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AIDS FOR ANALYTICAL CHEMISTS Multlpass Apparatus for Molten Salt Spectroelectrochemical Experiments Brisco L. Harward,l Leon N. Klatt,*2 and Gleb Mamantov'
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Department of Chemistry, T h e University of Tennessee, Knoxville, Tennessee 37996-1600, and Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 Spectroelectrochemical techniques (SEC) are useful in the study of redox systems (1). In conventional transmission thin-layer spectroelectrochemistry,the light beam is at normal incidence to the optically transparent electrode (OTE) resulting in a short path length, which frequently restricts the application of this method to the investigation of strongly absorbing species. Recently much effort has been focused upon the development of optical enhancement techniques to extend this methodology to the study of weak chromophores. These developments include specular reflection a t glancing incidence (2-4), multiple specular reflection (5), parallel absorption (6-8), external reflection from cylindrical microelectrodes (9, lo), diffractive spectroelectrochemistry (11,121, and the use of specialized cell designs (13-17). Although various spectroelectrochemical methods have been applied to studies in molten salt media (18),the development of techniques and apparatus to improve the optical sensitivity of such measurements is nonexistent. The corrosive nature, moisture sensitivity, and elevated temperatures associated with molten salts often preclude the use of sophisticated optical systems and fragile cell components. This note describes a simple apparatus for enhancement of the optical signal in molten salt spectroelectrochemical experiments. In this method, the optical beam is redirected through an OTE several times by a mirror assembly positioned outside the thin-layer cell. The gain in optical sensitivity is defined as the ratio of the response for n passes to that for a single pass.
EXPERIMENTAL SECTION Apparatus. The cell used is illustrated in Figure 1. This cell was modified from a previous design (19) to facilitate cleaning and reuse. Large ratio rectangular Pyrex tubing (1X 10 mm i.d., Vitro Dynamics, Rockaway, NJ) was used to form the thin-layer optical region. A platinum screen (80 mesh, 0.0762 mm wire diameter, Engelhard Corp., Iselin, NJ) served as the OTE. An aluminum wire reference electrode, isolated from the main cell compartment by a glass frit (4-8 gm porosity, Ace Glass, Inc., Vineland, NJ), a platinum foil counter electrode, and a platinum wire microelectrode (not shown for clarity)were also inserted into the cell. All electrodes were secured by threaded glass connectors equipped with Viton O-rings and Teflon bushings (Ace Glass, Inc.). An aluminum washer of appropriate dimension was inserted between the bushing and O-ring, resulting in an acceptable seal at the elevated temperatures without problems due to Teflon flow. An opaque mask was attached to the thin-layer region to prevent light not passing through the OTE from reaching the detector. The mirror assembly used to produce the multipass effect within the confines of the furnace is illustrated in Figure 2. The support assembly is constructed from aluminum and stainless steel and has been coated with an opaque high temperature paint to minimize stray reflections. Two 12 x 12 mm front surface mirrors (No. 31005, Edmund Scientific, Barrington, NJ) are attached to adjustable mirror mounts, each centered on a rectangular support plate. Displacement in the vertical direction is achieved by sliding 'The University of Tennessee. Oak Ridge National Laboratory. 0003-2700/85/0357-1773$01.50/0
the plate between parallel support rods. Two fine resolution socket head screws allow adjustment of the mirror angles. A 1/4-20 threaded hole in the center of the base permits a rod to be attached and removed, thereby allowing the insertion and removal of the mirror assembly from the furnace while operating at elevated temperatures. The optical furnace consists of a vertical Pyrex tube (10 cm ad., 45 cm length) wrapped with two nichrome heating elements. A second glass tube of slightly larger diameter, enclosed in 1-in. blanket (Koawool, Babcock & Wilcox Co., Augusta, GA) and a Teflon sleeve, fits over the heating tube to provide electrical and thermal insulation. Two optical ports with 50 mm diameter quartz windows, each fabricated from a threaded glass connector and Teflon bushing window holder assembly (Part No. 7896-30, Ace Glass, Inc.), are located 8 cm from the bottom of the furnace at 180' alignment. The furnace temperature is monitored by K-type thermocouples. Power to the heating elements is manually controlled by autotransformers. Multiwavelength optical measurements were performed with a silicon based vidicon rapid scanning spectrometer. A detailed description of the spectrometer and computer instrumentation is provided in the literature (18,20). Reagents. Reagent grade sodium chloride used in melt preparation was dried under vacuum at 400 "C for several days. Anhydrous aluminum chloride (puriss. grade, Fluka Chemical Corp., Hauppauge, NY) was purified by repeated sublimation in the presence of a small amount of sodium chloride. Sulfur (t5N5 grade, Alfa Products, Danvers, MA) was used without further purification. Niobium pentachloride (puriss. grade, Alfa Products) was separated from the corresponding oxychloride by repeated sublimations at 100 "C.Melt preparation and related procedures have been reported previously (21). Procedure. The molten salt cell was cleaned with concentrated HC1, followed by rinsing with distilled water and drying at 120 "C for 24 h. The cell and electrodes were transferred to the drybox, where the powdered A1C13-NaC1 mixture containing the analyte was added. The reference electrode and an absorbance cell (for 100% T measurements) were filled with a mixture of the same composition but with the analyte absent. Both cells were preheated in an auxiliary furnace to the experimental temperature. After the cell was positioned in the optical furnace, adjustment of the mirror assembly for alignment of the light beam was completed by briefly removing the furnace windows. Initiation of the experiment and all data acquisition and storage were controlled by the computer.
