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(IO) Kalvoda, R.;Anstine, W.; Heyrovsky, M. Anal. Chlm. Acta 1970, 50, 93-102. (11) Stankovich, Marian T.; Bard, Allen J. J. Electroanal. Chem. 1977, 75, 487-505.
RECEIVED for review June 9, 1980. Resubmitted January 23,
1981. Accepted April 17, 1981. A M Y . thanks Rutgers Research BiomedicalResearch Grants, and the National Institutes of Health (Grant No. GM 28125-01) for research support. R.S.L. thanks Rutgers University for the award of a University Fellowship.
Stopped-Rotat ion Volta mmet ry Joseph Wang Department of Chemistry, New Mexico State Universiiy, Las Cruces, New Mexico 88003
The technique of stopped-rotation voltammetry at a rotated disk electrode, whlch measures the current difference with the rotation speed switched on and off, Is described. Sensitlvlty, preclslon, mass-transport properties, and linearity of response are reported. Well-defined current-potential curves are obtalnable and compared with those obtalned by dlfferentlal pulse voltammetry. An asymmetric hydrodynamic modulatlon waveform and a relatively rapld stopped-rotation procedure (wlthout the achievement of steady states) are employed to shorten the analytlcal cycle. Due to its inherent sensitivlty and slmpllclty, the technique seems well suited for the measurement of low concentrations of electroactlve materials.
In recent years, the use of solid electrodes for electroanalytical purposes has gained popularity, one of the primary reasons being their applicability to anodic oxidations. However, practical difficulties have hampered the attainment of precise quantitative data employing classical or potential pulse voltammetry at solid electrodes. Hydrodynamic modulation at solid electrodes has been shown to be a feasible technique for obtaining reproducible current-voltage data a t low concentrations of electroactive species. Such modulation involves measurement of the current difference between two rates of convective transport, resulting with efficient discrimination against the nonconvective background current. Miller, Bruckenstein, and co-workers have done a great deal of work in developing the sinusoidal modulation of a disk electrode's rotation speed about a center value (1, 2). The analytical usefulness of this approach has been exploited for obtaining voltammograms a t submicromolar concentrations. The sinusoidal component of the current, which follows directly from the Levich equation i s given by
where n is the number of electrons transferred per molecule, F is the Faraday, A is the disk area, D is diffusion coefficient, v is kinematic viscosity, Cb is the bulk concentration of the electroactive species, Am is the peak to peak amplitude of the change in the rotation speed, and wo is the center value of the rotation speed. Aw is always much smaller than 00 (usually about 1-10% of coo), resulting with exploitation of only a very small portion of the steady-state analytical current of interest. In order to decrease the attenuation of the steady-state analytical response (i.e., increased current difference), Blaedel and Engstrom have suggested the pulsed-rotation voltammetry
(PRV) in which the rotation speed of a rotated disk electrode is switched between two values (3)
Ai = 0.62nFAD2/3v-1/6Cb(wH1/2 - wL1l2)
(2)
The subscripts H and L designate the high and the low rotation speeds, respectively. Since low and high rotation speeds of 500 and 1500 rpm, respectively, are usually selected as the two current states, more than half the steady-state analytical current is not exploited for the quantitation (eq 2). Further increased current difference has been achieved by incorporating the PRV approach with the high surface area of a rotated porous disk electrode (4). Analogous procedures have been developed for electrochemical flow detectors, with the solution flow rate being pulsed to the surface of a stationary electrode (5,6),or with hydrodynamically modulated rotating disk electrode in a flow cell (7). The purpose of the following work is to demonstrate the characteristics of stopped-rotation voltammetry (SRV), in which differential current measurements are made between zero and high rotation speeds. The main advantages of this procedure are its inherent sensitivity (Le,, exploitation of all the rotation-dependent analytical signal of interest for the quantitation (100% modulation)) and the simplicity of operation (turning on and off the rotational speed). Since the lower current (at w = 0) is not zero but a very small value, due to diffusion (linear and radial) and natural convection (8),an expression for the on-off current is not obtained simply by substituting wL = 0 into eq 2; instead, and because of the theoretical limitations to evaluate the zero rotation speed current (8), Ai may be described by the following relationship:
Ai = 0.62nFAD2/3v~1~6Cbw1/2 - L* = O
(3)
Compared to previously reported hydrodynamic modulated rotating disk electrode approaches (discussed above) the SRV does not require any programming circuity for changing the rotation speed, and thus it can be incorporated with every rotating disk assembly. These advantages and the various characteristics of the SRV are elucidated by application to the measurement of micro- and submicromolar concentrations of ascorbic acid and ferrocyanide.
