Renaissance in polarographic and voltammetric analysis

INSTRUMENTATION

Advisory Panel Jonathan W. Amy Richard A. Durst G. Phillip Hicks

Donald R. Johnson Charles E. Klopfenstein Marvin Ma rgoshes

Harry L. Pardue Howard J. Sloane Ralph E. Thiers

The Renaissance in Polarographic and Voltammetric Analysis JUD

B. FLAT0

Chemical Instrument Group, Princeton Applied Research Corp. P.O. Box 2565, Princeton, N.J. 08540

As more and more analysts adopt modern polarographic and voltammetric techniques, literature data will become more complete, and the use of such methods as differential pulse polarography, alternating current polarography, fast linear sweep voltammetry, direct anodic stripping voltammetry, and differential pulsed anodic stripping voltammetry for fingerprint purposes and analytical applications will increase HEYROVSKY discovered t h a t the current flowing between a counter electrode arid a dropping mercury electrode ( I ) , a t a particular potential, was related t’o the concentration of one of the species present, in the solution through which the current was flowing, polarography has been used as a n aiialytical technique. In 1927 when tlie initial work was performed, instrumeiital methods were a rarity, and tlie development’ of this technique, despite difficulties i i i using photographic recorders and mechanical galvanometers, provided a breakthrough in the analytical laboratory. For the first time, a method other than time-consumitig volumetric aiid gravimetric techniques was available for t h e determination of metals in solution. For the first time, solutions below millimolar level could be analyzed without using extremely elaborate techniques. Through the second quarter of this century, the growth of the technique was marked, aiid polarographic analysis found its way irit,o many laboratories. I n fact, the contribution of this technique to the advancemetit of analysis was considered sufficiently important to warrant the awarding of the Sobel prize for cliemistr>-to Heyrovsky in the late 1950’s. The basic dc polarographic technique, however, suffered from a number of “defects” irhich made it less than ideal for routine analytical purposes and made the results obtained somewhat difficult to interpret. The idealized ISCE

waveforms expected from dc polarography are quite easy to interpret, but the actual waveforms obtained from all but the best cases are considerably more difficult to use. T h e “Faradaic” current produced by the reduction or oxidation of the species of interest is only one of a number of currents flowing through the system, so t h a t very dilute solutions, where the contribution of this Faradaic current to the overall signal is small, yield de polarograms lacking in useful information. Problems such as these dampened enthusiasm for the technique somewhat and diminished its groivth. I n addition, i t was quite clear from the onset of iiolaroaranhic investiaa~tioiis that many applications to organic chemistry existed for the technique. However, early two-electrode equipment could not cope with the higher resistance of nonaqueous solvent systems, so t h a t the applications were additionally limited to systems which could be made up in water or water-alcohol systems. Beginning in the mid-1950’s when a degree of electroiiic sophistication entered tlie chemistry laboratory and, still more, with the advent’ of low-cost operational amplifiers in the late 50’s and early 60’s, modifications of the basic polarographic technique aimed a t overcoming the various problems associated $1-ith it began to nieet with success. Various workers applied waveforms different from and usually more complex than t,he simply varying de I

potential normally used in classical polarography and applied various modes of signal processing to the measured currents obtained. Each of these efforts was aimed a t overcoming one or more specific problems, and little by little, ways of overcoming each of these problems were developed. Investigations of such techniques as square wave polarography, pulse polarography, differential pulse polarography, and ac polarography began in various locations, and results began to appear in t h e literature. I n fact, the decade from 1955 t o 1965 might be characterized as t h e one single period during which the greatest advancement in the technical asnects of polarography took place, while, simultaneously, t h e greatest decline in the practical everyday usage of these techniques occurred. Great progress was being made in t h e research laboratory in solving the basic problems associated with de polarography and in applying the newer polarographic techniques to organic systems, but comrnercia1 instrumentation capable of exploiting this progress was either unavailable or extremely e x p e n h e . Thus, analysts who were unwilling or unable to build their oivn equipment were forced to rely upon primitive de instrumentation. Then, when other analytical methods which were either less prone to difficulty or more profitable for the instrument manufacturers began to be aggressirely promoted and when comniercial instrumentation for these

ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972

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instrumentation

Figure 1. Simple twoelectrode cell showing resistance

Figure 2. Potentiostatic three-electrode system

methods began to become readily available, the polarograph was shoved to the back of the bench; the analyses previously performed on it were transferred t’o the atomic-absorption spectrometer, the flame-emission spectrometer, the gas chromatograph, etc. New Techniques

