(10) Holtje, R., Geyer, R., 2. anorg. Chem. 246, 265 (1941). (11) Johnson, M. G., Robinson, R. J., -4NAL. CHEM. 24,366 (1952). (12) Laitinen. H. A,. Jennings. W. P., Parks. T. D.. IND. ENG.CH&.. A x . 4 ~ . ED. 18, 355 (1946). ( 1 3 ) Lindquist, Ingoar, Acta Chem. Scand. 5, 568 (1951). ~
(14) Page, J. A., Lingane, ,J. J., .anal. Chim. Acta 16, 175 (1957). (15) Roberts, I., IXD. Ex(; CIIEJI., ANAL.ED.8, 365 (1936). (16) Sacconi, L., Cini, R., J .Im. Chem. SOC.76, 4239 (1954).
alytical Chemistry, 139th Meeting, ACS, St. Louis, hlo., March 1961. Investigation carried out during the tenure of a Research Fellowship granted to G. A. Rechnitz by the National Heart Institute, United States Public Health Service. The financial support of the National RECEIVED for review May 10, 1961. .IC- Institutes of Health is gratefully accepted July 21, 1961. Division of Anknowledged.
EIectroa na lytical Techniques in Molten Lithium Sulfate-Potassium Sulfate Eutectic C. H. LIU' Brookhaven National Laboratory, Upton, Long Island, N. Y.
Three electroanalytical techniques for the determination of metal ions in molten lithium sulfate-potassium sulfate Couloat 625" C. were examined. metric titration with a potentiometric end point and chronopotentiometry with a solid electrode gave satisfactory results for the determination of copper (I), whereas determinations by direct potentiometry lacked precision. The chronopotentiometric technique involving the reduction of copper(1) to metallic copper gave much better results than the anodic process where copper(1) was oxidized to copper(l1).
E
LECTROANALYTICAL
TECHNIQUES
have been widely applied in molten salts in recent years including direct potentiometry (4, Q), polarography (3, I O , 13), chronopotentiometry (2, 8), and coulometric titration (7'). A list of literature references has been given b y Van Norman in a recent article ( I S ) . I n most instances, alkali halide melts were employed as the solvents. Electrochemistry in the lithium sulfate-potassium sulfate eutectic melt was investigated in this laboratory and reported in a previous publication (11). The electrolytic decomposition of the melt was examined, and the osidationreduction potentialq of several electrode syhtemh were established. The objective of the present work is a critical evaluation of a few electroanalytical methods whirh are applicable in this melt. Direct potentiometry, chronopotentiometry, and coulometric titration with a potentiometric end point were compared in the determination of copper(1). APPARATUS A N D CHEMICALS
Furnace. Hevi-Duty crucible furnace, Type 51-506 (Hevi-Duty Electric Co., Milwaukee, Wis.) . Present address, Department of Chemistry, Polytechnic Institute of Brooklyn, Brooklyn 1, N. Y .
