500
Anal. Chem. 1985, 57,500-504
(3) Engvall, E. I n "Methods in Enzymology"; Van Vunakis, H., Langone, J. J., Eds.; Academic Press: New York, 1980; Vol. 70, pp 419-439. (4) Green, N. M. Biochem. J. 1965, 9 4 , 23c-24c. (5) Green, N. M. Biochem. J. 1986, 101, 774-789. (6) Green. N. M. I n "Methods in Enzvmoloav": McCormich. D. B.. Wrioht. L. D.. Eds.; Academic Press: New York:.1970; Vol. 18A, pp 414-434. (7) Schultz, J. s.;Sims, G. Biotechnoi. Bioeng. symp.1979, 9 ,65-91. (8) Ikariyama, Y.: Aizawa, M. R o c . 2nd Sensor Symp. 1982. 97-100
(9) Yamakawa, T., Ed. "Data Book of Biochemistry": Tokyo Kagaku Dojin: Tokyo, 1980; Voi. 1. pp 94 and 100. (10) Beers, R. F., Jr.; Sizer, I . W. J. Bioi. Chem. 1952, 195, 133-140. (11) Ikariyama, Y.; Aizawa, M. Proc. 3rdSensor Symp. 1983, 18-20.
RECEIVED for review August 22, 1984. Resubmitted October 30, 1984. Accepted October 30, 1984.
Determination of Oxide in Aluminum Chloride-Sodium Chloride Melts via Electrochemical Methods T. M. Laher, L. E. McCurry, and Gleb Mamantov*
Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1600
The use of TaCI, as a probe solute for the differential pulse voltammetric determination of oxide In AICI,-NaCI,,,, melts has been successfully demonstrated. This method makes use AIOCI,- F= TaOCI,- 4of the equilibrium reactlon TaCI,AICI,-. The analogous TI( I V ) method, used previously for basic AICI,-BPC melts, was found not to be applicable to the AICI,-NaCI melt. The equilibrium constant for the above reaction was determined to be (1.9 f 0.4) X l o 5 at 200 'C. I n addltlon to the Ta(V) method, an electrochemlcal cell employlng a P-alumlna membrane was shown to be useful In the potentiometric determlnatlon of oxlde In AICI,-NaCl,,, melts.
+
In an earlier study, McCurry (7) investigated the electrochemical behavior of Ta(V) in A1C13-NaCl,a,d melts as a function of oxide concentration in the melt. At low oxide concentration only a single voltammetric peak a t +0.65 V vs. Al(III)/Al in AIC1,-NaCLd melt was observed. The electrode reaction proposed for this process is given in eq 1. As the TaC16- -k e-
0003-2700/85/0357-0500$01.50/0
TaC1,'-
(1)
oxide content of the melt was increased, a second voltammetric peak at +0.45 V was observed, with the peak height increasing with increasing oxide concentration. The second process was ascribed to the following reaction TaOC1,-
Several attempts to determined the oxide content of chloroaluminate melts via electrochemical, spectroscopic, and other physical methods have been reported in the literature. For example, Tremillon et al. (1)proposed the use of a Ni/NiO electrode for the determination of oxide in AlC1,-NaCl melts. Later studies showed that the potential of this electrode is dependent on the chloride ion rather than the oxide ion activity ( 2 ) . Berg et al. (3) recently described a method for the determination of oxide in AlC13-NaC1 melts in which the oxide content of the melt is determined by difference of the weighed amount of AlCl, in the melt and that determined via potentiometric measurements. Various electrochemical studies have shown that the behavior of Nb(V) ( 4 ) ,Mo(V) (5),Ti(1V) (6), and Ta(V) (7) in chloroaluminate melts is dependent on the oxide concentration present in these melts. Osteryoung and co-workers (6,8) devised a voltammetric titration technique for the determination of oxide in the chloride ion rich (basic) room temperature aluminum chloride-N-(n-buty1)pyridinium chloride (AlCl,-BPC) melt using TiC14 as a probe solute. When Ti(1V) was added to the 0.8:l.O AlC1,-BPC melt which contained oxide, the species Tic&'- and TiOC1,'- were found to exist in equilibrium. The chloro complex could be reduced to Tic&-a t -0.35 V vs. A1 in 2.0:l.O AlCl,-BPC, while the oxychloro complex was reduced to Tic&,- at -0.77 V. From normal pulse voltammetric measurements of the reduction of Ti(1V) to Ti(II1) as a function of Ti(1V) concentration, the initial oxide concentration of basic AlCl,-BPC melts could be determined. Because of many similarities between the basic AlCl,-BPC melt and the AlC13-NaClsad melt, the possibility of using the above method for the determination of oxide in the basic AlC1,-NaCl melt was briefly explored. However, these attempts failed because TiC14, when added to AlC1,NaClSatdmelts, was found to be quite volatile with the result that the total Ti(1V) concentration in the melt rapidly decreased.
