164
C. H, Lrv
Jt has been shown elsewhere2that if the Qoof the first complex ion has the expected value for a $1 ion of about 1, the effects on Pi and pg are negligible; and that even if this QO meie much larger, the resulting curvature would be so slight as to be indectable from data with the precision indices of Table 111. Since the data of Table I1 are fitted precisely by a straight line, there is no evidence for a third complex even at 0.05 M Ka2S04. However, nothing meaningful can oe said about assumed & values of less than 2 to 3 X IO3, and perhaps even larger. In view of the error in Qo,the recommended values of p1 and pz (and their subjectively estimated
Vol. ci6
uncertainties) at 25' and p = 0.50201M are 66 f 1 and 800 f 40, respectively. An attempt2 has been made to correlate these values with values (some of which were corrected) from other studies a t various ionic strengths. Acknowledgment.-The author wishes to acknowledge the helpful interest and criticisms of Dr. J. F. Lemons under whose general direction this work was done. He also is grateful to Prof. R. E. Connick of the University of California for his helpful comments and encouragement, and to Prof. G. Scatchard of the Massachusetts Institute of Technology for his helpful criticisms.
ELECTRODE POTENTIALS I N MOLTEN LITHIUNI SULFATE-POTASSIUM SULFATE EUTECTIC1 BY C. H. L I U ~ Bmokhavcn National Laboratory, Upton, L.I., N . Y . Received September 6, lg61
The lithium sulfate-potassium sulfate eutectic (80% lithium sulfate by mole; melting point 535') was shown to be an adequate molten solvent for electrochemical investigations a t 625'. A procedure for preparing the eutectic melt was eutablished. The silver(1)-silver(0) system was found to be a satisfactory reference electrode in this melt. The electrode systsems,whose standard potentials were measured by direct potentiometrg against the silver reference, are: copper(1)-copper( 0 ) ,copper(11)-copper( I), palladium( 11)-palladium( 0), and rhodium( 111)-rhodium( 0). The Nernst equation was applicable in a11 cases.
Interest in molten salt electrochemistry has been increasing and advances in this field have been rapid in recent years. Numerous attempts have been made to evaluate electrode potentials in fused solvents; however, experimental results are difficult to correlate because of the widely different molten solvents and experimental conditions employed by different workers. Comprehensive literature surveys on potentiometric measurements are available.3 A systematic compilation of electrode potentials in molten lithium chloride-potassium chloride has been made by Laitinen and coworker~.~Flengas and Ingraham have made a similar investigation in molten potassium chloridesodium chloride.5 The potentials of a number of metal electrode systems were measured and compared in fused potassium fluoride-sodium fluoride by Grjotheim.6 Very little has been reported in the published literature regarding chemical and electrochemical phenomena in molten sulfates. The oxygen-oxide potential on platinum was used to estimate the formation potentials of various metal oxides in lithium sulfate-potassium sulfate at temperatures up to 750"'; the results, however, (1) This work was done under the auspioes of the United States Atomio Energy Commission. (2) Department of Chemistry, Polyteohnie Institute of Brooklyn, 333 Jay Street, Brooklyn 1, New York. (3) (a) C. H. Liu, Ph.D. thesis, University of Illinois, 1957: (b) J. W. Pankey, Ph.D. the&, University of Illinoie, 1958. (4) (a) H. A. Laitinen and C. H. Liu, J . Am. Chem. Soc., 80, 1015 (1958); (b) H.A. Laitinen and J. W. Pankey, ibid., Si, 1053 (1959). ( 5 ) $. N. Flengae and T. R. Ingraham, J . Electrochem. ~ O C 106, ., 714 (1969)). (6) K. Grjotheim, Z. physik. Chsm. (Frankfurt), 11, 180 (1957). (7)D.C. Hill, B. Porter and R. 9. Gillespie, Jr., J. Etertrochsm. Boc., 106,408 (1968).
