AMI. Chem. 1003, 85, 237-237
231
Effect of Rotation Rate on the Response at PoIy(c hIorotrif IuoroethyIene) Composite Disk EIect rodes Joseph E. Vitt and Dennis C. Johnson* Department of Chemistry and Ames Laboratory, Iowa State University, Ames, Iowa 50011
Dennis E. Tallman Department of Chemistry, North Dakota State University, Fargo, North Dakota 58105
Microelectrode arrays were operated as rotated dlsk electrodes (RDEs) to examlne the effects of convectlon on their voltammetrlcresponse. The mlcroelectrodearray RDEswere Kelgraf (graphhe and Kei-F) and Au/Kelgraf (Au, graphlte, and KeCF) composHe electrodes. As expected for microelectrode arrays, a slgnlfkant enhancement In current dendty was observedin comparison to solld RDEs. For example, the current density for the oxidationof hydrazine at a rotationrate of 10.5 rad 8-' was 13 tlmes larger at a 2 % Au/lO% Kelgraf RDE than at a solld Au RDE. The relative current denslty (J) was defined as the ratio of the current dendty at a microelectrode array RDE in comparison to that at a solld RDE. The largest values of J were obtained for the lowest values of rotation rate and for ca. 2-3% active area; however, the values of J were dependent on the cholce of the test reaction.
solutions; however, their response under hydrodynamic conditions is not as well characterized. Theoretical descriptions and experimental results have been presented for the amperometric response of various microelectrode array detectors in flow ce11s,10J1~20-24 and the higher current density is attributed to (1) replenishment of the depletion zone for downstream electrode sites and/or (2) radial mass transport, i.e., the "edge effect". Microelectrode arrays also are e x p d to exhibit higher current densities under other hydrodynamic conditions, e.g., when operated as rotated disk electrodes (RDEs). Here, we report the application of Kel-F composite electrodes as RDEs, and the discussion of the voltammetric response is based on considerationsoriginally developed for partially blocked (fouled) RDEs. The amperometric response at microelectrode arrays can be characterized by the ratio ( p ) of the current observed at the microelectrode array to the current observed at a solid electrodewithan equalgeometricarea (eq 1). This parameter
INTRODUCTION Poly(chlorotrifluoroethy1ene)(Kel-F)composite electrodes function as microelectrode arrays' and have been used advantageously for v ~ l t a m m e t r y ~and - ~ as electrochemical detectors for HPLC and FIA."l0 The primary interest in microelectrode arrays results from the enhancement of the current response as compared with solid electrodes;however, substantial interest also has been focused on the possibility of decreased sensitivitytovariations in convection," i.e., pump oscillations. The theoretical description of the response at microelectrode arrays is relatively well developed for v ~ l t a m m e t r y *and ~ - ~chronoamperometry1~19 ~ in stationary
The inactive fraction of a partially blocked electrode was originally defiied as a blocking factor ( I ~ ) , ~ ' - ~ O therefore Aaetive
Corresponding author. (1)Tallman, D. E.;Petersen, S. L. Electroanalysis 1990,2,499-510. (2)Anderson. J. E.:Tallman. D. E.: Chesnev, D. J.; Anderson, J. L. Anal. Chem. 1978,50,1051-1056. (3)Petersen, S.L.;Tallman, D. E. Anal. Chem. 1988,60,82-85. (4)Petersen, S.L.;Tallman, D. E. Anal. Chem. 1990,62,459-465. (5)Anderson, J. L.;Chesney, D. J. Anal. Chem. 1980,52,2156-2161. (6)Chesney, D. J.; Anderson, J. L.; Weissharr, D. E.; Tallman, D. E. Anal. Chim. Acta 1981,124,321-331. (7)Weissharr, D. E.;Tallman, D. E.; Anderson, J. L. Anl. Chem. 1981, 53,1809-1813. (8)Tallman,D. E.;Weissharr,D. E. J.Liq. Chrornatogr. 1983,6,21572172. (9)Anderson, J. L.; Whiten,K. K.;Brewster, J. D.; Ou,T.-Y.;Nonidez, W. K. Anal. Chem. 1985,57,1366-1373. (10)Ou, T.-Y.; Anderson, J. L. Anal. Chem. 1991,63,1651-1658. (11)Caudill, W. L.; Howell, J. 0.; Wightman,R. M. Anal. Chem. 1982, 54,2532-2535. (12)Gueshi, T.; Tokuda, K.; Matauda, H. J. Electroanal. Chem. Interfacial Electrochem. 1979,101, 29-38. (13)Aoki, K.; Akimoto, K.; Tokuda, K.; Matauda, H.; Osteryoung, J. J. Electroanul. Chem. Interfacial Electrochem. 1984,171,219-230. (14)Sleszynski, N.; Osteryoung, J.; Carter,M. Anal. Chem. 1984,56, 130-135. (15)Cheng, I. F.; Whiteley, L. D.; Martin, C. R. Anal. Chem. 1989,61, 762-766.
