Anal. Chem. 1980 52, 1601-1606
1601
identified with pH(S) in the operational definition of pH. Nevertheless, too little is yet known concerning the residual liquid-junction potential in these solvents at low temperatures to permit one to assess the internal consistency of the pH scale defined by these reference points. Measurements of pH under these extreme conditions are rendered difficult by the extraordinarily high resistance of the usual glass electrodes. In addition, modified reference electrodes will doubtless prove a necessity.
LITERATURE CITED G. Bates, J . Solution Chem., 8, 887 (1979) (2) E. J Cohn, F. R. N. Gurd, D. M. Surgenor, B. A . Barnes, R . K. Brown, G. Deronaux. J. M. Gillesoie. F. W . Kahnt. W. F. Lever. C. H. Liu. D. Mittelman, R.'F. Mouton,~K:Schmid, and E. Urorna, J . Am: Chem. Sbc.. 72, 465 (1950). (3) R. G. Bates and E. A. Guggenheim. Pure Appl. Chem., 1, 163 (1960). (4) M. Paabo, R. A. Robinson, and R. G. Bates, J . Am. Chem. SOC.,87. 415 (1965). Faraday Trans. 7, 73, (5) H. P. Bennett0 and J. J. Spitzer, J . Chem. SOC., 1066 (1977). (6) H. S. Harned and R. W. Ehiers, J . Am. Chem. SOC.,54, 1350 (1932); 55, 652 (1933). (7) D. A. MacInnes and T. Shedlovsky, J . Am. Chem. SOC., 5 4 , 1429 (1932). (8) R. G. Bates and S.F. Fxree, J . Res. NaN. Bur. Stand., 34, 373 (1945). (9) H. 0. Spivey and T. Shedlovsky, J . Phys. Chern., 71, 2171 (1967). (1) M. Sankar, J. B. Macaskill, and R. \
0
Spivey and Shedlovsky
0
Present work
- I
47T 0
20
10
wt
30
X EIOH
Figure 3. pK(molality scale) for acetic acid in ethanol/water mixtures at 0 and 25 OC as a function of the composition of the solvent. Data in water are from Ref. 6 and 7
RECEIVED for review March 3, 1980. Accepted May 12, 1980.
of p H measurements in ethanol/water media at temperatures below 0 "C. The values of PQHfor the acetate and phosphate buffers given in Table I11 may fill this need. When used as reference solutions, the PQH of these buffer solutions is
This work was supported in part by the National Science Foundation under Grant CHE76 24556. M.S. thanks the following organizations in the Republic of South Africa for financial assistance: t h e C.S.I.R., Ernest Oppenheimer Memorial Trust, and Sentrachem Ltd.
Determination of Oxidation States of Uranium in Uranium Dioxide Pellets by Two-step Flow-Coulometry Sorin Kihara,' Zenko Yoshida, Hiroshi Muto, Hisao Aoyagi, Yuji Baba, and Hiroshi Hashitani Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki-ken, Japan
A chemical method for analysis of distribution of oxidation states of metals in solids has been developed as a reference to other methods based on physical measurements. A solid is dissolved with a continuous flow of strong phosphoric acid from surface to bulk, and oxidation states of the dissolved ions are determined consecutively by two-step flow-coulometry. Strong phosphoric acid dissolves such solids as uranium or iron oxides to Ions whose oxidation states are the same as those in solids. The oxidized surface of about 20 A on a UOz pellet which had stood for about 10 years at room temperature in alr was estimated to be U02.05,and the O/U ratio decreased toward the bulk until 0.1 pm. After the pellet was heated for 10 h at 300 or 430 O C in air, the oxidized surface layer increased to about 8 pm (400 8, of powdered U308 UO,+,, 0.66 > x > 0.01) or about 820 pm (370 prn of powdered U308 -t 3 prn of U,08 450 pm of UOZ+,, 0.66 > x > 0.01), respectively.
