Use of Chronopotentiometry to Determine the Solubility of Oxides in Molten Lithium Chloride-Potassium Chloride Eutectic SIR: Although many chemical and electrochemical studies have been carried out in LiCl-KCl eutectic, very few investigations have been done on the solubility of oxides in this melt. Laitinen and Bhatia (4) determined the solubility of CuzO, PtO, PdO, NiO, and BzOa in LiCl-KC1 at 450°C., from potential measurements. Delarue (8) carried out a voltammetric study of some oxides in LiC1-KCl and gave a qualitative estimation of their solubility. In this investigation chronopotentiometry is used to determine the solubility of oxides in LiC1-KCl. This technique is fairly simple compared to more classical methods such as E M F or analysis of saturated solutions. The value of the solubility of PdO obtained in this work and that reported in the literature agreed favorably. The equipment and the procedure used in this investigation have been previously described ( I , 5, 6). Medium porosity zirconia tubes were used as compartments for the indicator electrodes in the molten salt cell. The temperature was measured with a chromel-alumel thermocouple which was placed in the melt. The palladium oxide and palladium chloride were analytical
reagent grade and were dried under high vacuum before addition to the melt. The LiCl-KCl eutectic mixture (melting point 352OC.) was prepared and purified by Anderson Physics Laboratories, Inc., Champaign, Ill., according to the method proposed by Laitinen, Tischer, and Roe (8), except that no magnesium was added during the purification procedure. For a semi-infinite linear diffusion controlled process the Sand equation (9) can be considered.
where I is the current density in amp. cm.-2, T the transition time in sec., C the bulk concentration of the reacting species in mole c m . 4 , D the diffusion coefficient in cm.* set.-', and n, F , and T have their usual meanings. The chronopotentiogram of a saturated solution of PdO at 485" C. is shown in Figure 1. One reduction step was observed with a quarter wave potential of -0.25 volt us. Pt(I1) (1M)IPt reference electrode. Laitinen and Liu (7) reported the standard potential of -0.214 volt for the Pd(II)/ The second wave in Pd(0) couple.
r-------I
I
0.1
Figure 1. 4 8 5 0 c.
1588
I
0.2
I
0.3
I
I
0.4 0.5 TIME,SEC.
,
0.6
0.8
Chronopotentiogram of Pd(ll) in LiCI-KCI
ANALYTICAL CHEMISTRY
Table 1. Chronopotentiometric Data for Saturated Solution of Palladium Oxide in LiCI-KCI Eutectic at 4 5 0 " C. .1/2 I x 103 1 ~ 1 ' 2 x 103 sec.1'2 amp. cm.-2 sec.1'2 cm.+ amp. 7.29 1.215 6.0 7.90 1.275 6.2 7 .74 1.106 7.0 8.06 1.033 7.8 8.0 7.31 0,914 7.70 0.938 8.2 7.76 0.902 8.6 7.74 0.530 14.6 7.60 0.507 15.0 8.54 0.562 15.2 7.99 0.512 15.6 16.0 7.42 0.464 Av. 7 . 7 5 Std. dev. f 0 . 2 7
J
I
0.7
Figure 1 was due to the reduction of the solvent. The chronopotentiometric data are reported in Table I. For various current densities the product Z P remained constant within 3% showing that Equation 1 was applicable to the reduction process of Pd(I1). Equilibrium between solid and solution phases was considered to be attained when the transition time, a t a given current density, remained constant. From the Sand equation, it is possible to calculate the bulk concentration C
C.10' M O L E CM;'
at
Figure 2. W / 2 vs. concentration curve for Pd(ll) in LiCIKCI a t 4 8 5 " C.
of dissolved Pd(I1) if the diffusion coefficient, D, is known. If one considers PdO to be completely dissociated in the melt and the species which is reduced to be Pd(II), then the diffusion coefficient of Pd(1I) can be assumed as a good approximation to be equal to that observed for P d C k A chronopotentiometric study of PdClz dissolved in LiC1-KC1 was carried out; the values of Z+* us. concentration are plotted in Figure 2. From the slope of this curve, the diffusion coefficient of Pd(I1) was estimated to be (1.68 i. 0.05) X cm.2sec.-1, a value which is comparable to that found by Laitinen and Ferguson (6) for Cd(I1) (1.7 x 10-5 sec.-l) which would be expected to be similarly coordinated in the C1- eutectic. Csing 1.68 x 10-5 cm.2 see.-' as the diffusion coefficient, the solubility of PdO in LiC1-KC1 a t 485OC. was calculated from the data of Table I to be (11.1 =k 0.5) X lO-3.M, in good agree-
ment with the value found by Laitinen and Bhatia (4)from potential measurements (9.4 X 10-3hl a t 450' C.). Inman (3) developed a third kind electrode of the type Pd/PdO, CdO, NaNOpKN03 eutectic and reported the solubility of PdO to be, very approximately, 10-6M in nitrate melt a t 254' C. The higher value of solubility found in LiC1-KC1 eutectic is explainable if one considers the great complexing tendency of the chloride ions. Since accurate values of diffusion coefficients of many metal chlorides are available in the literature, the proposed method can easily be used to determine the solubility of the corresponding oxides.
