(2) Berzins, T., Delahay, P., J . A m . Chem. Soc. 7 5 , 2486 (1953). (3) Delahay, P., “New Instrumental Methods in Electrochemistry,” Interscience, New York, 1954. (4) Karaoglanoff, Z., 2. Elektrochem. 12, 5 (1906). ( 5 ) King, R. M., Reilley, C. N., J . Electroanal. Chem. 1 , 435 (1959/60).
(6) Kouteckey, J., Cizik, J., Collection Czech. Chem. Commun. 22, 914 (1957). (7) Macero, D. J., Anderson, L. B., J . Electroanal. Chem. 6 , 221 (1963). (8) McMaaters, D. L., Schaap, W. B., Proc. Indiana Acad. Sei. 67, 117 (1957). (9) Reinmuth, W. H., ANAL.CHEM.32, 1514 (1960). (10) Sand, H. J. S., Phil. Mag. 1 , 45 (1901).
(11) Snzad, W. K., Remick, A. E., J . A m . Chem. SOC.79, 6121 (1957). (12) Testa. A. C.. Reinmuth. W. H.. ANAL.CHEM.33,’1324 (1961): RECEIVEDfor review April 22, 1964. Accepted December 16, 1964. The au-
thors thank the Department of Chemistry, Syracuse University, for financial assistance in the form of a fellowship awarded to one of us (L.B.A.).
A n Instrument for Cyclic Voltammetry CHARLES K. MANN Department of Chemistry, Florida State University, Tallahassee, Flu. 32306 An instrument has been designed for cyclic voltammetry utilizing electronic DC-coupled switching which will accept either fast or slow sweep signals. Provision is made for onehalf-, one-, two-cycle, and freerunning operation. Controlled electrode D.C. level is adjustable over approximately a five-volt range. It can b e made to sweep in either the positive or negative direction from the rest position. A switching arrangement is provided which, in conjunction with a staircase sweep, will eliminate charging current effects a t a mercury electrode. This allows determination of reversibly reduced substances to a sensitivity of approximately 1 O-’M.
A
for cyclic voltammetry which have been recently described utilize operational amplifier circuits for sweep potential control and for current amplification. Davolio, Guerzoni, and Papoff (6) have described a n instrument suitable for oscilloscopic readout. It imposes a free-running triangular signal which can be varied over a wide range of sweep rates with accurately determined adjustable D.C. level. Buck and Eldridge (4) have designed an instrument capable of producing free running triangle or step potential or current signals. Alden, Chambers, and hdams (1) have developed a n instrument which provides a slow triangular signal to be used with recorder readout. It can be used for either single or repetitive sweeps. It is the purpose of the present paper to describe a n instrument which, like those mentioned above, makes use of operational amplifiers, but which incorporates all-electronic D.C.-coupled switching which makes it quite versatile in operation. I n addition, a sampling device is described which permits exclusion of charging current when fast sweeps are used. This involves the use of a staircase sweep, with current measurements made after decay of charging current (9). 326
PPARATUS
ANALYTICAL CHEMISTRY
The function of the major components of the apparatus can be understood from the block diagram of Figure 1. The basic sweep signal is generated as a sawtooth wave form. This may be either a linear or staircase signal. Its period may be short as a few milliseconds for very high sweep rates, or may be as long as desired for slow sweep experiments. The basic positive going signal is inverted a t unity gain. Both positive and negative signals are presented to the sweep gate. The function of the sweep gate is to synthesize the desired sweep signal by correct choice of positive and negative signal segments. The following types of sweep are convenient and are provided: single sweep, positive or negative; cycle, positive then negative; one one cycle, negative then positive; two cycle, positive-negative sequence; two cycle, negative-positive sequence; continuous operation with either positive or negative initial sweeps. D.C. level adjustment of the controlled electrode potential is also accomplished in the sweep gate. Provision is made for adjustment over approximately a fivevolt range, making available the range of potentials, anodic and cathodic, which is of interest in electrochemical experiments. The function of the gate signal generator is to provide the command signals
to the two sides of the sweep gate. I n continuous operation, for example, this amounts to two square wave trains, 180’ out of phase. I n the gate signal generator, the decision is made about the duration of the sweep-Le., half cycle, one cycle, etc. The signal from the sweep gate is accepted by the potential control amplifier and is imposed on the electrolysis cell in a conventional operational amplifier arrangement. The sweep generator and gate produce a sweep signal which initially moves positive, then negative. This is the sequence usually desired for work with mercury electrodes, since the sweep is imposed on the counter electrode while the controlled electrode remains at virtual ground. This will be called the reducing mode of operation. For anodic voltammetry, it is convenient to have the controlled electrode start a t a negative rest potential and move positive. This sequence, termed the oxidizing mode, is conveniently obtained by including an optional unity gain inverter in front of the potential control amplifier. The current amplifier uses a stabilized operational amplifier with the controlled electrode held at virtual ground. Output sensitivities of 1, 10, 100, and loo0 pa. per volt are provided, When a linear sweep is used, the current
(UTE SIONAL OENERATOR
Figure 1 .
