Volumetric Determination of Uranium in the Presence of Niobium Utilizing a Bismuth Metal Reducto r RAYMOND J. JAWOROWSKI and WILLIAM D. BRATTON Pratt & Whitney Aircraft, CANEL, Middletown, Conn.
uranium in the presence of large amounts of niobium without separation would be advantageous. A procedure based on the work of Sill and Peterson (8),using a lead reductor, was in use for the assay of unalloyed uranium. When mixtures of uranium and niobium were analyzed, the niobium was partly reduced with the uranium and was subject to hydrolysis. Attempts to reoxidize the niobium completely by the passage of air or oxygen through the solution were unsuccessful. Bismuth metal n as then substituted for the lead in the reductor column to take advantage of its lower reducing potential, thereby avoiding the reduction of niobium while still reducing uranium quantitatil-ely to the uranium(1V) state.
The use of bismuth metal as a reducing agent during the determination of uranium in the presence of large amounts of niobium was studied. Other metals or metal amalgams reduce both uranium and niobium, prohibiting a simple volumetric determination. When a strongly acid solution of uranium and niobium is passed through a column of bismuth metal, the uranium(V1) is quantitatively reduced to uranium(1V) with no uranium (111) or niobium(ll1) being formed and without encountering the problem of niobium hydrolysis. The uranium is determined indirectly b y reaction with excess iron(lll), producing iron(ll) which is titrated with standard cerium(lV) solution. The end point of the titration is determined potentiometrically. Interferences were encountered with iron, molybdenum, vanadium, and tungsten in mixtures of uranium and niobium but not with nickel, zirconium, chromium, beryllium, copper, titanium, and aluminum.
ARIOUS
EXPERIMENTAL
Uranium Standard Solution. T h e standard solution contained 5.8997 grams of NBS U308, which had been ignited a t 900' C. for 1 hour prior to weighing. The U308 was dissolved by heating in 25 ml. of 20% H2S04and 1 ml. of 30% HzOz. The solution was taken to fumes and cooled and the sides of the container were rinsed. It was again taken to fumes, cooled, and diluted to 500 ml. This solution was used to standardize the 0.02N sulfatoceric acid titrating agent according to the procedure of Sill and Peterson (8). Ferric Stock Solution. Fifty grams of Fe(NH4)(S04)* 12H10 was dissolved in 500 ml. of concentrated hydrochloric acid. Apparatus. -4 Beckman Model K automatic titrator was used for all potentiometric measurements. The electrode system consisted of a Beckman calomel electrode and a ulatinum helix. The bismuth reductor was made by filling a 3/a-inch-diamcter Jones reductor column to a depth of 8 inches with granular bismuth metal (Fisher KO. B-319 2@60 mesh). Glass wool plugs
methods hare been used
for the assay of uranium (6, IO).
Most procedures call for the reduction of uranium(V1) t o uranium(1V) and the subsequent oxidimetric titration back to uranium(V1). Metal or metal amalgams such as zinc ( 5 ) , zinc amalgam (S), cadmium ( I I ) , cadmium amalgam lead ( 2 ) , mercury ( I ) . copper ( 7 ) , aluminum ( b ) , magnesium ( b ) , and bismuth amalgam (9) hare been used to reduce uranium. Kiobium is hydrolyzed, reduced, or both under the conditions of the procedures using these reducing metals. Extraction, precipitation, or ion exchange techniques may give quantitative separation of uranium from niobium, but all are time-consuming. -2 method which would allow for the determination of
(e).
were placed a t both ends of the column. The reductor was stored in 170HzSO, and activated each day with 100 ml. of 20% H2S04. Procedure. Dissolve a sample containing not more than 200 mg. of uranium in 5 ml. of concentrated hydrofluoric acid, adding concentrated nitric acid dropwise t o aid in dissolution. Add 10 ml. of concentrated sulfuric acid, fume strongly, and allow t o cool. Cautiously dilute t o 50 ml. with water, keeping t h e solution cool. 24110w the sample solution to pass through the bismuth reductor a t a rate of approximately 25 ml. per minute. Wash the column with 100 ml. of 20% H2S04. Catch the sample and washings in a 400-ml. beaker containing 10 ml. of the ferric stock solution. Set the automatic titrator at 750 mv. and titrate the reduced solution with 0.02N sulfatoceric acid solution. RESULTS
Table I shows the effect of niobium on the determination of 100 mg. of uranium, Table I1 shows the effects of other commonly used alloying metals on the determination of 100 mg. of uranium. Iron, molybdenum, and vanadium are reduced by the bismuth reductor and produce high results by consuming the ceric solution. Tungsten precipitates in the highly acid solu-
Table II. Effect of Alloying Metals on Uranium Determination
(Solutions all contained 100.0 mg. of U, 100 mg. of Nb)
Element Zr Ni ~~
Be A1
Cr
cu
cu Table
I.
cu Ti Mo
Precision and Accuracy of Uranium Determination
Mo _. hIo ~
U Added,
Xb Added,
Mg. 100.0 100.0
Mg.
No. of Detns.
Av. U Found, Mg.