I n classical de polarography, i t is assumed that t’he measured current is “diffusion-limited.” This simply means that diffusion is the only mode of transport by which the electroactive particles whose reaction will produce the current can reach the electrode and that when the applied potential is noticeably more negative than the halfwave pot,ential, any particle which arrives a t the electrode surface by diffusion immediately and essentially instantaneously undergoes a n electron transfer reaction. I n such a process, the current that flows is determined by the diffusion rate of the mat’erial in question, which is determined by its concentration, its diffusion coefficient, and by the electrode area. One can derive a theoretical model for the diffusion-limited process by making assumpt’ions about the area and shape of the electrodes, and in pure, relatively concentrated solutions, the results can be made to agree with esperiment to a good approximation. Unfortunat’ely, the actual current that one measures contains contributions from a number of other current Yources : in addition to this diffusion-controlled Faraddie current. Such sources include the current necessary to charge the capacitance of the eleclrical double layer at the electrode surface, currents 76A

e

produced by the reduction of other species in the solution, and background or noise currents from either the electrode .?stem or the inqtrument. The problem of diqtinguishing the current of interest from the various other signals is the one which each of the “modern” voltammetric techniques seeks to solve. In the remainder of this article we shall discus. the way in which these techniques can provide such information and indicate some of t h e applications nhich have developed in the paqt five years for these techniques. It is only with the advent of low-cost modern polarographic instrumentation that these techniques can be routinely applied, and the renaiqqance in polarographic analvsis R-hich forms the subject of thic article can be directly attributed to the availability of such instrumentation. Potentiostatic Control

One key characteriqtic common to all modern polaronraphic inctrumentation is “potentioqtatic” (e) control of the working electrode potential. Classical polarograph. did not possess w c h “potentiostatic” capabilities and were thus n o t usable in high-resistance solution. and organic solvents. They applied the potential acrocq the entire cell, rather than acro-c the working electrode-qolution interface. and thuc vielded data n hich ll-ere considerably in error if qolution rvistanceq and the rewltant voltage dropc through them made an appreciable contribution to the recorded data (Figure 1). Modern instrumentc incorporate a potentioqtat which controls the potential right a t the working electrode-colution interface,


eliminating errors owing to solution resistance, and are thus usable in a much wider range of systems. A potentiostat accomplishes this end by making use of a three-electrode system, as shown in Figure 2. Here, a reference electrode of constant potential, inserted in the system and positioned as closely as pos>ible to the working electrode, is connected to the instrument through a circuit which draws essentially no current from it. There is thus no current f l o ~between the tip of the reference electrode (or its connecting bridge) and the instrument and thus no voltage drop. The output of the circuitry nithin the instrument is then t h e voltage right a t t h e tip of.the reference electrode. T h e operational amplifier control loop will then apply sufficient compensating potential to the counter electrode to insure t h a t t h e potential a t the reference electrode tip is the desired one, even if solution resistance is sufficiently high as to cause appreciable voltage drops when currents flow through the solution. This procedure is in contrast to earlier instruments using only tn-o electrodes. They either employed a simple metallic counter electrode with, a t best, mediocre potential reference abilities or used the reference electrode as the only other electrode in the \ysteni, so that current was forced to flow through it. Since reference electrodes are often sensitive to current flow and can undergo potential change owing to polarization under theqe circumstances, the use of tem. even nhen the electrode is positioned closely to the n orking electrode, can introduce qignificant errors. Modern Voltammetric Techniques

For the remainder of this article, we shall assume that all the instrumentation discussed possesses potentiostatic capabilities. and vie shall limit our discussion to those techniques n-liich have found the greatest degree of analytical application. Commercial in.trunientation for all these techniques is available. The techniques to be discussed are as follom : Differential pulse polarography ;IC polarography Fast linear sweep voltammetry Direct anodic stripping voltammetry Differential pulsed anodic stripping voltammetry All these techniques are united in their applicability to electrodes other than the standard dropping mercury electrode employed in most normal de polarography, and all these technique.. are alike in their ability t o detect and

Instrumentation

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Figure 3a. Differential pulse polarogram. 1.3 X 10-jM chloramphenicol in 0.1M acetate buffer. PAR Model 174 polarographic analyzer, dropping mercury electrode, 50 mV pulse amplitude, 1-sec drop