Temperature Controller. Wheelco indicating controller, Model 403 (Barber-Coleman Co., Rockford, Ill.). Constant Current Source. All-electronic current regulator constructed by the Instrumentation Division, Brookhaven National Laboratory. It is capable of delivering constant currents from 0.001 to 100 ma. with a precision of *0.5%. Potentiostat. Electronic controlled potential coulometric titrator built by the same Instrumentation Division according to the design of Kelley. Jones. and Fisher (6). Potential Recorder. High-speed Dynamaster recorder, Model 590 (The Bristol Co., Waterbury, Conn.). The pen speed is 0.4 second full scale, and the maximum chart speed is I inch per second. Solvent. The lithium sulfate-potassium sulfate eutectic (80'% lithium sulfate by mole; melting point 535" C.) was used at 625' C. The solvent was prepared in accordance with a procedure previously described (11). The eutectic mixture was first dehydrated and then filtered through a quartz frit while molten. Electrolytic Cell. The cell consisted of Vycor and quartz parts (11). Within the main cell, the melt was compartmented into separate portions by small quartz test tubes with fritted bottoms which served as salt bridges. Metal ions of interest were usually put into the melt by controlled anodization of the corresponding metals in foil or wire form. All experiments were performed under a dry argon atmosphere. The sulfate content of each compartment was determined at the end of a n experiment by converting sulfate into chloride through ion exchange and titration of the generated chloride b y conventional argentometric methods. The concentrations of the solutes in the experiment could then be calculated. Reference Electrode. The reference electrode consisted of a silver coil in equilibrium with a solution of silver(I), generated b y anodizing the silver coil in a fritted compartment a t a constant current for a measured period of time
(11). The silver(1) concentration was made approximately 0.05m, the exact value being calculated after analysis of the melt content of the compartment a t the end of the experiment. Indicator Electrodes. Copper foil indicator electrodes were used in direct potentiometry, and a palladium wire or foil served as both the generator and the indicator electrode in coulometric titration. Rectangular platinum indicator electrodes constructed from platinum foils welded to thin platinum contact wires were used in chronopotentiometry. Their areas, including the edges, ranged from 0.259 to 2.22 sq. em., measured micrometrically. The areas of the immersed portions of the contact wires were considered negligible. Chemicals. All chemicals were reagent. grade. EXPERIMENTAL RESULTS A N D DISCUSSIONS
Direct Potentiometry. Copper(1) solutions of various concentrations were prepared b y a n electrolytic method which had been shown to be satisfactory (11). A copper foil electrode was anodized in a fritted compartment a t a constant current for a measured period of time. The current density was about 5 ma. per sq. cm. The potential at the electrode after generation was measured against a silver(1)-silver(0) reference electrode. The copper(1) concentration corresponding to each potential measurement was calculated after the determination of the salt content of the compartment. The copper(1)-copper(0) system obeyed the Nernst equation in the copper(1) concentration range from 4.8 X 10-6 to 4.7 X 10-2m. The measured potentials in each experiment vere corrected relative to the lm silver(1)-silver(0) electrode with the aid of the Nernst equation to facilitate the construction of a working curve. The logarithm of the concentration of copper(1) was plotted against VOL. 33, NO. 1 1 , OCTOBER 1961
1477
4 Figure 1. Coulometric titration curves A.
1.87
low4
X
mole of
copper(l)
x x 4.66 x
E. 9.33
c. D.
w
1
200
400
600
800 Io00
1
_
L
6.22
10-5 10-5
10-5
I
1200 1400 1600 I800 2000
TIME OF E L E C T R 0 L Y S I S . S E C O N D S
ing from 1 to 4 ma. per sq. cm. These electrodes were placed in the solution with their planes vertical to the surface plane of the solution. Reproducible results were obtained with all four electrodes for transition times shorter than 7 seconds. At longer transition times, behavior was erratic. With smaller electrodes, transition times were abnormally long. Similar behaviors were reported by Laitinen and Ferguson and were attributed to adverse diffusive conditions around the electrode surface @), I n the present work, both the oxidative process
Cu(1) + Cu(I1)
-
+ e-
and the reductive process potential vs. Im silver(1)-silver(0). Fifteen potential measurements in this concentration range showed a n average deviation of 1 4 mv., corresponding to an error of 5% for a Nernst slope of 0.1782 a t 625' C. The maximum error for a single measurement was 14 mv. or 20%. Coulometric Titration. A known amount of copper(1) was anodically generated with a copper foil electrode in a fritted compartment in the same manner as described previously. T h e copper foil electrode was then removed, and a palladium wire or foil electrode introduced into t h e compartment for t h e coulometric titration. T h e palladium electrode was anodized a t a constant current (usually 10 ma. with an electrode area of approximately 1 sq. cm.) for successive measured periods of time. The potential a t the electrode was measured against the silver reference electrode after each period of anodization; the titration was continued until well beyond the end point. At first, copper(1) was electrolytically oxidized to copper(I1). When the copper(1) concentration became too low to maintain the constant current, palladium(I1) would be generated. The palladium(I1) formed would oxidize the remaining copper(1) to maintain the titration efficiency a t 100%. Before the equivalence point, the potential in volts, E, at the electrode is given by the equation
Cu(1)
E& and Eid have been evaluated to be 0.051 and 0.541 volt, respectively vs. Irn silver(1)-silver(0) (11). Figure 1 shows the titration curves for various amounts of copper(I), in good agreement with these equations. The titration curves were then annlyzed by conventional methods to determine the end points. The following table summarizes the results.
