condenser and a gas inlet tube whose tip extends below the surface of the phosphoric acid. Argon gas is bubbled in a t the rate of one to two bubbles a second to stir the viscous liquid adequately as well as to maintain an inert atmosphere over the acid bath. Any other high purity inert gas such as nitrogen, helium, or carbon dioxide with an oxygen content not exceeding 20 p.p.m. may be used in place of argon. A heating mantle controlled by a Variac allows good control of the heating. The rate of heating is adjusted so that the acid boils and condensation is restricted to the lower third of the reflux column. With the equipment described, 100 mg. of -100 to +325 mesh oxide can be dissolved in about an hour. When dissolution is complete, 50 ml. of deoxygenated 0.2M perchloric acid are added to the hot solution in about three portions. The solution is stirred vigorously after each addition by increasing the gas flow rate, aided by manual shaking, until the gel that forms a t the interface of the phosphoric acid and dilute perchloric acid redissolves. K h e n the solution is cool, the volume is made up to 100 ml. with deoxygenated water, and the solution is made homogeneous by bubbling the gas through for a few more minutes. The polarographic determination is made as soon as possible following the dissolution. Standard curves prepared by polarographing known amounts of U(V1) in a phosphate-perchloric acid medium of
DISCUSSION
The validity of this method depends theore tically upon the deduced equivalence of excess oxygen to uranium in valence states higher than 4 and experimentally upon the preservation of the oxidation states of uranium when the oxide is dissolved in phosphoric acid. The U(V1) content of freshly reduced UOz (3) which had been kept under inert atmosphere from the time of reduction to dissolution was 0.02 and 0,03% for argon and hydrogen reduced oxides. The analysis of 100 mg. of Us08 dissolved in phosphoric acid gave 67.0% U(VI), in contrast to the theoretical 66.7% where the analysis was performed coulometrically. Any reduction or oxidation seems to be within the limits of experimental error and very little bias is observed. The relative standard deviation is 4% or better for the excess oxygen and is fairly constant as long as a measurable wave height is obtained. This procedure, therefore, is most effective when used to determine small amounts of excess oxygen, on the order of 3% or less. The relative accuracies that can be obtained by some of the methods currently used are given in Table I. Methods which determine the deviation from stoichiometry are best used when the deviation is small, and assay methods become better as the deviation increases. Solutions of U(1V) slowly oxidize upon exposure t o air. When a series of solutions initially containing about 2 mg. of V(V1) per 100 ml. was alloxed t o stand overnight in contact with air in the neck of the volumetric flasks, an average increase of 27% in the U(V1) content was observed. Protection against air oxidation is necessary, especially with solutions of samples containing very small amounts of excess oxygen. The use of a calibrated boiling flask in which the solution can be made t o volume without the necessity of transferring to a separate volumetric flask, the deoxygenation of the diluents, and the polarographic determination immediately following dissolution minimize exposure of the solution to air. Because the standard addition method involves the possibility of exposing solution to air, the use of predetermined standard curves for interpolating U(T’1) is recommended.
Figure 1 . Polarograms of uranium(V1) in phosphoric acid UM).
20 y per mt.
the same concentration are used for interpolating U(V1) concentrations.
Calculations. The abundance of U(VI), A , in the sample is calculated from the following relation: A
=
CV/FW
where
C
= U(V1) content, mg. per ml.
V = volume of solution, ml. F = abundance of U in UOz W = sample weight, mg.
The value for F is 0.8815 for the pure and stoichiometric oxide made from naturally occurring uranium. This factor changes with variation in U(V1) content as well as with enrichment. When natural uranium products are being analyzed, a -0.1% error per unit per cent of excess oxygen results from using the value for stoichiometric UOZ. This error is small compared to the over-all precision of 4%; however, F values of 0.8789 for U(V1) content of 1to 5% and 0.8738 for U(V1) of 5 to 10% are recommended. For more exact work the U(V1) content can be recalculated by using the adjusted factor obtained by the first approximation. The maximum change in F with enrichment is on the order of 1%; therefore, F should be reevaluated when the enrichment exceeds 10%. From the equivalence of U(V1) to excess oxygen the oxygen to uranium ratio is calculated by: O/U
Table I.
