current due to niobium was masked by large molybdenum current. Thallium(1) mas also reducible under these conditions and gave a reversible wave a t -0.48 volt tu. S.C.E., well ahead of the niobium wave. Thus, niobium and thallium(1) could be deterrnined in a misture of t h e two. Therefore, by taking certain precautions, niobium was determined polarograpliically in the presence of phosphate
ion, tantalum, tungsten, molybdenum, and thallium.
LITERATURE CITED
(1) Brindley, D. T., Analyst 85, 877
,
/i
nanj
Iav",.
ACKNOWLEDGMENT
The author thanks V. L. *ltemus~ Jr., for performing much of the esperimental work reported in this paper and H. M. Hubbard and G. R. Coraor for their helpful comments.
( 2 ) Elson, R.E., J . Am. Chem. Sac. 75, 4193 (1953). (3) Ferrett, D. J., Milner, G. W. C., J . Chem. soc. 1956, 1186. (4) Ferrett, D. J., Milner, G. \V. C., S a t w e 175, 477 (1955).
R~~~~~~~ for review ~~~~~b~~ 7, 1 9 6 ~ . .-lccepted February 24, 1961.
Anodic Chronopotentiometry at a Liquid Bismuth Electrode in Fused Lithium Chloride-Potassium Chloride JOHN D. VAN NORMAN Brookhaven National laboratory, Upton, N. Y.
b The use o f liquid bismuth as a coolant and as a fuel matrix in reactor technology has created a need for analyzing directly for small amounts of metals, such as corrosion products, dissolved in liquid bismuth. Anodic chronopotentiograms obtained for zinc and lithium dissolved in a liquid bismuth electrode in contact with the lithium chloride-potassium chloride eutectic at 450" C. show the theoretical relationship of current density, concentration, and transition time. The diffusion coefficients obtained for zinc and lithium in liquid bismuth are 6.5 and 2.2 X sq. cm. per second, respectively. The anodic chronopotentiometric analysis of small amounts of these metals in liquid bismuth was performed with an accuracy of +4%.
T
HE lithium chloride-potassium chloride eutectic has been used as a solvent system by many investigators using a wide variety of techniques such as potentiometry (5, 6, 13, 16), polarography (4, 5> I d ) , coulometry (5, I O ) , and chronopotentiometry (11, 12) to study the behavior of metal ions in solution. Most of these investigations have been performed with solid electrodes. However, Heus and Egan (4) utilized a dropping bismuth electrode to study cadmium, lead, and zinc ions in the chloride eutectic. They demonstrated that bismuth is a good choice for a liquid metal electrode as i t is fairly noble, has a low melting point, a low vapor pressure, and can be easily obtained pure. Their study was limited to cathodic processes a t the dropping bismuth electrode. The use of liquid bismuth as a coolant and fuel matrix has been and is still of active interest in the field of reactor technology. Fused salt technology is
946
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ANALYTICAL CHEMISTRY
currently being studied in a number of related fields-e.g., the fused salt reactor, fused salt processing of nuclear fuel elements, and production of metals. It is noteworthy that a liquid bismuth electrode, a noble electrode, is compatible with most fused salt systems, Consequently, the study of anodic processes a t a liquid bismuth alloy electrode is of great interest in the development of methods of analysis for small amounts of metals dissolved in bismuth. The technique of chronopotentiometry was selected as being perhaps the most versatile, the advantages having been pointed out by Laitinen and Ferguson (11). They applied this technique to the study of the cathodic behavior of metal ions in the fused lithium chloride-potassium chloride eutectic using solid electrodes. There have been no published investigations regarding chronopotentiometry using liquid metal electrodes in fused salts. I n chronopotentiometry, the potential of a polarizable electrode is measured as a function of time while a constant current js passed through an unstirred solution containing a depolarizer and supporting electrolyte. The concentration of depolarizing species a t the electrode surface, which determines the electrode potential, is limited only by semi-infinite linear diffusion. During the electrolysis, the depolarizing species is removed a t the electrode surface and the time necessary for the concentration to approach zero is termed the transition time, 7, characterized by a large shift in the potential of the electrode. The analytical significance of the transition time has been demonstrated by Gierst and Juliard (3) while the name chronopotentiometry was suggested by Delahay and Namantov ( 1 ) .
