Anal. Chem. 1988, 60,88-90
88
was observed for larger proteins such as bovine serum albumin even though these species could not be separated via liquidliquid extraction in a related system (17, 18). This may be due to the fact that membrane-based systems can utilize facilitated or coupled transport. Under the conditions of this study, no significant protein conformational changes or denaturation was observed for solutes on either side of the membrane (see Experimental Section). Further work is currently under way involving more complex protein mixtures and matrices as well as on the use of different membrane configurations and types. Registry No. AOT, 577-11-7;cytochrome c, 9007-43-6; lysozyme, 9001-63-2.
(7) Armstrong, D. W.; Boehm, R. E. J. Chromatogr. 1984, 288, 15. (8) Blanquet, R. S.:Bui, K. H.; Armstrong, D. W. J. Liq. Chromatogr. 1986, 9, 1933. (9) Drager, R.; Regnier, F. E. J. Chromatogr. 1986, 359, 147.
(10) Regnier, F. E. LC-GC 1987, 5 , 100. (11) Regnier, F. E. LC-GC 1987, 5 , 234. (12) Regnier, F. E. LC-GC 1987, 5 , 392. (13) Cohen, S.A.; Benedek, K. P.; Dong, S.;Tapuki, T.; Karger, B. L. Anal. Chem. 1984, 5 8 , 217. (14) Luisi, P. L.; Henninger, F.; Joppich, M.; Dossena, A.; Casnati, G. Biochem. Biophys . Res. Commun, 1977, 74 1384. (15) Luisi, P. L. ; Bonner, F. J.; Pellegrini, A,; Wiget, P.; Wolf, R. Heh. Chim. Acta 1979, 62, 740 (16) Menger, F. M.; Yamada, K. J. Am. Chem. SOC. 1979, 101, 6731. (17) Gijklen, K. E.; Hatton, T. A. Biotechnoi. Prog. 1985, 7 , 69. (18) Giiklen, K. E,; Hatton, T. A. Sep. Sci. Technoi., in press. (19) Armstrong, D. W. S e p . Purif. Methods 1985, 14, 213. ~
Daniel W. Armstrong* Weiyong Li
LITERATURE CITED (1) Rivier, J. E. J. Liq. Chromatogr. 1978, 7 , 343. (2) Rubinstein, M. Anal. Biochem. 1979, 98, 1. (3) Hern, M. T. W.; Grego, 6.; Hancock. W. S. J. Chromatogr. 1979, 185, 429. (4) Van Oss, C. J.: Absolom, D. R.; Newman, A. W. S e p . Sci. Technoi. 1979, 14, 305. (5) Pearson, J. D.; Lin, N. T.; Regnier, F. E. Anal. Blochem. 1982, 724,
217. (6) Hancock, W. S.;Sparrow, J. T. Mgh-Performance Liquid Chromatography; Horvath, Cs., Ed.; Academlc: New York, 1983; Vol. 3, pp 49-85.
Department of Chemistry University of Missouri-Rolla Rolla, Missouri 65401 RECEIVED for review June 1, 1987. Accepted September 14, 1987. Support of this work by the National Institute of General Medical Sciences (BMT 1 ROI GM 36292-01) is gratefully acknowledged.
