Anal. Chem. 1982, 5 4 , 1005-1008
BENZENE
Figure 8.
t
Raman spectrum of neat benzene.
OBSERVATIONS ON ELECTRICAL NOISE We have at least three excellent (but troublesome) noise generators in our laboratory. The laser supply itself has a current pulse of 600 A at 3 1500 V. The rise time is only a moderate 1-5 pclu. Igniting the flashlamps at the startup of the laser uses a 20-kV pulse of very low current. The single pulse selector used in the picosecond experiments generates a low current, 4 kV pulse, but with a rise time of 2 ns. Thus the frequency content is very high and this makes it hard to shield. Any or all of these signals is sufficient to send the electronics and computer into chaos. As mentioned in the layout section, the avoidance of ground loops is essential in helping to eliminate interference problems. All signal lines are shielded and all connections from the computer to the outside world are opto-isolated. We have not found it necessary to use isolation transformers for all the electronic equipment.
1005
The elimination of ground loops is not enough to eliminate interference. Long signal lead length (4),especially where magnetic (or far field) interference is important, is enough to generate spurious voltages and currents. Such leads are generally unavoidable. The one thing we have found very useful is to wrap all noise sources and signal lines in grounded aluminum foil. This almost completely eliminates noise problems. It is also quite inexpensive. Recently, we added a pair of 8-in. floppy disk drives to our computer. When the laser was operating, the computer would crash every minute or so. Since we have wrapped with foil the 50-wire cable, which connected the disk drive to the computer and the dual disk drive itself, which was in a wooden box, crashes are very rare. The moral is that careful layout and shielding of both sources and detectors/computers can eliminate noise problems. Planning ahead is better than patching up after where possible. Reference 4 is a very good source of information and suggestions for tackling noise problems.
LITERATURE CITED (1) "Linear Applications, Vol. 1"; Natlonal Seminconductor Cosp.: Santa Clara, CA, 1973. (2) "Cromemco Tuart Digital Interface"; Cromemco Inc.: Mountain View,
CA.
(3) Langhoff, C. A,; Moore, E.; DeMeuse, M. Symposium on Lasers In Chemistry, IUPAC Meeting, Vancouver, E. C., Aug 18, 1981, unpub-
(4)
llshed results. Ott, Henry W. "Noise Reduction Techniques in Electronic Systems"; Wlley: New York, 1976.
RECEIVED for review July 29,1981. Accepted February 8,1982. Funds for this work were provided by the Research Corporation, the donors of the Petroleum Research Fund, administered by the American Chemical Society, NSF (CHE7908628), and IIT.
Anaerobic Thin-Layer Electrochemical Cell for Planar Optically Transparent Electrodes Edmond F. Bowden, David J. Cohen, and Fred M. Hawkridge" Department of Chemlstty, Virginia Commonwealth Unlversity, Richmond, Virginia 23284
Optically transparent thin-layer electrochemical (OTTLE) cells have been designed for many purposes (I,2),but an easily assembled version for anaerobic measurements with planar plate-type optically transparent electrodes (OTE) has not appeared. Our interest in studying protein electrodic processes at metal oxide semiconductor OTEs (3-5) as well as viologen surface phenomena (6-8)has stimulated the development of the cell design reported here. This design allows straightforward utilization of m y of the numerous glass and quartz based planar OTEs (9,10)while realizing the advantages of thin-layer spectroelectrochemistry. The mesh-type metal minigrid has proved its worth as the most popular working electrode in OTTLE cells (I,9).For numerous applications, minigrids are completely satisfactory and can be used in proven, economical cell configurations. Planar OTEs, however, possess certain characteristics not found in minigrids which can be of overriding experimental concern. By virtue of their geometry,planar OTEs are suited to optical probing of surface localized processes such as adsorption and chemical modification, whereas minigrids are not. Determination of molar absorptivity values for electrogenerated adsorbed species is one example of this. Another important feature is that planar OTEs can be fabricated by using noinmetallic electrode materials as well
as metals. In particular, the ability to routinely use doped metal oxide semiconductor OTEs in a thin-layer configuration is inviting. These OTEs have seen considerable use in nonthin-layer cells and are well-known for their wide potential windows, ready availability, and ease of handling (9).Their recent use in numerous chemically modified electrode (CME) applications (11,12), as electrocatalysts for the direct reduction and oxidation of biological molecules (3-5,13) and as substrates for viologen electrochromic devices (14-I6),is well established. The OTTLE cell described in this paper was designed to meet the following criteria: (1)usable with any rigid planar OTE; (2) capable of introducing and maintaining anaerobic solution conditions (less than ca. 1pM oxygen); and (3) ease of assembly and disassembly. Other designs have dealt with these considerations individually. Pt and Au OTEs have previously been incorporated into thin-layer configurations (I7,18), but fabrication of specially patterned electrodes was required in each case. Anaerobicity has not been addressed often in the OTTLE cell literature. Norris et al. (19)first developed a practical anaerobic version for minigrids which eliminated oxygen via vacuum/nitrogen cycling (20). A modification of their original design has also proved successful (8).