were not definitive. In a series of articles,* Lux reported the use of an oxygen electrode on platinum to measure the oxide activities in molten potassium sulfate-sodium sulfate at 950". The volatility loss of metal oxides was found to be rapid, and it was necessary to extrapolate the measured potentials to zero time to obtain reasonable results. The objectives of the present study are an evaluation of the oxidation-reduction potentials of various electrode systems in molten lithium sulfate-potassium sulfate eutectic at 625" and an examination of the electrolytic decomposition of this melt. Apparatus and Chemicals Furnace. Hevi-Duty crucible furnace, Typc BT-506, Hevi-Duty Electric Co., Milwaukee, Wisconsin. Temperature Controller. Wheelco indicating controller Model 403 (Barber-Coleman Co., Rockford, Illinois). Constant Current Generator. An all electronic current, regulator built by the Instrumentation Division, Rrookhaven National Laboratory. It is capable of delivering constant currents from 0.001 to 100 milliamp. with a precision of f0.5%. Electrolytic Cell.-The main cell consisted of a quartz beaker placed in an outer Vycor jacket (3 in. o.d. and.13 in. long with a sealed flat bottom) the lower 8 in. of whmh were placed in the heating chamber of the furnace. The furnace then was insulated with asbestos plates and refractory insulating bricks. The outer Vycor jacket was equipped with a ground glass flange. Ground glass joints, sealed vertically to a borosilicate glass top with a similar ground flange, permitted the insertion of an argon inlet tube, 8 thermocouple tube, and various electrodes. Addition and removal of materials into and from the main cell also could be accomplished through these joints. A stopcock sealed to the side of the top served as the argon outlet and the (8) H. Lux, 2, Blektroehsm., 46, 303 (1939); 62, 220 (1948); 54, 224 (1948); 58, 41 (1949): 68, 43 (1949): 63, 45 (1949).
ELECTRODE POTENTIALS IN MOLTEN LITHIUM-POTASSIUM SULFATE EUTECTIC
Jan., 196'2
165
vacuum connection. The main cell was compartmented with smell quartz tubes with sealed-in quartz fritted discs
for bottoms. In this fashion, eachpreparation of the eutectic sulfate melt could be used in several separate experiments; the fritted discs also acted as salt bridges. The cell was evacuated at the start of each molten salt experiment to facilitate the filling of these fritted compartments. A dry argon atmosphere was maintained during each experiment; tank argon was passed through a heated Vycor column containing copper screens to remove oxygen and then through a column of anhydrous magnesium perchlorate to remove moisture before use. Salt Filtration Assembly.-The eutectic melt was filtered into a quartz beaker placed in a Vycor outer jacket similar to the main electrolytic cell. The filter consisted of a fritted quartz disc, sealed to one end of a quartz tube which in turn was ring-sealed to a flanged quartz top. The eutectic was added to the fritted tube through a standard taper joint sealed vertically to the center of the top. An argon inlet, tube was introduced through the same joint by means of an O-ring seal, and an argon vent also was provided. A stopcock sealed to the side of the top permitted connection to vacuum during filtration. This assembly was a modification of the type used by Van Norman.* Chemicals.-All chemicals used were reagent grade. Metal Electrodes.-All metal electerodesused were chemically pure and were in either foil or wire form.
0.4
0.3
0.2
0.1
0.0 -0.1 -0.2 -0.3 -0.4 -0.5
-0.6
P O T E N T I A L . VOLTS, VERSUS I M O L A L A g ( J ) - A g REFERENCE.
Fig. 1.-Nernst Rh(II1)-Rh(O),
Cu(1)-Cu(0).