Simple Jet Electrodes for Kinetic and Synthetic Purposes Sir: A jet electrode (JE) consists of a stationary working electrode with hydrodynamic mass transport achieved by impingement of a jet stream normal to the electrode surface. The theory and use of jet electrodes (wall jet electrode, wall tube electrode) has been well-documented (1-9).Techniques involving JE's are analogous to those involving rotating disk electrodes. Jet electrodes with nozzle diameters greater than the electrode diameter have uniform accessibility and are thus suitable for mass-transport-controlled analytical applications. Previous accounts describe the use of JEs that have employed elaborate pump systems and sealed electrochemical cells. We describe a JE that employs simple and inexpensive equipment. We have used this JE to perform kinetic measurements on semiconductor electrodes (IO). The pump is a commercial veristaltic pump. The electrochemical cell used was nonsealed and of low volume. The cell geometry could be easily adapted for use with almost any solid electrode. The system might be of interest to electroanalytical laboratories for routine use as an efficient stirring device or, with modifications, as an analytical device. EXPERIMENTAL SECTION Materials. Acetonitrile (ACN, Burdick and Johnson, UV grade, dried by refluxing over phosphorus pentaoxide) and 0.1 M tetrabutylammonium fluoroborate (TBAFB, Southwestern, ground and dried under vacuum for 2 days) was the electrolyte solution. Ferrocene (Aldrich) was purified by sublimation. Equipment. The core of the system was a veristaltic pump or 3/8-in.-o.d.silicone (Junior Model, Manostat) fitted with rubber pump tubing. Nozzle and exit tubes to the cell were made of Teflon tubing. Plumbing made use of HPLC fittings. A glass tubing to HPLC fitting connector (Alltech, no. 20060) was modified with a raised ridge and inserted into the silicone tubing. Heat-shrink tubing was used to keep the tubing from slipping off. No pulse dampener was used in our studies. Two cells of different sizes were used. They were both of similar design, consisting of a glass beaker (50 or 100 mL) with a Teflon electrode cap (Figure 1). The total cell volumes (includingtubing) for the two cells were about 10 or 25 mL. The larger cell, which had a diameter of 3.6 cm and a solution depth of 1.5-2.0 cm, was
used to obtain the voltammetric and electrolysis data reported here. Electrodes were all constructed with 6-mm glass tubing. The tubing fit snuggly into the holes of the Teflon cap to prevent rotation of the electrodes during jet operation. The reference electrode was Ag/AgN03 (0.01 M), TBAFB (0.1 M), ACN. The frits were made of Vycor disks. The inlet and outlet tubes were also 6-mm glass tubing. The nozzle inlet tube had a 90" bend on the end so the jet streamed horizontally into the electrode. The nozzle of the jet was the unmodified end of the Teflon tubing. Nozzle Velocity Determination. Mean flow rates (cm3/s) for the veristaltic pump obtained with exit tubes of diameters 0.086,0.17, and 0.27 cm were measured with a graduated cylinder and stop watch while using water as a solvent. Mean nozzle velocities were calculated from the flow rates divided by the cross sectional area of the nozzle. Voltammetry and Electrolysis. All electrochemical experiments utilized a PAR 1731179 potentiostat/coulometer and positive iR compensation. Current-potential curves were recorded by using a sealed platinum-wire working electrode having a radius (R)of 0.067 cm. The wire was sealed in the end of a bent glass tube a8 depicted in Figure 1 and polished with alumina. Bulk electrolysis were performed with a platinum-gauze working electrode with dimensions of 1.5 X 4.5 cm. The current was monitored with an x-y recorder (Houston, 200), and the total charge was recorded when the current decayed to 1%of ita initial value. The jet was aimed directly at the working electrode with the pump at the highest speed for both the I / E curve and the bulk electrolysis. RESULTS AND DISCUSSION The most rema+able feature of the system is the nonsealed, low solution vokme cells employed. The solution did not vortex head gas except in instances where the nozzle stream was directed at the outlet. It was possible to position the outlet to within 1mm of the electrode without vortex problems. The cell and pump geometry is shown in Figure 1. Flow rates were measured with water for the different tubing diameters, d, listed in Table I and range from 1 2 to
0003-2700/88/0360-0088$0 1.50/0 0 1987 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 1, JANUARY 1, 1988
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0’71 F A
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0.6
0.5
4
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i
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Figure 1. Cross sectional view of an electrochemical cell employing
the jet electrode configuration and a working electrode appropriate for voltammetric experiments.