The equation of chronopotentiometry which is of analytical significance is:
where r is the transition time, C the bulk concentration of depolarizer having a diffusion coefficient D,and io is the constant current density. The other terms have their usual electrochemical significance. APPARATUS A N D PROCEDURE
Cell. The cell consisted of a Vycor container 7 5 X 350 mm. with a ground flange sealed with Picein wax t o a borosilicate glass head which had a matching ground flange, a side a r m and six standard taper joints; modified Kilson O-ring seals made of aluminum were sealed into the standard taper joints rvith Picein was. T h e O-ring seals permitted vertical pqsitioning of various electrodes, bubbling tubes, and thermocouple sheaths. A borosilicate glass inner crucible was used to contain the molten chloride eutectic. Electrodes. T h e reference electrode consisted of a borosilicate glassfritted compartment with a platinum foil suspended in t h e chloride melt; t h e melt was filtered u p through t h e frit into t h e compartment while t h e cell was under vacuum. T h e frit provided electrical contact between t h e reference electrode and the melt, its resistance being less than 10 ohms. The Pt foil was anodized a t a constant current for a measured length of time, and the concentration of Pt(I1) in the reference electrode was calculated by weighing the eutectic in the compartment. Laitinen and Liu ( I S ) have demonstrated the utility of such a reference electrode. The working electrode consisted of a similar fritted compartment containing a plat'u i m m
I
I
-06
i
I I
6
8
SECONDS
Figure 1 . Experimental curves for oxidation of zinc in bismuth a t 450" C. A. 8. C.
3.18 X 6.36 X 9.16 X
low6
mole/cc., mole/cc., mole/cc.,
5 mo./sq. 6
ma./sq. 10 ma./sq.
cm. cm. cm.
foil or a silver foil. I n some instances, solid silver chloride was added to the compartment to prevent electrodeposition of lithium metal on the foil which resulted in attack of the borosilicate glass where the foil was in contact with the grass. The bismuth pool electrodes were contained in borosilicate glass cups which had areas of approximately 1 to 2 sq. cm. Electrical contact was made by a 0.02-inch tungsten wire sealed through a 6-mm. capillary side arm attached to the cup. Various foil and needle electrodes were also used in this investigation. Furnace. T h e furnace Consisted of a 3-inch stainless steel tube 1/8-inch thick, 8 inches in height, closed at one end with a l/d-inch steel plate, covered with asbestos tape and wound with No. 18 Nichrome V resistance wire. The wire was covered with Alundum refractory cement and the entire unit insulated. A rear-lighted view port was built a t such a level in the furnace that visual observation of the salts, the electrodes, and bubbler was possible. Temperature Controller. T h e tempeiaturc of t h e furnace was controlled by a Minneapolis-Honeyn-ell, Brown Elecktronik controller 15 hich also recorded the trmperature of the furnace; the Chromel-Alumel thermocouple was placed between t h e furnace tube and t h e Vycor cell. T h e temperature of the salts was maintained within ~ 3 C." as measured by a thermocouple placed in the borosilicate glass thermocouple well in contact n-ith the melt. Constant Current Source. T h e Lons t a n t Lurreni source vaq a n all electronic cuirent repiilnt )r built b y t h e lnstrumenlatiori Lhvisioii of Biookhaven Kational I,abor:itory. Controlled Potential Source. T h t control~cc pcitential source \vas a n electroil,i ci,*itrol;ed potential coulometric t i t i a t m designcd by Kelley, Jones, and Fisher (9)and built b y the Instruinmtation Division of Brookh x ~ Sw a t i o n d Laboratory
Recorder. This \vas a Bristol HighSpeed Ilynaniaster Recorder lllodel 590 with a pen speed of 0.4 second full scale. C h a r t speeds u p t o 1 inch per second were obtained by using various gear ratios. Chemicals. T h e lithium chloride and potassium chloride used as t h e solvent were of reagent grade and werc obtained from Baker Chemical Co. T h e bismuth metal used was obtained from Cerro de Pasca and ~ m either s center cut or filtered, in t h e molten state, through a coarse borosilicate glass frit; spectrographic analysis indicated purity greater than 99.99%. All other chemicals and metals were of reagent grade and were not subjected t o further purification. Solvent Preparation. T h e eutectic solvent was purified b y treating t h e appropriate mixture of lithium chloride and potassium chloride (59 mole yo LiCl) in a borosilicate glass filtration unit with d r y H C l while t h e temperature of the salts was raised to 450" C. The passage of HC1 was continued for 30 minutes after the salts became molten, and was followed b y a 20-minute argon purge to remove gross amounts of HC1 dissolved in the eutectic. A vacuum was then applied below the frit while the passage of argon was continued and the molten eutectic was filtered through the frit and collected in a borosilicate glass crucible. The filtered salt was allowed t o cool and solidify in the crucible, then was slid out of the crucible and quickly placed in the Vycor cell. The head was sealed
Figure 2. Relation of io zinc in bismuth a t 450°&.