Results of Coulometric Titrations
CoppedI) Present, Mole 1 1 1 9 9 6 6 4
87 x 10-4 87 x 10-4 87 X 33 x 10-5 33 x 10-5 22 X loe6 22 X 66 X
Copper(1) Found by Coulometric Titrations, Mole 1 85 1 80 1 80 9.40
9 6 6 4
x x x x x
10-4 10-4 10-4 10-5 10-5
20 10 X 22 X 50 x 10-5
Chronopotentiometry. Since Gierst and Juliard first pointed out the analytical utility of this method ( 5 ) , numerous applications have been reported. I n a discussion of chronopotentiometric relationships (1), Delahay and Mamantov have given the fundamental equation for a single depolarizer.
+ e-
Cu(0)
were examined. I n the absence of a depolarizer, electrolysis in the sulfate melt with platinum electrodes yields a mixture of sulfite and sulfide at the cathode and oxygen at the anode. I n all determinations, a known amount of copper(1) was anodically generated in a fritted compartment, and its concentration was calculated at the end of the experiment after the determination of the salt content of the compartment. The platinum indicator electrode was cleaned by electrolyzing a t a controlled potential of 0.00 volt vs. a silver reference electrode before each transition time measurement, an isolated platinum foil serving as the counter electrode. An electrolysis a t a chosen constant current density was then started between the indicator and the counter electrodes. The potential of the indicator electrode us. the silver reference electrode during the electrolysis was recorded with the potential recorder a t a chart speed of 1 inch per second. The constant current was turned off immediately after the transi-
+ 0.1782 log lrx T.
E = E$
A
where E& is the standard potential of the copper(I1)-copper(1) system, and X is the per cent of copper(1) titrated. After the equivalence point, the potential is determined by
where E& is the standard potential of the palladium(I1)-palladium(0) system, and mcucr,is the initial molality of copper(1). At the equivalence point, 1478
ANALYTICAL CHEMISTRY
where C is the bulk concentration of a depolarizer whose diffusion coefficient is D, T is the transition time, and iois the constant current density in the electrolysis. F , a, and T have their conventional electrochemical meanings. The feasibility of chronopotentiometry as an analytical tool in the eutectic sulfate melt was tested in the determination of copper(1). Four platinum foil indicator electrodes with areas of 2.22, 1.08, 0.510, and 0.259 sq. cm. were used a t current densities rang-
Figure 2. ogram
Cathodic
chronopotenti-
Current density, 3 ma./rq. cm. Copper(1) concentration, 5.79 X 10-3111
//
3
5
- YE
Figure 3.