+
+
Analysis of oxidation states of metals in solids based on physical measurements gives abundant information on ma0003-2700/80/0352-1601$01 .OO/O
terial science. These physical methods can be classified broadly into methods (1) for analysis of very thin layers a t the surface such as by X-ray photoemission spectrometry (XPS) or Auger electron spectrometry (AES) and methods ( 2 ) which give overall information on a solid such as measurements of conductivity or electromotive force (EMF). There are many difficulties still remaining with these methods. In the former, for example, pretreatment of the surface and composition of the matrix greatly affect the result of analysis, and, in the latter, interaction between sample and electrode and the presence of impurities often give serious trouble. Therefore, calibration by reference materials and careful evaluation of data obtained are required. Obviously, data obtained by chemical analyses offer many suggestions for the interpretation of results obtained by physical methods. Although analyses of oxidation states by chemical methods give rather quantitative results, most of the work using these methods has been done to obtain the overall oxidation state of a sample. Few attempts have been made to analyze oxidized films of solid surfaces or to determine distribution of oxidation states in a solid. The objective of our research was to develop a new independent method ;o investigate the distribution of oxidation 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980
1602
He Gas Rotator ( d 1
Quartz Capillary
Buret ( m l
Gas - leok valve ( P 1
20
(b)
+I
......
i
u (VI) --U(IV).
*Drain
Strong Phosph
(SPA1
Flow Coulornurv (
I
1
.......
..I
gz;;'
Sample (Powder)
Figure 1. Schematic diagram of apparatus. (A) Sample holder for
powder sample states of metals in solids. This new method is based on completely different principles from those of physical methods and does not require reference materials. Such a method will be a reference or referee method to the physical methods mentioned above. As is well-known, some kinds of solvents can dissolve solids, producing ions whose oxidation states are the same as those in solids. For example, such dissolution can be attained by strong phosphoric acid ( 3 ) for iron, uranium, or plutonium compounds; by molten salt solutions of mixtures of aluminum chloride a n d potassium chloride ( 4 ) for actinide oxides; and by diluted sulfuric acid (5) for iron compounds. Dissolution of uranium oxide powder in strong phosphoric acid was employed for t h e polarographic (6-8) or the coulometric ( 3 ) determination of uranium(V1). However, no attempt has been reported for the determination of U(V1) distribution in a solid of uranium oxides. In our work, analysis of radial distribution of uranium(V1) a n d -(IV) in uranium dioxide pellets was successfully carried out by dissolving these pellets from surface to center in a flow of strong phosphoric acid, followed by the continuous determination of dissolved ions using two-step flow-coulometry (9).
The distribution of oxidation states of uranium in a uranium oxide pellet can be converted into the distribution of the ratio of oxygen to uranium, O/U ratio, in t h e pellet, which is an important factor in nuclear reactor safety or fuel economy ( I O ) . EXPERIMENTAL A p p a r a t u s a n d Procedure. A schematic diagram of the apparatus used is shown in Figure 1. The sample room (a) is made of quartz and has an inner space of 8 mm i.d. x 10 mm. It consists of a tapered joint which makes it convenient to replace samples. For the powder sample, a holder (A in Figure 1) is used. Quartz capillaries (b) have an inner diameter of 1 mm i.d. Quartz ball joints (c) are used to allow the rotation of the sample room. The typical procedure for the state analysis of a uranium oxide pellet is as follows. After setting the pellet in the sample room, it is rotated at a rate between 60 and 100 rpm using rotators (d) and a synchronous motor (e). Strong phosphoric acid, deaerated, flows through a micro pump (f) (Mini Micro Pump type KHU-16, manufactured by Kyowa Seimitsu Co.) as indicated by broad arrows. Upon heating of the sample room at 180 to 200 "C with a heater (g), dissolution of the pellet begins with yielding U(1V) and U(V1) ions. After the resulting solution is diluted 3 to 6 times in a mixing room (h) with diluted strong phosphoric acid (20% vol), which has been deaerated and is supplied by a micro pump (i) (the same type as (f)), the solution is introduced into the two-step flow-coulometric column electrode (j).Uranium(V1) in the solution is reduced to U(1V) at the first-step column electrode, E-I, whose working electrode potential has been controlled at 4 . 6 V vs. Ag-AgC1 (SSE). Then at the second column electrode, E-11, of +0.85 V U(IV), both the reduction product at E-I and the dissolution product of U O pin the pellet are oxidized to U(V1). Both currents at E-I and E-I1 are recorded with dissolution time on a graph. The ratio of the current at E-I to that at E-I1 at a given time is calculated to the ratio of the amount of U(V1) to the total amount of uranium ions, U(1V) + U(VI), in the flowing
Figure 2. Electrode reactions of uranium in phosphoric acid at the column electrode. Sample: 10 pL of 5.86 mg U,O,/mL. Electrolyte solution: diluted strong phosphoric acid (20% vol). Flow rate: 0.5 mL/min