RESULTS AND DISCUSSION Sulfur Oxidation in Molten A1Cl3-NaC1 (63-37 mol %). Our interest in the electrooxidation of sulfur in molten chloroaluminates results primarily from the development of the rechargeable low temperature molten salt cell (22, 23). Na/Na+ ion conductor/S(IV) in molten A1CI3-NaCl The oxidation of sulfur in molten A1C13-NaC1 has been investigated extensively by electrochemical methods (24-27). Recently, the application of thin-layer transmission spectroelectrochemistry in the UV-visible region has illustrated the complexity of this redox system (25). Although the mechanism for this oxidation is not fully understood, we selected this system to evaluate the multipass apparatus because 0 1985 American Chemical Society
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the first oxidaton wave yields several producta whose absorptivities are low; due to the limited solubility of sulfur, one is restricited to low concentrations. A secondary ojective was to obtain additional spectral information which may be useful in understanding this sytem. The results from a multipass investigation of the first oxidation wave of sulfur in molten AlClpNaCl(63-37 mol %) a t 150 OC are shown in Figure 3. The spectrum for a single pass experiment using the same cell and sulfur concentration is included. Comparison with the single pass measurements showa that the expected gain of three is obtained. As reported in an earlier study using a higher sulfur concentration and longer path length (%), absorption bands with maxima a t 600 and 730 nm appear when the potential applied to the OTE (Ern) is varied between 1.6 and 1.8 V vs. the Al(III)/AI reference eled.de. In addition. a band a t 440 nm is observed for Ern = 1.7 V. This spectral feature was not apparent from the previous work. The Em values used in thin study are 0.1 V more positive than those used in the earlier study which multed in a similar optical response; this result is attributed to the high iR drop of the SEC cell, which was verified with aqueous ferricyanide in KCI. I t is important to note that, although the spectra are steady state, current continues to flow due to edge effects a t the top of the OTE. The previously obtained W-visihle absorption and electron spin resonance spectroelectrwhemid results for the first oxidation step of the system (25)were rationalized in terms of a complex electrochemical and chemical reaction sequence involving species such as Sa+(or S , B ~ +Sa2+, ) , SS+, and S,gzf. In a more recent publication Fehnnann et al. (28)concluded
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F ~ P N9. AbsapaDn specbn of Um poduas of the hst sulh.oxldstion wave 10 mln afler E, stepped horn 1.3 V (-), to 1.6 V (-4. 1.7 V (---), and 1.8 V (---) lor n = 3 sulfur concanballon. 0.01 m(as Um maraner)h 63-37 mol % A&-NaU at 150 O C : dpa* length. 1.0 mm. Slngla pass where E , Is stepped hom 1.3 V (-) to 1.8 V (-.-). At E, = 1.3 V. Um sulfur is reduced 10 S ,.
from the steady-state W-visible absorption and ESR spectra and potentiometric measurements that the important lowoxidation sulfur species in acidic chloroaluminate melts are &,+, S4+,and Si:+. It was found from computer deconvolution of our steady-state absorption spectra, using the molar absorptivity data from ref 28 that the spectra in Figure 3 may be interpreted in terms of Sa+,S,+, and SlZz+on'y. Thus for Em = 1.6 V, the dominant sulfur species is Sa+(90%)with only a small fraction present as SIz2+ (10%). There is no evidence for S4+or Saa t this OTE potential. Further oxidation a t Ern = 1.7 V results in the presence of all three sulfur species. SI?*and S', are present a t approximately the same concentration and account for most of the sulfur (85%). The remaining sulfur is present as Sa+ (15%). At Erne = 1.8 V, S4+is the major species (83%) with SiZ2+also present (17%); there is no evidence for Sa+. A t Em = 1.9 V. essentially all
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iment. In the study of redox systems with multiple steps, especially those involving species with greatly varying absorptivities, this allows the study of more than one electrochemical wave from the same cell without a change in solute concentration. This advantage is more fully appreciated when operating with systems such as molten salts, where such a routine change in solute concentration usually encompasses a period of several days.