-
+ 2c1- + e-
-
TaCls2-
+ 0'-
(2)
Two other reduction peaks were also observed at +0.23 V and +0.15 V; these peaks were assigned to the further reduction of TaC&'- to lower oxidation states of tantalum. This type of behavior was also noted earlier by Ting ( 4 ) for Nb(V) in the AlC13-NaCLa melt in which the Nb(V) oxychloro species was found to be stable. This behavior is similar to the behavior of Ti(1V) in AlC1,-BPC (6,8). Furthermore, TaCl, is a relatively nonvolatile solid at typical working temperatures (175-200 "C) for A1C13-NaCl,a,d melts. Hence, TaCl, was believed to be a good probe solute for oxide determinations in AlCl,-NaCl,,d melts. Tremillon and co-workers (9) successfully employed an yttria-stabilized zirconia membrane electrode as an oxide ion selective electrode in the NaC1-KCl eutectic. However, attempts to use this electrode as an oxide indicator electrode in A1C13-NaCLd melt failed (10). Instead, this electrode was found to be chloride-ion selective in this melt. At present no known oxide ion selective electrodes for use in chloroaluminate melts exist. The exact nature of the oxide species in chloroaluminate melts has been the subject of much debate. The formula for the oxide species in basic chloroaluminate melts has been assigned in most of the reported literature as A10C12- (11). However, in a very recent study of the phase diagram of the A1C13-NaC1 system and the effect of oxide impurities on the freezing point depression of the melt, Berg et al. (3)proposed that the aluminum oxychloro species exists as a solvated dimer, A1302C&-.Their results did not rule out the possibility that this solvated dimer species was in equilibrium with other aluminum oxychloro species, most notably Al,OC&'- (solvated AlOClZ-). EXPERIMENTAL SECTION Drybox System, Because of the air- and moisture-sensitive nature of the compounds used, all handling of materials and 6 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 2, FEBRUARY 1985
501
indicator electrode were carried out with a Keithley 616 digital electrometer. Temperature control of the furnace was maintained with a variable autotransformer; cell temperature was measured with a chromel-alumel thermocouple equipped with an Omega Model 199 digital temperature readout.
Figure 1. Electrochemical cell employing the @-aluminamembrane.
experiments involving the Ta(V) method for the determination of oxide were conducted under a nitrogen atmosphere inside a Vacuum Atmospheres drybox equipped with a dry trainfoxygen removal column. The moisture level of the atmosphere was monitored with an Ondyne Model 1440 digital hygrometer. The moisture level was typically less than 5 ppm. Materials. The chloroaluminate melts were prepared from aluminum chloride (Fluka, anhydrous),which was sublimed twice, and from sodium chloride (Fisher) dried under vacuum at 300 "C for 24 h. Melt preparation is described elsewhere (12). Tantalum(V) chloride (99.995%), used as a probe solute in oxide determinations, was obtained from Johnson-Matthey, and used without further purification. The (3-alumina tube (GE, 10 mm o.d.), used in oxide determinations was prebaked at 900 OC under vacuum before use. Electrochemical Cell for Ta(V) Method. The electrochemical cell employed for oxide determinations via the Ta(V) method used a typical three-electrodeconfiguration. The working electrode consisted of glassy carbon (Tokai,area 0.0707 cm2)sealed in a Pyrex tube with electrical contact being achieved via a nichrome spring and a tungsten rod. Aluminum and tungsten wires (both Alfa, m5N, 0.75 mm diameter) served as reference electrode and counterelectrode, respectively. Aluminum wires were etched in HNO3-HZSO4-H3PO,(3030:40by volume), washed with distilled water, and dried in an oven before use. Tungsten wires were electrolyzed in 50% NaOH solution containing 5% NaN02, washed with distilled water, and dried in an oven before use. Both counterelectrode and reference electrode compartments were separated from the working compartment by means of fine porosity (4.5-5.0 wm pore size) frits. Electrochemical Cell for Potentiometric Oxide Measurements. A schematic diagram of the electrochemical cell employing a @-aluminatube as a membrane is shown in Figure 1. The reference compartment consisted of a @-aluminatube attached to a Pyrex tube via a graded seal. A platinum wire, dipped in the AlC13-NaClsaa melt containing a known amount of oxide and also in contact with oxygen (1 atm) above the melt, served as the indicator electrode for the reference compartment. A similar platinum wire served as an indicator electrode for the working compartment with the exception that the oxide concentration of the melt in contact with the electrode was variable. A special addition chamber, which could be purged with very high purity (99.9995%) oxygen, was used for additions of AlOCl to the melt in the working compartment. A pressure release valve was employed to maintain 1 atm O2 pressure within the entire cell. Instrumentation for Electrochemical Measurements. Differential pulse voltammograms were obtained with a PAR 174 polarographic analyzer equipped with a Houston 2000 X-Y recorder. Potentiometric studies involving the @-aluminaoxide