EXPERIMENTAL SECTION Apparatus. The rotating electrode assembly (Model PIR, Pine Instruments Co., Grove City, PA) with a 0.75-cm diameter glassy carbon disk electrode (Model DDI 15, Pine Instruments Co.) was used in conjuction with a cell of 200-mL capacity made from Pyrex glass. The salt bridges of the reference electrode (Ag/AgCl, Model RE-1, Bioanalytical Systems Inc., West Lafayette, IN) and of the counterelectrode (a Pt coil immersed in 0.1 M phosphate buffer) join the cell through two holes in its Plexiglas cover. The working
0003-2700/81/0353-1528$01.25/00 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981
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Flgure 1. Stopped-rotation response for the oxidation of 3 p M K,Fe(CN),: rotation speeds, 1600 (on) and 0 (off) rpm; cycling times (A) 15 (on) and 30 (off) s; (B)5 (on) and 5 (off) s; (C)3 (On) and 3 (off) s; applied potential, 4-0.7V; supporting electrolyte, 0.1 M phosphate buffer (pH 7.4).
electrode was initially polished with a O.l-wm alumina slurry, until a mirrorlike finish was obtained. Current-voltage data were recorded with a Princeton Applied Research Model 364 polarographic analyzer and a Houston Omniscribe strip-chart recorder. Reagents. All solutions were prepared from deionized water and analytical grade chemicals. The supporting electrolyte was 0.1 M phosphate buffer i[pH 7.4), prepared from a 1:4 mixture of KHzPOl and K2HPOI. Millimolar stock solutions of K4Fe(CN),-3H20 and ascorbic acid were made up fresh each day. Aliquots of the stock solutions were added to the supporting electrolyte to give the desired concentration. Procedure. A 200-mL aliquot of the buffer was pipetted into the cell and pretreatment of the electrode begun. This consisted of cycling the applied potential between $1.0 V and -1.0 V for 10 min, allowing 2 min at each potential. Following pretreatment, the desired working potential in the mass-transport limited (plateau) region was applied. Stopped-rotation measurements were performed only a short time (15 s to 2 min) after the working potential was applied. These were achieved by switching manudy the rotational speed on and off, employing different cycling periods (as described later in thls paper). SRV measurements of the analyte solution followed those of the background solution. All data presented were corrected for background. The stopped-rotationvciltammograms were developed pointwise by making 100-mV changes in applied potential and waiting about 30 s before applying the rotation pulse. Between analyses, the working electrode was stlored in the empty cell.
RESULTS AND DISCUSSION The Characteristics of the Stopped-Rotation Current Amplitude. Stopped-rotation current-time responses for 3 pM ferrocyanide, employing different cycling times, are shown in Figure 1. The stopped-rotation response times are around 3 s (on) and 25 s (off) (Figure 1A). For this reason, and in order to shorten the analytical cycle while still achieving the two steady-states, asymrnetrical modulation waveforms (with the rotation “on9’times shorter than the “off’ times) have been employed throughout this study. Previously reported hydrodynamic modulation procedures, which based on pulsing the rotation speed or the flow rate (e.g., 3 , 5 ) have used symmetrical square-wave hyclrodynamic pulsings, with equal times for the high and low convection rates (Le-,without trying to reduce the length of the analytical cycle). Another way to shorten the analytical cycle is not to wait for the zero current steady state (Figure lB,C). As the frequency is increased the decay to the zero current state is cut off, and the stoppedrotation current amplitude decreases. It is noteworthy that the 4.5- and 7.5-fold reductions in the cycling period results in current diminutions of only 16% and 20%, respectively. The change in current amplitude as a function of the cycling period reflects the time-dependent fall-off in the concentration gradient a t the electrode surface (i.e.. Cottrell current transient-i t-’/’) as the rotation is stopped. Mathematical
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APPLIED POTENTIAL, VOLTS
Figure 2. Stopped-rotation and differential pulse voltammograms of 6.25 pM ascorbic acid in 0.1 M phosphate buffer: SRV conditions, 1600 rpm for 15 s, 0 rpm for 30 s; differentlal pulse conditions, scan rate, 5 mVls; amplitude, 50 mV; rotation speed, 1600 rpm. The dotted lines represent the blank solution.
