Otto H. Muller State University of N e w York Upstate Medical Center Syracuse
The Development of Polarography and Polarographic Instruments
I n our rapidly advancing world of scientific instrumentation, one is tempted to start a historical discussion of polarography with the words "once upon a time," even though it is not a very old technique. Just forty years ago, Dr. Jaroslav Heyrovskj., of the Charles' University of Prague, published his first paper on studies with a dropping mercury electrode (I). This led to the development of polarography, a basically new method of analysis, that eventually won him a Nobel Prize in 1959.' However, the origins of polarography may be traced back to 1873 when Lippmann (2) designed his capillary electrometer. A somewhat later form of this instrument is shown in Figure 1. It consists of a column of mercury in the capillary, B, in contact with dilute sulfuric acid which serves as one electrode, while the pool of mercury in the other arm, E, of the vessel serves as the other electrode. Any change in potential hetween these two electrodes produces a change in surface tension of the mercury and thus a change in the position of the meniscus of the mercury in the capillary. Presented as part of the Symposium on History of Equipment and Instrumentation before the Division of History of Chemistry at the 145th meeting of the American Chemical Society, New York, N. Y., September 1963. Supported in part by Grant No. GM 09811-02 from the Division of Research Grants of the National Institutes of Health, U. S. Public Health Servioe. 'For a brief biography of Prof. Heyrovskf, see Z ~ A NP., , m n ELYINQ, P. J., J. CHEW.EDUC., 37, 562 (1960).
320
/
Journal of Chemical Education
Between measurements the electrodes are short-circuited by means of switch, C. This electrometer responds to potential changes of as little as 1 mv and thus can serve and has served as a rapidly responding, sensitive null-point instrument for potential measurements in place of a galvanometer. If the height of the mercury column, representing the surface tension, is plotted against the applied voltage, a so-called electrocapillary curve is obtained in the form of a parabola. A similar curve can be obtained when one plots the weight of the drops of mercury that fall from a fine capillary under a given head of pressure when these drops are polarized to different potentials as one electrode of an electrolytic cell. The size of such drops obviously is also a function of the surface tension. However, when K u k r a (4) compared the electrocapillary curves obtained by means of this "dynamic" method with those obtained by the "static" method of Lippmann, he found in certain instances a distinct difference in the ascending limb of the electrocapillary curves. I n solutions of sodium or potassium sulfate, the dynamic method yielded higher values over a certain potential region which then abruptly changed to the same values as the static method. Having no explanation for this phenomenon, KuEera suggested its study to Heyrovskj. when he was a candidate for the PhD degree. Heyrovsk? eventually showed that dissolved oxygen was responsible for these anomalies, but he also found that the currents flowing
during the polarization of the mercury drops were far more interesting and informative than the drop weights. I n one of his earliest papers on the dropping mercury cathode, published in 1924, he clearly demonstrated that by plotting current-voltage curves he obtained curves with steep portions followed by a horizontal plateau which he called "waves" (5). The height of these waves gave an indication of the concentration of the reacting material, while the "deposition potential" was characteristic of the material reduced and thus served for qualitative analysis.
Figure 1.
tiometric wire is connected to the positive pole of a battery, the other to the negative pole. The positive pole is also connected t,o the mercury layer anode a t the bottom of the electrolysis vessel by means of platinum wire, sealed in glass. The little contact trolley is connected to the dropping mercury electrode (DME), again by means of a platinum contact, in series with a sensitive, low-resistance galvanometer. As the contact trolley travels from left to right on the potentiometric wire, the potential difference between the layer anode and the DME is gradually changed. If the layer anode is large, its potential will remain unchanged while that of the DME will gradually become more negative. The photographic paper is rotated during this procedure past the slit in the light-tight box and the light reflected from the galvanometer then draws on it a currentvoltage curve.