Block diagram of apparatus
amplifier output can be taken directly to the readout device. If the staircase sweep is used, the current signal must be subjected to switching in the sampler to eliminate the charging current spikes. The sampler accepts the current signal from the current amplifier and a pulse train synchronized with the steps of the staircase signal. It samples the current signal a few microseconds before a step is applied to the cell. This sample is stored in a holding circuit and is applied continuously to the readout device during almost the entire duration of the succeeding step. Just a t the end of the next step, the signal holder is discharged and a new sample is accepted. A brief description of each of the sections of the instrument is given below. A detailed schematic diagram can be obtained by mail from the writer. Power Supplies. D.C. power in the instrument is furnished by two Philbrick R-100B i300-volt supplies and one larger i.250-volt supply. Because the 300-volt supplies are more stable and offer less hum, they have been used throughout where stability and freedom from noise is critical. The 250-volt supply is used in the less critical pulse circuitry. Nearly all parts of the instrument powered by the 250-volt supply would operate without modification from a 300-volt Supply. Filament current is obtained by stepping down the output from a Sola 110-volt regulating transformer. The considerable line voltage fluctuations in our laboratory make this necessary; quite likely it would not be necessary in a building with better wiring. Filaments in the sweep gate are run from a D.C. supply. Sweep Generator. The instrument has been used exclusively with oscilloscope readout; accordingly the sweep generators which are described provide relatively fast sweeps with short periods. Slower sweeps with longer periods would he entirely compatible with the other components of the instrument if recorder readout were desired. The sweep generator is an operational amplifier integrator (Philhrick K2W). Sweep rate is in part determined by selection of input resistance to the integrator. Current being integrated is supplied from the stabilized dual power supply. A sawtooth wave form is generated by discharging the integrating capacitor with a thyratron. To obtain the thyratron discharge pulse, the sweep wave form is scanned by a Schmitt comparator. This also serves as the time marker pulse in the gating circuitry. Sweep length can he adjusted by adjusting the comparison voltage in the Schmitt circuit. I n order
TIME
MARKER
START OPER4lE -RESET OPERATE RESET POSITIVE SWEEP QATE SWEEP DELAY DELAYED START OATE NEGATIVE SWEEP QATE SWEEP TERMINATE
Figure 2. Timing signal generator
sequences,
gate
to keep the thyratron filaments away from the K2W input to avoid pickup of 60 C.P.S. noise, it is necessary to generate a negative-going sweep. It is convenient, however, to arrange the sweep generators so that the linear and staircase sweeps are entirely interchangeable. Since the basic staircase signal is positive-going, the basic linear sweep is inverted, using a unity-gain operational amplifier (K2W). By combining selection of sweep input resistance with variable attenuation in the sweep gate, sweep rates from 2 to 100 volt per second are available. The staircase sweep generator is a modified diode pump circuit, described by Kramer ( 8 ) . The necessary pulse train input for staircase generation is obtained from a General Radio Type 1217B pulser. The discharge pulse formed in the linear sweep generator is used to terminate the staircase sweep also. This ensures that current samples will be taken a t randomly chosen potentials in free running polarograms to avoid loss of detail due to the periodic sampling process. The signal produced has a