rads, and a 0.001-in1 thick polycarbonate. The 0.002- and 0.005-in. silicone membranes were unsatisfactory because they produced more peak distortion while transmitting the same amount of helium carrier gas. The irradiated membrane was unsatisfactory because it was mechanically unstable and tore readily. The polycarbonate membrane was also unsatisfactory because it developed pin hole leaks when heated above 100 "C. There are several advantages inherent t o the two-stage separator containing a silicone membrane and a silver frit. First, the outlet of the chromatograph is at atmospheric pressure, thereby permitting a one-to-one correlation between GC alone and GCiMS studies. Second, the silicone membrane can be designed to transmit more than 90% of most organic compounds by proper control of its surface area and operating temperature. Third, the separator can be used with widely varying carrier gas flow rates permitting GC/MS operation with flow programming (15).
The yield of the first stage ranges from 90 t o 100% at flow rates from 5 t o 60 cma/min provided that the temperature of the separator is 50 t o 75 "C below the boiling point of the material t o be transmitted. The yields of the second stage are fixed between 40 and 60%. The yields of the two stages opperating together vary from 40 to 60 %. The enrichments of the first stage are between 2 and 20 depending upon the flow rate of the chromatograph. Enrichments of second stage are between 1 and 2. Enrichments of the two stages operating together are between 2 and 40. Peak broadening is not a major problem as the width of most chromatographed peaks only increases 10 t o 20z upon passage through the dual-stage separator.
RECEIVED for review November 3, 1969. Accepted January 12, 1970. Research conducted under the McDonnell Douglas Independent Research and Development Program.
Simple Electrode for Thin-Layer Electrochemistry James C. Sheaffer and Dennis G . Peters Department of Chemistry, Indiana Unioersity, Bloomington, Ind.
IN RECENT YEARS many papers have discussed the great versatility and wide applicability of thin-layer electrochemistry (1-9). A variety of electrode designs have appeared, including micrometer electrodes ( 2 , 3), thin-metal-film sandwich electrodes ( 4 , 6), a gold electron-microscope-grid electrode (7), and metal cylinder capillary electrodes ( I , 2,8). While the advantages of each type of electrode have been well documented, most designs are either difficult to construct or require unusual manipulative techniques. One finds that vapordeposited films are fragile and tend to peel, whereas liquiddeposited films may contain significant impurities (IO). Furthermore, the thin-layer cavity may be quite inaccessible for removal of contaminants from the electrode surface, for examination of electrolysis products, or for changing electrodes after construction. Since no commercial thin-layer electrodes are presently available and since each worker must devise his own, it is desirable to construct the simplest possible electrode consistent with thin-layer behavior and quantitative measurements. We describe a thin-layer electrode which may be easily assembled from readily available materials and used compatibly with ordinary electrochemical apparatus, but which requires only a few microliters of sample solution. (1) C. R. Christensen and F. C. Anson, ANAL.CHEM.,35, 205 (1963). (2) A. T. Hubbard and F. C. Anson, ibid., 36, 723 (1964). (3) J. E. McClure and D. L. Maricle, ibid., 39, 236 (1967). (4) L. B. Anderson and C . N. Reilley, J. Electroanal. Chem., 10, 295 (1965). ( 5 ) A. T. Hubbard and F. C. Anson, ANAL.CHEM., 38, 58 (1966). (6) A. Yildiz, P. T. Kissinger, and C. N. Reilley, ibid., 40, 1018 (1968). (7) W. R. Heineman, J. N. Burnett, and R. W. Murray, ibid., p 1970. (8) L. P. Zajicek, J. Electrochem. SOC.,116, 80C (1969). (9) C. N. Reilley, Rev. Pure Appl. Chem., 18, 137 (1968). (10) B. S. Pons, J. S. Mattson, L. 0. Winstrom, and H, B. Mark, ANAL.CHEM., 39, 685 (1967). 430
ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
EXPERIMENTAL
Construction and Use of Thin-Layer Electrode. As shown in Figure 1, the thin-layer electrode is fabricated from a glass tube into which is inserted a short length of Teflon (DuPont) needle (Hamilton Co., Whittier, Calif.), the latter being flush with or protruding slightly beyond the end of the tube. The glass-Teflon interface must either be a solutiontight pressure fit or be sealed with a suitable material such as epoxy resin. An interchangeable r-shaped wire electrode extends the entire length of the glass tube (and slightly below the bottom of the Teflon needle) and is held in a reproducible position by a notch in the top of the tube. Thus, the thin-layer cavity is the space between the wire and the inner surface of the Teflon needle. It must be recognized that the thickness of the thin-layer cavity is not necessarily uniform. Although this latter characteristic does not preclude the use of the thin-layer electrode for a wide variety of conventional applications, specialized studies of electrodeposition phenomena and of electrode kinetics require invariant solution thickness. In use, the electrode is positioned so that approximately one fourth of the thin-layer region is immersed into pure supporting electrolyte solution contained in a conventional electrochemical cell ( I I ) with a platinum auxiliary electrode and a saturated calomel reference electrode (SCE). To remove and exclude oxygen from the system, the thin-layer electrode may be purged with nitrogen before each filling, and nitrogen can be passed continuously through a plastic tent built around the cell. To clean the thin-layer cavity, one withdraws the entire thin-layer electrode from the electrochemical cell, removes solution from the cavity by using a water aspirator, and rinses the cavity with water, supporting electrolyte solution, and the sample solution. Final filling is accomplished in one to two seconds by capillary action from a separate airfree solution containing the electroactive species. Although the depth of immersion of the electrode into the sample solution does not affect the filling, the top of the Teflon (11) J. J. Lingane, J. Electroanal. Chem., 1, 379 (1960).