Lippmon capillary electmmeter 131
The First Polarograph
The plotting of these current-voltage curves was tedious and took from one to two hours per curve since the current was observed at 5-mv increments of the applied voltage. This led Heyrovsk9 and his co-worker Shikata to the development of a machine, the polarograph, that would automatically plot these curves, which were called polarograms (6). Results obtained with such a polarograph were first published in 1925. A special May issue of that year, of Recueil des Trauaux Chimiques des Pays-Bas was published in honor of Pmfessor Brauner, an eminent Czech chemist. I n this issue appeared many papers by Czech chemists, among them also the first eleven truly "polarographic" papers. A picture of the first polarograph is shown in Figure 2. It was a home-made machine with a wooden drum of about 40 cm in diameter around which was wound a0.6-mm thick nickelin resistance wire in 20 turns. The ends of this wire were connected to the metal shafts attached t o each side of the drum. Different voltages could then be taken off the potentiometric wire by means of a small trolley on the spring stand. The drum waa rotated by a phonograph motor with a rubbertired wheel that was pulled against the potentiometric drum by means of a spring. Geared to this drum, in a l:20 ratio, was a cylinder, to which was fastened a sheet of photographic paper, inside a light-tight box. The latter had a small slit through which light from a galvanometer could pass. The whole setup is shown diagrammatically in Figure 3. One end of the poten-
Figure 3.
Schematic drawing of polarographic setup 161
The first published polarogram that was thus obtained is shown in Figure 4. It demonstrates that a 0.254' cerium(II1) sulfate solution contained lead as an impurity in a concentration of 5 X 10-= M. To illustrate that organic substances are also subject to polarographic analysis, this same paper contained the polarogram of nitrobenzene in an ammonia buffer, reproduced in Figure 5. I n this case the wave was quite large and had a peak, a so-called "maximum," at its beginning, so that the galvanometer sensitivity had to be reduced to one third of its maximum value, in order to get the whole curve on the polarogram. Heyrovskfs new polarographic method attracted many students and the publications from his laboratory came with ever increasing frequency. By 1929 a most convenient outlet for these publications was the newly Volume 41, Number 6, June 1964
/
321
founded C'ollection of Czechoslovak Chemical Communications which was edited and published jointly by E. Vototek and J. Heyrovskj., until 1947. This journal had its troubles, expecially during the German occupation and later during the conversion of Czechoslovakia to communism when publications had to be temporarily suspended. However, it has survived and is now in its 28th volume which has a larger format and 3000 pages per volume, or about five times as many pages than the early volumes. I n this journal can be found nearly all the important Czech contributions to polarography, published mainly in English, besides German and Russian. Many of these articles are translations of original articles publiihed in the Czech language in Chemicke ListQ.
Figure 4. Fird published polorogram (61.
applied voltage by illuminatiug the whole slit of the camera for an iustant at each complete revolution of the potentiometric wheel. With more polarographs available, there followed a n intensive study by Heyrovskf's school aimed to find the extent and limits of application of the new method. Besides the reduction of the various metals, either free or in bound form, investigations were extended to include the reduction of oxygen, hydrogen ions, proteins, and organic materials; also studies were made of the effects of temperature, of resistance, of capacity effects of the mercury drops, and of the cause of maxima. Although much of the spade work had been done by the end of 1932, the method was hardly known outside Czechoslovakia. However, in the spring of 1933 Heyrovskp altered this situation by a lecture tour through the United States on a Carnegie Visiting Professorship. He spent considerable time a t the California Institute of Technology, the University of California, and Stanford University, where he not only gave a series of lectures but also demonstrated his machine, which was now in the form shown in Figure 6. The improvements in this machine were better contact rings and certain built-in resistances which permitted "anodic and cathodic polarization," a feature that puts zero applied voltage into the middle of the potentiometric wire. I happened to do some undergraduate research on the passivity of metals a t Stanford at the time of IIeyrovskfs visit and became very enthusiastic about the polarograph when I was allowed to try it out on my problems. I n a few minutes, I thus got more information than in hours of tedious work before. My enthusiasm and Professor Heyrovskfs kind encourag~mentinduced me to follow him to Prague.
Figure 6. Figure 5.
One of Heyrovskfs students, V. NejedlfS together with his brother J. Nejedlj., decided that polarography was here to stay and went into the bwiness of manufacturing polarographs and other scientific instruments. I n their more compact instrument, the potentiometric wheel was only about 18 cm in diameter and was made of nonconducting synthetic material instead of wood. The polarograms were now about 10 X 20 cm instead of the 20 X 40 cm for the original polarograph. This machine was driven by phonograph motor, either spring wound, or electric. It also had the new feature of automatically indicating increments of 322
/
Model 1932 polarogroph
First polorogram of an orgonic reduction 161.