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quantitate analytes a t levels orders of magnitude below those normally associated with dc polarography. In addition, most make some attempt to separate capacitive and Faradaic currents, and all ?-ield presentations where the signal arising from each individual aiialyte is iii the form of a peak of some sort, rather thaii the step usually associated with tic polarograpliy. Difrerential Pulse Polarography. Figure 3a is the differential pulse polarogram of a 1.3 X 1O-jJI solution of the antibiotic chloramphenicol in a 0.1U acetate buffer supporting elec-

trolyte. Figure 3b is a de polarogram of the identical solution. Both curves ITere run a t the same instrument sensitivity-1 p A full scale. This concentration represents the approximate lower limit for the de polarographic determination of the material. T h e wave is clearlj- discernible, but quantitation of the n a v e height would be diffikult, and precise location of the half-wave 110tential almost impossible. In the pulse case, however, the clearly defined sharp peak allon-s precise measurement of peak height and exact location of peak potential. CIRCLE 193 O N READER SERVICE CARD ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972 77A

Instrumentation

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I n fact, differential pulse polarography can be used to determine antibiotics and many other materials a t concentrations well below the ppm level. Figure 4a, for example, is the differential pulse polarogram of a 0.36 ppm tetracycline solution in a similar 0.1M acetate buffer solution. The curve, run a t a n instrument sensitivity of 200 nA full scale, gives a peak analytically useful. By contrast, however, Figure 4b is a de polarogram of a 180 ppm solution of the same material-500 times more concentrated. The curve is too poorly defined to be of use. The differential pulse polarographic technique, originally developed as a n offshoot of square wave polarography in Britain during the 1950's ( 3 ) , consists of superimposing a fixed-height potential pulse a t regular intervals on the slowly varying potential associated with de polarography (Figure 5 ) . The pulse is repeated a t intervals of perhaps 1 see and is synchronized with the maximum grorrth of the mercury drop, if a dropping electrode is used. Differential pulse polarography instrumentation then samples the current 0

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Figure 4a. Differential pulse polarogram. 0.36 ppm tetracycline HCI in 0.1M acetate buffer, pH 4. PAR Model 174 polarographic analyzer, dropping mercury electrode, 50 mV pulse amplitude, 1-sec drop

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Figure 4b. Dc polarogram. 180 ppm tetracycline,HCI in 0.1M acetate buffer, pH 4, similar conditions

floving into the working electrode twice during each operating interval, by use of electronic switching. The first current sample is taken just before the application of the potential pulse and is a sample of a current which is essentially equivalent to that which would be obtained in the normal de polarographic case. Immediately after the conclusion of the sample-taking process, a sudden pulse of potential, usually between 5 and 100 mV, is applied to the electrode. T h e application of this sudden change in potential produces a concurrent sudden change in the current flowing, which comes from two primary sources. The first is the additional current which must flow to charge the double-layer capacitance of the electrode to the new applied potential, and this current decays eaponentially a t a rate governed by the magnitude of the capacitance and the series resistance of the system. Simultaneously, an additional current may flow if the applied potential has suddenly changed to a potential where the equilibrium between the reduced and oxidized forms of the electroactive species involved is shifted.


X h e n both the potentials involved lie either before or after the rising portion of the polarographic wave, no change in the Faradaic current nieasured will be observed. However, when a t least one of the two potentials is on the rising portion of the polarographic step, a significant change in the Faradaic current' flowing takes place when the potential is suddenly stepped. A larger current' is required than by the diffusioncontrolled system, and this current is further increased by t,he fact t h a t static equilibrium conditions do not' prevail because of the sudden change of poteiitial. The pulse potential is niaint'ained for a period of time long enough to allow the capacitive current to decay to a low value, but' during which the Faradaic current, although it also decays somewhat, still does not reach the diffusion-controlled level. At the end of this period, a current sample is again taken. The difference b e h e e n these two samples, developed by applying the two signals stored in the memories to a differential amplifier, is then amplified and presented to the output of the system. This difference current-curve,

