678
Anal. Chem. 1984, 56,678-681
CONCLUSIONS On the basis of this work, the RVC coulometric detector is very competitive with state-of-the-art amperometric detectors available for FIA and HPLC. The large mass transfer coefficient, coupled with complete electrolysis and the nature of the decrease in concentration along the length of the electrode in the direction of flow, is such that the response volume of the electrode is only 5 to 6% of its void volume. Operationally, this RVC detector is equivalent to a 1 0 - ~ L UV cell in terms of band broadening, and the response time is quite adequate for FIA work. Detection limits are also as good as most amperometric detectors suggesting that the larger noise associated with the large surface area of the RVC electrode is compensated by the larger Faradaic signal obtained by complete conversion. There are other advantages of the RVC detector. The pressure drop across the electrode is negligible. The electrode surface requires no mechanical preparation such as polishing prior to being placed in service. The electrodes are readily replaced, and the electrode material is inexpensive. Like other coulometric detectors, the flow dependence of the measured current can be eliminated by measuring the current-time integral. Because of larger currents, hydrodynamic voltammograms of rather dilute solutions are easily obtained by the scanning technique. This provides a rapid and convenient way to select the control potential. Registry No. Carbon, 7440-44-0.
LITERATURE CITED (1) Newman, J. S.;Tledemann, W. I n "Advances in Electrochemistry and Electrochemical Englneerlng, Vol. 1 l", Tobias, C. W., Gerlscher, H., Eds.; Wlley-Interscience: New York, 1977;pp 355-438. (2) Siode, R. E.; Keating, K. B. I n "Electroanalytical Chemistry, Vol. 12"; Bard, A. J. Ed.; Marcel Dekker: New York, 1982;pp 19-27.
(3) Johnson, D. C.; Larochelle, J. Talanfa 1973,2 0 , 959. (4) Stullk, K.; Pacakova, V. J. Necfroanal. Chem. 1981, 129, 1. (5) Brunt, K. Trace Anal. 1981, 1, 47. (6) Strohl, A. N.; Curran, D. J. Anal. Chem. 1979,5 1 , 353. (7) Ruckl, R. J. Talanta 1980,2 7 , 147. (8) Weber, S.(3.; Purdy, W. C. Ind. Eng. Chem. Prod. Res. Dev. 1981, 2 0 , 593. (9) Wang, J. Electrochlm. Acta 1981,2 6 , 1721. (10) Brooks, M. A.; Hackman, M. R.; Mazzo, D. J. J . Chromatogr. 1981, 210, 531. (11) Klssinger, P. T.; Bruntlett, C. S.; Davis, G. C.; Felice, L. J.; Rlgglns, R. M.; Shoup, R. E. Clln. Chem. (Winston-Salem, N . C . ) 1977,23, 1449. (12) Van Zee, J.; Newman, J. J. Nectrochem. SOC. 1977, 724, 708. (13) Adams, R. N. "Electrochemistry at Solid Electrodes"; Marcel Dekker: New York, 1989;pp 365-367. (14)Hawley, M. D.; Tatawawada, S. V.; Plekarskl, S.;Adams, R. N. J . Am. Chem SOC. 1967,8Q,447. (15) Sternson, A. W.; McCreedy, R.; Felnberg, 6.; Adams, R. N. J . Nectroanal. Chem. 1973,46, 313. (16) Perone, S.: Kretlow, W. J. Anal. Chem. 1986,38, 1760. (17) Rulz, J. J.; Aldaz, A.; Dominguez, M. Can. J . Chem. 1977,5 5 , 2799. (18) Anderson, J. L.; Welsshaar, D. E.; Tallman, D. E. Anal. Chem. 1981, 53, 908. (19) Sloda, R. E. Nectrochlm. Acta 1988, 13, 375. (20) Lankelma, J.; Poppe, H. J. Chromatogr. 1978, 125, 375. (21) Stullk, K.; Pacakova, V. J . Chromatogr. 1981,208, 289. (22) Johnson, E. L.; Stevenson, R. "Basic Liquid Chromatography"; Varlan Associates: Palo Alto, CA, 1978;p 274. (23) Poppe, H. Anal. Chlm. Acta 1980, 774, 59. (24) Strohl, A. N.; Curran, D. J. Anal. Chem. 1979,5 1 , 1050. (25) Takata, Y.; Muto, G. Anal. Chem. 1973,45, 1864. (26) Hanekamp, H. 6.; Bos, P.; Brlnkman, U. A. Th.; Frie, R. W. Fresenlus' 2.Anal. Chem. 1979,297, 404. (27) Blaedei, W. J.; Wang, J. Anal. Chem. 1979,5 1 , 799. (28) Chu, A. K. P.; Flelschmann, M.; Hills, 0.J. J . Appl. Necfrochem. 1974,4 , 323.