solution at the time. From this value the ratio of U 0 3 to the total uranium oxides, UOz + U03, at the dissolving surface of the pellet, and hence the distribution of U 0 3 in a UO, pellet, can be estimated. If the pellet contains uranium metal it is dissolved, generating hydrogen gas according to the reaction, U(meta1)
+ 4Hf
-
U(1V) + 2H,
(1)
The hydrogen gas moved to part (k) is released with the aid of an electromagnetic gas-leak valve (1). The volume of the gas is determined by a gas burette (m) and recorded with dissolution time in order to estimate the distribution of the metal in the sample. T w o - s t e p Flow-Coulometric Column Electrode. The column electrode used was identical to that described in a previous paper (9) and glassy carbon fibers of 10- to 12-pm 6 packed in a porcelain cylinder of inner diameter of 5 mm were used as the working electrode. The saturated KC1-Ag/AgCl electrode (SSE) was employed as the reference electrode and all potentials in this paper are referred to SSE. The temperature of the sample solution was controlled at 25 & 1 "C. Potentiostats and a two-pen recorder used were the same as those described previously (9). Chemicals. Strong Phosphoric Acid. Reagent grade phosphoric acid was heated in a quartz flask under reducing pressure with the aid of a water-jet pump until the temperature of the acid rose to 300 OC (7). Uranium Oxide Samples. Uranium dioxide (UO,) powder was prepared by reducing triuranium octoxide (U,O,) powder in a hydrogen atmosphere. It was dried in a desiccator with phosphoric acid anhydride for 2 days before use. The size of the UO, powder was determined by microscopic observation to be between 0.1 and 1 pm, Uranium dioxide pellets (5.6 mm 6 X 8 mm) used were prepared from UOz powder by addition of 1%poly(viny1 alcohol), molded to be pellets of 5.9 mm 6 X 9 mm at a pressure of 3 tons/cm2 and sintering at 1500 to 1600 "C for 2 h under a hydrogen atmosphere (11).
The UOz powder and pellets used were prepared about 10 years ago and have been allowed to stand at room temperature in air. Triuranium octoxide used was NBS SRM 950a, and it was heated at 900 O C for 1 h in air before use. Other chemicals were of reagent grade. Recommended Conditions for t h e Analysis of Uranium Dioxide Pellets. Electrode Potentials of the Column Electrode for the Determination of Uranium(Vl)and -(ZV).Diluted strong phosphoric acid (20%) flowed through the column electrode at a rate of 1 mL/min. From the sample inlet located in front of the column electrode, 10 WLof uranium solution was injected and the integrated electrolytic current (coulomb) obtained at various potentials was plotted against the electrode potential (coulombpotential curve, see Ref. 9). Figure 2 is the coulomb-potential curve of uranium in diluted phosphoric acid solution. The uranium solution (5.86 mg U308/mL)was prepared by dissolving 293.0 mg of U,08 powder in 10 mL of strong phosphoric acid followed by dilution to 50 mL with water. The quantitative reduction of U(V1) to (IV) and the
* 2
0
ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980
3
0
Time ( h r )
Flgure 3. Analysis of U308 powder. Curve 1: reduction current of
U(V1) to -(IV) at E-I; curve 2: oxidation current of U(IV) to -(VI) at E-11. Sample: 143.0 mg of U308 powder. Dissolution: at 180 "C. Flow
rate: strong phosphoric acid, 0.1 mL/min; diluted strong phosphoric acid (20% vol), 0.4 mL/min. Electrode potential: E-I, -0.6 V; E-11, +0.85 V vs. SSE Table I.
total coulombs, C
2.27 64.98 12.0 6.68 i , b , mA 6.76 4.10 97.17 17.8 10.1 i,c, mA 10.1 0.6687 0.661 0.671 0.669 0.670 i,li2 a Conditions are the same as those given in Figure 3. Reduction current at E-I. Oxidation current at E-11. quantitative oxidation of U(IV) to (VI) were performed at potentials more negative than 4 . 5 V and at potentials more positive than +0.75 V, respectively. Temperature for Dissolution. The time required for complete dissolution of a U 0 2 pellet was about 13 h at 180 "C, 9 h at 200 "C, and 4 h at 230 "C, respectively. At temperatures lower than 150 "C dissolution was negligible and at temperatures higher than 210 "C the dissolving surface of the pellet was considerably rougher as shown in F of Figure 6; thus the optimum temperature was between 170 and 200 "C. Dilution of Strong Phosphoric Acid. This process at part (h) in Figure 1is required to reduce the viscosity of the solution and, consequently, to make the diffusion of ions fast and the flow of the solution through the column electrode easy. When the dilution was carried out with a solution of strong phosphoric acid whose concentration was less than 10% vol, however, the precipitation of polymerized strong phosphoric acid and/or U(1V) compound was often formed. Therefore, the dilution should be done with a solution of strong phosphoric acid whose concentration is higher than 15%. The optimum strong phosphoric acid concentration of the solution after dilution was between 20 and 50%. Flow Rate of the Solution. Smooth and uniform dissolution of the pellet was attained when the flow of strong phosphoric acid was fast. Also, the resolution of the stratified analysis of the pellet became better with faster flow rate. On the other hand, the electrolytic efficiency at the column electrode decreased with increasing flow rate. For our research, the flow rate of the diluted strong phosphoric acid solution flowing through the column electrode was controlled at between 0.3 and 2 mL/min.