T ' / ~to
C for
Electrodes used, 1.76 a n d 1.34 rq. cm.
to the cell which was evacuated and maintained at a pressure of less than mm. of mercury and at a temperature of 200' C.for 18 hours. The criterion for dryness and purity of the molten eutectic was that used b y Laitinen, Osteryoung, and Ferguson (15), based o r the polarographic behavior of a metal electrode in the pure eutectic. Polarograms of the molten eutectic a t 450" C. obtained at tungster, and platiriur: needle electrodes shon-ed diffusior, current densieies of
4
I
-20
u w W W Gz
ie W
-1.4
- 1.0 , -0.8
-06
c i
I
0
1
2
5
4
5
SECONDS
Figure 3. Experimental curves for oxidation of lithium in bismuth at
450" C. A. 8. C.
D.
1.1 9 X' 10-6 mole/cc., 5 ma./rq. 1.78 X 1 0-6 mole/cc., 7 ma./sq. 2.37 X mole/cc., 8 ma./sq. 2.95 X mole/cc., 10 ma./sq.
low5
cm. cm
cm. cm.
less than 2 pa. per sq. mm. at -2.0 volts us. a 0.03iV P t ( I I ) / P t reference electrode, indicating a high purity of the molten chloride eutectic. Procedure for Measurement. The circuit for measuring transition times consisted of t h e constant curren? regulator, a three-electrode systen consisting of t h e reference electrode t h e working electrode, and the bismuth pool electrode, a voltage divider, a n d t h e high speed recorder. A constant current was passed between t h e bismuth pool electrode (the indicator electrode) a n d t h e working electrode while the potential between the bismuth and reference electrodes was recorded as a function of time. A voltage divider was used to make the 10-mv. full scale of the recorder correspond t o 2 volts. I n all cases, the recorder was allowed to achieve maximum speed by running for a t least 1 second before initiation of the electrolysis current Following the current initiation, a t to the potential of the liquid bismutb electrode assumed the potential of thc metal-metal ion couple until the surfacf concentration of the metal in the bi? muth approached zero. a t which timc the potential rapidly shifted to the dis solution potential of bismuth. Thf transition time was measured from tc t( that point on the final potential b r w l where the potential shift achi linearity. This potential s a s ui.1 1 mined for several anodic chrono-iotrr :Igrams in each system studied and sub-quent transition times measurrcl iron : to this potential. The metal-bismuth alioys s erc 1 rt pared by depositing tne nitwl frcii solution of the metal ior ir, the fu eutectic a t a rontrolled potentia! acd t! integrating the current pnsser: wtr, t b VOL.
33 NO. 7,JUNE 1 9 6 '
8
$47
electronic contiollcd potential coulometric titrator. The metal concentration in moles per cubic centimeter was calculated from the weight of bismuth, the density of bismuth a t 450" C. ( 7 ) , and the number of coulombs passed. I n each case, the salt was preelectrolyzed a t a potential more negative than the estimated bismuth oxide-bismuth couple to reduce any bismuth oxide present on tlie surface of the bismuth pool or in the salts, but more positive than the deposition potential of the metal being studied. RESULTS A N D DISCUSSION
ior1/2/C, was evaluated as 4.04 X l o 2 amp. cm. see.'" per mole with an average deviation of &4'%. The diffusion coefficient of lithium in bismuth at 450' C. was calculated from the slope of the plot of i0r'I2us. C shown in Figure 4 and was 2.2 X 10-5 sq. cm. per second. Each point represents the average of five or more measurements. This plot extrapolates through the origin, as in the previous case of zinc in bismuth. Similarly, this technique is capable of analysis for lithium in bismuth with an accuracy of *4%. The diffusion coefficient of lithium in liquid bismuth is considerably smaller
The anodic stripping of zinc and lithium from liquid bismuth into a LiC1-KC1 eutectic a t 450" C. was 01 2 1 studied in detail. Figure 1 shows typical anodic chronopotentiograms obtained for zinc dissolved in liquid bismuth. The transition times could be N 8 measured to 10.05 second at the chart E speed used. It is obvious from Equa4 tion l that the product io 7'11 C is a constant dependent only upon the square root of the diffusion coefficient; this product is accordingly called the .transition time constant. The transition time constant for zinc in bismuth n at 450' C. was 1.38 X lo3 amp. cm. 0 10 20 30 sec.'