8i
I
L 2 5
or the presence of convection around the electrode surface for an electrode
2c
25
30
35
SEC3rr^S
Anodic chronopotentiogram
Current density, 4 ma./rq. cm. Copperll) concentration, 5 . 7 9 X 1 0
tion tiriie was observed and the indicator electrode again cleaned by electrolyzing at 0.00 volt 1’s. the silver reference electrode. The transition time measurement could then be repvated after waiting for a short period of time to let the system regain equilibrium I n all experiments, the residual tiansition time (that is, the blank) was small. A typical chronopotentiogram involving the reduction of copper (I) to copper(0) is shown in Figure 2. To determine the transition time, the vrrtical linear portions of the chronopotentiogram were extended, resulting in two points tangmtial to the curve. The time elapsed between these two tangential points was taken as the transition time. The cathodic transition tinir was reproducible to Tithin =t5 to 6%. h typical anodic chronopotentiogram is given in Figure 3. The anodic transition time n-as considerably longer than the cathodic transition time for the same copper(1) concentration The chronopotentiogram did not shon a very sharp potential change, and deviations up to 20% wcre observed for replicate measurements of the transition time. A posqible evplanation is the formation of a surfact! film of platinum oxide nhen the
ladium(I1) &es results accurate’ to =k2 to 3% and is a useful technique for in situ determinations of the amount of copper(1) present in the eutectic sulfate melt. This accuracy is comparable to t h a t reported by Laitinen and Bhatia for coulometric titrations of vanadium(I1) in molten lithium chloride-potassium chloride (’7). Chronopotentiometry with platinum foil electrodes in the reduction of copper(1) to copper(0) is capable of analyses with similar accuracy and is useful for determining the concentration of copper(1) in the melt. The diffusion coefficient of copper(1) in the melt at 625’ C. was evaluated from the slope of plot A in Figure 4 which is equal to nFR1i2D”2. The density of the melt needed for the evaluation of the diffusion coefficient was 2.1 grams per cc., determined b y a pycnometric method with calibrated quartz flasks. The diffusion coefficient in square centimeters per second is calculated to be 2.0 X 10-5 compared to 3.5 X reported by Laitinen and Ferguson for the same ion in lithium chloride-potassium chloride eutectic at 450’ (8). The oxidation of copper(1) to copper (11) is a poor process for chronopotentiometric determinations, and a n error of A10 to 15% may be expected. Direct potentiometry, although simple and rapid, does not yield precise results. ACKNOWLEDGMENT
The author thanks H. L. Finston, Clemens Auerbach, J. D. Van Pl’orman, and Leonard Newman for their helpful suggestions and encouragement. He
I
2
3
C^’ltEY79h70
6
5
?F L L
c
6
7
mo 0 1
Figure 4. Working curves for chronopotentiometry A. B.
Cathodic process Anodic process
also thanks George Kissel for his suggestions and assistance in the experimental part of the work. LITERATURE CITED
(1) Delahay, P., Mamantov, G., ASAL. CHEM.27,478 (1955). 1 2 ) Delimarskii. Y . K.. Gorodvskii. A. V.. Kuz’movich, t.V., Collection Czechoslov: Chem. Communs. 25,3056 (1960). (3) Delimarskii, Y . K., Panchenko, I. D., Shilina, G. Y . ,Zbid., 25,3061 (1960). (4) . . Flengas, S. N., Inaaham, T. R., J. EZectr&hem. Soc.’106;714 (1959). (5) Gierst, L., Juliard, A., J . Phys. Chem. 57. 701 - - - IlSFi.?). (Si Kelley, 11.1. T., Jones, H. C., Fisher, D. J., .ANAL. CHEN.31,488 (1959). (7) . . Laitinen, H. A., Bhatia, B. B., Ibid., 30,1995 (ig58). ’ (8) Laitinen, H. A., Ferguson, W. S., Thid.. 29. 4 (1957). \
I
’
I
\ - - - - / .
T
~
\ - - -
(9,-Lai%nen, H. A.,Liu, C. H., J . Ani.
Chem. SOC.80, 1015 (1958). (10) Laitinen, H. A,, Liu, C. H., Ferguson, W.S., ANAL.CHEM.30, 1266 (1958). (11) Liu, C. H., J . Phys. Chem., t o be
aublished.
( l i ) Steinberg,
M,,Xachtrieb, S . H., J . A m . Chem. SOC.72,3558 (1950). (13) Van Norman, J. D., ANAL.CHEM. 33,946 (1961). RECEIVEDfor review July 18, 1961. Accepted August 16, 1961. Division of Analytical Chemistry, 139th Meeting, ACS, St. Louis, Mo., March 1961. Work performed under the auspices of the U.S. Atomic Energy Commission.
END OF SYMPOSIUM
VOL. 33, N O . 1 1 , OCTOBER 1961
1479