equation plots: 1, Pd(I1)-Pd(O), 3, CU(II)-CU(I), 4, Ag(1)-Ag(O),
2,. 5,
per square centimeter. One hundred per cent. current efficiency was established by the weight loss of a silver wire after the passage of a known number of faradays of electricity in a coulometric measurement of the "n" value of the electrode reaction. I n some cases, weighed portions of silver sulfate were added to the melt. The exact silver(1) concentration was calculated at the end of each experiment after the determination of the sulfate content of each compartment. The applicability of the Nernst equation to Experimental Results and Discussion Preparation and Analysis of the Melt.-Lithium sulfate the silver(1)-silver(0) system is shown by curve 4 in the monohydrate was mixed with potassium sulfate in the figure. The line is theoretical, based upon the standard correct pralportions ( 8 0 9 lithium isulfate by mole) and potential 0.000 volt and a one-electron Nernst slope of This temperature was main- 0.1782; the circles are experimental points from concentragradually heated to 200 tained for a few hours to complete the preliminary drying. tion cells of silyer(I), one of which was the reference elecThe dehydrated eutectic (melting point 535") was melted trode. The experimental standard otential calculated in the filtration assembly under a dry argon atmosphere and from each potential measurement by t f e use of the Nernst filtered by vacuum suction through the quartz frit at ap- equation showed an average deviation of i 0.002 volts for proximately 625". In this manner, solid particles, pre- 40 measurements over the concentration range from 0.0016 sumably carbonaceous and siliceouffi materials, were re- to 0.25 molal. At lower concentrations, positive deviations moved, and a clear melt resulted. After cooling, the were observed. One possible cause for this deviation was filtered eutectic was crushed and stored in a desiccator to the presence of oxidizing impurities in the melt which would react to yield silver(1). Several experiments were be used in molten salt experiments. In order to make quantitative measurements, it. was performed to check the validity of the silver(1)-silver(0) essential to know the amounts of salt present in the fritted system as a reference electrode. Replicates of this eleccompartments. The sulfate content of each compartment trode were constructed in each experiment. The drift of was determined by argentometric titrations after the s ~ l - the potential between any two of these electrodes was never fate had been converted to chloride by ion exchange, Dowex greater than two millivolts for observation periods up to 15 hr. Furthermore, the potential of the electrode a t 1being the anion exchanger used. Electrode Potential Measurements. N e F s t E uation'o sufficiently high silver( I) concentrations was not shifted by and Standard State.-When molality, m, IS use! as the the passage of a small current except for the iR correction. reference function in the eutectic melt at 625', the Nernst Thus, the silver(1)-silver(0) electrode was established as a satisfactory reference electrode in the eutectic sulfate equation is melt. The potential of this electrode in the standard state is arbitrarily assigned the value 0.000 volt to be used as the reference point in the standard potential measurements. where I n each experiment, a fresh reference electrode was made E is the electrode potential in volts by anodizing a silver coil of ap roximately two square centimeters in area in the manner Xescribed. The silver(1) EO is the ritandard potential in voltri n is the no. of electrons in the electrode process, and ox concentration was made approximately 0.05 m with the and red designate the oxidized form and reduced form, exact concentration calculated at the end of the experiment. respectively Limiting Electrode Processes of the Melt.-When the melt was electrolyzed with two platinum electrodes, the The activity coefficients within the concentration limits to electrode reaction at the anode appeared to be be reported are shown to be either unity or constant. -+ -k SO8 2eThus, either the concentration is identical with the activity or the activity coefficient can be incorporated as a constant Continuous and vigorous gas evolution occurred at the electerm into the standard potential. An electrode system is trode with no observable solid deposits during an electrolysis said to be in its standard state when the ratio mox/mrd at a current density of approximately ten milliamperes per is equal to unit For a pure metal, the standard state is square Centimeter. The potential at the electrode at the bedefined as its pgysical state at 625' under one atmosphere ginning of electrolysis was about 0.7 v. after i R correction of pressure. The standard state for a metal ion in a metal v8. the standard reference one molal silver(I)-silver(0) elecion-metal syiatem is unit molality. trode and drifted to a stable value of 0.9 v. after fifteen Reference Electrode.-The silver(X)-silver(0) electrode minutes. I n the cathode compartment, both sulfite and at various concentrations of silver(1) was examined in detail. sulfide were found by chemical analysis, accounting for over I n most cases, silver(1) was produced by anodizing a silver 95% of the electricity passed. The cathode process was coil electrode in a fritted compartment at a constant current complicated and probably involved the total reactions for a measured period of time against a platinum working cathode. The current density was about five milliamperes SOa-e 2e- -+ S08-s 0-a SO4-% 6e- --ic S 40-2 (9) J. D. Van Normsn, Ph.D. thema, Rellsselaer Polytechnic Institute, 1069. and (10) The sign convention recommended by the IUPAC in the
.