PULSE AMPLITUDE

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Potential excitation waveform used in differential pulse polarography and pulsed stripping voltammetry

which is proportional to the concentration of material, all the other usual polarographic parameters, and the amplitude of the pulse, has t'he appearance of a peak rather than the usual polarographic step. It is also not complicated by significantly sloping baselines, since t h e capacitive current contribution is minimized by the delay in the sampling process. Further refinements in the data can be obtained by synchronizing the timing of the mercury drop with the power line frequency so that t h e entire sampling process takes place on equivalent portions of t h e power line sinusoid, and by making the width of the sampling gate equal t o a known multiple of the power line frequency so t h a t slight vari a t'ions in the signal produced by the pickup of power line signals are minimized. These precautions are often necessary because reference electrodes and high-resistance organic solutions may act as antennas which pick up significant amounts of power line signal; this can cause a great deal of noise on the output unless it is removed. T h e differential pulse technique provides solutions to just about all the

problems which plague polarographers. T h e influence of capacitive currents is minimized by the pulsing and sampling process, and peaks are obtained rather than steps, so t h a t resolution can be improved. Such problems as polarographic maxima, poorly defined waves, and severely sloping background baselines are all a t least partially attacked by the differential pulse technique, and all yield to its effects to some degree. Instrumentation for differential pulse polarography requires various timing and sampling circuits, low-drift analog memories, good differential amplifiers, etc. Until recently, available commercial instrumentation was quite old and expensive, used tubes with their attendant service problems, and was nonpotentiostatic. These factors precluded the rapid development of a body of applications information and served to prevent widespread adoption of the technique. However, with the recent advent of instruments employing lowcost, highly stable integrated circuit amplifiers and high-impedance junction field-effect transistors, differential pulse polarography has been brought within the reach of every laboratory and is

being applied to many different analytical areas. Subject only to limitations on the duration of the applied pulse, the technique is applicable to both reversible and irreversible reactions and yields peak shapes which closely approximate the theoretically predicted derivative of the de waveform in most cases. It thus permits one to obtain the maximum possible resolution between closely spaced waves, while permitting examination of large and small signals during the same scan. I n addition, when used with a modern potentiostatic instrument, solutions containing low concentrations of supporting electrolyte will yield curves as well-shaped as those obtained with solutions containing high concentrations of supporting electrolyte. Ac Polarography. Figure 6a is a phase-sensitive ac polarogram of hydrolysis products in acetonitrile containing 0.1V tetraethylammonium perchlorate as supporting electrolyte. The relatively smooth upper line is obtained by using drop-synchronized current sampling, with a single sample being taken just before the mercury drop is dislodged. Using this approach per-


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Instrumentation

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Figure 6a. Phase-sensitive ac polarograms. Hydrolysis products in acetonitrile. PAR Model 174 polarographic analyzer, Model 174/50 ac polarographic interface, and Model 124 lock-in amplifier. Dropping mercury electrode, 30 mV modulation, 1-sec drop

160 Hz

mits the analyst to obtain smooth curves from which the usual oscillations of drop growth are removed. The lower curve is the usual ac polarogram x i t h the drop oscillat,ionsshown. Figure 6b is the second harmonic, phase-sensitive ac polarogram of the same solution, where the system has been modulated a t the same frequencies, but detection has been carried out a t twice the modulation frequency. The second harmonic technique, by virtue of the fact t h a t its signal crosses through zero at the half-wave potential, is useful for resolution problems, especially in complex mixtures. Each of the experiment'al curves has been run both in 80A

the full-signal mode and by using dropsynchronized current sampling. I n the ac polarographic technique (Q), a small amplitude sinusoidal modulation is superimposed upon the slowly varying direct potential used in normal de polarography. The instrumentation used employs circuitry which permits detection and presentation of only the alternating components of the total current flowing into the working electrode, and in the more sophisticated instruments, the signal presented is further selected by being limited to that signal which maintains a specific preselected phase relationship with the phase of the modulation of the applied


potential. By looking only a t t h e alternating portion of the current t h a t flows and detecting its amplitude, we are in effect looking a t the difference in current t h a t flows between the minimum and maximum applied potentials during the modulation period. We thus get a peak waveform, rather than a de step, and thus operate in a n environment where the rate of arrival of electroactive material a t the electrode surface is different than i t would be in the dc case. I n differential pulse polarography, the signal arising from capacitive current is suppressed by sampling the f l o ~ of current after the capacitive contribution has decayed. I n ac polarography the signal can be suppressed by using a phase-sensitive detector to measure only that portion of the alternating current which maintains a specific phase relationship with the applied potential. The capacitive current, just as n-ould be the case in any other capacitor, will differ from the applied voltage by 90' in the ideal case, whereas the ideal reversible Faradaic current differs from the applied potential by 45'. Using a phase-sensitive detector thus permits one to measure only the Faradaic current or, as can be extremely important in studies of kinetics and adsorption, only the capacitive current. It has been postulated that ac polarography can be employed successfully in some cases without the removal of dissolved oxygen from the system. This is because the reduction of oxygen on mercury is highly irreversible, and the signal from the oxygen reduction is thus quite small and does not interfere rrith t h e measurement of signals arising from more favorable reactions. This is a significant analytical advantage in cases where it is applicable. However, the electrochemistry of oxygen produces products which can in themselves severely affect the overall course of reactions lvithin the system. When such effects are observed, outgassing is necessary even in ac polarography. T h e peak position, height, and shape obtained in ac polarography are quite dependent on the specific electrode reaction taking place. The signals may be moved around, and their interrelationships changed significantly by varying the frequency of the applied ac modulation. This makes ac polarography, especially when phase-sensitive detection and second harmonic capabilities are available, particularly useful for analyzing complex systems n here many different electroactive substances give signals. Some substances give well-defined ac waves M hose phase characteristics correspond to ideal reversible behavior a t a particular frequency but may completely change