/
Figure 1. Thin-layer electrode
sleeve must be cut off square since rounded edges allow the sample solution to creep above the thin-layer cavity. Next, the thin-layer electrode is carefully replaced in the cell and the electrolysis initiated immediately or, in the case of the diffusion-loss study described later, after a specified time lapse. Two different thin-layer electrodes were used in this study and were calibrated from chronopotentiometric measurements (1) with known concentrations of K3Fe(CN)6 in 1 F potassium chloride medium. From the experimental data listed in Table I, the volume of the thin-layer cavity was computed from the relation (12) ir = nFCV
where i is the constant current (amperes), r is the transition time (seconds), n is the number of faradays per mole of electrode reaction, F is the Faraday constant (96,487 coulombs), C is the concentration of the electroactive species (molesiliter), and V is the volume (liters). For each electrode the thin-layer region extended 1.15 cm in height, as determined by measurement with a low-power microscope equipped with a reticle; the smaller electrode employed a 20-gauge (0.032-inch diameter) platinum wire and contained 1S O pl of solution with an average thickness of 42 p, and the larger electrode prepared from 14-gauge (0.064-inch diameter) wire held 6.39 p1 with an average thickness of 63 p , Reagents. Reagent-grade chemicals were used without further purification. Stock solutions of 0.0100F K3Fe(CN), in 1F potassium chloride, 0.0100F K2PtC16 in 1F hydrochloric acid, and 0.0100F K2PtC14in 1F hydrochloric acid were prepared by weight in deionized distilled water and kept in the dark under a nitrogen-gas atmosphere, and were diluted with supporting electrolyte solution for some of the experiments. Instrumentation and Procedure. Single-sweep voltammetry was performed with a solid-state cyclic-scan generator (13) and a conventional three-electrode potentiostatic circuit. Chronopotentiometric equipment was of a standard design (11). Ultraviolet spectra were obtained through use of a Cary 14 recording spectrophotometer and 0.1- and 0.01-mm cylindrical quartz-plate cells (Hellma Cells, Inc., Jamaica, N. Y.). All measurements with ferricyanide ion were performed with a reduced platinum wire electrode preconditioned in boiling concentrated hydrochloric acid for 20 minutes and potentiostated at 0 V us. SCE in 1F hydrochloric acid for 24 hours (14). Successive reductions could then be made without additional pretreatment. Between runs the electrode was stored in nitrogen-saturated 1F potassium chloride solution. In experiments involving reduction of PtC16'-, the platinum working electrode was coated with platinum (12) A. T. Hubbard and F. C. Anson, ANAL. CHEM.,38, 1887 ( 1966). (13) J. DiSalvo and W. B. Schaap, Abstracts, 153rd National ACS Meeting, Div. of Anal. Chem., Miami Beach, Fla., April 1967. (14) A. A. Chodos and L. Meites, ANAL.CHEM., 41, 846 (1969).
Table I. Chronopotentiometric Calibration of Thin-Layer Electrodes by Reduction of Ferricyanide" i, PA r , sec ir, pC X low3 Small electrode (volume = 1.50 pl, thickness = 42 p ) 276.8 186.1 140.8 93.78 56.72 31.90 25.24 18.58 14.80 11.72 5.82 3.86
4.48 7.03 10.17 15.67 25.07 44.6 57.1 78.6 96.1 126.0 252.5 382.0
Large electrode (volume
=
6.39 p l , thickness
1.24 1.31 1.43 1.47 1.42 1.42 1.44 1.46 1.42 1.48 1.47 I .46 =
63 p )
277.0 16.5 4.57 140.8 39.5 5.56 93.6 64.2 6.00 70.1 89.4 6.27 56.5 112.7 6.37 47.38 130.5 6.16 40.84 148.4 6.06 35.72 172.6 6.17 30.64 193.5 5.93 19.76 293.4 5.80 14.76 383.7 5.66 9.68 509.0 4.90 0.0100F K3Fe(CN)ein 1 F potassium chloride medium.