Std. Dev., %J
0 100
7 11
100.0 100.1
0.2 0.2
hf o
V
w
Fe
Amount Added, Mg. 100 100 100 100 100 100
1 .o
0.01 10 10 1 .o
0.i
0.01 10 1.0 10
70Theoretical Titer
100.2 100.1 100.0 100.3 99.8 100,3 99.7 100.0 100.0
Reduced 101.2 100.1 100.0
Reduced Ppt. Reduced
VOL. 34, NO. 1 , JANUARY 1962
111
tion and gives low results possibly due to entrapment and slow release of the uranium(1V). DISCUSSION
Ferroin was tried initially as a n indicator for visual determination of the end point of the titration. The visual end point was unsuitable because of difficulty in obtaining a stable end point in this system. Determination of the end point by potentiometric means eliminates this difficulty. Niobium is not reduced by the bismuth column and the high concentration of sulfuric acid prevents its hydrolysis. Zirconium, beryllium, nickel, aluminum, chromium, titanium, and copper in the amounts shown in Table I1 do not interfere in the determination of uranium. Copper and titanium are not reduced by bismuth metal, thereby eliminating the interference from these elements which is encountered when some of the other reductors are used.
Chromium and trace amounts of molybdenum and copper have been reported to form active states which induce the oxidation of uranium(1V) to uranium (VI) in the presence of air (6, 8, IO). The combination of the low reducing potential of the bismuth metal and the high sulfuric acid concentration apparently inhibits the formation of these active states, thereby allowing the determination of uranium in the presence of these elements with little danger of induced oxidation, as indicated by the results in Table 11. The standard deviation, as shown in Table I, for the determination of uranium by this method with or without 100.0 mg. of niobium is 0.2%, adequate for most assay work. ACKNOWLEDGMENT
The authors acknowledge the assistance of Everett W. Hobart in the preparation of this manuscript and the
support of the Atomic Energy Commission under Contract AT(ll-1)-229. LITERATURE CITED
(1) Caley, E. R., Rogers, L. B., J. A m . Chem. SOC.68, 2202 (1946). (2) Cook, W. D., Hazel, F., McNabb, W. B., ANAL.CHEM.22,654 (1950).
(3) Furman, N. H., Schoonover, I. C., J . Am. Chem. SOC.53,2561 (1931). (4) Kano, N. J., J. Chem. SOC.Japan 43,
333 (1922). (5) Kern, E. F., J . Am. Chem. SOC.23, 685 (1901). (6) Rodden, C. J., “Analytical Chemistry of the Manhattan Project,” p. 65, McGraw-Hill, New York, 1950. (7) Scagliarini, G., Pratesi, P., Ann. chim. appl. 19,85 (1929).
(8) Sill, C. W., Peterson, H. E., ANAL.
CHEM.24,1175 (1952). (9) Someya, K., 2.anorg. Chem. 152, 368 (1926). (10) Steele, T. W., Taverner, L., Proc. 2nd U.N. Conf. 3, 510 (1958). (11) Treadwell. W. E.. Helv. Chim. Acta 5,732 (1922):
RECEIVEDfor review August 9, 1961. Accepted November 13, 1961.
Identification of Precipitates in Diffusion Zones Using the Electron Probe Microanalyzer R. E. SEEBOLD and 1. S. BlRKS U. S. Naval Research laboratory, Washington 25, D. C.
b Small precipitates often form in intermetallic diffusion zones due to interaction with impurities such as carbon, nitrogen, oxygen, phosphorus, or sulfur contained in the parent metals. With the electron probe microanalyzer it is possible to identify and analyze individual precipitates and determine which impurity element is responsible for the interaction. When iron of the same quality was diffused in one case with niobium and in another case with chromium, the precipitates formed in the iron were Nb& for the Nb-Fe system and CrNz or Cr2Ns for the Cr-Fe system. This indicates that impurity activity varies with the diffusing metal atoms.
iron matrix as far out as the leading edge of the niobium, where the concentration of niobium is less than 1%; thus the interaction may be between two very low-level compositions. To understand the diffusion reactions better, it is necessary to identify and analyze such precipitates and to determine which of the impurity elements is responsible. The procedure reported in this paper was first to attempt identification by comparison of measured x-ray intensities from the precipitates with intensities calculated for jikely compounds of the metal element’and, second, to prepare various impurity compounds in the laboratory for direct measurement of their x-ray intensities in the electron probe (1) and observation of their physical appearances.
T
HE purest metals obtainable are used in solid-solid diffusion studies and yet one often observes precipitates of the order of 1 to 3 microns in size in the diffusion zone. These precipitates result when impurities, present in one of the parent metals, react with incoming atoms of the other parent metal to form stable stoichiometric compounds. It was shown previously (3) that, in the Nb-Fe system, precipitates form in the
112
e
ANALYTICAL CHEMISTRY
EXPERIMENTAL
All diffusion couples and laboratory compounds were prepared for examination in the electron probe by sectioning and subsequent metallographic polishing with diamond paste. No etchants were used before this electron probe examination.
Standards of the appropriate pure metals (Nb, Fe, or Cr) were mounted with each specimen so that the x-ray intensity from the 100% standard could be measured under precisely the same conditions as the unknown. The ratio of intensities from unknown and standard is what is used herein as relative x-ray intensity. It is usually not the same as Eeight per cent because of matrix absorption and enhancement. The calculation technique to go from weight per cent to relative intensity has been described (8). Nb-Fe System. I n previous experiments on the Nb-Fe system (S), niobium containing less than 100 p.p.m. of impurities was diffused with three grades of iron containing impurities in the amount of 65, 560, and 1400 p.p.m. I n all three cases, scattered precipitates of the order of 1 to 3 microns in size were observed in the parent iron near the NbFez intermediate phase. The number of precipitates for the three grades of iron was approximately 5 x 106, 4 X 107, and 2 X 108 per cc. Figure 1 shows the diffusion couple with the intermediate grade of iron. The precipitates are the black particles in the parent iron near the NbFez