0003-2700/82/0354-1005$01.25/00 1982 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 6, MAY 1982
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--
n
~0
Flgure 1. Thin-layer spectroelectochemical cell with demountable planar OTE: (a) solution inlet valve; (b) reference electrode valve; (c) upper Lucite plate; (d) copper wire for OTE contact: (e) silver wire auxiliary electrode: (f) rubber compression sleeve: (9) brass OTE contact; (h) 0-rlng; (i)Teflon tape; (1) OTE: (k) Teflon tape; (I) OTE O-ring: (m) perimeter 0-rlng: (n) lower Lucite plate; ( 0 ) aluminum retainer. The quartz wlndow, which is located on the underside of the upper plate opposite of OTE, is not shown.
Nearly all OTTLE cells, and in particular the anaerobic ones, require epoxy sealing. If the working electrode must be removed for pretreatment or replaced after each experiment, the process of epoxy removal, reapplication, and drying becomes a hindrance. With much OTE work, e.g., CMEs and bioelectrochemical studies, the ability to avoid epoxy use during repetitive electrode installation and cell assembly is a decided practical advantage. The OTTLE cell described herein does not utilize epoxy for either installation of the OTE or sealing of the cell. EXPERIMENTAL SECTION The cell, shown in Figure 1, is of the double Lucite plate design (6-8). Both plates are 3/8 in. (0.95 cm) thick for good rigidity and are fastened together with 10 4/40 screws and a metal retainer which contains threaded holes for the screws. The underside of the lower plate is recessed around its perimeter to accommodate the retainer. This feature avoids threaded holes in the Lucite, which tend to fail, and provides more uniform compression of the plates. The lower plate contains a cavity for the OTE, a perimeter O-ring groove which defines the solution volume, and 10 clearance holes for the screws. The '/E in. (2.22 cm) X 9/16 in. (1.43 cm) OTE cavity is milled to a depth equal to the thickness of the OTE being used. A rectangular O-ring groove (3/32 in. (0.24 cm) wide X 0.035 in. (0,089 cm) deep) is then cut into the cavity floor for use with a 6/8 in. (1.59 cm) i.d. X '/le in. (0.16 cm) thick Viton O-ring. This O-ring fits into the rectangular O-ring groove described above. To pass the light beam, a in. (0.64 cm) X 3/8 in. (0.95 cm) slot is cut through the plate at the center of the OTE cavity. The perimeter O-ring is 1.75 in. (4.45 cm) i.d. X l/ls in. (0.16 cm) thick Viton and rests in a 3/32 in. (0.24 cm) X 0.035 in. (0.089 cm) deep groove. Inside dimensions of this rectangular groove are 1.75 in. (4.44 cm) X 1.00 in. (2.54 cm). Contacts for all three electrodes are made through the upper plate. This feature avoids the sealing problems associated with making working and auxiliary electrode contact between the Lucite plates. The method for contacting the OTE is shown in detail in Figure 2. First a copper wire is inserted in a small
d
Figure 2. Cross-sectional detail of OTE contact region. Letter designatlons refer to Figure 1. The OTE O-ring and details of the OTE cavity are not shown.