RESULTS A N D DISCUSSION Ta(V) Method for the Determination of Oxide in AlC13-NaClwd Melts. In an initial experiment small quantities of TaC1, were added to an AlC13-NaCl,a,d melt, and differential pulse voltammograms were obtained for the reduction of Ta(V). A single voltammetric peak at +0.45 V vs. A1 in the AlC13-NaClsad melt was observed for the reduction of Ta(V) to Ta(1V) until a Ta(V) concentration of ca. 10 mM was reached (Figure 2a). Once this concentration was reached, a small shoulder a t +0.65 V was observed, which increased in size as the Ta(V) concentration was increased (Figure 2b). At a Ta(V) concentration of ca. 20 mM this shoulder became a distinguishable peak of roughly equal height as the first peak (Figure 2c). The peak a t +0.65 V continued to increase as further additions of Ta(V) were made. The half-widths of the two peaks were found to be 140 mV, in good agreement with the theoretical value of 143 mV expected for a one-electron process (13). In addition to the voltammetric peaks at +0.65 V and +0.45 V, other peaks were noted a t +0.23 V and +0.15 V; these peaks were previously assigned to the reduction of Ta(1V) to species of lower oxidation state (7). The above behavior for the reduction of Ta(V) to Ta(1V) can best be explained by the following equilibrium reaction:
TaC1,-
+ AlOC1,- & TaOC1,- + AlC14-
(3)
The aluminum oxychloro species in this equation is given as A10C12-; the equation is similar in form to that previously assigned to the Ti(1V) chloro-oxychloro complex equilibrium in basic A1C13-BPC melts (6, 8). Equation 3 can also be written in terms of the aluminum oxychloro species appearing in the equation as A1302C16-or A120C16'- ( 3 ) . However, the expression, derived from eq 3 (see below), for the concentration of oxide in the melt is basically of the same form regardless of the nature of the oxide species in the melt. The above evidence for the reduction of Ta(V) to Ta(1V) indicates that TaCl, might be used as a probe solute for the electrochemical determination of oxide in AlC13-NaCLd melts, provided that the reduction processes of the Ta(V) chloro complex and the oxychloro complex are diffusion controlled (6, 8). T o determine if the electrochemical reactions given by eq 1and 2 are diffusion controlled, chronoamperometric experiments were performed first on a melt containing only TaOC1,- and later on the same melt to which enough TaClj had been added to result in a solution containing a large excess of Tach- as compared to the oxychloro species. In both cases the product it1Iz was found to be constant over the entire time range (10 ms to 2 9). The average values of it1/' together with the diffusion coefficients, calculated from the average it1/' values, are given in Table I for the reduction of both TaC16and TaOCl,-. The above results are confirmed by double step chronoamperometric measurements made by using the same melts. Figures 3 and 4 show plots of -ir/if vs. t , / T for the reduction of TaC16- and TaOC14-, respectively, where t , is tf - 7, if is the reduction current a t time t,, i, is the reoxidation current a t time t,, and 7 is the time a t which the potential is reversed. Both of these curves agree well with the theoretical working curve obtained for a diffusion-controlled electrochemical reaction where both the oxidized and the reduced species are stable during the time span of the chronoamperometric experiment (14). On the basis of the above observations of the electrochemical behavior of Ta(V) in A1Cl3-NaClnadmelt which contains oxide, it is possible to determine the oxide content of an
i I
I
1 1I
1.5
1.3
1.7
1.9
t r / 7 ( t , = t' t 7 )
Flgure 3. Double step chronoamperometric results for the reduction of TaCI,- in AIClt;NaCI,, melt at 200 OC: glassy carbon electrode 44.02 mM. The solid area, 0.0707 cm , €,, 0.56 V; E,. 1.0 V; CTaC,,-, ~ the range circles represent average values of - i r / i f at each t , / over T = 2 s to 20 ms.