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Dynatrac’s exclusive front end trading filter eliminates extreme interference and harmonic responses, without the instability problems associated with manual filters. W Only Dynatrac has an isolated front end that eliminates grounding problems. W Only Dynatrac has built-in RFI protection at all inputs and outputs. W Only Dynatrac operates to .1 Hz W Only Dynatrac has full-scale sensitivity to 100 picovolts with optional remote preamp. Only Dynatrac has all of the following: Quadrature phase accuracy of 2 degrees Built-in variable frequenc) sine oscilla tor 12 dbloctave output filter-essential in critical applications Full-scale sensitivit) to 100 n a n o i o l t s without preamp Signal frequencies of 1 Hz to 200 KHz in optimized ranges-tracks external ref erence Noise mode for measuring narrow-band noise versus frequency Multi-turn \ ernier controls for phase frequency and zero suppress Unsurpassed noise performance 01 erload capability, and output stabilitv

their behavior a t higher frequencies. Others give no detectable ac wave under any conditions, so t h a t analyses of mixtures in which they are present can be accomplished by combining ac determination of other components with pulse studies of these. The second harmonic technique can also be useful when resolution is the primary problem, since i t permits resolution of much more closely spaced waves than other techniques. It has been applied to a large number of complex analytical problems, especially in the pharmaceutical field, for this reason. Fast Linear Sweep Volturnmetry. Figure 7 shows the polarograms which can be obtained with a variety of techniques from the well-known insecticide carbaryl (“Sevin”), which has been nitrosated to render it polarographically active. The polarographic determination of nitrosated carbaryl is an accepted technique for determining trace residues of this material. Curve I shows the normal dc polarogram of the material whose concentration in this solution is 6.4 ppm. Yo detectable wave is observed. Curve B s h o m the fast linear sweep voltammogram of the identical solution. The t x o curves were obtained on the identical system, a t the same current sensitivity of 2 p h full scale, and on the same dropping mercury electrode. The difference, however, is that curve -1n a s run a t a scan rate of 10 mV/sec by use of many drops, whereas curve B was obtained by applying a 500 mV,sec scan to a

single drop. I n curve B the wave is clearly discernible and easily quantitated. (For reference, curves C and D, which are differential pulse polarograms of the same system, are included.) Clearly, the fast sweep technique provides a marked increase in sensitivity over dc polarography and offers the added advantage of giving rapid results. I n the particular example shown, the differential pulse technique proved to be approximately five times more sensitive with the specific instrument settings used, but fast slveep voltammetry was much more rapid. I n this technique the normal slowly varying scan potential used in dc polarography is replaced by a fast sweep M ith no modulation of any sort (6). Potential change rates in de polarography are usually of the order of 1-50 niV/sec, but fast linear sweep voltammetry is carried out a t 100 mV/sec and higher. The technique was originally developed to obtain higher sensitivities than could be obtained by dc polarography without requiring the extremely sophisticated circuitry of the square wave or pulse polarographs. It succeeded in this endeavor because the application of a rapidly changing potential to a n essentially stationary electrode produced a nonequilibrium condition around the working electrode. I n any electrochemical system, an equilibrated diffusion layer is established between the electrode and the bulk of the solution-a layer in which, once the

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Figure 7. Nitrosated carbaryl (“Sevin”). 6.4 pprn. analyzer, Model 174/51 linear sweep accessory

PAR Model 174 polarographic

Dc polarogram, 2 PA full scale, 1-sec drop, E vs. SCE Fast linear sweep voltammogram, 2pA full scale, 500 mV/sec sweep, E vs. SCE Differential pulse polarogram, 10 +A full scale, 50 mV pulse, 1-sec drop, E vs. Pt D. As in C, but E vs. SCE

