Anal. Chem. 1986, 58, 2479-2481
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Spectrophotometric Determination of Nonstoichiometry in Hyperstoichiometric Uranium Dioxide M. K. Ahmed* a n d N. L. Sreenivasan Radiochemistry Programme, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India
Uranlum dioxide is widely used as a nuclear fuel and usually exists as hyperstolchlometrlc UO,+,. I n the descrlbed method UO,+, Is dissolved in warm concentrated phosphork add saturated with sodium sulfate at room temperature to prevent the alr Oxidation of U(IV). The concentratlonsof U(V1) and U( I V ) are directly determined by means of the absorbance of the two species at two speciflc wavelengths 420 and 665 nm, respectively. The results obtalned are predse to f0.002 O/U units and have been compared with those obtalned from gravimetric tltritnetry employing potentiometricend point detectlon. The O/U ratio determined by the two procedures agrees within 0.004 O/U units. The method has been employed to determine O/U ratio In near stdchlometric U02 (2.039) as well as in U30s(2.670). The developed spectrophotometric method is simple and fast and does not require inert atmosphere.
Uranium dioxide is well-known as a nuclear fuel; its composition is close to the stoichiometric ratio of oxygen-touranium of 2:1, but the exact stoichiometry is seldom attained. Generally uranium dioxide exists as hyperstoichiometric U02+z because of the presence of interstitial oxygen. Knowledge of oxygen to uranium ratio is essential because it affects certain physical and chemical properties of the fuel that are very important to fuel performance under reactor conditions (1). All wet chemical methods for the determination of O/U ratio are based on the determination of either U(1V) or U(V1) since hyperstoichiometry can be directly related to the concentration of U(V1). Thus the determination of the U(V1) serves as an exact measure of the concentration of oxygen above 2:l ratio and, coupled with total uranium determination, enables determination of the exact composition of the oxide. The choice of the most convenient analytical procedure for the determination of U(V1) is related to the degree of hyperstoichiometry of UO,,,. For the near-stoichiometric UOz pellets, where a minute quantity of U(V1) has to be determined in the presence of a large excess of U(IV), polarographic methods are preferred (2). The potentiometric titration of trace amounts of U(V1) in the H3P04solution is also possible, but special methods of end point detection, e.g., bipotentiometry or biamperometry, are required (3). Gravimetric titrimetry coupled with potentiometric end point detection has been employed in the case of U308where U(V1) is present in larger quantity. A weighed quantity of U308powder is added to a standard solution of Ce(1V) in 2 M H2S04 and is dissolved by keeping the beaker in a thermostated water bath a t 70 "C for 15-20 min. After complete dissolution the solution is cooled t o room temperature and a known excess of standard Fe(I1) solution is added. The remaining Fe(I1) is back-titrated against standard potassium dichromate solution ( 4 ) . The recently published method of Khatoon and Rao ( 5 ) , which involves the dissolution of U30s in an excess of Ce(1V) in 2 M perchloric acid a t room temperature followed by potentiometric determination of the remaining Ce(IV),seems to be promising but the dissolution behavior of sintered UO, pellets
is yet to be established. The automation of expensive test preparation, titration, or electrochemical methods needs elaborate instrumentation and expertise and is time-consuming. The weight of the oxide sample should be precisely known in the titrimetric methods, which is not the case with the present spectrophotometric procedure. Kuhn e t al. (6) proposed a spectrophotometric method for the determination of O/U ratio in UOz+x. Though the method is simple, it was not used extensively, probably because of the following two reasons. I t involves the dissolution of U02+xin hot (130 "C) concentrated phosphoric acid under an inert atmosphere to prevent air oxidation of U(1V). Secondly, the precision obtained is not adequate compared to other methods. The present communication describes improvements over the previously reported spectrophotometric method.