Journol of Chemical Educofion
Recollections of Heyrovsky's Laboratory
I n Prague I enjoyed the company of a most active and pleasant group of young scientists, some working for the doctorate, others doing postdoctoral research. At 5 P.M. we met every day informally, while Dr. Heyrovskj. served tea, discussing any new problems that had come up during our work. Figure 7 shows a small portion of this group with Dr. Heyrovskj. and his niece. On the far left is Dr. IlkoviE who contributed so much to the theory of polarography. Next to him is Dr. Gosman who worked on sulfurous acid, anions, and adsorbable substances. The smiling geutleman
is Dr. BrdiEka who had discovered the catalytic effect of proteins and cysteine in cobalt-containing ammonia buffema Next is Dr. Smoler who had studied the reduction of acetaldehyde and fructose. Directly above Dr. Heyrovskj. is Dr. Majer who was developing micropolarographic techniques for the analysis of 0.005 ml of solution. He later developed the technique of polarometric titrations which is probably better known as amperometric titratious. Between the heads of Drs. Smoler and Majer is a portion of a placard on the wall. These placards were typical of Heyrovskj.'~ laboratories. There were a number of slogans, in fairly large letters, posted in every laboratory. They were: "Work, finish, publish!" by Michael Faraday; "A problem solved is dead." by Frederick Soddy; ''Progress is made by trial and failure." by Sir William Ramsay; and "A man must eithw resolve to put out nothing new or to become a slave to defend it." by Isaac Kewton.
Figure 7.
Polorogropherr in Proferror Heyrovrky'r laboratory.
Seoled,
"
H. K ~ n w e i l o v a , ~ JHeyrowky; .
standing (left to right] D. Ilkovic, 8. A. Gorman, R. Brdicka, I. Smoler, V. Mojer, 0.H. Miiller.
My years in Prague were exciting ones because at that time some of the most fundamental discoveries in polarography were made. First, Heyrovskq analyzed the factors which determine the diffusion current, and Ilkovir developed the equation which bears his name. The paper in which this was published (7) is of special interest, because it demonstrates Heyrovskfs generosity concerning publications of his coworkers. Of the hundreds of papers coming from his laboratory, only those few bear his name as co-author in which he contributed more than half the actual work. Reading Ilkovi6's paper on the diffusion current, one 6nds that almost half of it was, in fact, a report of Heyrovskj.'~ work, yet Heyrovsk?~ name does not appear as 00-author. The second great discovery of that time had t o do with the theoretical significance of the half-wave potential. It was published jointly by Heyrovskj. and Ilkovir (8). Growth of Applications
When I left Prague in 1935, I was so convinced of the future of polarography that I bronght an instrument with me as my personal property. I also had hopes Dr. BrdiEka assumed the direction of the Polarographic Institute of the Caeehoslovak Academy of Sciences which Prof. HeyrovskJ.unfortunately had to relinquish because of ill health in t,he spring of 1963.