hole in one end of a 3/32 in. (0.24 cm) diameter X 3/32 in. (0.24 cm) long brass cylinder and soldered in place. The other end of the cylinder is mechanically polished. A rubber sleeve of ca. 3/32 in. (0.24 cm) outside diameter is then slipped over the wire against the brass cylinder, and the assembly is inserted into the plate. When the two Lucite plates are screwed together, the sleeve compresses and holds the brass contact firmly against the OTE surface. The length of the rubber sleeve is chosen to provide satisfactory compression for making good contact. A 1/8 in. (0.32 cm) i.d. X '/le in. (0.16 cm) thick Viton O-ring isolates the brass contact from the solution. This assembly is located at one corner of the OTE away from the reference electrode and occupies ca. 10% of the OTE surface. For the auxiliary electrode, a shallow trough in. (0.32 cm) X 1/16 (0.16 cm) X 3/4 in. (1.90 cm)) is milled in the upper plate. A platinum or silver wire can then be inserted through a small hole drilled through the plate, and the hole is then sealed with epoxy. A 0.075 in. (0.191 cm) thick quartz window is epoxied in place flush with the inner surface of the upper plate. Because of the ability of Lucite to absorb oxygen (vide infra) it is best for the window to be as large as the OTE. It will also be somewhat irregular in shape because of the OTE contact. Approximately 0.050 in. (0.13 cm) of unmachined Lucite was left between the quartz window cavity and the brass contact O-ring cavity. An optical slot was cut through this plate to match the one in the lower plate. Provisions for the reference electrode and the vacuum degassing bulb followed previous designs (8,19).Square Lucite blocks with 0.166 in. (0.422 cm) diameter through holes were solvent welded to the upper plate and inert valves from Hamilton Co., Reno, NV, were epoxied in place. A no. lMMl ( B O o ) valve was used for the filing port and a no. 2MM2 ( W O ) valve was used for the reference electrode. The 0.110 in. (0.28 cm) diameter hole drilled through the upper plate for the reference electrode was centered 3 mm from the edge of the OTE. Cell assembly is as follows. The OTE O-ring, the OTE, and the perimeter O-ring are installed in the bottom plate. Next, strips of 0.005 in. (0.013 cm) thick Teflon tape (Dilectrix Corp., Lockport, NY) are positioned on the four sides of the lower plate between the O-ring and the screws. To set the path length, two 3 mm wide strips of the same tape are then positioned on the OTE at its two short edges (see Figure 1). The upper plate, with contacts installed, and the lower plate are then aligned with the retainer, and the assembly is fastened together with screws. Disassembly, cleaning, and reassembly with a new OTE requires about 15-20 min. The vacuum/nitrogen train, the degassing bulb, and the Ag/AgC1(1.00 M KC1) reference electrode have been described
ANALYTICAL CHEMISTRY, VOL. 54, NO. 6, MAY 1982
- - - II
I
T
0.3
1007
0.4
0.5 0.6 0.7 - E I V vs Ag/AgCI
0.8
0.9
Figwe 3. Cyclic voltammograms of 1.O mM MV2+, 0.10 M phosphate, 0.10 M NaCI, pH 7.0. Geometric surface area of tin oxide OTE WE was 2.7 cm2. Scan rates are in mV/s: (a) 1; (b) 2; (c) 5. Cell path length was 0.16 nim.