0
Table I. Flow Characteristics of the Jet
l/&.
mean flow rate, cm3/a mean nozzle velocity, cm/s % variation Reynolds no. Rld
0.17
0.27
20
/I
0
50
150
100
200
250
;
-0.2
0.0
0.2
0.4
0.6
0.0
E/V
tubing 3.01 519 21 4500 0.78
+t
TIME/S
nozzle i.d., cm 0.086
o.2 0.1
30
6.95 306 33 5200 0.39
7.33 128 33 3500 0.28
10.7 473 8 8000 0.39
11.2 196 8 5300 0.28
Figure 2. (A) Current-potential curve for the oxidation of ferrocene (2.0 mM In CH3CN) at the platinum-wire electrode: d = 0.086 cm, H = 1-4 cm. The potential scan rate was 10 mV/s; positive and negative scans were superimposible. (e) Current-time curve for the constant potential electrolysis of 9.1 mg of ferrocene in 25 mL of CH3CN (1.96 mM) at the platinum-gauze electrode.
3/s-in. tubing
mean flow rate, cm3/s mean nozzle velocity, cm/s % variation Reynolds no. Rld,
3.26 562 23 4800 0.78
45 cm3/s. Also listed are the calculated nozzle velocities, U, which range from 128 to 519 cm/s. The percent variation corresponds to the variation in flows at the highest and lowest pump speeds. It is apparent that the pump would need flow-control modifications if anticipated for use as an analytical tool. Chin and Tsang (2) have investigated the type of jet electrode configuration described here. They presented a semiempirical solution to a convective diffusion model which they compared to experimental data. The critical parameters in their analysis were the nozzle Reynolds number, Re = U d / u ( u is the kinematic viscosity), the ratio of electrode radius to nozzle diameter, R / d , and the ratio of the distance of the nozzle from the electrode to the nozzle diameter, H / d . For turbulent jets and H l d ratios ranging from 0.2 to 6, it was found that the limiting current density varied by less than 10% for R / d ranging from 0.1 to 1.0, i.e. the electrode surface appeared to be uniformly accessible within these regimes. The data presented here were used to calculate Reynolds numbers of 3500-8000 (Table I), thus the flow is well into the turbulent region. Values of R / d are also included in Table I and indicate that the surface of the Pt-wire electrode should be uniformly accessible, based on Chin and Tsang’s analysis. Figure 2A contains a voltammogram for the oxidation of ferrocene in acetonitrile at the jet electrode while using the highest pump speed and 0.086-cm exit tubing. Since the rate of this process becomes mass-transport limited, the limiting current contains noise associated with the pulsating action of the pump. It should be possible to remove this noise by using a pulse dampener. As predicted by Chin and Tsang’s
work (2),the currents were not sensitive to the distance between the nozzle and electrode surface when the H / d ratio ranged between 1 and 4. The limiting current density for the wave in Figure 2 is 48.2 mA/cm2. With the reported diffusion coefficient of ferrocene in acetonitrile (11)of 2.6 X cm2/s and a v of 4.4 X cm2/s, the Levich equation (12)can be used to calculate the rotation rate of an RDE that would be required to achieve the same current density. The calculated equivalent rotation rate is 314000 rpm. This type of calculation merely illustrates that extremely high mass-transfer coefficients are achievable with the JE. The current densities obtained at the J E cannot be related to those at an RDE in a rigorous manner because the Levich equation applies to laminar flow while the flow at the J E is turbulent. We have utilized these high mass-transport rates to simplify investigations of dark kinetic current flow at semiconducting electrodes, specifically WSez. Observed current densities at these electrodes are less than 10% of the mass-transportlimited values even when the electrode potential is several hundred millivolts from the equilibrium potential. Since the currents are not limited by mass transport, equations describing kinetic current flow can be applied directly (13). It should be noted that the WSez electrode depicted in Figure 1,which has been sealed with epoxy, would not display flow characteristics identical with those of the flush, polished electrode described above. The high mass-transfer rate also facilitates rapid bulk electrolyses. Figure 2B contains the current-time profile for electrolysis of 9.1 mg of ferrocene at a platinum-gauze working electrode and exit tubing with d = 0.17 cm. The total charge passed was 4.6 C or 97% of the calculated value. The current is limited by incomplete i R compensation during the first 100 s of the electrolysis. At the later times, the electrolysis half-time becomes about 25 s. No attempt was made to optimize the electrode area to volume ratio in this experiment.