.
RECEIVED for review August 25, 1983. Accepted December 20, 1983. This paper was taken in part from the Ph.D. dissertation of T. P. Tougas and was presented in part at the 186th National Meeting of the American Chemical Society, Washington, DC, Aug 1983.
Background Compensation in Fast Scan Square Wave Voltammetry and Other Pulse Techniques at the Dropping Mercury Electrode Adina Lavy-Feder and Chaim Yarnitzky*
Department of Chemistry, Technion-Israel Institute of Technology, Haifa, Israel 32000
The calculatlon of the charge Injected Into the double layer, after a potentlal Is applied to the worklng electrode, has led to the lntroductlon of a new background compensatlon method. Thls method Is useful for all pulse polarographlc technlques such as fast scan square wave voltammetry and pulse polarography, normal or dlfferentlal, when the common dropplng mercury electrode Is used. The deslgn of slmple electronlc unlts whlch can be attached to a one drop square wave analyzer (ODSWA) and the PAR Model 174 polarographic analyzer Is descrlbed. The performance of the unlts Is demonstrated by recording voltammograms at low current sensltlvltles. The Influence of the compensation on the Faradalc peak currents Is also glven.
The use of polarographic instruments in analytical and research laboratories has increased considerably in the last
decade. However, problems concerning electronic noise and background current due to drop growth still exist and limit the use of pulse voltammetric techniques to concentrations above lo-' M. In normal or reverse pulse methods the situation is even worse due to the gradually increasing potential steps applied. The use of a simple averager for signal to noise enhancement has been demonstrated (1). While the averager was shown to yield distinctive peaks, measurable with higher accuracy, the sensitivity of the method remains unaffected; these peaks are superimposed on an unconstant, nonlinear background current, which must be compensated before averaging can take place. Ultimate compensation may be achieved by adopting the alternate drop method (2) which, unfortuantely, suffers from three inherent disadvantages: (a) the recording time is doubled; (b) the electronic noise is increased; (c) the peak current is decreased due to the subtraction of the Faradaic current measured in the staircase mode from the current
0003-2700/84/0356-0678$01.50/00 1984 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984
i
i
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i
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1
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Flgure 1. Wave form and current measurement tlmes in square wave
W.E.*
voltammetry.
INPUT
measured in the square wave mode. Background compensation in normal or reverse pulse polarography may also be achieved by using the double measurement mode (mode D, ref 3). Again, the electronic noise is increased and the Faradaic signal is decreased. The use of computers has also been demonstrated (4). Unfortunately, this approach is too costly. The static mercury drop electrode (SMDE; Model 303, EG and G Princeton Applied Research, Princeton, NJ) offers the ultimate solutions to the problem; it is however still too expensive for routine work. The compensation method described in the present study is based on the measurement of the charge injected into the double layer after the application of the potential step. This charge is stored on a capacitor and used later for the compensation, without resorting to the use of alternate drops; the reduction of peak current is also negligible.
--
IK
Figure 2. Electronic diagram of the compensator attached to the PAR analyzer Model 174. C, and R, are the compensating capacitor and resistor, respectively. C,-3 are electronic switches (1/4 SCL 4016 or 113 SCL 4053 each). Current range resistor is 1 M.