analysis of the current decay as a function of time results with it1I2 values of 0.346hio and 0.357Aio for the 3 and 5 s nonsteady-state operations, respectively (Aio refers to the steady-state current amplitude (Figure 1A)). The dependence of the stopped-rotation current amplitude upon ferrocyanide concentration has been studied for the two cycling periods, shown in Figure 1A,B. Linear correlations between the current amplitude and the concentration are obtained. With the steady-state stopped-rotation operation the sensitivity is 167.2 nA/pM, while for the non-steady-state operation (5 s (on) and 5 s (off)) it is 140 nA/pM. When these data are normalized to the electrode surface area, values of 379 and 317 nA/(pM cm2) are obtained, respectively. A lower sensitivity value of 222 nA/(pM cm2) was calculated from data reported for PRV between 500 and 1500 rprn (3). The linear correlation between the steady-state current amplitude and the concentration indicates that io=o (see eq 3) is a concentration-dependent component (asexpected for diffusion and convective processes; a flow rate independent component was found to be linearly dependent on the analyte concentration in the case of a flow-through tubular electrode (5)). The dependence of the steady-state stopped rotation current amplitude upon the difference in rotation speeds was evaluated by using 5.0 pM ascorbic acid, over Aw range of A400-A2500 rpm (conditions: applied potential, +0.65 V; 15 (on) and 35 (off) 9). A plot of Ai vs. A d 2 (not shown) was linear, with zero intercept and a slope of 39.5 nA/rpm1lZ. When replotted on a log-log scale these data gave a straight line, with a slope of 0.51. This indicates, that under the experimental conditions of this work, the effect of the analyte flux at zero rotation speed is very small, and thus the current amplitude (eq 3) is approaching the Levich equation. Analytical Applications. Stopped-rotation and differential-pulse voltammograms, obtained for the oxidation of 6.25 pM ascorbic acid under identical experimental conditions are compared in Figure 2. Such an experimental comparison between an hydrodynamic modulation procedure and a potential pulse technique has not been described yet. The differential pulse analytical response at this concentration level is almost completely obscured by the large (and irreproducible)
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Figure 3. Current-time trace for 7 pM ascorbic acid: applied potential, +1.2 V; cycling times, 15 (on) and 15 (off) s; rotation speeds and
supporting electrolyte, as in Figure 1. i l i
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Stopped-rotation response for 0.6 pM ascorbic acid: applied potential, +0.85 V; conditions as in Figure 1A. Figure 4.
background transient currents, associated with the abrupt potential excitations. In contrast, the stopped-rotation voltammogram is characterized by its well-defined (and reproducible) wave and plateau regions. The SRV background (dotted line) is very low, indicating good correction for nonconvective currents and the absence of electroactive contaminants. The stopped-rotation half-wave potential is +0.05 V. Overall, the data of Figure 2 indicate the advantages of the hydrodynamic modulation procedure over the potential pulse technique for quantitative trace analysis using solid electrodes. It should be noted that the SRV data were taken in about 18 min, compared to 4 min for the differential-pulse voltammogram. The incorporation of the non-steady-state SRV (Figure 1B,C) with linear potential scan and its automated control would permit the achievement of rapid current-potential data; this possibility is now under investigation. The nonconvective nature of solvent decomposition permits measurements to be made over a wider voltage range than for
conventional scanning voltammetry. Figure 3 demonstrates the feasiblity of performing micromolar measurements at relatively extreme anodic potentials, well into oxygen generation; it shows the SRV response for 7 pM ascorbic acid obtained at applied potential of +1.2 V. The blank stopped-rotation current (shown on the left) is too low to be measured at this current scale, indicating minimized modulation response to oxygen generation. (The current spikes on both sides of the blank current amplitude are electrical noises due to rotation stepping.) Figure 4 illustrates the sensitivity and precision obtained employing the SRV technique. The five current amplitudes shown are part of a series of 12 successive measurements of 0.6 pM ascorbic acid in 0.1 M phosphate buffer for which a relative standard deviation of 2.71 % has been calculated. Different noise levels, of 1 2 and 4 nA, are observed for the rotation “on” and “off” current states, respectively. This is due to the carbon brush electrical contact of the Pine rotating electrode which is electrically noisier than the direct mercury contact used in various PRV studies ( 3 , 4 ) . If the limit of detection is taken to be equal to the noise level at the rotation “ony7operation, a value around 4 X M is obtained. The use of a rotated electrode with a mercury contact (for which the noise level is around 1nA ( 3 , 4 ) )would result with further lowering the detection limit. Overall, due to its significantly increased current amplitude (i.e., analytical signal) and using the same electrode assembly (i.e., same noise level), the SRV would have much lower detection limits than those of the sinusoidally modulated or the pulsed-rotated disk electrodes. The improved sensitivity and the simple instrumentation and procedure indicate considerable promise for SRV as a method of measuring low concentrations of electroactive materials,
LITERATURE CITED (1) Miller, B.; Bellavance, M. I.; Bruckensteln, S. Anal. Chem. 1972, 4 4 ,
1983-1992. (2) Miller, B.; Bruckensteln, S. Anal. Chem, 1974, 46, 2026-2033. (3) Blaedel, W. J.; Engstrom, R. C. Anal. Chem. 1978, 50, 476-479. (4) Blaedel, W. J.; Wang, J. Anal. Chem. 1980, 52, 76-80. (5) Blaedel, W. J.; Iverson, D. Anal. Chem. 1979, 49, 1563-1566. (6) Blaedel, W. J.; Wang, J. Anal. Chem. 1981, 53, 78-80. (7) Blaedel, W. J.; Wang, J. Anal. Chlm. Acta 1980, 116, 315-322. (8) Adams, R. N. “Electrochemistry at Solld Electrodes”; Marcel Dekker: New York, 1969;p 90.
RECEIVED for review January 26,1981. Accepted May 20,1981.