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ANALYTICAL CHEMISTRY, VOL. 00, NO. 1, JANUARY 1, 1988
A versitile feature of the jet electrode for electrolyses is that the entire solution volume is turned over in &lo s and the flow can be directed a t working electrodes with a variety of shapes and sizes. The pump does cause heating in the cell making temperature regulation necessary for certain applications such as kinetic measurements. The pump used in this study is not suitable for extended operation; thus the higher quality pumps available are suggested for use. For use with solutions that react with or corrode silicone rubber, a thin-wall Teflon tube inserted into a larger bore silicone rubber tube will function. However, lower flow rates are obtained with this configuration. The use of J E s allows forced-convection (hydrodynamic) techniques to be performed with almost any solid electrode, thus eliminating the need for RDE fabrication. The simple J E design and geometry reported here, coupled with its effectiveness, suggest that others may find additional applications. Registry No. WSe2, 12067-46-8; Pt, 7440-06-4; ferrocene, 102-54-5.
LITERATURE CITED Giauert, M. E. J. Fluid Me&. 1958, 7 , 625. Chln, D. T.; Tsang, C. H. J. Electrochem. Soc. 1978, 725, 1461. Chin, D. T.; Chandran. R. R. J. Electrodrem. Soc. 1981, 728, 1904. Hsueh, K. L.; Chln, D. T. J. Electrochem. Soc. 1988, 733, 75. Yamada, J.; Matsuda, H. J. Elecfrfflnal. Chem. Inferfaclal Elecfrochem. 1971, 3 0 , 261.
Yamada, J.; Matsuda, H. J . Electroanel. Chem. Interfacial Elecfrochem. 1973, 4 4 , 189. Coeuret, F. Chem. Eng. Sci. 1975, 30, 1257. Dalhuljsen. A. J.; Van Der Meer, Th. H.; w n d o o r n , C. J.; Hoogvllet, J. G.; Van Bennekom, W. P. J. Elecfroanal. Chem. Interfacial Nectrochem. 1985, 782, 295. Aibery, W. J. J. Nectroanal. Chem. Interfacial Electrochem. 1983, 744, 105. Olson, J. B. Ph.D. Thesis, University of Colorado, 1987. Sharp, M.; Petersson, M.; Edstrom, K. J. Elecfroanal. Chem. Interfacia/~techochem.1980. 109, 271. Levlch, V. G. Phy.slochemical Hydrodynamics ; Prentice-Hall: Englewood cws, NJ. 1962. Bard, A. J.; Faulkner, L. R. Necfrochemlcal Methods; Why: New York, 1980;Chapter 3.
John B. Olson Department of Chemistry Texas A&M University College Station, Texas 77843
Carl A. Koval* Department of Chemistry and Biochemistry Campus Box 215 University of Colorado Boulder, Colorado 80309 RECEIVED for review December 1, 1986. Resubmitted April 6, 1987. Accepted September 28,1987. This work was supported by the U.S. Department of Energy (Division of Chemical Sciences) Grant No. DE-FG02-84ER13247.
CORRECTION Self-Training, Self-optimizing Expert System for Interpretation of the Infrared Spectra of Environmental Mixtures Li-Shi Ying, Steven P. Levine, Sterling A. Tomellini, and Stephen R. Lowry (Anal. Chem. 1987,59, 2197-2203). On p 2199, Figure 3 should be changed to the following:
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