step rise time of about 4 psec. and a tilt of about 0.4%. The basic sweep signal chosen, linear or staircase, is inverted (K2W) and the D.C. level of both positive- and negative-going signals is adjusted by a cathode follower arrangement to permit them to be put together to form a coherent triangle. Gate Signal Generator. As indicated above, this component provides two signals. One of these controls the positive sweep transmission gate, the other controls the negative gate. These signals are generated by a pair of bistable multis, each capable of furnishing an Open or a Close gating signal to the corresponding transmission gate. When the sweep is not in use, both gating multis are in the Close condition. To illustrate the operation of this component, consider the action to produce a one-cycle sweep. Timing sequences are shown in Figure 2. A train of pulses, produced in the sweep generator and synchronized with the basic sweep signal, designated Time
Marker pulses, serves as the basic timing reference. When the sweep is being used, a logic gate passes them as Operate pulses to operate the gating multis. When the sweep is not in use, another logic gate uses these pulses as Reset pulses to hold all components in their rest conditions. When the operator closes the switch to start the sweep, his action probably will not coincide with the start of one of the sweep signals. This is indicated by the position of the Start pulse relative to the Time Marker pulses. Accordingly, the Start command must be accepted and held until the start of the next sweep signal. At this time, an Operate pulse is directed to the positive gating multi to put it in the Open condition. The second Operate pulse must be directed to the positive gating multi to Close it and simultaneously to the negative gating multi to Open it. Since this generates one complete sweep cycle, the third Time Marker pulse, as well as successive Time Markers, appearing after the Start pulse is directed in the Reset circuit to return all components to their rest conditions. I n Figure 2, two Operate pulses are shown appearing a t the time of a two-pulse vacancy in the Reset train. The operator’s action generates a Start pulse which triggers a bistable multi to form the Operate-Reset gating signal. The Operate condition of this signal is the third line of Figure 2. The Reset is the reciprocal and is taken from the other side of the multi. These two signals actuate logic gates to provide Operate and Reset pulses, respectively. The positive gating multi is connected directly to the Operate logic switch and is opened by the first Operate pulse and closed by the second. When sweeps longer than one cycle are used, the positive gating multi is opened by all odd numbered pulses and closed by all even numbered pulses. To avoid scrambling the halves of the sweep signal, the negative gating multi must be opened by even numbered pulses and closed by odd numbered pulses. To get the negative gating multi out of phase from the positive gating multi, the Operate pulses actuating the negative gating multi are passed through an additional logic gate which is itself opened by the first Operate pulse. This is done by using the Operate pulses to trigger a univibrator set to produce a pulse quite a bit wider than the Operate pulses. The trailing edge of this Sweep Delay pulse opens the Delayed Start Gate which passes the second and all successive Operate pulses to the negative gating multi to produce the Negative Sweep Gate signal. When a one-cycle sweep is desired, the trailing edge of the Positive Sweep Gate triggers a univibrator to produce the Sweep Terminate pulse which VOL. 37, NO. 3, MARCH 1965
327
STEP
PULSE
MOWLATED WLSE OELAYING PULSE DISCHARGE PULSE SAMPLING FULSE SPACER PULSE 2 AXIS PULSE
Figure 3.