(16)Gueshi, T.;Tokuda, K.; Matauda, H. J. Electround Chem. Interfacial Electrochem. 1978,89,247-260. (17)Weissharr, D. E.;Tallman, D. E. Anal. Chem. 1983,55, 11461151. (18)Shoup, D.; Szabo, A. J. Electroanal. Chem. Interfacial Electrochem. 1984,160,19-26. (19)Scharifker, B. R.J.Electroanal. Chem. InterfacialElectrochem. 1988,240,61-76. (20)Moldoveanu, S.;Anderson, J. L. J.Electroanal. Chem.Interfacial Electrochem. 1985,185,239-252. (21)Fosdick, L. E.;Anderson, J. L.; Baginski, T. A.; Jaeger, R. C. Anal. Chem. 1986,58,2750-2756. (22)Fosdick, L. E.;Anderson, J. L. Anal. Chem. 1986,58,2481-2485. (23)Cope, D. K.; Tallman, D. E. J . Electroanul. Chem. Interfacial Electrochem. 1986,205,101-123. (24)Magee, L. J.; Osteryoung, J. Anal. Chem. 1990,62,2625-2631. (25)Landsberg, R.; Thiele, R. Electrochim. Acta 1966,11,1243-1259. (26) Scheller,F.;Muller, S.;Landsberg,R.;Spitzer,H.-J.J.Electroanal. Chem. Interfacial Electrochem. 1968,19,187-198. (27)Scheller, F.; Landsberg, R.; Wolf, H. Electrochim. Acta 1970,15, 525-531. (28)Filinovsky, V. Yu. Electrochim. Acta 1980,25,309-314. (29)Levart, E.J. Electroanal. Chem. Interfacial Electrochem. 198S, 187,247-263. (30)Contamin, 0.;Levart, E. J . Electroanal. Chem. Interfacial Electrochem. 1982,136,259-270.
0003-2700/93/0365-023 1$04.00/0
was originally introduced to describe the amperometric response of partially blocked electrodes and, therefore, is described as an attenuation The relative current density (J)is also useful in describing microelectrode arrays and is defiied here as the ratio of the current density observed at a microelectrode array to the current density observed at a solid electrode (eq 2). J = (i/AactiVe)~.y/(i/Aactive)soiid = P/(l - 0)
0 1993 American Chernlcal Society
(2)
232
ANALYTICAL CHEMISTRY, VOL. 65, NO. 3, FEBRUARY 1, 1993
= (1- &A,,,. For situations in which the background current is proportional to Aactive,the signal-to-background current density is enhanced by the factor J. The effect of rotation rate on the voltammetric response of microelectrode arrays is easily predicted for two limiting conditions. For low-density microelectrode arrays (0 1) with active sites which behave as isolated microelectrodes, the current is expected to be independent of the rotation rate. However, since the transport-limited current at a solid RDE is proportional to the square root of rotation rate31( u ~ / ~ ) , p is expected to be proportional to d 2For . high density arrays (0 0), the diffusion zones of adjacent active sites can overlap extensively. In the limit of total overlap of adjacent diffusion zones, the microelectrode array has a virtually uniform diffusion layer thickness across the face of the electrode. Under these conditions, the current is expected to be proportional to d 2 and p is expected to be independent of rotation rate. This behavior has been predicted by models which neglect the effect of radial mass For microelectrode arrays of intermediate density (0 250 "C as quickly as possible (ca. 4000 psi) was applied continuouslywhile the mixture cooled to