THEORY Consider a simple electrochemical cell in which a dropping mercury electrode (DME) and a reference electrode are connected to a pulse generator. The upper and lower potential limits of the pulse are E2 and El, respectively (Figure 1). It is assumed that the RcCdl constant of the cell (R, is cell resistance and Cdl is the absolute double layer capacitance) is much shorter than the pulse period. As the potential changes, the fast charging current decreases rapidly to the low charging current accompanying the growth of the mercury drop. Assume that the current is measured just prior to and a short time, t, after pulse application (arrows in Figure 1). Let iE, and iE, be the slow charging currents flowing through the cell at E , and Et, respectively. The recorded charging current is Ai = iE2 - iEl. The absolute charge at any time and potential is qMEA = q ~ ~ K 7 ~ 1 ~
(1)
where q M E is the electrode charge per unit area at the potential
E, A is the area of the drop, and K equals 0.8515m2J3 cm28 1 3 , where m is the mercury flow rate. The charging current is
and the recorded charging current is
(the time difference between the two measurements is negligible). In effect, the last equation explains the shape of the voltammograms recorded with normal or reversed pulse; the charging current at the fixed potential is constant (sometimes it is not measured at all) and the current difference recorded resembles the charge-potential curve. With the ODSWA (I), however, a large background current flows at the beginning of each scan, decaying significantly as the drop grows. To compensate for this current, a capacitor, C,, inserted in the I to E converter, accumulates the high fast charging
Flgure 3. Timing diagram of the electronic gates of the compensator shown in Figure 2.
IOK
... born
I t o E CONVERTER SBH INVERTER Flgure 4. Electronic diagram of the background current compensator used in conjunction with the ODSWA: C,, compensating capacitor; GI-,, electronic gates (1/4 SCL 4016 each); R,, input resistor; R,, feedback resistor; RM, current measuring resistor.
current, by means of electronic gates (Figure 2). A resistor, and an electronic switch connects the capacitor to the ground and to the working electrode. The timing diagram of the gates is given in Figure 3. As will be shown below, the current flowing through this resistor, compensates for the slow charging current. At the beginning, G1goes “high” and the capacitor C, is shortened. Then, G2goes “high” and the capacitor is connected in parallel to the measuring resistor R,. The pulse is applied to the cell and the charge is stored on the capacitor. Some of the charge is lost due to current flow through R,. A t last, G3 goes “high” and the compensating current is measured along with the Faradaic and the slow charging current. Additional circuitry is needed for use with the compensating system in conjunction with the ODSWA (Figure 4). The potential of the capacitor, V , is stored by S&H amplifier and R, is replaced by a resistor which is propor-
R,,,,
880
ANALYTICAL CHEMISTRY, VOL. 56, NO. 4,APRIL 1984
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-400
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Figure 5. Timing diagram of the electronic gates of the compensator 05
shown in Figure 4.
tional to drop life. The timing diagram of the gates is shown in Figure 5. Assuming that the fast charging current is integrated and stored in the compensating capacitor, C,, the potential V, across the capacitor is
v, = K T 2 / 3 ( q ~-EqME1)/c, z and the current flowing through R,, 2) is closed is:
15
IO
vs S.CE
-Volts
Flgure 6. Normal pulse polarogram of M Zn(I1) in 0.1 M KCI: scan rate, 2 mV s-'; flow rate, 0.991 mg s-'; (a) uncompensated, (b) fully compensate, (c) 20 % overcompensated.
(4)
when the gate G3 (Figure
For complete compensation this current should be equal to Ai calculated previously
K r 2 / 3 ( q ~-EqME1)/C&Om= z 2/3KT-'/3(q~Ez - qME1) ( 6 ) or In other words, the R,,C, constant must be equal to drop age multiplied by 3/2, When using techniques such as normal (and reverse) or differential pulse, the compensating capacitor is constant (1pF) and the resistor is equal to 3 / 2 ~( r and R are expressed in seconds and megohms, respectively). In fast scan voltammetry,R,, must follow drop age. This is achieved by using a counter which is advanced by the square wave generator and reset to zero at the drop fall time. The counter is connected to a simple eight-bit DAC, with the ladder resistors of the DAC replacing the feedback resistor of the inverter shown in Figure 4. The input resistance is also roplaced so that R, = Ri, at 1 s, and R,C, = 1.5 s (5). As the counter counts up, the feedback resistance decreases; the net compensating current decreases as expected. Some Faradaic current, however, stored at the same time, may interfere later by causing some decrease in the recorded peaks. This source of interference must be noted and the extent to which it affects results calculated. The diffusion controlled current is given by
iF = KFt-1/2Kr2/3
=~KF~'/~KT~/~
(9)
I
04
-Volts
"
"
'
10
08
06
vs S C E
Flgure 7. Normal pulse polarograms of 4 X M and 1.4 X lo-' M C q I I ) in 0.1 M KCI: (a) uncompensated: (b) 20% overcompensated. All conditions are given In Flgure 6.
0.5
0.