RESULTS Preliminary Studies. With U308Powder. Triuranium octaoxide is considered to be a mixture of U(1V) and -(VI) which corresponds to the composition U02.2U03. Powder of U B 0 8(143.0 mg) was taken in the sample holder a n d analyzed under conditions described in the legend to Figure 3, according to the procedure given in the experimental part. T h e reduction current a t E-I, whose potential was controlled to 4 . 6 V, and the oxidation current at E-I1 of +0.85 V were recorded with time from the beginning of dissolution and are shown in curves 1 and 2 in Figure 3. The ratio of the current at E-I, il, to the current at E-11, ip, at a given time (il/iz ratio) is summarized in Table I. T h e ratio of il/i2 in Table I is nearly constant at any time and close to 213 which coincides with the ionic ratio of U(V1) t o [U(IV) + U(VI)] in U308. This fact indicates that the rate
3
Figure 4. Analysis of UO, powder. Curve 1: reduction current of U(V1) to -(IV) at E-I; curve 2: oxidation current of U(IV) to -(VI) at E-11. Sample: 185.8 mg of UO, powder. Other conditions are the same as those given in Figure 3
Table 11. Analysis of UO, Powdera
Analysis of U,O, Powder time for dissolution, h 0.7 1.0 1.8 2.3
2 Time(hr)
1603
0.35
time for dissolution, h 0.7 1.3 2.0
total coulombs, 2.7
C
i l b , mA 0.31 0.98 0.73 0.42 0.13 6.068 i , C , m A 4.15 15.1 11.9 11.1 6.15 133.8 0.0454 il/i2 0.074 0.065 0.061 0.037 0.021
Conditions are the same as those given in Figure 4. Reduction current a t E-I. Oxidation current at E-11.
a
of dissolution of U O p is identical to t h a t of U 0 3 and t h a t oxidation states of uranium in the powder remain unchanged even after dissolution, transportation, and dilution. It is also evident that the reduction of U(V1) a t E-I and the oxidation of U(1V) at E-I1 are quantitative. With UOzPowder. T h e powder (185.8 mg) was analyzed by the same procedure as that for tJ308 powder. The relation between currents at E-I and E-I1and time from the beginning of dissolution is reproduced in Figure 4. A rather large reduction current of U(V1) was obtained a t E-I (curve l), indicating the existence of U 0 3 in the powder. Because the ratio of ill& given in Table I1 decreased with time, i t is concluded that the powder had been oxidized from the surface and was not a homogeneous mixture of U 0 3 and UOz. T h e U 0 3 content in the powder was calculated t o be 4&% by comparing the Coulomb under curve 1with that under curve 2. This value agrees well with the value, 4.870, obtained by polarography (7) and corresponds to the O / U ratio of 2.0456. With U308Powder in the Presence of Uranium Metal. In the sample holder, 448.3 mg of U308powder (thought to be a mixture of 304.5 mg of U 0 3 and 143.8 mg of UOz) and 56.7 mg of uranium metal were taken, and the simulated sample was analyzed by the proposed method. From the hydrogen gas volume, the amount of the metal was calculated to be 55.5 mg. The amount of U 0 3 in the sample was determined to be 302.8 mg from the Coulomb obtained by integrating the current a t E-I with respect to the time. The amount of U 0 2 was estimated to be 144.8 mg from the amount of U(1V) in the sample, which was calculated by subtracting the amount of uranium metal and the amount of uranium in U 0 3 from the total amount of uranium obtained from the Coulomb a t
E-11. These analytical results indicate that the state analysis of uranium oxides is possible by the proposed method even in the presence of uranium metal. Stability o f U ( I V ) in Diluted Strong Phosphoric Acid Solution. After dissolving U 0 2 powder in strong phosphoric acid, the solution was diluted to 20% strong phosphoric acid. Then U(V1) was reduced to U(1V) by controlled potential electrolysis with a glassy carbon working electrode. Deaerated solutions containing various concentrations of U(1V) were allowed to stand for various periods of time a t 25 "C and the concentration of U(V1) generated during the standing time was determined by using the column electrode. The results are summarized in Table 111. Although the rtitio
1604
ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980
Table 111. Stability of U(1V) in Diluted Strong Phosphoric Acida concn of U(IV), M
standing time at 25 "C, h
4.8 x 10-7 2.2 x l o - $ 4.6 X 4.6 X 2.0 x 1 0 - 6 3.6 X 4.3 x 10-7 1.1x 2.7 x l o - $
0.5
10-4
10
24 0.5
10-3
10
24 0.5 10 24 a
271 x Io-' c
U(V1) found, M 1532C 1
0
2
8
7
Time(hr)
Figure 5. Analysis of UOp pellet. Curve 1: reduction current of U(V1) to -(IV) at E-I; curve 2: oxidation current of U(1V) to -(VI) at E-11. Sample: UO, pellet (5.6 m m C$ X 8 mm, 2.167 9). Dissolution: at 200 O C . Other conditions are the same as those given in Figure 3 c-----1
20 vol %, deaerated.