EXPERIMENTAL SECTION Instrumentation. Absorbance measurements were recorded on a Cary Model 17D spectrophotometer. Matched quartz cells of 1 cm path length were used. The base line and the digital absorbance readings were adjusted to 0.000 by keeping the reagent medium (phosphoric acid saturated with sodium sulfate at room temperature) in both the reference and the sample chambers before each set of measurements. Gravimetric titrimetry was performed by using platinum wire, indicator, and saturated calomel as the reference electrode. The change in the potential during the course of titration was observed with the help of a digital voltmeter ( 7 ) . Reagents. All chemicals used were of analytical reagent grade. In a quartz vessel, to 1000 mL of phosphoric acid (AnalaR/BDH, 85%) 5 mL of concentrated HNO, (AnalaR/BDH grade) was added and temperature was gradually raised to 275 "C. This temperature was maintained for 30 min while a gentle stream of C 0 2 was passed through the solution for complete removal of the nitric acid (8). Twenty grams of Na2S04was slowly dissolved in 500 mL of purified phosphoric acid by gentle warming and this was used as the dissolution medium throughout the experiments. Procedure. From 50 to 150 mg of U02+, powder was added to 10 mL of warm phosphoric acid saturated with sodium sulfate at room temperature with the help of a glass spatula and dissolved by gentle heating under an infrared lamp. The solution was then cooled to room temperature and the absorbance was measured at 665, 544, 516, and 420 nm. Determination of Molar Absorptivity of U(V1) a t 420 nm. A weighed quantity of U308 was dissolved in 1:l HNO, and evaporated to dryness three times with concentrated HNO, to make sure that all uranium was present in the U(V1) state. To this, 5 mL of concentrated H2S04 was added, and the solution was heated until dense white fumes of SO3appeared. This was then dissolved in phosphoric acid medium. The concentration of uranium in this stock solution was determined by gravimetric titrimetry employing a modified Davies and Gray procedure (9) with potentiometric end point detection. Aliquots of the stock solution on weight basis were then introduced into 25-mL standard flasks and made up to volume with purified phosphoric acid. Subsequently the absorbance was measured at 420 nm and a calibration curve was plotted. The molar absorptivity was calculated from the slope of the curve. Determination of Molar Absorptivity of U(1V) at 420,544, and 665 nm. Nuclear grade uranium metal turnings were washed successively with acetone, nitric acid, and triple distilled water. Moisture was removed by purging with argon gas. The turnings
0003-2700/86/0358-2479$01.50/00 1986 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 12, OCTOBER 1986 550
500
400 I
I
I
600 I
I
I
roo I
75 I
Table I. Determination of O/U Ratio in Hyperstoichiometric Uranium Dioxide, UOz+,,
potentiometric titration
t
w
UOz 2.039
0
z
f 0.002
(DF= 5)
4
m
a
UOz
a
Us08
sm
WAVELENGTH nm
-.-C
Figure 1. Absorption spectrum of U(VI), 17.471 mglmL and U(IV), 4.776 mg/mL, in H3P04saturated with Na,SO,.
were then dissolved in purified phosphoric acid medium, and dissolution was completed by gentle heating under an infrared lamp. In another approach, UO,+, powder was equilibrated with Ar/H2 mixture at 990 "C for 24 h. This was then cooled to room tempetature under the flow of the gas mixture and immediately poured into the phosphoric acid niedium. Dissolution was again completed by gentle w .The concentration of U(IV)in stock solution was determined by adding a saturated solution of ferric ammonium sulfate followed by dilution with a solution of vanadyl sulfate (1 mg/mL of VOSOl in 1M HzS04). The liberated ferrous ions, equivalent to U(1V) pre'sent in the solution, were titrated with a standard solution of potassium dichromate (7).Aliquots were then taken and absorbance was measured in the abovedescribed fashion. It was observed that U(1V) at 544 nm and U(V1) at 420 nm obey Lambert-Beids law up to a concentration of 35 mg/mL (6),which is equivalent to 7 mg/mL for U(Iv) at 665 nm.
RESULTS AND DISCUSSIONS For the accurate determination of O/U ratio in UOz+, it is essential to preserve ita oxidation states. In solution, U(V1) can react with impurities like Fe(II), and U(1V) is prone to air oxidation particularly a t higher temperatures. This is the reason why inert atmosphere operation is required. In the present method phosphoric acid is saturated with 50:- ions, which in turn forms a mixed ligand complex with U(1V) thereby preventing air oxidqtion. Consequently the need to maintain inert atmosphere throughout the dissolution step and the difficulties associated with it are avoided. In the selected medium, even after 48 h, no change was observed in the U(W) concentration. Fop the preservation of U(VI) species it is essential to remove reducing impurities from phosphoric acid. For this, the procedure of Tolk et al. (8)was followed. The absorption spectra of U(1V) and U(V1) in concentrated phosphoric acid saturated with sodium sulfate at room temperature are reproduced in Figure 1. The maximum absorbance peak of U(V1) is at 420 nrh. For U(1V) it is a t 665 nm. The absorbance of U(1V) a t 544 nm has also been used for its determination. I t is clear from Figure 1 that U(V1) does not absorb in the region of 544 and 665 nm, whereas U(1V) does absorb at 420 nm. Accordihgly the calibration curve for U(VI) at 420 and for U(1V) at 420,544,and 665 nm have been drawn. In this medium the determined molar absorptivity of U(V1) at, 420 nm is 14.010 Lemol-km-'. The uranyl ion absorption in the visible region is considered (IO) as a singlet to triplet transition, a charge transfer process from ligand to metal ion within U022+. This spin-forbidden process gets sufficient intensity due to spin-orbit coupling, giving rise to a molar absorptivity normally less than 50. Although, the equitorial ligation on UOz2+does not significantly alter the transition energies of this band, the change could be observed