Timing sequences, sampler
returns t,he Operate-Reset multi to its original condit'ion. As shown in Figure 2, the next Time Marker pulse appears in the Reset train and returns all multis to their original conditions. If a single sweep polarogram is desired, the Sweep Terminate pulse is triggered by t'he first Operate pulse. For two-cycle operation, the trailing edge of t'he Positive Gate signal is divided by two to produce a Sweep Terminate pulse on appearance of the fourth Operate pulse. I n continuous operation, the Sweep Terminate pulse is produced by the operator pressing the start switch a second time. Sweep Transmission Gate. This consists of two unidirectional diode transmission gates operating into a common load resistor. Signal attenuation to the level suitable for the cell is accomplished by taking a portion of t,he drop across this resistor. The attenuated signal is taken to a long tailed cathode follower fitted with potentiometer in its cathode circuit. Resistance values are chosen to permit adjustment of the D.C. level of the controlled electrode over the desired range without appreciable variation in signal at,tenuation. Potential Control Amplifier. The signal from the sweep gate is applied to the noninverting input of a n operational amplifier (Philbrick SK2V) if the reducing mode is desired. The output of this amplifier is applied t o the auxiliary electrode. The reference electrode signal is brought to the inverting input of the amplifier and to t h e X-axis of the readout. Using the reference electrode signal this way imposes a limitation on the input impedance of the readout device, but eliminates the problem of removing the offset of the potential control amplifier. I n practice, there has been no difficulty with aqueous systems using either a n X-Y recorder for conventional polarography or an oscilloscope for cyclic voltammetry. Dimethylsulfoxide and acetonitrile have been used with fiber type reference electrodes filled with silver in the appropriate solvent. S o difficulties are experienced when the reference electrode is taken to t'he X-axis of a n oscilloscope with one-megohm input impedance. When the oxidizing mode of operation is desired, the sweep signal is first' taken to an operational amplifier inverter 328
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.ANALYTICAL CHEMISTRY
(SK2V) and then to the potential control amplifier described above. Current Amplifier. h stabilized operational amplifier (Philbrick US.1-3) is used in the conventional way for a current amplifier for the 1000, 100, and 10 pa/volt ranges. To obtain 1 pa./ volt sensitivity with the staircase sweep, it is necessary to use a second stage (SK2V) with a gain of ten because of the frequency response demands imposed by the raw current signal from the staircase electrolysis. Sampler. Figure 3 illustrates the timing sequences in the sampler. The Step pulse, from the General Radio 1217B pulser, is used as the time marker for the sampler. This pulse actuates a relaxation oscillator, Schmitt trigger time modulation arrangement with univibrator pulse shaping to produce a time-Modulated pulse which occurs a t a n adjustable time near the end of each step. The variable time modulation arrangement is required to permit a change in step width in the signal generator. The leading edge of the Modulated pulse triggers a univibrator to generate a 240-psec. Delaying pulse. The trailing edge of the Delaying pulse is similarly used to trigger a 20fisec. Discharge pulse, the trailing edge of which triggers a 40-psec. Sampling pulse. The Modulated pulse also is used to trigger a 350-psec. Spacer pulse, the trailing edge of which triggers the 90psec. 2-Axis pulse. This sequence of events produces three trains of pulses which are fixed in time with respect to each other, the Discharge pulse followed almost immediately by the Sampling pulse, followed 50 psec. later by the Z-Axis pulse. These pulses can be adjusted as desired relative to the steps of the staircase signal. The raw current signal appears a t the input of a diode transmission gate. During the period of the Sampling pulse, the gate is open to the current signal. At this stage, the sampled current signal carries the desired information as the maximum D.C. level of quite large pulses. If this signal were taken directly to the oscilloscope, it would be impossible to operate at very high sensitivity without saturating the scope amplifier. To avoid this difficulty, the sampled signal is taken to a boxcar holder. As described above the Discharge pulse is timed to discharge the holder sharply just before a new sample is taken. The resulting signal is taken to the scope Y-axis after D.C. level adjustment in a long-tailed cathode follower with glow tube coupling. The oscilloscope trace is turned on once during each step for a period of 90 psec. by the Z-Axis pulse. This arrangement prevents the scrambling of the polarogram by either the shifting of the potential with the appearance of the