10mm
of generated U(V1) to U(1V) in the original solution increased with decreasing U(1V) concentration, the amount of U(V1) generated was negligible and does not interfere with the state analysis of uranium oxides by the proposed method. In this connection, the time required to transfer the uranium ions from sample room to the electrode in the proposed method is less than 10 min. A n a l y s i s of D i s t r i b u t i o n of Oxidation States of U r a n i u m in U O z Pellets. A uranium dioxide pellet (5.6 mm 4 x 8 mm, 2.16 g) was analyzed and the result is reproduced in Figure 5 , T h e small reduction current a t E-I (curve l), which was observed only a t the beginning of the dissolution, indicates that only a thin layer at the surface of the U 0 2 pellet had been oxidized and that uranium in the bulk of the pellet remained as UOz. Because no hydrogen gas evolution was observed, there was no uranium metal in the pellet. The dissolution of the UOz pellet was performed smoothly and uniformly as shown in the picture (A, B, C, D, and E in Figure 6). For example, the diameter of what remained of t h e pellet after dissolution a t 200 "C for 3.2 h (I3 in Figure 6) was 4.83,4.83, and 4.83 mm a t the top, the middle, and the bottom, respectively. Values of the original pellet were 5.61, 5.60, and 5.60 mm. At temperatures higher than 210 O C , dissolution was more rapid but less smooth (F in Figure 6). After the UOz pellet was heated at 300 O C for 10 h in 1 atm air, the pellet was analyzed (Figure 7). Results given in Figure 7 are summarized in Table IV. In this table, the depth from surface was calculated from the amount of dissolved uranium obtained by integrating i2 with respect to time.
[F]
[AI
Figure 6. Aspect of dissolbtion of UO, pellet. Sample: UO, pellet (5.6 I$ X 8 mm, 2.16 f 0.04 9). Flow rate of strong phosphoric acid:
mm
0.1 mL/min. [ A ] : not dissolved: [B], [ C ] , [D], and [E]: dissolved at 200 O C for 3.2, 5.9,7.1, and 7.5 h, respectively: [F]: dissolved at 230 OC for 3.1 h
W
Time ( h r )
Flgure 7. Analysis of UO, pellet heated at 300 O C . Curve 1: redtiction current of U(V1) to (IV) at E-I; curve 2: oxidation current of U(1V) to (VI) at E-11. Sample: UO, pellet (5.6 m m 4 X 8 mm, 2.17 g) heated at 300 O C for 10 h in 1 atm air. Dissolution: at 200 O C . Other conditions are the same as those given in Figure 3
When the UOz pellet (5.60 mm 4 X 8.03 mm, 2.119 g) was heated a t 430 "C for 10 h in 1 atm air, 0.664 g of U 0 2 a t the surface was oxidized to powder. The composition of the powder was determined to be U308by the proposed method. .____I _I_.____.
_ I
Table IV. Analysis of UO, Pellet Heated at 300 " C for 1 0 h in 1 atm Aira time for dissolution, h i l b , mA izC,mA ill& depth from surface, pm a
0.080
0.125
0.25
0.375
0.50
0.75
0.0076 0.0113 0.67 0.009
0.034 0.052 0.651 0.048
0.102 0.333 0.306 0.122
0.203 3.38 0.060 0.362
0.232 13.4 0.017 2.65
0.161 48.6 0.0033
Conditions are the same as those given in Figure 7.