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Figure 8. Differential pulse anodic stripping voltammetry. PAR Model 174 polarographic analyzer, Model 9319 wax-impregnated graphite electrode (mercury-plated). 2 X IO-gMZn, Cd, Pb, and Cu

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reaction potentials have been reached, the concentration of the electroactive species of interest changes from the “bulk” concentration value a t positions far removed from the electrode surface to essentially zero a t the electrode surface, because any electroactive particle arriving a t the surface instantly reacts. When slow scans are employed, the slope of the concentration gradient within the diffusion layer is that which is dictated primarily by the rate of depletion of the electroactive species. This gradient varies from essentially zero, at potentials significantly more positive than the reduction potential, to a value governed by the concentration and diffusion coefficient of the substance in


question, a t potentials well past the reduction potential. When the process in question uses rapid changes in potential, however, either because the scan rate is fast or because a pulse modulation of some sort is employed, the slope of the concentration gradient a t any particular potential will be greater than in the slow-scan case, and the bulk concentration will be present closer to the electrode surface. Thus, the number of electroactive particles arriving a t the surface per unit time will be greater, and larger signals will result. When single drops are used, the typical concentration gradient discussed previously prevails a t potentials more

Instrumentation ~~~

cause a n intermetallic compound forms between the copper and the zinc under the specific deposition conditions used for this experiment, and much of the copper is then not stripped properly when its oxidation potential is reached. For contrast, Figure 9 is a similar curve run on a solution approximately 10 times more concentrated, where intermetallic compound formation is not a problem, and the copper is easily seen. Intermetallic compound formation should not be taken as a problem, however, in that, if the deposition in this experiment was carried out at, for example, -0.9 V vs. SCE instead of the -1.4 V which was employed, the zinc would not deposit, and $he copper wave can easily be studied. The procedure for the analysis would then simply be to run it twice, once depositing a t - l .4 V and a second time with deposition a t -0.9 V. Figure 10 is a direct anodic stripping voltammogram of the identical solution, run on the identical electrode. The pulsed curve was obtained by depositing, with stirring, for 3 min. After 5 min of stirred deposition, the dc case was stripped from the same electrode. Clearly, the data are there, but the allowed deposition time did not permit adequate stripping currents to be obtained. I n addition, the severely sloping baseline obtained a t these sensitivities makes the data somewhat more difficult to interpret. Pulsed and direct anodic stripping voltammetry are techniques which are applicable almost exclusively to metals analysis (6-8). They are essentially offshoots of their polarographic counterparts which make use of the properties of a stationary mercury electrode to increase sensitivity, based on the

positive than the reduction potential. However, once this point has been passed, more and more material is used up, and the diffusion layer extends further and further into the solution. Unlike the case with dropping electrodes, this process is not periodically reversed by the stirring associated with drop fall, so t h a t the decay of signal continues and peak-like readouts are obtained. I n addition, since all the data are obtained on a single drop, the annoying mercury drop oscillations superimposed on the usual polarographic waveform are not present. Linear sweep voltammetry is usually performed on a n instrument which either incorporates or is connected to an oscilloscope, since until recently, recorders with adequate response speeds were unavailable. The technique suffers from sloping baselines caused by the increase in both capacitive and Faradaic currents with drop size but still yields useful data, and dc differentiators can be used to overcome the baseline problems. A great deal of data has been published by using this method, and now that low-cost instrumentation which incorporates this capability is available, much of the published data can be applied to routine analytical problems. Pulsed and Direct Stripping Voltammetry. Figure 8 is a pulsed stripping voltammogram of a solution containing 2 x 1 0 - 9 ~zinc, copper, lead, and cadmium (corresponding to 0.1 ppb, 0.1 ppb, 0.4 ppb, and 0.2 ppb, respectively). Clearly, the zinc, cadmium, and lead peaks are welldefined and easily quantitated. However, note that the copper peak is not as well-defined. This is primarily beI

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Figure 10. Direct anodic stripping voltammetry.