(20). Although sample deoxygenation and introduction were initially performed after the manner of Norris et al. (19),a simpler procedure was found to work equally well (vide infra). The fluorine-doped tin oxide OTEs (PPG Industries, Pittsburgh, PA) were cut on a glass saw and then cleaned by successive 5-mill ultrasonic treatments in Alconox, ethanol, and doubly distilled water (twice). Gold minigrids (200 wires/in.) from Buckbee-Mears Co., St. Paul, MN, were cleaned in a Harrick plasma cleaner. Diquat (DQ2+;6,7-dihydrodipyrido[1,2-~:2',1'-c]pyrazinediiumdibromide), a bipyridinium salt, was from Chem Service, West Chester, PA, and was recrystallized three times from a water-acetone mixture. Methyl viollogen (MV2+; l,lf-dimethyl-4,4'-bipyridinium dichloride) from Aldrich was recrystallized three times from a methanol--acetonemixture. All other chemicals were reagent grade.
RESULTS AND DISCUSSION Cyclic voltammetric responses of the cell are shown in Figure 3. These cyclic voltammograms of 1 mM methyl viologen are typical of those observed in thin-layer cells and reveal the well-known uncompensated iR drop effect (I). This cell also exhibits satisfactory diffusion controlled response to potential step perturbations. Stepping the applied potential from -0.30 V to -0.80 V resulted in quantitative reduction of a 2.46 mM diquat sollution in 1.5 min as determined by monitoring the 378-nm absorption band of the diquat radical. A standard OTTLE experiment, spectropotentiostatic determination of Eo'and n (21),was performed with a solution of 2.46 mM DQ2+,0.101 M phosphate, 0.10 M NaC1, pH 7.8. A t 378 nm, a linear Neirnst plot with a correlation coefficient of 0.9999 was obtained and yielded E"' = -0.372 V vs. NHE and n = 1.00. The E"' value is in excellent agreement with that determined undep. identical solution conditions with a gold minigrid OITTLE, -0.371 V (8). The anaerobic capability of this cell was evaluated using cyclic voltammetry, Y = 2 mV/s, with 0.10 M phosphate, 0.10 M NaC1, pH 7.8 solution. As is well-known, many plastics, including Lucite, absorb oxygen. This proved to be the major source of unwanted ox:ygen in these studies, but fortunately this source can be minimized with the precautions subsequently described. Scan a of Figure 4 is a cyclic voltammogram obtained for the air equilibrated solution at the tin oxide OTE. Forty-five minutes later scan b was acquired. Both show the morphology of linear sweep reduction of oxygen at this electrode. If deoxygenated solution is introduced into the cell and a cyclic voltammogram is immediately acquired, scan c is the result. Oxygen is essentially undetectable. However, 45 min later, scan d reveals that oxygen has diffused into the vicinity of
0.2
0.4
0.6
0.0
- E I VvrAglA$I
Flgure 4. Oxygen exclusion capability of cell shown in Figure 1: solution, 0.10 M phosphate, 0.10 M NaCI, pH 7.8; CV scan rate, 2 mV/s; initial potential, 0 V; electrode area and path length are as given in Flgure 3; for undeoxygenated solution, (a) initla1 CV, (b) 45 mln later; for deoxygenated solution, (c) initial CV, (d) 45 rnin later; for deoxygenated solution after evacuating cell for 20 h, (e) initial CV, (f) 45 rnin later, (9) 3 h after (e).