21.1
1.00
0.056 63.2 0.0009 130
Reduction current at E-I.
7.00
total coulombs, C
0 52.6
0.493 1541 3.19 x 10-4
0
-
Oxidation current at E-11.
_-__
.____
Table V. Analysis of What Remained of the Pelleta after Heating at 430 " C for 10 h in 1 atm Air& time for dissolution, h
_ _ ~ _ _ _ _ _
i l C , mA i z d ,mA i, li, depth from surface, pm
0.17
0.33
0.50
0.58
1.41 2.11 0.67 0.93
5.58 8.46 0.66 2.48
12.1 22.8 0.53 10.6
12.3 34.1 0.36 17.8
_- -- .__-
0.67
0.83
1.00
2.00
3.00
10.8
5.68 57.1 0.099 55.8
2.47 62.4 0.040 102
1.03 57.4
0.55 55.6 0.0099 453
43.5 0.25 35.4
0.018
27 0
Analytical conditions are the same as those a 370 pm of the surface of the original pellet was oxidized t o U,O, powder. Oxidation current at E-11. ___ .I_.._I__ given in Figure 7. e Reduction current at E-I. .____._I_
ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980
principles and on a primary standard, Le., Faraday constant. The UO:! pellet used in our work is very close to the stoichiometric composition of UOz as demonstrated in Figure 5. Marin and Contamin (15)reported Equation 2 as the diffusion coefficient of oxygen in stoichiometric UO,;
06
05
D = 0.26 exp(-59300/RT) (cm2.s-'), R; 1.987 ca1.K-l (2)
04
03
02 01
0001
1605
001
01
1
Depth from surface
10
100
1000
(pr)
Figure 8. Variation of the i , / i , ratio with the depth from surface of UO, pellets or powder. Curve 1: UO, pellet heated at 300 OC for 10 h (Table IV); curve 2: remaining UO, pellet after heating at 430 OC for 10 h (Table V, 370 pm of the surface of the original pellet was oxidized to U,08 powder during the heating); curve 3: UO, pellet (Figure 5); curve 4: UO, powder (Table 11, calculated by assuming the powder as spheres with radius of 0.25 p m )
The thickness of the powdered layer was estimated to be 370 pm from the amount of the powder and the size of the remaining pellet (4.86 mm @ X 7.30 mm). Results of state analysis obtained with the remaining pellet by the proposed method are summarized in Table V. T h e variation of the il/i2 ratio of heated pellets given in Tables IV and V is plotted against the depth from the surface in Figure 8. About 400 8, a t the surface of the pellet heated a t 300 "C had been oxidized to U308 (ill&= 0.67), and the ill&ratio decreased toward the center until 8 pm (ill&= 0.01) and the bulk was UO,. About 370 pm a t the surface of the pellet heated a t 430 "C had been oxidized to U306powder and 3 pm of the surface of the remaining pellet was U30s Beyond the U30s layer the ill&ratio decreased until about 450 pm (ill&= 0.01) and the bulk was UOz. In this connection, the O / U ratio is expressed by 2 + il/i2.
DISCUSSION Among physical methods for the state analysis in situ (XPS, AES, IMA, etc.), X-ray photoemission spectroscopy (XPS) has been most widely used for the study of uranium oxides (12, 13). Even by X P S , however, it is difficult to determine oxidation state distributions in surface films of uranium oxide pellets because the chemical shift between 4f binding energies in U 0 2 and U03, 1.8 eV, is not large enough for the state analysis. Also, alternative in-situ approaches are essentially qualitative or semiquantitative a t best ( I , 14). Chemical methods for state analysis of uranium oxides, such as gravimetric, gasometric, coulometric ( 3 ) ,or polarographic (6-8) methods, are quantitative in their principles. But these methods have been proposed only for the investigation of overall oxidation state in a sample, and no chemical approach has been reported for the distribution of oxidation states. In this work, a chemical method for the analysis of the distribution of oxidation states of uranium in uranium oxide pellets has been discussed. The method is based on the dissolution of the pellet from surface to bulk in a small sample room with a flow of strong phosphoric acid and flow-coulometric detection of uranium ions of an oxidation state a t the dissolved layer. Rotation of the sample room during the dissolution ensures rapid and smooth dissolution, and the use of flowing solution is advantageous not only to the consecutive determination but also to the rapid dissolution. Coulometry used for detection has the unmatched advantage of being truly quantitative because it is based on first