LITERATURE CITED
(1) Bankert, R. D., Ph.D. Thesis, Vniversity of Illinois, Urbana, Ill., 1966.
( 2 ) Delarue, G., J . Electround. Chem. 1,
285 (1959).
(3) Inman, D., Electrochim. Acta 10,
11 (1965). (4) Laitinen, H. A., Bhatia, B. B., J. Eleclrochem. soc. 107, 705 (1960). (5) Laitinen, H. A., Ferguson, W. S., ANAL.CHEM.29, 4 (1957). (6) Laitinen, H. A,, Ferguson, W. S., Osteryoung, R. A., J . Electrochem. Soc. 104, 516 (1957). (7) Laitinen, H. A., Liu, C. H., J . Am. Chem. Soc. 80, 1015 (1958). (8) Laitinen, H. A., Tischer, R. P., Roe, D. K., J . Electrochem. SOC.107, 546 (1960). (9) Sand, H. J. S., Phil. ?dag. 1, 45 (1902).
BRUNO SCROSATI~ Department of Chemistry and Chemical Engineering University of Illinois Urbana, Ill. 61801 WORKsupported by U. S.Army (Grant USDA-ARO-DG-586 1.
1 Present address, Istituto di Chimica Fisica ed Elettrochimica, Universita di Roma, Roma, Italy.
Application of Electroanalytical Techniques to the Study of Flash Photolysis Processes SIR: The application of electrochemical techniques for the study of transient photolytic reactions was prompted by the observation that most photochemical processes appear to involve free radical and other electroactive intermediates (19, 21). Furthermore, it appeared that many photolyticallyinduced chemical processes were similar to electrolytically-induced chemical processes (14, 17, 21). Thus, not only could electrochemical techniques be applied to the study of photolytic intermediates, but their qualitative characterization could be simplified by purely electrochemical studies of the chemical processes in question. Furthermore, electrochemical measurements have several analytical advantages for flash photolysis studies: sensitivity is available for dilute concentrations of a wide range of compounds; nearly the same detection limit exists for all electroactive compounds, since response depends primarily on mass transfer; and time-resolution in the microsecond range is available (6,18). Several workers have been active recently in the area of photoelectrochemical phenomena (3, 10,fa, 13, 19). Most recently, Delahay and Srinivasan (10) have investigated photocurrents generated a t a mercury/electrolyte solution interface during flash irradiation. Most pertinent to the work reported here are the studies of Berg (2-5) who has applied polarized electrode techniques to the study of flash photolytic processes in solution. Berg's
approach has been primarily exploratory and has involved conventional polarographic instrumentation and technique. I n addition, he has reported the observation of transient photo-product currents during the drop-life of individual drops at the dropping mercury electrode. Thus, his studies of rapid kinetics have involved analysis of current-time behavior at individual expanding mercury drops. This quantitative approach was admittedly inaccurate and insensitive, however, because of the general difficulty in developing theory for kinetic currents a t the dropping electrode, and because of the additional complications of handling second-order kinetic processes. It was the purpose of the work reported here to introduce modern electroanalytical techniques and instrumentation to the study of transient processes in flash photolysis. To do this, a chemical system was selected which had been well-characterized both photochemically (2&26) and electrochemically (14, 17). That system was benzophenone in very alkaline ethanolwater solution. Upon flash irradiation of this system, several electroactive transient photo-products result. I n this work, the overall photolytic process was followed qualitatively and quantitatively by placement of an accurately polarized hanging mercury drop electrode in the region of greatest photolytic activity. By utilizing potentiostatic measurement techniques, the flash products could be monitored con-
tinuously or discontinuously with time. The development of a time-delayed potentiostatic measurement technique allowed the acquisition of kinetic data. EXPERIMENTAL
Instrumentation. Figure 1 is a schematic diagram of the flash and electrochemical apparatus. T o provide the flash radiation a modified FX47A Xenon flash tube (Edgerton, Germeshausen, and Grier, Inc., Boston, Mass.) was employed. The tube operated a t 1000 volts into a 225 mf. capacitor bank (yielding better than 100 joules of energy) with a nominal pulse duration of 40 psec. when triggered externally. h measured pulse duration of 200 psec. was obtained when the flash tube was triggered by series injection (11). Although a longer flash time resulted, series injection triggering was used since it was possible to minimize generation of radio frequency noise. The potentiostatic circuit used in this work was discussed by Schwars and Shain (Figure 4b in reference $3). The controlling unit was a high-gain differential operational amplifier, SK2V, with a current booster amplifier, SKSB, (Philbrick Researches, Dedham, Mass.). Currents were measured across a floating load resistor in the feedback loop. The iR signal was sampled at the differential input of a model 536 Tektronix oscilloscope with a Type D plug-in unit. A Dumont model 2620 Polaroid camera was used to record traces from the oscilloscope screen. An adder-controller circuit with a current-following operational amplifier VOL 38, NO. 1 1 , OCTOBER 1966
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