the OTE. The major source of this oxygen is the Lucite adjacent to both the OTE and the quartz window. When this experiment was repeated after first subjecting the assembled cell to vacuum for 20 h and then introducingthe deoxygenated solution, scan e is the immediate result and scan f is the result 45 min later. The cyclic voltammogram which resulted after waiting an additional 21/4 h, scan g, was indistinguishable from scan f. This cell is therefore suitable for maintaining solution oxygen concentration a t 51 pM for a minimum of 3 h. The 1 pM figure was arrived at as follows. Because of the coulometric nature of slow potential scan rates in thin layers the area under a voltammetric peak, such as shown in Figure 4d, can be considered to represent the charge required for the four-electron reduction of oxygen in the OTE thin-layer volume, V. Converting this charge to moles and then dividing by V affords the original oxygen concentration. It was conservatively estimated that 1 WMoxygen was the upper limit for being able to detect oxygen reduction in these experiments. By evacuation of the cell as just described, it was also found that a somewhat simpler procedure for sample deoxygenation and introduction was possible. After oxygen was eliminated from the reference electrode compartment (19),three cycles of vacuum and nitrogen applied to the cell interior and degassing bulb resulted in performance identical with that obtained with a more elaborate procedure (19). A previously reported anaerobic minigrid OTTLE cell (8) was used to further establish the fact that Lucite-absorbed oxygen is the major consideration in maintaining anaerobicity. This cell was assembled with 10 mm wide gold minigrids as previously described (8)and then subjected to vacuum for ca. 16 h before filling K t h deoxygenated solution. The electrodes were then connected with the auxiliary electrode (AE) and working electrode (WE) leads reversed relative to normal operation. In this configuration, the minigrid situated between Lucite surfaces acts as the WE and the minigrid between quartz surfaces acts as the AE. For the cyclic voltammograms shown in Figure 5, scan a, obtained immediately, and scan b, obtained 45 min later, are evidence for the influx of oxygen into the region adjacent to the electrode. After scan b, the electrode leads were returned to their normal configuration so that the minigrid sandwiched between the two quartz windows is then the WE. Upon connection a t 0 V, a momentary cathodic current flowed at the WE to reduce the oxide formed during its role as an auxiliary electrode (scans a and b) and then scan c was recorded. Scan d, otained l L / z h later on the same electrode, was essentially indistinguishable from scan c. These data confirm the fact that Lucite acts as
1008
Anal. Chem. 1982, 5 4 , 1008-1011
r---, I
I
1 I
I
I
0
I
I
0.2
I
I
1
I
1
0.4 0.6 - E / V vs Ag/AgCi
I
0.8
Figure 5. Oxygen retention capability of Lucite: cell, minigrid OlTLE (8);minigrid dimensions, 10 rnm X 23 mm; path length, ca. 0.16 mm; CV 5can rate, 2 mV/s; solution, deoxygenated 0.10 M phosphate, 0.10 M NaCI, pH 7.8; for minigrld sandwiched between Lucite. (a) initial CV, (b) 45 min later; for minigrid sandwiched between quartz window, (c) initial CV (ca. 1 h after (a),(d) 75 min later.
an oxygen sink/source in these experiments. Although these practical aspects of anaerobicity are not the main focus of this paper, they do warrant attention. The overriding question is, of course, what level of anaerobicity does an experiment demand and for what duration? The OTTLE cell described here should prove suitable for a broad spectrum of applications. The fact that very low oxygen levels can easily be maintained for a t least 3 h in a thin-layer cell with a demountable OTE is particularly outstanding. It is also clear that even this performance can be bettered through design (e.g., by further enlarging the quartz window area or by decreasing the OTE area) and procedure (e.g., storing the cell under Nz or vacuum for extended periods). Although a cell of all-glass construction may provide improved performance in this regard (22), it is not obvious that an easily constructed, easy-to-assemble and disassemble, rugged all-glass OTTLE for use with interchangeable OTEs is a practical undertaking. The design shown in Figure 1could, however, be executed with a machinable ceramic such as Macor (23) which has extremely low oxygen permeabiltiy. Such a cell could also be used with a wide variety of nonaqueous solvents as well as with fused salts. After using two of these Lucite OTTLE cells for ca. 2 months, we have encountered no operational problems. For good contact to the OTE, the face of the brass contact should be lightly polished before assembly. For more critical applications, a silver contact or silver paste could be used. When
fastening the two cell plates and retainer together, more force is required than with other screw-type OTTLE designs (6-8). This results from the intentionally shallow O-ring grooves (0.035 in. deep) and the consequent significant compression of the O-rings. We feel that the grooves could be cut somewhat deeper if desired while still retaining the sealing quality demonstrated here. In conclusion, the OTTLE cell described here meets the basic design goal of being usable with any plate-type OTE. Fabrication of specially patterned OTEs is not required. Fast and simple assembly and disassembly are other important practical advantages. Excellent anaerobicity characteristics have been experimentally demonstrated.