(8)
where KF includes all parameters controlling the pulse of square wave current. If this current is integrated for a time, t , and sampled after, the charge q F stored in the capacitor c, is QF
1 2 0 0 L " ' ' 00 02
1.5
1.0
-Volts
VS.
S.C.E
M Zn(1I) in 0.1 M KCI; Figure 8. Differential pulse polarograms of amplitude, 50 mV. All other conditions are given in Figure 6.
for t = 0.01 s and
r = 1 s,
DF is ~ 0 . 9 9 .
EXPERIMENTAL SECTION
Normal and differential pulse polarograms were recorded with the PAR 174 polarographic analyzer. Fast scan square wave voltrammograms were recorded with the ODSWA ( i ) ;the units above had been added to the instruments. All exicomF= ~ K F ~ ~ / ~ K ~ 4/3KFt1/2K/r1/3 ~ / ~ / C & !(10) ~ ~ ~ described = periments were carried out at 25 i 1 "C in a common threecompartment cell. Deaeration was carried out by bubbling niand the diminution factor (2) is trogen through the solution in the usual manner. KCl (0.1 M) DF = (iF - icom)/iF= ( r - 4 t / 3 ) / r (11) solutions were prepared with triple distilled water and Analar The compensating Faradaic current flowing through R,,, is given by
ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984
1
.
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01
,
'
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05
03
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,
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Figure 9. Polarograms of lo-' M Pb(I1) and 5 X lo-' M Zn(I1) in 0.1 M KCI: step level, 8 mV; delay, 1 s; square wave amplitude, 50 mV p-p; flow rate, 0.374 mg s-'; (a) uncompensated, (b) compensated.
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Figure 10. The dependence of the absolute double layer capacltance of the mercury drop on potential; delay times, number on curves. All other condltions are given in Figure 9. Dashed lines are based on Grahame's data.
grade KC1. The reference was a SCE.
RESULTS AND DISCUSSION Figure 6 shows uncompensated (a) fully compensated (b), and 20% overcompensated (c) normal pulse polarogram. It can be seen that some residual current contributed by the "capillary noise" remains and is recorded even when the
681
charging current is fully compensated; similar results were reported with the alternate-drop technique (2);overcompensation may yield a flat base line (from zero to -1.0 V vs. SCE). Figure 7 shows normal pulse polarograms of (0.4 and 1.4) X lo* M of Cd2+in 0.1 M KC1. At more negative potentials the residual current increases very rapidly and can be compensated only partially. Figure 8 demonstrates the compensation effect on differential pulse base line. Again, a full compensation does not yield a flat base line. It was also expected and found that when the drop life is increased, the compensation is more effective and vice versa. Square wave polarograms are shown in Figure 9. The results obtained fully substantiate the effectiveness of the background current compensation. It should be noted that compensation works most effectively for the normal pulse method; it is less significant with other methods and it does not affect peak height. As a rule, the change is within the limit of the experimental error of the instrument. Differential double layer capacitance could be also measured by the unit connected to the ODSWA. The recorder was connected to the output of the S & H amplifier. This output is proportional to the difference in metal charge q M E z - q ~ (eq~ 3).l Since E2 - El is constant (50 mV), the output divided by 0.050 V must be proportional to the absolute double layer capacitance. The dependence of the double layer capacitance on potential and time is shown in Figure 10. The dashed line is the differential double layer capacitance as obtained by multiplying Grahame's data (6) by the drop area. Registry No. Hg, 7439-97-6.
LITERATURE CITED Yarnltzky, Ch.; Osteryoung, R. A.; Osteryoung, J. Anal. Chem. 1980, 52, 1174-1170. Turner, J. A.; Osteryoung, R. A. Anal. Chem. 1978, 50, 1496-1500. Klein, N.; Yarnitzky, Ch. Elecfroanal. Chem. 1975, 6 1 , 1-9. Bond, A. M.; Garbaric, B. S. Anal. Chem. 1879, 51, 337-341. "Intersll Data Book"; Intersll Inc.: 1981; pp 4-77. Grahame, D. C. J . Am. Chem. SOC. 1949, 7 1 , 2975-2978.
RECEIVED for review March 23,1982. Resubmitted June 27, 1983. Accepted December 6,1983. This work was supported by the Fund for Application of Industrial Research at Universities No. 061-031 and the Fund for the Promotion of Research at the Technion No. 063-056.