+
.
-
compt. rend. of the XVILth Conference, Stockholm. 1963, in umd.
+
+
+
+
Vol. 66 IIowever, eleinental sulfur, if prcscnt a t all, appeared to be a miiior product,. The cat,hodc potential a t the start was about - 1.8 v. aftderiR correction us. one molal silver(Itsi1-
ver(0) and drifted to a f i n d stable value of -2.0 v. after 35 minutes. Standard Potentials.-For the metal ion-metal electrodes, which included palladium(II)-palladium(0), rhodium( 111)rhodium(O), and copper(1)-copper(O), the metal ions were formed by anodizing a coil (or foil) electrode of the respective metal in an isolated compartment of the melt: in a manner analogous to that described for the silver electrode. The "n" value of the electrode reaction was cheeked in each case by t,lie weight loss of the metal electrode and the number of faradays of electricity passed. The potentials a t the electrode after successive measured periods of anodization were measured against a silver(1)-silver(0) reference electrode. The molal concentrations of the metal ion for each potential measurement were calculat,ed from the number of faradays of electricity passed, the "n" value, and the weight of melt present after the determination of the melt content of each compartment a t the end of the experiment. The coppcr(I1)-eopper(1) potential was evaluated by first anodizing a copper foil to form a known amount of copper(1) in a fritted compartment. After the removal of the copper foil, a platinum or pttlladium coil (or foil) electrode was lowered into the solution and anodized a t a lower current densit.y (approximately two milliamperes per square centimeter) to convert some of the copper(1) to copper(I1). The potentials a t the electrode after successive periods of eIectrolysis were measured against a silver( I)-silver(0) reference. The copper( I) and copper( 11) concentrations were in the range from 0.005 to 0.1 m, and the same potential values were observed on platinum as on palladium. All measured potentials were corrected to values us. one molal silver(1)-silver(0) by the use of the Nernst equation in order to facilitate comparison and correlation of the results from different experiments. Extrapolation of these values to potentials of the electrode systems in the standard state yielded the standard potentials, which were computed as the averages of all measurements. The experimental results are presented as curves 1, 2 , 3 and 5. The lines are based upon the experimental standard potentials and the theoretical Nernst slopes. The circles are experimental points. The Nernst equation is obeyed in all eases. The experimental standard potentials, which are averages of large numbers of replicate measurements, are reproducible to within & 0.002 to 0.003 v. These values are tabulated below
Electrode aystcm
Standard potentials, v.
I'd( 11)-Pd( 0) Rh(111)-Rh( 0) Cu( 11)-Cu(1) MI)--MO) Cu( I)-Cu( 0)
0.541
-
,387 .051 ,000 .202
Conclusions The lithium sulf atr-po t assium su If ate eutectic a t 625" may be used as a molten solvent for electrochemical investigations. It is easy to prepare and convenient to handle compared with the halide melts in the same temperature range. However, its potential span is much shorter than the alkali chloride m e h a t 5 The more activc metal elrctrodes cannot be studied conveniently bccause of chemical reduction of sulfate. For example, nickel was observed to react with the melt to form nickel sulfide. Some metal ions tend t o precipitate as their oxides, releasing sulfur trioxide. Rismuth(111) was observcd to behave in this manner. A similar reaction between aluminum(II1) and molten sodium sulfate has been reported." Thus, competing acid-base equilibria also play an important role in molten sul€ates. Displacement of potential values and reversal in order in the electromotive force scries are observed between the sulfate and chloride melts. Apparently, stabilization of some valence states of metals by interaction with the solvent anions is the main cause for this phenomenon. Acknowledgment.-The author wishes t o acknowledge Drs. H. L. Finston, C. Auerbach, J. D. Van Norman and Z. Newman for their helpful suggestions, discussions and encouragement. He also would like to thank hlr. G. Kissel for his as.;istnncc in the experimental part of the work. 3v5
(11) I