1I
1.5
1.3
t,/r
1.7
1.9
(t,= t, * 7 )
Figure 4. Double step chronoamperometric results for the reduction of TaOCI,- in AICI,-NaCI, melt at 200 OC: glassy carbon electrode area, 0.0707 cm2; E,, 0.40 V; E,, 1.00 V; CTaa,-,14.46 mM. The solid circles represent average values of - i r / i , at each t r / T over the range T = 2 s to 20 ms. 1.0
0.8
I
I
I
I
0.6
0.4
0.2
O:O
E (V)
Flgure 2. Reduction of Ta(V) in AICI,-NaCI, melt: temperature, 200 OC; scan rate, 5 mV/s; pulse width, 57 ms; time between pulses, 0.5 s; glassy carbon electrode area, 0.0707 om2; C& (a) 7.19 mM, (b) 13.09 mM, and (c) 23.06 mM. T a b l e I. C h r o n o a m p e r o m e t r i c R e d u c t i o n of T a C l C and TaOC1,- in AlCl,-NaCl,,,.d M e l t a t 200 "C (Glassy C a r b o n Electrode A r e a = 0.0707 cm2)
electrochemical reaction
+ e- + TaC1:44.02 mM TaOCIL + A1C14- + e- + TaCb2- +
TaC1,-
105it1/2, Ad2
IO~D,,, cm2/s
3Llni 0.5 3.4 f 0.5
CTaCb- =
8.8"i 0.2
2.5 i 0.2
pulse voltammetry as an analytical technique since the voltammetric peak separation between the two peaks of interest is rather small. According to eq 3, it is possible to write the following equilibrium expression:
K=
K' =
A1C13-NaCl,a,d melt employing a voltammetric titration procedure involving TaC1, as a titrant. For this determination differential pulse voltammetry is more suitable than normal
(5)
CTaOC14~/CTaC~~CA10C12~
where
K' = K / C A l C l i During the course of the titration procedure, Ta(V) is assumed to exist as either TaOC1,- or TaC1,CTaCh-
= 14.46 mM
Average values.
(4)
Since the concentration of AlC14-, CAIc4-,is large and relatively constant, eq 4 can be rewritten as follows:
AlOC12CTeOC14-
~TaOC14~CA1C14~/CTaC~~CA10C1~~
= c'%(V) -
(6)
CTaOC14-
By substitution of eq 5 into eq 6 and after rearrangement of that expression, the following equation results: CTot Ta(V)/CTaOCl;
=
( c % f V ) - CTaOC14-) /CoAIOC12-
+
(K'COA~OCI,+ I ) / (K'coA10C12-)
(7)
ANALYTICAL CHEMISTRY, VOL. 57,NO. 2, FEBRUARY 1985
503
150
--
100
4,
x
.-I a
50 / / / /
/ / /
-4
,
-2
0
[s2-1
I
I
2.5
I 50
I 7.5
I 10.0
4%)- C n c q Figure 5. Results or an oxide determination of a typical AICI,-NaCI, 6.59 mM; melt via the Ta(V) method. Inverse of the slope (CoAw12-), intercept, 1.007 (K’ = 2.3 X lo4);temperature, 200 OC.