As is described in Matthews' review (16),for measurements of oxygen self-diffusion in uranium dioxides (UOz+,), the only work in which UOz of sufficiently stoichiometric composition (x = 0) was used is that of Marin and Contamin. According to Equation 2, D in the pellet heated a t 430 "C is expected to be 1.7 X lo4times greater than that in the pellet heated a t 300 "C and, therefore, the depth of the oxidized layer in the former is expected to be about 130 times larger than that in the latter. In our research, the thickness of the oxidized layer in the pellet heated at 430 "C, about 820 pm (370 pm of powdered U30E+ 3 pm of U308 + 450 pm of UOZ+,, 0.66 > x > 0.01), was about 100 times larger than that in the pellet heated a t 300 "C, about 8 pm (400 8, of U30E+ 8 pm of UOz+,, 0.66 > x > 0.01). The gravimetric study of Scott and Harrison (I 7) demonstrated t h a t the oxidation of low surface area UOz powder (100-Fm 4, surface area, 0.006 m2.g-') starts a t temperatures between 335 and 390 "C and is complete by 500 "C yielding U306. I t is believed that the great increase of the U30Elayer (powdered) observed in our research with the pellet heated a t 430 OC corresponds to the phenomena observed by Scott and Harrison. Similar to curves 1 and 2 in Figure 8, results obtained with the unheated UOz pellet (Figure 5 ) are added in the same figure (curve 3). Though only semiquantitative information is given in this plot because il observed is not large enough, it is estimated that the surface layer of about 20 8, is near t o UOz,mand x in UOZ+,is less than 0.01 a t the depth of 0.1 pm. The radial distribution of oxidation states of uranium in the UOz powder is also realized in Figure 8 (curve 4). This curve is obtained from the results given in Figure 4 and Table I1 under t h e assumption t h a t each particle in the powder is a sphere of radius of 0.25 pm though the radius in the actual sample was between 0.05 and 0.5 pm. Qualitatively the ill& ratio (= x in UOz+,) is constant, about 0.06, until 0.05 pm from the surface and is less than 0.01 a t the depth of 0.1 pm. In conclusion, the proposed method can be applied not only to the state analysis of uranium oxides but also to that of other metal oxides. For example, the state analysis of iron oxides, which are believed t o be mixtures of FeO and Fez03,is expected to be possible, because the dissolution rate of FeO in strong phosphoric acid is identical to that of Fez03 and the coulometric detection of ferrous and ferric ions is easy. In fact, when Fe304 (FeO.FezO,) powder was dissolved in strong phosphoric acid by the proposed method and ferrous and ferric ions in the resulting solution were determined a t E-I of 4 . 2 V and E-I1 of +0.5 V, respectively, the results showed that the ratio of the amount of ferrous ions to that of ferric ions were unity a t any dissolution time.
ACKNOWLEDGMENT The authors thank A. Hoshino of Japan Atomic Energy Research Institute for discussions.
LITERATURE CITED (1) Fiermans, L.; Vennic, J.: Bekeyser, W. "Electron and Ion Spectroscopy of Solids"; Plenum: New York, 1978. (2) Putley, E. H. "The Hall Effect and Related Phenomena"; Butterworth and
(3) (4) (5) (6)
Co. Ltd.: London, 1960. Stromatt, R. W.; Connally, R. E. Anal. Chem. 1961, 33, 345-346. Lyon, W. L.; Moore, R. H. USAEC-report HW-59147, 1959. Kihara, S., unpublished results. Burd, R. M.: Goward, G. W. USAEC-report WAPD-205, 1959
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Anal. Chem. 1980, 52, 1606-1610
(7) Motojirna, K.; Hoshino, A. J . A t . Energy SOC.Jpn. 1960, 2, 1-5. (8) Kubota, H. Anal. Chem. 1960, 32. 610-612. ( 9 ) Kihara, S. J . Electroanal. Chem. 1973, 4 5 , 31-44 and 45-58. (IO) Olander, D. R. "Fundamental Aspects of Nuclear Reactor Fuel Elements", TID-2671 I - P I , US-ERDA: Virginia, 1976. (1 1) Hausner, H. H.; Schumer, J. F. "Nuclear Fuel Elements", Reinhold: New York, 1959. (12) Veal, 6.W.; Lam, D. J. Phys. Lett. A . 1974, 4 9 , 466-468. (13) Allen, G. C.: Crofts, J. A.; Curtis, M. T.; Tucker, P. M.; Chadwick, D.;
Hampson, P. J. J . Chem. Soc., Dalton Trans. 1974, 1296-1301. (14) Ibach, H. "Electron Spectroscopy for Surface Analysis", Springer-Verbg:
New York, 1977. (15) Marin, J. F.: Contamin, P. J . Nucl. Mater. 1969, 30, 16-25. (16) Matthews, J. R. AERE-M 2643, AERE Harwell, 1974. (17) Scott, K. T.; Harrison, K. T. J . Nucl. Mater. 1963, 8, 307-319.