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Figure 4. Absorption spectra of the second Nb reduction wave for E, stepped from 0.6 V to 0.1 V: NbCI, concentration, 0.01 M in 65-35 mol % AICI,-NaCI at 175 O C ; cell path length, 1.0 mm; time, inltlal (-), 4 min (--), and 15 min (---).
sulfur is present as S4+.The unequivocal confirmation of this simpler product distribution and the assignment of an oxidation mechanism should probably await the ESR hyperfine splitting measurements of the sulfur radical cations formed; such measurements to date have been unsuccessful (25,28). Nb(II1) Reduction in Molten AlC1,-NaCl (65-35 mol %). The electroreduction of niobium pentachloride in molten chloroaluminates has been studied (29). In acidic melts two reduction waves are observed; the first wave involves an ECE mechanism to yield Nbze+with the second wave involving a higher reaction order to yield Nb38+. While thin-layer SEC investigations in the U V region have been reported (19),results in the visible region have provided little information concerning this system. The multipass technique was used to obtain spectral information in the visible region associated with the second reduction wave. The results obtained for a three-pass and a single-pass investigation of the second reduction wave of NbC& in AlC13-NaC1 at 175 OC are shown in Figure 4. The initial spectrum is believed to be that of the Nbz6+ion and the final spectrum that of the Nbas+ ion. The spectral features are much more discernible with the three-pass experiment. From steady-state spectra, a broad band at 540 nm and two welldefined isosbestic points at 600 and 750 nm are obtained at EoTE = 0.1 V vs. Al(III)/Al reference electrode. The band present initially at 630 nm disappears at this potential. The visible spectra of these cluster ions in solution have not been previously reported. CONCLUSION The multipass apparatus has been shown to be a useful optical enhancement method for studies in molten salts or similar media whose very nature precludes the application of many other approaches introduced to increase optical sensitivity. Due to the use of external optics, the enhancement is unaffected by changes in electrode reflectivity and therefore is considerably more tolerant of highly corrosive systems than internal reflectance methods. The multipass apparatus permits one to use a low power continuum source, thereby allowing simultaneous multiwavelength measurements. While the net gain in optical sensitivity is directly proportional to the number of passes through the cell, the presence of stray reflections with the accompanying attenuation of the light beam, resulting from the unavoidable presence of interfaces between media of different refractive indexes, ultimately limit the gain to a practical value of 3-5. A significant advantage of the use of external optics is the ease with which one can convert from a high optical gain to a low optical gain exper-
ACKNOWLEDGMENT Appreciation is expressed to L. H. Norman for construction of the cell used in this work and to C. Brooks for advise during the construction of the mirror assembly. Research was sponsored by the Office of Energy Research, U.S. Department of Energy, under Contract DEAC05840R21400 with Martin Marietta Energy Systems, Inc., and from the University of California Subcontract 4502810 with the U.S.Department of Energy. B.L.H. acknowledges support from the U.S. Department of Energy in the form of a Laboratory Graduate Participation Fellowship awarded by Oak Ridge Associated Universities, Oak Ridge, TN. This work was presented in part at the 188th National Meeting of the American Chemical Society, Philadelphia, PA, August 27-31, 1984. Registry No. NbClS, 10026-12-7; Sa+, 11062-42-3; S4+, 12597-08-9;SI$+,80263-39-4;AIC13, 7446-70-0;NaCl, 7647-14-5; sulfur, 7704-34-9; platinum, 7440-06-4. LITERATURE CITED (1) Heineman, W. R.; Hawkrldge, F. M.; Blount, H. N. I n "Electroanalytical Chemistry"; Bard, A. J., Ed.; Marcel Dekker: New York, 1984;Vol 13. (2) McCreery, R. L.; Pruiksma, R.; Fagan, R. Anal. Chem. 1979, 57, 749-752. (3) Ahiberg, E.; Parker, D. P.; Parker, V. D. Acta Chem. Scand., Ser. 8 1979, 833, 760-762. (4) Skulk J. P.: McCreerv, R. L. Anal. Chem. 1980, 52, 1885-1689. (5) Baumgartner, C. E.; Marks, G. T.; Aikens, D. A,; Richtol, H. H. Anal. Chem. 1980, 5.7, 267-270. 1 Pruiksma, R.; McCreery, R. L. Anal. Chem. 1979, 51, 2253-2257. Pruiksma. R.; McCreery, R. L. Anal. Chem. 1981, 53, 202-206. Tyson, J. F.; West, T. S. Talanta 1980, 27, 335-342. Robinson, R. S.; McCreery, R. L. Anal. Chem. 1981, 53,997-1001. Robinson. R. S.;McCurdy, C. W.; McCreery, R. L. Anal. Chem. 1982, 54. 2356-2361.
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RECEIVED for review October 16,1984. Resubmitted March 25, 1985. Accepted March 25, 1985.