/l per mole with an average deviaCONCENTRATION - moles / c m 3 a lo6 tion of +4%. The diffusion coefficient Figure 4. Relation of io r'/' to C for of zinc in bismuth a t 450" C. was evaluated from the slope of the plot of i ~ r ' / ~ lithium in bismuth at 450' C. us. C, shown in Figure 2 . The diffusion Electrodes used, 1.23 and 0.99 sq. cm. coefficient calculated from this slope was 6.5 x 10-5 sq. em. per second. The than any diffusion coefficient deterpoints on this plot are a n average of five mined by Kiwa et al. (17). There is, or more measurements. The plot is however, a reasonable explanation for linear and extrapolates to zero a t a zinc this rather small diffusion coefficientconcentration of zero. Zinc dissolved i.e., the formation of a n intermetallic in liquid bismuth can be determined compound. Since the diffusing species using this technique with a n accuracy of in the case of a n intermetallic com*4%. pound is much larger than a single The diffusion coefficient evaluated for metal atom, i t is reasonable to expect a zinc in liquid bismuth is larger than smaller diffusion coefficient. diffusion coefficients found for metal The activity coefficient of lithium in ions in the fused chloride eutectic, as one bismuth at 450' C. was estimated during would expect from analogy to mercurythis study by measuring the potential of aqueous systems. hloreover, Niwa et the bismuth pool containing various al. (17) have evaluated diffusion coefficoncentrations of lithium and by subcients of metals dissolved in other stitution of these values into the Nernst liquid metals and have found that the equation. The value used for Eo of diffusion coefficients are indeed larger the lithium-lithium ion couple was that than those found by other investigators determined by Laitinen and Liu (13). for metal ions in the fused chloride The standard states of the lithium and eutectic. They report a diffusion colithium ion were the metal a t 450" C. efficient, D,for tin in liquid bismuth at and one atmosphere, and the lithium 450' C. of 5.5 X lO-5sq. cm. per second, ion at the prevailing activity in the comparable to the diffusion coefficient eutectic mixture. The activity coof zinc found in this investigation. efficient thus obtained was 5 X The anodic stripping of lithium from somewhat smaller than the value of 1 X liquid bismuth was also studied in de10-5 reported by Egan and Wiswall (2) tail; typical anodic chronopotentiofrom the study of concentration cells. grams obtained are presented in Figure This low activity coefficient of lithium 3. The transition time constant, N
rl
A
0
zt/
948
ANALYTICAL CHEMISTRY
1
in bismuth can also be explained by the formation of a n intermetallic compound. It is noteworthy that Katz, Hill, and Speirs (8) estimated a small diffusion coefficient, 1.93 X 10-6 sq. cm. per second, for samarium in bismuth at 500' C., using a falling drop method. The rare earths are known to have low activity coefficients in liquid bismuth. Again, the small diffusion coefficient and low activity coefficient suggest the formation of a n intermetallic compound. This technique has been demonstrated to be feasible for analysis of dilute alloys of electropositive metals in bismuth. It should be possible to extend this technique to analysis of metal ions in fused salts by electrodeposition a t a liquid metal electrode followed by anodic chronopotentiometry. ACKNOWLEDGMENT
The author thanks H. L. Finston for the advice and kind encouragement he has given and Clemens Auerbach for numerous and stimulating discussions. The aid of George Kissel in the construction of apparatus and in performing analyses is also gratefully acknowledged. LITERATURE CITED
(1) Delahay, P., Mamantov, G., ANAL.
57. 701 '(1953).
,
I
(6) Inman, D., Hills, 1 Bockris, J. O'M., Tr 5 5 , 1904 (1959). (7) Jouniaux, A., Bull. SOC. chim. 51, 677 (1932). 1 - - - - 1
(8) Katz, H. M., Hill, F. R., Speirs, J. L., Trans. Met. SOC.A I M E 218, 770 (1960). (9) Kelley, M. T., Jones, H. C., Fisher, D. J., ANAL. CHEW31, 1475 (1959). (10) Laitinen, H. A., Bhatia, B. B., Ibid., 30, 1995 (1958). (11) Laitinen, H. A., Ferguson, W. A., Ibid., 29, 4 (1957). (12) Laitinen, H. A., Gaur, H. C., Anal. Chim. Acta 18, 1 (1958). (13) Laitinen, H. A., Liu, C. E., J . Ana. Chem. SOC.80, 1015 (1958). (14) Laitinen, H. A., Liu, C. H., Ferguson, W. A., ANAL.CHEM.30, 1266 (1958). (15) Laitinen, H. A., Osteryoung, R. A,, Ferguson, W. A., J . Electrochem. SOC. 104, 516 (1957). (16) Laitinen, H. A., Pankey, J. W., J. Am. Chem. SOC.81, 1053 (1959). (17) Niwa, K., Shimoji, M., Watanabe, Y., Yokokawa, T., Trans. A I M E 9, 96 (1957). RECEIVED for review December 27, 1960. Accepted March 3, 1961. Work done under the auspices of the U. S. Atomic Energy Commission.