Table 11. Oxide Concentrations of Several Batches of AICl,-NaCl,,,d Melt, Determined via the Ta(V) Method, as a Function of the Number of Sublimations of AlCl, Used in Making These Melts melt batch no. 1 2 3 4 5 6 7
a
9 10
no. of AlC1, sublimations
oxide concn found, mM
1 1
16.34 17.65 5.05 6.59 7.42 11.10 9.54 11.39 20.62 22.91
2 2 2 3
3 3 5 5
In eq 7, @Alocl,-represents the original concentration of oxide in the melt as AlOCl; before the addition of TaC1, to the melt. This equation is similar in form to that used for oxide determinations of basic AlC13-BPC melts using TiC1, as a titrant (8). A plot of CE~vJCTaOClrvs. (q& - CTaOC4-)is shown in Figure 5. The inverse slope of this plot, which is equal to the amount of oxide initially present in the melt as an aluminum oxychloride species, was found to be 6.59 mM. It is interesting to note that, while the average oxide concentration was ca. 10 mM, oxide concentrations varied greatly from batch to batch. An attempt was made to determine the source of oxide impurities in these melts. A series of melts was made by using the same batch of NaCl and AlCl, which had been resublimed a variable number of times. Table I1 shows the results of this study. In general it was found that most of the oxide present in the AlCl, is removed after two sublimations. In fact, it appears that further sublimations introduce oxide into the melt made from AlCl, that has been sublimed more than twice. This result is in agreement with the argument made recently by Berg et al. (3) which stated that attack of AlCl, vapor on the Pyrex glass containers typically used for the sublimation of AlCl, may occur with the result that oxide is introduced into the “purified” AlCl, rather than removed. However, these results should be taken with caution, since water adsorbed on the glassware used in the cell construction, which can react with the melt to form oxides
2
4
added ( m M )
Figure 6. Peak current at 4-0.45V vs. added oxide concentration for the reduction of TaOCI,- in AICI,-NaCI, melt at 200 “C; = 7.59 mM.
CEiv,
in the melt, may be responsible for the variation of oxide content observed. Once the oxide concentration of an A1C13-NaClMd melt has been determined, eq 7 can be used to determine the value of K’and, thus, the equilibrium constant, K , for eq 3. The values of K’ and K for the reaction given in eq 3 in A1C13-NaCl,,,d melt at 200 “C were found to be (2.2 f 0.4) X lo4 and (1.9 f 0.4) X lo5, respectively. By comparison, the equilibrium constant for the Ti(1V) oxychloro-chloro complex equilibrium in basic AlCl,-BPC at 40 “C was found to be 1.0 X lo3 (6,8). The results for the oxide determination of AlC13-NaCl,a,d melts via the Ta(V) method were verified by the results obtained from standard addition experiments. In these experiments oxide was added as AlOCl to a sample of a given batch of AIC1,-NaCl,,td melt containing a significant excess of TaCl,, and the peak height for the TaOC1,- reduction as a function of added oxide concentration was observed by use of differential pulse voltammetry. A plot of the standard addition results for a typical A1C13-NaCl,,,d melt is given in Figure 6. The oxide content of this melt was found to be 2.25 mM via the standard addition method. This result is in agreement with the value of 2.19 mM found for another sample of the same batch of melt via the Ta(V) method. Attempts were also made to compare the results obtained for oxide determinations of A1C1,-NaClSatdmelts via the Ta(V) method with a nonelectrochemical method (e.g., neutron activation analysis). It was found that melts of high oxide content (300-350 mM) were necessary to make such a comparison since the neutron activation analysis is applicable only at these concentrations (15). Higher oxide concentrations could not be used due to solubility limitations. For a melt analyzed via the Ta(V) method to be 347 mM in oxide, neutron activation analysis yielded a value of 800 mM, more than twice that observed via the Ta(V) method or by the standard addition method. Potentiometric Measurements w i t h t h e @-Alumina Membrane. Frequently it is desirable to monitor the oxide content continuously. The Ta(V) method, while useful for determining oxide in A1C13-NaClsa,d melts, has two major disadvantages: (1) it is a sample-destructive method, Le., contamination of the melt by Ta(V) prevents further use of the melt, and (2) the method involves a batch process. Therefore, we decided to explore the continuous monitoring of oxide in A1C13-NaCl,,td melts. With AlOCl as a titrant, potentiometric titration experiments were performed using the cell diagramed in Figure 1. The oxide concentration of the melt in the reference com-
504
ANALYTICAL CHEMISTRY, VOL. 57, NO. 2, FEBRUARY 1985
- 2.5
0 -2.0
1
I -1.5
-1.0
I
I
- 0.5
0.0
lop 10.1
Flgure 7. Plot of observed cell voltage vs. oxide concentration for the cell employing a P-alumina membrane in AIC13-NaCI,,d melt at 200 OC.