= 2.0000 f
A
Relative Accuracy of Methods for Determination of Excess Oxygen in Uranium Dioxide Error of Method (Units of Oxygen Atom)
Composition of
Sample UOma U02.10
UOz.67
Pyrolytic or gravimetric UaOs 10,005 10.002
fO.OO1
Volumetric oxygen
Combustion
f0.009 1 0 006
10.001 10.003
10.01
10.02
I
Polarographic fO.OO1 f O .004 10.03
VOL. 32, NO. 6, MAY 1960
61 1
Uranium(1V) forms a number of complexes with phosphate, depending upon the relative concentration (6),and also forms agarlike gels which are sometimes called precipitates. These solids are formed at the interface when the concentrated acid solution is diluted with water and can be removed by sufficient agitation. There is a critical value of dilution beyond which the solids do not redissolve, and the resulting suspension often turns into one solid gel upon standing. It is best to dilute the solution slowly and with constant stirring to prevent the formation of large gel aggregates which take a long time to redissolve. The critical level above which permanent gel form does not occur is about 20% phosphoric acid; however, the final dilution in this method is 35% phosphoric acid, at which concentration any gel that is first formed redissolves rapidly. There is a smell decrease in the sensitivity of the polarographic procedure at the higher phosphate concentration. The sintered pellets often require several days to dissolve when left in the bulk form. Prolonged heating of the phosphoric acid often causes the solution to deposit a white crust which is
difficult to redissolve. By using material broken into pieces weighing 10 to 50 mg,, dissolution time can be reduced to 10 to 20 hours without materially altering the surface-weight ratio of the oxide. A better procedure is to grind the sample under a n inert atmosphere in a glove box before dissolution. Specifications for reactor grade uranium dioxide usually allow a maximum of 200 p.p.m. of iron, which is the only impurity of sufficient magnitude to interfere. This is equivalent to a maximum error of d=0.0005 in the oxygenuranium ratio. The results of several months of routine determinations indicate that the iron is retained in a state that does not bring about any oxidation of uranium during the dissolution. A recently developed fuel element is composed of 4 to 7% of UOZ dispersed in a Tho2 matrix. The oxygen-uranium ratio of the UOZalso can be determined by this procedure. The pellet is ground under a n inert atmosphere prior t o dissolution. Three drops of dilute (1 to 20) hydrofluoric acid are added to the hot phosphoric acid solution to hasten the dissolution. To circumvent nonhomogeneity of the sample, both (UVI) and total U are determined.
The polarographic method is simple in both procedure and instrumentation and is best when the U(V1) concentration is on the order of 5 to 50 y per ml. Solutions more concentrated than the upper limit can be determined with no loss in precision; however, titrimetric methods can be used here with perhaps better results. LITERATURE CITED
(1) Chi, T. Y . , Hua Hsaeh Hsueh Pao 23,
79-89 (1957). (2) Hoekstra, H. R., Katz, J. J., ANAL. CHEM.25, 1608 (1953). (3)?Katz, J. J., Rabinowitcli, E., Satl. huclear Energy Series, Vol. VIII-5, p. 306 McGraw-Hill, New York, 1951. (4) data, J. J., Seaborg, G. T., “The Chemistr of the Actinide Elements,” p. 139, Metguen & Co., London, 1957. (5) Roberts, L. E. J., Harper, E. A,, Atomic Energy Research Establishment, Harwell, England, AERE C/R-885 (1952). (6) Schreyer, J. M., J. Am. Chem. SOC.77, 2972 (1955). (7) Simmler, J. R., U. S. At. Energy Comm., Rept. NYO-5218 (1948). RECEIVEDfor review November 13, 1959. Accepted February 10, 1960. Division of Analytical Chemistry, 135th Meeting, ACS, Boston, Mass., April 1959.
Spectrophotometric Determination of Uranium with 3-(2-Arsonophenylazo)-4,5-dihydroxy-2,7napht halenedisuIfonic Acid (Trisodium Salt) H. PERRY HOLCOMB and JOHN H. YOE Pratt Trace Analysis laboratory, Department o f Chemistry, University o f Virginia, Charlottesville, Vu,
b A more comprehensive study of the Fritz and Johnson-Richard arsenazo method (4) for the spectrophotometric determination of uranium is presented. Arsenazo instantaneously forms a stable blue complex with uranyl ions that has a maximum absorbance a t 590 mp. The mole ratio of uranyl ion to compound is 1 to 1. The color reaction conforms to Beer’s law and has a sensitivity of 1 part of uranium in 30,000,000parts of solution. Tolerances to diverse ions, effect of temperature, and the separation of uranium from interfering ions by ether extraction of uranyl nitrate have been investigated. The accuracy of the new method compares well with the dibenzoylmethane method. The sensitivity of arsenazo is about 50% higher than that of dibenzoylmethane. The method has been applied to the analysis of synthetic samples containing
612
ANALYTICAL CHEMISTRY
uranium and various interfering ions, as well as to a number of uranium ores.
T
HE INCREASED use
of uranium due to its application in the many phases of atomic energy has resulted in an intensive study of this element and its compounds. Many methods have evolved in the past ten to fifteen years for the colorimetric (or spectrophotometric) determination of uranium with organic reagents. Many organic compounds react with uranium, usually as the uranyl ion (UOz+z), to give colored complexes or precipitates as shown in the complications by Rodden (16) and Ware (22). Sandell (17) gives procedures for using thiocyanate, hydrogen peroxide, ferrocyanide, and diethyldithiocarbamate as colorimetric reagents for uranium. H e states, how-
ever, that the colorimetric methods for uranium are insensitive and not well suited for the determination of small amounts of the element. Yoe, Will, and Black (24) were the first to develop a method for the colorimetric determination of uranium using dibenzoylmethane, CsH&OCH,COCeH6. This reagent is highly sensitive (0.05 p.p.m. of uranium), but i t has one disadvantage in t h a t it is only slightly soluble in water. For this reason it is necessary to use a 57% (by volume) ethyl alcohol solution for the determination of uranium by their method. Modified procedures with dibeneoylmethane were used by Adams and Maeck ( I ) , Francois (S), and PEibil and Jelfnek (IS). Other methods for the determination of uranium have been reported by Silverman, Moudy, and Hawley (20) and Nietzel and D e Sesa (12).