Solution and equipment as in Figure 8

Company Address City

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CIRCLE 53 ON READER SERVICE C A R D


85A

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Instrumentation

premise t h a t significant increases in signal strength can be obt,ained by concentrating the material of interest into a n electrode before studying it. If a potential more negative than the half-wave potential is applied to a suitable electrode, the metal ions wi!l be continually reduced a t the electrode surface. h s time passes, the concentration of the material of interest in the amalgam electrode will groiv and, after a suitable interval, will be significantly higher than the concentration of the same material available for reduction in the body of the unknown solution. Application of a positive-going current will then cause the metals to oxidize back out of the mercury, and their higher concentration in the amalgam will give rise to much higher currents than would have been obt,ained in the reduction process. It would seem a t first glance that any electrode might be usable for this purpose, but severe complications arise, except in special cases when anything other than mercury is employed. This is because the potential a t which the reactions take place is strongly dependent on the nature of the electrode surface, and this surface is changing significantly during the course of the experiment, as it becomes covered with reaction products. If mercury is used, however, metallic reaction products will dissolve in the electrode to form amalgams, and the electrode surface will thus only gradually be converted from pure mercury to amalgam. In addition, if more than one material is depositing on the electrode surface, intermetallic compounds may form among the various constituents, but this phenomenon will be minimized (though not eliminated) when the various metals are “diluted” with mercury. In the basic dc anodic st’ripping process, a suit,able mercury-containing electrode is initially maintained at’ a specific known potential, more negative than the reduction potential of the metal which is most difficult to reduce, for a known length of time. The material deposits into the electrode a t a rate governed by its concentration and its rate of arrival a t the electrode surface (either by diffusion or via stirring), and a t the end of a knon-n length of time, a known amount of material has deposited onto the electrode surface. At the end of this process, a potential increasing in the positive direction is applied t,o the electrode, and the current Obtained is measured as a function of potential. 111 the normal dc case, the most well-defined waves and the highest’ sensitivities are obtained when the rate of change of this potential is reasonably fast, so that peak waveforms are obtained. Baseline slopes are encoun-


tered but may either be compensated or ignored. The sensitivity of this technique is theoretically limited only by the length of t’ime over which one may keep the instrument and electrode system stable so t h a t deposition may continue, the degree of reproducibility and control one can exercise over the deposition process, especially when stirring is employed, and the degree of one’s patience. It is theoretically possible to analyze solutions well below 10-11JI concentrations by this technique through the use of long deposition times. However, instrument instabilities and electrode malfunc$ions may be a problem when experiments last many minutes, and more importantly, the excessive times involved can cause all of the analyte to disappear as i t is adsorbed on the walls of the vessel. Sensit,ivities of the order of l O - 9 M can be obtained, and the technique is often used for metals analysis a t these levels. The original electrode used for this technique, and the one most commonly associated with it, is the hanging mercury drop electrode. Such an electrode in the modern syringe form is convenient and reproducible. I t suffers from some problems, however, in t h a t it is difficult to maintain a stable drop for times of one-half hour and in that diffusion of the species of interest into the electrode takes place to a significant degree during the course of the deposition, so that not all the material which has been reduced is available for oxidation when t,he scan is applied. Recent efforts have aimed a t developing mercury film electrodes of various types, which can overcome these problems because they are mechanically stable and employ thin films, so t h a t diffusion into the body of the electrode is not a problem. Various degrees of success have been reported, and such electrodes as mercury-plated n-ax-impregnated graphite, mercury-plated pyrolytic graphite, and mercury-plated carbon have all been employed for the purpose. One must be careful, however, in using thin-film electrodes, in that the time scales of the de stripping technique involve massive depletion of the solution, when extremely dilut’e solutions are being investigated, and exhaustive stripping of the mercury film during the detection process. This means that reproducibilities will be subject to modification by factors which can affect such massive, exhaustive processes! such as the degree of constaricy of the stirring rate, the exact thickness of the mercury film, and the amount of exposure, if any, of the substrate. Dij’erential pulsed anodic stripping voltammetry differs from dc anodic strip-

Instrumentation

ping voltammetry only in t h a t the osidat,iori process is studied through the use of the same pulse-modulated ramps discussed under differential pulse polarography. T h e deposition process is carried out in exactly the same way (except t h a t commercial practice often involves leaving the pulsing on during the deposition process since pulses of 25 or 50 mV amplitude have no appreciable effect on the deposition rates). I n differential pulsed stripping voltammetry, all the considerations of the de stripping voltammetry case apply. However, the far greater sensitivity and signal processing capabilities of the pulse-modulated detection technique permit significantly higher instrument sensitivities to be used and thus allow either much shorter deposition times or much lower instrument gains. Under these circumstances, deposition times are kept to a few minutes so t h a t electrode instabilities are less of a problem, and diffusion into t h e body of t h e electrode is less important. Similarly, the differential pulse scan rate of 5 mV,/sec or so is such t h a t the system remains in equilibrium wit,h t'he electrode, escept for t h e effects of t h e pulse potential change, so t h a t greater reproducibilities may be obtained. B y use of the pulsed stripping technique on both hanging drop and film electrodes, sensitivities similar to those obtainable by dc strippink volt'animetry can be obtained with much shorter experiment times. T h e same limitations which apply t o t h e de stripping voltammetry technique become t h e limits of sensitivity; t h a t is, electrode stability, instrument stability, and loss of analyte t o the vessels. However, the shorter times involved mean t h a t the limits of sensitivity can be pressed perhaps one or two orders of magnitude further by extending deposition times. I n addition, since the signal-processing circuitry of the pulse modes permits the use of higher instrument gaiiis, sensitivities may be further improved. Prognosis