spectrophotometry absorbance absorbance method of of U(1V) at of U(1V) at Kuhn et al. 665 nm
544 nm
(6)
very high 2.038 f 0.002 1.988 f 0.005 (DF= 10) (DF= 10) absorbance
2.225 f 0.001 2.221 f 0.002 2.223 f 0.002 2.207 f 0.003 (DF= 11) (DF= 11) (DF= 5) (DF= 11) 2.670 f 0.001 2.673 f 0.002 2.673 f 0.004 2.688 f 0.003 (DF= 13) (DF= 13) (DF= 5 ) (DF= 13)
in the relative intensities of vibrionic peaks and the overall absorptivity (11). The observed molar absorptivity is slightly higher than the reported (6) value of 12.817 L.rnol-'.cm-' in pure phosphoric acid medium. In the case of U(IV), internal k f trarisitions are basically Laporte forbidden as electric dipole radiation of the metal ion is at a site of inversion symmetry. But the destruction of center of symmetry due to vibrionic interaction can give rise to a weak spectrum apart from magnetic dipole radiation induced transitions. The f-f transitions associated with higher oxidation state are sensitive to environment (11)but by shifting from phosphate to mixed medium there was no significant change observed either in transition energies or in molar absorptivity as far as the above-mentioned bands are concerned. The reported molar absorbtivity value (6) at 420 and 544 nm for U(1V) are 5.755 L.mol-l.cm-l and 12.271 L.mol-l.cm-', respectively. The molar absorptivity a t 665 nm for U(1V) is found to be 52.053 L. mol-'-cm-' in this medium. In the reported procedure (6) the valley at 516 nm in the U(1V) spectrum has been taken as reference point and absorbance a t this wavelength has been subtracted from the absorbance a t 420 and 544 nm. The reasons given for this are mismatching of cells and darkening of solutions. The determined O/U values recorded in Table I clearly indicate that the values obtained based on this assumption vary from the true value. This analytical approximation, though not accurate, could provide results precise to 10,005 O/U units, because the molar absorptivity values at 420 nm for U(V1) and at 544 nm U(1V) are close to each other. Since the results were not compared with the other existing wet chemical methods for the determination of O/U ratio the difference was not noticed. Near stoichiometry, the concentration of U(V1) constantly goes down and, in case of U02.05,a very minute quantity of U(V1) has to be determined in the presence of a large excess of U(1V). The absorbance a t 516 nm increases with the concentration of U(1V) and is independent of UtVI) concentration. So when U(1V) is in large excess as in case of near-stoichiometric UOz, the absorbance at 516 nm due to U(1V) becomes greater than the absorbance due to a minute quantity of U(V1) at 420 nm. Now if the method of Kuhn et al. is applied for the calculation of O/U ratio, the resulting value will be lower than the stoichiometric ratio of 2:l. Hence this empirical method is applicable to the situation where both U(1V) as well as U(V1) are present in relatively large concentrations as in case of U,08 but would result in unacceptable error as stoichiometry is approached. Surprisingly the maximum absorption peak of U(1V) at 665 nm has been ignmdd. Since the molar absorptivity is 4-fold higher a t 665 nth than a t 544 nm for the same species, it is advantageous to work at this wavelength, patticularly in case of U308*here U(1V) concentration is low. Since the concentrations of U(V1) and U(1V) are independently and directly determined, weight of sample aliquot is not required. Hence the error introduced a t this step particularly due to moisture and oxygen pick up is avoided. The results presented in Table I show that the O/U values obtained from the described
Anal. Chem. 1980, 58, 2481-2485
method and those obtained from the titrimetric procedure (7) are in agreement within 0.004 O/U units. The overall precision achieved by the proposed spectrophotometric method is &0.002 O/U units. In the case of highly sintered UOz+,, dissolution at high pressure becomes essential.
.
CALCULATIONS The observed value of the slope at 420 nm for U(V1) is 0.051 89. For U(1V) it is 0.024 18 a t 420,0.05156 a t 544, and 0.21871 a t 665 nm. Suppose absorbance at 665 nm is AI, a t 544 nm is A2, and a t 420 nm is A3 Cu(Iv) (mg/mL) = A,/0.21871 = A2/0.05156
A, - (A,/0.21871)0.02418 CU(V1)
(mg/mL) =
-
0.05189 A3 - (A2/0.05156)0.02418 0.05189
Then
ACKNOWLEDGMENT The authors are thankful t o C . K. Mathews, Head, Radiochemistry Programme, IGCAR, Kalpakkam, for his keen
2481
interest and help extended during the preparation of the paper. Thanks are also due to S. K. Nayak and R. B. Yadav for useful discussions. Registry No. U308, 1344-59-8;U,7440-61-1; 02,7782-44-7.