RECEIVED for review August 14, 1979. Accepted May 27, 1980.
Faradaic Ion Transfer across the Interface of Two Immiscible Electrolyte Solutions: Chronopotentiometry and Cyclic Voltammetry D. Hornolka, L e Quoc Hung, A. Hofrnanov5, M. W. Khalil,' J. Koryta," V. MareEek, 2. Samec, S. K. Sen,' P. Vanqsek, J. Weber, and M. Btezina J. HeyrovskY Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Opletalova 25, CS- 7 10 00 Prague 1, Czechoslovakia
M. Janda and I. Stibor Department of Organic Chemistry, Institute of Chemical Technology, Suchbdtarova 1905, CS- 166 28 Prague 6, Czechoslovakia
The interface of an aqueous and an organic electrolyte can be electrochemically polarized from an external voltage or current source. The faradaic ion transfer taking place as an effect of the polarization has been studied by means of chronopotentiometry and cyclic voltammetry. These methods are suitable for determination of semihydrophobic ions present in one of the phases. Cation transfer is faciliated by macrocyclic complex formers (ionophores) present in the organic phase. This can be used for determinatlon of the complexing agents and for stability constants of the complexes formed In the organic phase.
In the present paper we intend to show the main features of electrolysis a t the interface between two immiscible electrolyte solutions (ITIES) in the chronopotentiometric and cyclic voltammetric mode. The processes determining the characteristics of electrolysis take place exclusively a t ITIES or in its vicinity while the metallic electrodes immersed in both phases (one aqueous, the other one organic) play only a role of conducting leads to these phases. We shall give examples of transfer of semihydrophobic ions across ITIES. As a process of analytical importance, the complex formation of alkali metal ions with macrocyclic ionophores in the organic phase will be sht?wn which can be utilized for quantitative determination of iiese substances. The interface between two immiscible electrolyte solutions, for example, with water (w) and an organic liquid ( 0 ) as solvents, has several basic features analogous to the interface metallic electrode/electrolyte solution ( I ) (see Figure 1). Between both liquid phases there is established an electric potential difference which in cases where both phases contain a common ion I with a charge number z follows the NernstDonnan equation 1
UNESCO Scholar. Present address: Department of Chemistry,
Cairo University, Giza, Cairo, Egypt.
UNESCO Scholar. Present address: 32, Baghbazar Street, Calcutta. India.
where the p's are inner electrical potentials, $s standard chemical potentials and ais activities of the ion I in each phase. These activities are defined, for example, on the molar scale; in the case of limiting dilution they approach molar concentrations (units m ~ l - d m - ~ )The . quantity AG$-" is the standard Gibbs energy of transfer from the organic to the aqueous phase for the ion I. In order to attribute a definite value to this quantity an extrathermodynamic approach has to be used, for example, the "TATB assumption" stating that the standard transfer Gibbs energies of tetraphenylarsonium cation and of tetraphenylborate anion are equal for any pair of solvents ( 2 ) . On the basis of this assumption, scales of standard Gibbs transfer energies and of standard electrical potential differences il,Wpocan be composed (3, 4 ) . On introduction of a charge to one of the phases from an external source (and, simultaneously, of a charge of opposite sign to the other phase-see Figure l),two processes can take place. One of these is transfer of ions whose equilibrium potential (Equation 1) due t o charge injection does not coincide with the actual value of A,"(.. The other one is the charging of the electrical double-layer present at ITIES. When the electrolyte of the aqueous phase only consists of very hydrophilic ions (ATP:, very positive for the cation and very negative for the anion) while the organic phase only contains very hydrophobic ions (Arpp very negative for the cation and very positive for the anion) then there exists a potential range ("a potential window") where ITIES behaves like an ideally polarized electrode, i.e., the injected charge is used only for double-layer charging ( 4 , 5 ) . If there are semihydrophobic ions present in the system a t a low concentration, the introduction of charge to a phase can cause their transfer to occur in the potential window. Thus, they can give an effect analogous to those shown by the electroactive species in methods like voltammetry or chronopotentiometry .
0003-2700/80/0352-1606$01.00/0 1980 American Chemical Society