next sweep step or by the discharge spike from the holder. Since the trace is illuminated for only 90 psec. during the course of a step, this prevents possible damage to the scope phosphor when the sweep is stationary between polarograms. I t also provides a convenient way to synchronize the sampling circuit with the sweep circuit. The sweep is displayed on the scope and the sampler modulation is adjusted till t h e Z-axis spot falls a t the foot of the step. The pulse circuitry is sufficiently stable that adjustment is rarely needed except when it is desired to change the step width. EXPERIMENTAL
Reagents. Solutions were prepared with water from the metal house still B-hich had been run through a mixed bed deionizer. The KC1 used was reagent grade, once recrystallized from deionized water. Cd(I1) stock solutions mere standardized by electrogravimetry. Solutions were deaerated with tank nitrogen from which traces of oxygen were removed by contact with reduced alkaline anthraquinone sulfonate. At the highest sensitivity, it m-as necessary to add a small amount of Na2S03 to remove the last traces of oxygen. Instrument Performance. The major sections of the instrument have been in satisfactory routine use for about a year. I t s stability is such that it is normally unnecessary t o make a n y adjustments except when the sweep signal is changed. T h e only d a y to d a y calibration required is of the oscilloscope and of t h e polarization rate, if a n accurate value is required. The precision of measurements with this instrument is comparable to that of similar instruments. As an example of the precision obtained in the midportion of the normal operating range of concentrations, the measurement of peak heights obtained with 0.1mM Cd(I1) at a hanging mercury drop electrode may be cited: at 40 volt/sec., the standard deviation was 5.7%; a t 20 volt/sec., 2.1a/00; a t 10 volt/sec., 2.4%; a t 4 volt/sec., 2.9%; at 2 volt/sec., 0.8%. Similar figures for anodic voltammetry at platinum electrodes have been published elsewhere (10). The sensitivitv of the instrument is quite dependeni upon the nature of the electrode and the electrode reaction being used. It is, of course, greatest with a mercury electrode and a reversible system. In this case, the limiting factor is the noise level in the sweep signal generator. This noise, predominately 60 c.P.s., has a peak-topeak value of approximately 0.5 mv. When this is impressed across the double layer capacitance of the mercury electrode, an -1.C. current signal is obtained which is not compensated by the staircase switching scheme. Its magnitude varies with the electrode area and the potential imposed. With a
501-
i
Figure 4.
Comparison of linear and staircase sweeps
0.102mM Cd(ll1 in 0.10M KCl; hanging mercury drop electrode’ 0.031 area; 25.0’ C. A. Solid line, theoretical curve; solid circles, corrected linear B. C.
D. E.
sweep peok currents Uncorrected linear sweep p e a k currents Uncorrected staircase sweep peak currents, 1 -msec. step Uncorrected staircase sweep p e a k currents, 2-msec. step Uncorrected staircase sweep p e a k currents, 4-msec. step
single DME drop of approvimately 0.03 cm.2 area in 0.1M KCI, the noise current varies from 0.1 to 0.5 ua., depending upon the potential imposed. I n practice this limits the range of detection to about 10-7JI when Cd(I1) is being electrolyzed. An attempt was made to carry out determinations a t the to lO-’JI level by electrolyzing Cd(I1) solutions a t a DME, using a continuous cyclic sweep of 10 volt/sec. with 2-msec. step widths. To assess the validity of the results, the assumption is made that the pak-to-peak height for a cyclic sweep arranged symmetrically about the half-wave potential is equal to twice the peak height for a single linear sweep. Knowing the diffusion coefficient for Cd(I1) in 0.1.31 KC1, the electrode area and other necessary constants, the peak height to be expected for a single sweep experiment can be calculated from Equation 1 and doubled according to the assumption being made. For 2 X 10-7JZ Cd(II), the value expected is 0.08 ua. The measured value was 0.06 ua. For 6 X 10-7.1f, 0.24 pa. would be expected and 0.3 pa. was measured. At the potential of the Cd(I1) peak, the noise level is approximately 0.15 pa. A measurement is feasible because a number of cycles during the latter part of the lives of qeveral drops can be photographed with the camera stopped back somewhat. This permits a sufficient bandwidth reduction to surmount the difficulty of the unfavorable siqnal to noise ratio. The lower liniit for analysis of reversibly reduced substances with thir instrument is approxiinately 10-7.11. For comparison, Cooke, Kelley, and Fisher have been able to determine cadmium to about the same level with their controlled potential derivative polarograph ( 5 ) . Barker and Jenkins have stated that the limit of detection for the square wave polarograph is 2 x