partment and the initial oxide concentration of the melt in the working compartment were determined by using the Ta(V) method. AlOCl was added to the melt, and the potential of the cell was observed while the melt was stirred. It was noted that the @-aluminamembrane required aging in the melt overnight to obtain reproducible initial potentials. It was also noted that the potential of the cell after each addition was somewhat unstable initially, requiring as much as 3-4 h to reach an equilibrium value. A typical plot of cell voltage vs. oxide concentration of the melt in the working compartment is shown in Figure 7 . The slope of this plot is 42.9 mV per unit log [Ox] (Ox is the oxide species in the melt), reasonably close to the value of 46.9 mV per unit log [Ox] expected for a two-electron process at 200 OC. The final oxide concentration (Figure 7 ) was checked by the Ta(V) method. The oxide concentration of the melt, obtained by using the Ta(V) method, was within 1% of that obtained by using the @-alumina membrane. CONCLUSIONS The Ta(V) method, which employs a voltammetric titration procedure using TaC1, as a titrant, has been used successfully to determine oxide in AlC13-NaClmd melts. By contrast, the
analogous Ti(1V) method ( 6 , 8 ) ,used for the determination of oxide in AlCl,-BPC melts, could not be applied to the determination of oxide in AlC13-NaCl,a,d melts. It would appear that the Ta(V) method should be applicable to basic low-temperature chloroaluminate melts. The cell employing the p-alumina membrane has been found to respond to the oxide concentration in A1Cl3-NaCLd melts. This cell is basically a concentration cell in which @-aluminaacts as a membrane. If the membrane potential remains essentially constant (i.e., there is no change in the sodium ion activity on either side of the membrane), the emf variation must then correspond to the working oxygen electrode potential and, hence, to the oxide content of the melt in the working compartment. The observed slow response time of the cell potential after each addition of oxide to the melt in the working compartment is probably a result of the system a t these experimental irreversibility of the 02/02temperatures. Although the nature of the behavior of the P-alumina membrane in this cell is not fully understood, the presence of interstitial oxygen ions in @-alumina(16) may be responsible for the observed response. The use of the @-aluminamembrane for the determination of oxide in A1C13-NaClBatdmelts results in a nondestructive method; however, it is plagued by a very slow response time. In contrast, the Ta(V) method, while being a sample-destructive method, is a relatively fast method of analysis for oxide in chloroaluminate melts. ACKNOWLEDGMENT We wish to thank J. E. Strain of the Oak Ridge National Laboratory for performing neutron activation analyses on oxide-containing melts. LITERATURE CITED (1) Tremillon, B.; Bermond, A,; Molina, R. J . Nectroanal. Chem. 1976, 74,53. (2) Gilbert. B.; Osteryoung, R. A. J . Am. Chem. SOC. 1978, 700, 2725. (3) Berg, R. W.; Hjuler, H. A.; Bjerrum, N. J. Inorg. Chem. 1984, 23, 557. (4) Ting, G. PhD. dissertation, University of Tennessee, 1973. (5) Scheffler, T. B.; Hussey, C. L.; Seddon, K. R.;Kear, C. M.; Armitage, P. D. Inofg. Chem. 1983, 22, 2099. (6) Linga, H.; Stojek, Z.; Osteryoung, R. A. J . Am. Chem. SOC. 1981, 103,3754. (7) McCurry, L. E. Ph.D. dissertation, University of Tennessee, 1978. (8) Stojek, Z.;Linga, H.; Osteryoung, R. A. J . Electroanal. Chem. 1981, 7 79,365. (9) Combes, R.;Vedel, J.; Tremillon, B. Electrochim. Acta 1975, 20, 191. (10) Taulelle, F.; Piolet, C.; Tremillon. B. J . Electroanal. Chem. 1982, 134, 131. (11) Letisse, G.; Tremillon. B. J . Electroanal. Chem. 1968, 77,387. (12) Marassi, R.; Chambers, J. Q.; Mamantov, G. J . Nectroanal. Chem. 1976, 69,345. (13) Parry, E. P.; Osteryoung, R. A. Anal. Chem. 1960, 3 7 , 1634. (14) Bard, A. J.; Faulkner. L. R. "Electrochemical Methods"; Wiley: New York, 1980; pp 136-212. (15) Yonco, R. M.; Maroni, V. A,; Strain, J. E.; Devan, J. H. J . Nucl. Mater, 197g3 79,354. (16) Steele, B. C. H. Solid State Ionics 1984, 72,391.
RECEIVED for review July 6, 1984. Accepted October 29, 1984. This work was supported by a grant from the Atlantic-Richfield Corporation.