Each of the above techniques provides a significant improvement in such key parameters as resolution arid sensitivity over de polarography. Many analytical problems which could never have been attacked by the de technique fall within the possible operating realm of one or more of the advanced techniques, and many analytes which gave essentially useless signals in the de case now yield useful data. I n the metal analysis area for wliicli polarography was originally developed, t h e pulsed stripping technique is one of the most sensitive techniques available and certainly offers the most economical way of obtaining these

sensitivities. The equipment is simple and easy to operate, and operator skill and knowledge are not required. I n the area of organic analysis, the specificity of voltammetric techniques permits these techniques to be applied to t h e analysis of relatively complex systems and to the unequivocal identification of complex substances. I n any particular supporting electrolyte-solvent combination, only certain organic functional groups will exhibit electroactivity in a particular potential region, and fairly small changes in the nature of t h e molecule can provide significant shifts in t h e reduction potential. I n addition, even in a relatively complex molecule, only a few peaks mill be obtained since only a few of the functional groups nil1 be electroactive. As a result, the worker using organic polarographic techniques for analysis or identification can usually identify t h e electroactive species n i t h relative ease by comparison with published data. If a sample of pure substance is available, unequivocal identification can often be made by selective addition, but even if this is not the case, t h e simplicity of the recorded data makes interpretation much more simple. As more and more analysts adopt modern polarographic and voltammetric techniques, more and more data will find its way into the literature, and t h e use of the techniques for fingerprint purposes and analytical applications will increase. Similarly, as more and more investigations of the electroactivity of compounds of interest take place, these techniques will become even more widespread, and their applicability will be accepted to a n even higher degree. Thus, n e see a technique, which is one of the oldest instrumental techniques available and which fell into disuse as the newer techniques came along, now returning to a place of prominence in the analytical laboratory, finding its way into new applications where i t had previously not been tried, and owing to the availability of modern instrumentation, returning to its rightful place in the laboratory. References (1) J. Heyrovsky, Trans. Faraday SOC., 19,785 (1924). ( 2 ) A. Hickling, ibid., 3 8 , 27 ( 1 9 4 2 ) . ( 3 ) G. C. Barker and A. W. Gardner, 2. Anal. Chem., 1 7 3 , 7 9 (1960).

(4)B. Breper and F. Gutmann, Trans. Faraday Soc., 4 2 , 6 4 5 (1946). ( 5 ) J. E. B. Randles, zbid.,44, 3 2 2 ( 1 9 4 8 ) . ( 6 ) I. Shain and It. D. DeLIars, Anal. Chem., 2 9 , 1823 (1937). ( 7 ) W.Kemula, Z. Galus, and Z. Kuhlik, Bull. Acad. Pol. Sci., Ser. Sci. Chini., 7, 723 (1959). ( 8 ) G. D. Christian, J . Electroanal. Chem., 23, l ( 1 9 6 9 ) .

measurements of uncommon accuracy, automat.mlls

Because our light-scattering photometer has a uniquely designed index vat which keeps stray light at a negligible level. For example, pure solvent or low molecular weight solutions will give a dissymmetry lower than 1% in the angular range 30-150.And you get unmatched stability, because of a true double-beam system. Operates over the temperature range from - 10°C. to 3OOOC. Fully automatic operation gives calculation corrections of the rough scattering values. No errors. Saves on labor. Over 10 years of improvement and modification has made the FICA 50 line the recognized leader among lightscattering photometers. For more information, call Duane Harmon collect at (716) 385-1000, or complete the coupon below.

It all adds up.

Let u s demonstrate. Bausch & Lomb, Analytical Systems Division, Dept. AC.950.820 Linden Avenue, Rochester, N.Y.14625

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87A

Renaissance in polarographic and voltammetric analysis

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