LITERATURE CITED Pinkerton, T. C.; Hajizadeh, K.; Deutsch, E.; Heineman, W. R. Anal. Chem. 1980, 52, 1542-1546, and references therein. Rhodes. R. K.: Kadish. K. M. Anal. Chem. 1981. 53. 1539-1541. Bowden, E. F.; Wang,' M.; Hawkridge, F. M. J . Nectrochem. SOC. 1980, 127, 131'2. Bowden, E. F.; Hawkridge, F. M.;Blount, H. N. Adv. Chem. Ser., in press. Bancroft, E. E.; Biount, H. N.; Hawkridge, F. M. Blochem. Siophys. Res. Commun. 1981, 101, 1331-1336. Landrum, H. L.; Salmon, R. T.: Hawkridge, F. M. J. Am. Chem. SOC. 1977, 99, 3154-3158. Staraardt. J. F.: Hawkridae, F. M.: Landrum, H. L. Anal. Chem. 1978, 50,330-932. Bowden, E. F.; Hawkridge, F. M. J . Electroanal. Chem. 1981, 125, 367-386 - - . - - -. Kuwana, T.; Heineman, W. R. Acc. Chem. Res. 1978, 9 , 241-248. De Angelis, T. P.; Hurst, R. W.; Yacynych, A. M.; Mark, H. 6.; Heineman, W. R. Anal. Chem. 1977, 49, 1395-1398. Murray, R. W. Acc. Chem. Res. 1980, 13, 135-141. Armstrona, N. R.: Sheoard, V. R. J. Nectroanal. Chem. 1980. 115. 253-265Yeh, P.; Kuwana, T. Chem. Lett. 1977, 1145-1148. Bruinink, J.; Kregting, C. 0. A. J. €/ectrochem. SOC. 1978, 125, 1397-1 40 1
F&her,S.; Duff, L.; Barradas, R. G. J. Nectroanal. Chem. 1979, 100, 759-770. Cieslinski, R. C,; Armstrong, N. R. J . Electrochem. SOC.1980, 127, 2605-2610. Ylldlz, A.; Kissinaer. P. T.; Reillev, C. N. Anal. Chem. 1988, 4 0 , 1018- 1024. Srlnlvasan, V. S.; Anson, F. C. J . Nectrochem. SOC. 1973, 120, 1359-1 - .. .360. - -. Norrls, B. J.; Meckstroth, M. L.; Heineman, W. R. Anal. Chem. 1978, 48. 630-632. Hawkridge, F. M.; Kuwana, T. Anal. Chem. 1973, 45, 1021-1027. Heineman, W. R.; Norris, B. J.: Goelz, J. F. Anal. Chem. 1975, 47, 79-84. Stankovlch, M. T. Anal. Biochem. 1980, 109, 295-308. Hawkridge, F. M.; Pernberton, J. E.; Blount, H. N. Anal. Chem. 1977, 49, 1646-1467.
RECEIVED for review October 2,1981. Accepted February 4, 1982. This work was supported in part by grants from the National Science Foundation (PCM 79-12348) and the National Institutes of Health (GM27208-02).
Characterization of Phosphorite Ores Dennis R. Jenke Montana Applied Research Group, Division of Copper City Enterprises, Inc., 120 West Park Street, Butte, Montana 5970 1
Frank E. Diebold" Chemistry and Geochemistry Department, Montana College of Mineral Science and Technology, Butte, Montana 5970 1
The characterization of phosphorite ores with respect to their major components is of considerable importance with respect to the efficient utilization of this valuable resource. Classical analytical techniques, including gravimetric and
colorimetric procedures, for obtaining this characterization, while precise, are difficult and time-consuming to perform and suffer from a multitude of potential matrix interferences. The resistance of particular phosphorite samples to acid digestion
0003-2700/82/0354-1008$01.25/00 1982 American Chemical Society