lO-7M (3). Csing the pulse polarograph, Barker and Gardner have been able to carry out determinations with the normal pulse technique to 10-7M and with the derivative pulse technique, to 10-8M (D. Use of Staircase Sweep. A discussion of the use of a staircase sweep to reduce t h e effect of double layer charging current has been presented previously ( 3 ) . This is quite effective when sweep rates greater than one volt/sec. are used with mercury electrodes. I t is not very helpful with platinum electrodes because of the large noncapacitative background currents. A comparison of the use of a linear sweep and of a staircase sweep with theoretically expected behavior on mercury electrodes is summarized in Figure 4. The line A represents peak current expected from theory. I t is calculated from Equation 1 , using the conditions of the experiments being conipared ( 7 )
ments of pcak height from zero current using a staircase sweep with 2-msec. steps. Line B represents similar nieasurements with a 4-nisec. staircase. It is evident that t,he use of a staircase sweeli is effective in reducing capacitance current’ effects and that, within limit,s, results obtained with the staircase sweep are in agreement with the theoretical relationship. One nieasure of effectiveness in eliminating capacitance effects is the difference between lines A and B in Figure 4 since results with the staircase sweep are in good agreement with the theoretical curve. From another point of view, the capacitance component of peak current at a sweep rate of 10 volt’/sec. is about 8 pa. For a cyclic sweep, this would be about’ 16 ua., about 200 times the peak current measured for 2 X lO-’.II Cd(I1) with a staircase sweep under similar conditions. It. can be seen in Figure 4 that as the sweep rate is increased, a negative deviation from the theoretical relationship is observed. Furthermore, the linear relationship between (sweep and peak current is lost quicker as the st,ep width is increased. This is attributed to the fact that as the steps are made larger, the staircase brcomes progressively a poorer approximation to the linear sweep assumed in the derivation of Equation I . The point at which the deviation becomes large is apparent’ly partly dependent upon the widt’h of the polarographic peak. With the broad peaks encountered in volt’amnietry with irreversible syst,enis, a 10 volt/sec. sweep with 3-msec. steps causes no appreciable error. I n practice, with reversible syst,ems, it is convenient’ to use 2- or 3-nisec. step widths a t low sweep rates and t,o use 1-msec. steps a t rates of 15 volt,’src. and higher. LITERATURE CITED
(1) Alden, J. R.,Chambers, J. Q., Adams, R. N., J . Electround. Chem. 5, 152 11963). (2) Barker, G. C., Gardner, A. \T., A t . Energy Research Estab. (Gt. Brzt.) X e p t . AERE C/R 2297 (1961). (3) Barker, G.C., Jenkins. J. L., Analyst 77. 685 11952). (4)Buck, R. P.,Eldridge, 11. W., AXAL. CHEY. 35. 1829 (1F)6x). ( 5 ) Cooke, iV. I)., Kellev, 11.T., Fisher, 11. J., Ibid., 33, 1209 (i96lj. (6) Ilavolio, G., Guerzoni, W., Pauoff,, P.., Electrochim. Acta 5, 29’1 (1961j. (T) Ilelahay, P., “ S e a Instrumental Methods in Electrochemist’ry,” p. 119. Interscience, New York, 1954. ( 8 ) Kfamer, S. I., Electronics 29 (2)192 \ - - - -
where ip is peak current in amperes; A , area is C, concentration in n i o l e ~ / c m . ~ ;n , number of electrons, niole for the couple involved; u , sweep rate in roltlsec.; D , diffuqion coefficient in cm2/sec. .A value of 7.15 X cm.*/sec. was used for diffuqion coefficient of Cd(I1) in 0.1OJI KCI (11). Line B represents measured values, using a linear sweep ‘and measuring peak heights from zero current. The solid circles represent measurements taken with a linear sweep, but with graphical correction for the blank. The solid squares of C represent peak currents measured with a staircase sweep with I-nisec. steps, peak height. measured froin zero current, t h r same procedure used to get the data for line B. Line D is made up of measure-
(1Sa6’i.
(9)AIann, C. K., 1484 (1961). (10)Ibzd., 36, 2424 11964). (11)von Stackelberg, JI., Pilgr am, I f . , Toome, XI., Z . Elektrochem. 57, 347 (1953).
REPEIVEI) for revien September 2, 1064 Accepted Derernber 26, 1964 Work supported by researrh grant 536-4 from the Petroleum Research Fund and h \ re-earch grant GRI 10064 from the l-nited States Puhlir Health Service VOL. 37,
NO. 3,
MARCH 1965
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