LITERATURE CITED (1) Metz, C. F.; Dahlby, J. W.; Waterbury, G. R. In Proceedings of a Symposium on Analytical Methods in The Nuclear Fuel Cycle; 1972; IAEASM-149/33, pp 35-44. (2) Buldini, P. L.; Ferri, D.: Paluzzi, E.; Zambianchi, M. Anawst (London) 1984, 109, 225-227. (3) Kuvik, V.; Krtli, J.; Moravec, A. Radiochem. Radioanal. Lett. 1982, 5 4 , 209-220. (4) Yadav, R. B.; Ahmed, M. K.; Nagarajan, K.; Kaliappan, I.;Rao, P. R. V. I n Proceedings of The Nuclear Chemistry and Radiochemistry Symposium, Nov 1981; pp 537-539. (5) Khatoon. F.: Rao. C. S. Radiochem. Radioanal. Len. 1985. 95, 241-246. (6) Kuhn, E.; Baumgartei, G.; Schmieder, H.; Gorgenyi, T. 2.Anal. Chem. 1973, 267, 103-105. (7) Nagarajan, K.; Saha, R.; Yadav, R. B.; Rajagopalan, S.; Kutty, K. V. G.; Saibaba, M.; Rao, P. R. V.; Mathews. C. K.; J . Nucl. Mater. 1985, 130, 242-249. (8) Tolk, A.; Lingerak, W. A. Proceedings of a Panel on Analytical Chemistry of Nuclear Fuel; IAEA: Vienna, 1972; STI/PUB/337, pp 51-58. (9) John, M.; Vaidyanathan, S.; Venkataramana, P.; Natarajan, P. R. Convention of Chemists; Indian Chemical Society, 1976; Anal-58. ( I O ) McGlynn, S. P.; Smith, J. K. J . Mol. Spectrosc. 1961, 6 , 104-187. (11) Ryan, J. L. "Absorption Spectra of Actinide Compounds" in MTP I n ternational Review of Science : Inorganic Chemistry, Series One ; Bagnall, K. W., Ed.: Butterwotths: London, 1972; Vol. 7, pp 323-367.
RECEIVEDfor review February 28, 1986. Accepted May 23, 1986.
Optimization of Microelectrode Array Geometry in a Rectangular Flow Channel Detector Lawrence E. Fosdick and James L. Anderson*
Department of Chemistry, The University of Georgia, Athens, Georgia 30602
The optlmum geometry was investlgated theoretically for a mlcroelectrode array flow detector operated at a constant applied potentlal on one wall of a rectangular channel under andmOns of laminar Row. Concentration proflles and current were calculated by uslng the backward Impllclt flnlte dlffere w e nunerical procedure for electrode arrays In whlch either the spacing between actlve electrode elements was systematically varied from one end of the array to the other while malntalnlng a constant electrode length parallel to flow or the lengths of individual electrode elements In the array were systematically varied whlk malntalnlng a constant gap length. Both studies indlcate that the optlmum electrode response Is obtained wlih a mloroelectrode array having active sites of constant length, separated by uniform gaps. Experimental data for an electrode wlth uniform electrode and gap lengths and an electrode with uniform electrode and varying gap lengths support the theoretlcal predictions.
The amperometric response of a microelectrode array in a flow-through channel is a subject of interest due to the ever increasing role of electrochemical detectors in chromatography and continuous flow analysis (1-4). Several studies indicate that microelectrode arrays operating a t a single, constant applied potential offer significant advantages over solid 0003-2700/86/0358-2481$01.50/0
electrodes of the same geometric area in regard to signal/noise enhancement, thus improving analytical detection limits and sensitivities per unit active area (2-5). Previous studies for characterization of microelectrode arrays have focused primarily on regularly spaced arrays (1, 5 ) ,although preliminary consideration has also been given to nonuniform arrays (5). However, techniques such as photolithography, which show promise as methods of microelectrode array fabrication, are capable of producing electrodes of a wide variety of geometries (6-8). Also, any microelectrode array fabrication technique is subject to some degree of variance in the size of the active sites and/or the size of the gaps between active sites. An understanding of the effects of fabrication tolerances is necessary to reconcile properly any theoretical treatment of microelectrode array response to experimentally acquired results. This paper will examine the effect of varying electrode size or interelectrode gap size on the response to a microelectrode array, using the backward implicit finite difference method. This study assumes that the microelectrode array consists of strip electrodes on one wall of a rectangular channel, oriented perpendicular to the direction of flow, with the electrode width equal to the channel width. Mass transfer limited response is assumed under laminar flow conditions, neglecting migration and diffusion parallel to flow. The validity and limitations of these assumptions for very fine array elements are presented elsewhere (5). 0 1966 American Chemical Society