black (15) and pretreated before each trial to form a halfreduced surface (16). In all work, the temperature was maintained constant at 27.0 -i: 0.3 OC. RESULTS AND DISCUSSION
Chronopotentiometric reduction of 0.0100F K3Fe(CN)6 in 1 F potassium chloride medium provided data for the calibration of both the small and large electrodes and demonstrated that a useful range of thin-layer behavior (1) existed for each electrode. In Table I, one observes that is is constant (within experimental error) for currents from at least 3.9 to 141 p A for the small electrode and from 30 to 100 pA for the large electrode. It is apparent, however, that the effects of incomplete electrolysis at short transition times and diffusion losses at long transition times tend to narrow the region of thin-layer behavior for the large electrode. It should be noted that the electroactive species is confined only to the thin layer itself, whereas the electrochemical cell into which the electrode dips contains only supporting electrolyte solution. This feature is advantageous if one wishes to conserve a sample solution which is difficult to prepare or which contains an expensive substance. In addition, since the electroactive species is not present in the bulk solution, any problem due to edge effects is minimized, at least for the 1.50pl electrode. If the bulk solution does contain electroactive species, the tip of the wire electrode must be covered with an insulating material to prevent extraneous electrolysis. Our electrode arrangement allows only outward diffusion of the substance of interest from the thin-layer cavity into the bulk solution. However, designs with electroactive species in both the bulk solution and thin-layer cavity may suffer from two(15) F. C. Anson and D. M. King, ANAL.CHEM., 34,362 (1962). (16) S.W. Feldberg, C. G. Enke, and C. E. Bricker, J. Electrochem. Soc., 111, 826 (1963). ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
rn
431
--I
’
’
I
I
I
I
I
1
1
0.8,
1
,
I
I
I
1
W 0
z a
m LT
0 v)
m a TIME, MINUTES
Figure 2. Plot showing loss of electroactive species by diffusion from thin-layer cavity as a function of time for small (1.50 pl) electrode Experimental points were obtained by means of voltammetric reduction of Fe(CN)e3-, initially at a concentration of 0.0100M in 1F potassium chloride solution, at a scan rate of 2.75 mV/sec
way diffusion which can permit electrolysis products and original substance to mix during an electrochemical experiment, Although filling of the present thin-layer design is fast and reproducible to within 2 or 3 diffusion losses occur as soon as the tip of the electrode contacts the bulk electrolyte solution. For the large electrode, this effect is evidenced in Table I by the decrease of ir for long transition times. Employing single-scan voltammetry, we performed a more complete study of diffusion losses from the 1.50-111electrode with a solution of 0.0100F K3Fe(CN)ein 1F potassium chloride. Figure 2 indicates that diffusion losses are not serious for experiments of short duration and that it should be feasible to apply a simple correction for diffusion losses occurring over longer time periods. It is often desirable to examine the products of electrochemical reactions spectrophotometrically. Although our design does not permit concurrent spectral monitoring (6, 7, IO), it is possible to recover the electrolysis products and to record an ultraviolet spectrum. We obtained reasonably good results by collecting electrolysis products from the 1.5O-pl electrode in a 0.01-mm quartz-plate cell and by installing the 0 to 0.1 absorbance-unit slide wire on the Cary 14 spectrophotometer. However, use of the larger (6.39 pl) electrode allows one to obtain ultraviolet spectra with a 0.1-mm quartz-plate cell, so that, when the usual 0 to 1 absorbance-unit slide wire is employed, much less noise appears in the spectrum. Figure 3 shows the ultraviolet spectrum of PtC162- in 1 F hydrochloric acid and compares the spectrum of its two-electron reduction product to that of authentic PtCI42- in 1 F hydrochloric acid. After reduction of PtClsz- was accomplished by means of linear-sweep voltammetry (IZ),the contents of the thin-layer cavity were blown out onto the 0.1-mm quartz-plate cell, the quartz cover-plate was placed in position, and the spectrum was recorded.
x,
432
ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
WAVELENGTH,
nm
Figure 3. Comparison of ultraviolet spectrum for 2.37 x lO-3M PtC162- (curve 1) with spectra for the reduction product of PtCl6’- (curve 2) and for authentic 7.19 X 10-3M PtC1,Z- (curve 3) in 1F hydrochloric acid and in a quartz cell with a path length of 0.1 mm
It was mentioned earlier that the wire electrode may be easily removed and interchanged with a different one. This is of particular value if the electrode surface should become contaminated by impurities. In addition, one can exchange a platinum electrode of one surface condition for an electrode of an entirely different electrochemical history. Since a standard-gauge wire is used, different metals such as gold or silver may be substituted for platinum. One might take advantage of this to use a mercury-coated platinum wire or the more easily formed amalgamated-gold electrode in performing thinlayer electrochemistry at a mercury surface. If one wishes to remove an electrode wire and replace it with another between the anodic and cathodic scans of a cyclic voltammetric experiment, the thin-layer assembly must first be removed from the electrochemical cell, the electrode wire interchanged, and the thin-layer assembly repositioned in the cell. Although capillary action keeps most of the sample solution in the Teflon needle during this manipulation, solution losses of between 10 and 2 0 x do occur, which seem to arise from breaking and making solution contact between the thin layer and the bulk electrolyte as well as from adherence of small amounts of solution to the electrode wire as it is withdrawn.
RECEIVED for review September 10, 1969. Accepted December 15, 1969. Presented in part at the Third Great Lakes Regional ACS meeting, DeKalb, Ill., June 1969. Work supported by National Science Foundation Grant GP-8563.