that it might find me a job somewhere, However, in the United States polarography was accepted very slowly and with much reservation. I was told by one eminent physical chemist that I had been foolish to have spent my time studying polarography since this method had been tried in his laboratory and proved a failure. I was fortunate, however, to find a physiologist, Dr. Baumberger of Stanford University, willing to give me a job since he was anxious t o apply polarography to the study of organic and biological oxidationreduction systems. Jointly we found that the cathodic wave of quinone and the anodic wave of hydroquinone in well-buffered solutions had the same half-wave potential and that other oxidation-reduction systems behaved similarly. I n 1937, we thus gave the first experimental proof of the significance of the half-wave potential and showed the identity of polarographic and potentiometrically determined values. Yet we had considerable di5culty getting this paper (9) accepted for publication. I n those years only few polarographic articles were coming from American laboratories. Ironical as it may seem, the first polarographic report by American authors to appear in an American journal was by Jurs and Noller (10) who never had done any polarographic studies themselves but reported analyses done by a friend. The situation altered rapidly after 1939. A symposium on polarography was held at the annual meeting of the ACS in Dallas, which was subsequently published in Chemical Reviews. Kolthoff and Lingane had carefully reinvestigated the various factors that determine polarographic currents and presented their findings in this review (If), while I had been asked t o present the organic applications of polarography (13). Subsequently the work of Kolthoff and Lingane was published in more detail in numerous papers in the Journal of the American Chemical Society. Eventually all of this material was collected into their book "Polarography," published in 1941 (IS). This book was so well written that it has been called the "polarographic bible." A second enlarged edition was published in 1952 and a third revised edition is in preparation. I n 1941 also several articles I wrote for the Journal of Chemical Education were collected into a book entitled "The Polarographic Method of Analysis" (14). It was designed to serve as a primer in polarography. I n this same year, Heyrovskj. also published a book on polarography in German (15). Only few copies reached this country because of the war, but fortunately Edwards Brothers lithoprinted this book under License from the Alien Property Custodian so that it became readily available. After the war it was the only source for Heyrovskj. to get copies of his book since the stock in Europe had been bombed out. Since the war, publications have appeared at an ever increasing rate. Fortunately bibliographies of these papers have been compiled by Heyrovskj. and published annually in the Collection of Czechoslovak Chemical Commtcnicatims. The schools of Kolthoff and his students, notably Lingane, Laitinen, and Wawzonek, have produced many excellent polarographers so that this new type of analysis is now well known throughout this country, and one no longer needs to introduce a lecture on polarography with the words: 'LPolarographyhas nothing to do with polarized Light." Volume 41, Number 6, June 1964
/
323
Improvements of Design
I n the polarograph I brought with me from Prague in 1935, the contact trolley had been replaced by a sliding spring contact and the connection between potentiometric wire and battery cable was accomplished by multiple strong springs sliding over special slip rings. (In later models, these were replaced by mercury cups.) Later models of photographically recording polarographs demonstrate the newer trend of putting apparatus in boxes with knobs on the outside. I n 1938 even the Czech polarographs, now no longer made by the Nejedlfs, started to go underground with the socalled micropolarograph which is shown in Figure 8.
Figure 8.
Micropoiarograph.
I n this instrument any possible contact troubles have been minimized by the use of a single turn of spiral wire on the potentiometric drum, with its ends permanently soldered to flexible wires. After one complete turn of this drum, it has to be brought back to the starting position manually, a move that automatically rewinds the spring of the driving motor. I n this polarograph for the first time the galvanometer and shunt, as well as its illuminating light, are all enclosed in the same black box. A similar polarograph is manufactured in this country, but in most instruments on the market today photographic recording has been replaced by pen-and-ink recorders. An early design in which the advantages of the mirror galvanometer and strip chart recorder are combined is shown in Figure 9. Here a photocell mounted on the pen carriage activates the pen drive motor whenever the galvanometer is deflected (16). Most pen-and-ink recorders depend on the amplification of very small voltage drops across fixed resistances placed in the circuit that includes the dropping mercury electrode. For instance one such instrument in my laboratory has a recorder with a maximum sensitivity of abont 9 X v/mm. It is equipped with !?xed resistances varying from 5 to 3000 ohms, any one of which can be placed in series with the DME to produce "current" records varying from 0.003 to 1.5 @a/mm. Current measurements obtained in this way will be the same as those obtained directly with a galvanometer, as long as the fixed resistances are small. One can readily calculate that with a resistance of 324
/
Journol of Chemical Education
10,000 ohms and a current of 1 Fa one would have to correct the applied voltage by 10 mv for the IR drop across the resistor. Saayer, Pecsok, and Jensen (17) and others have shown that the use of a third electrode can provide automatic correction for the I R drop in the circuit. They have constructed a n apparatus that plots directly the electrode potential, rather than the applied voltage against the current. However, even after such corrections are made either automatically or by calculations, one still har, other resistance effects to consider. It is known that polarographic maxima are deformed by the presence of resistance because of some metastable potentials. This has been discussed
REG CW
-PWER
DRWE MOTOR
POTENTIOMETER
Figure 9. Pen-and-ink recording polorogroph employing minor galvanometer ond photocell (16).
Figure 10. Peirker orrongement for effect. in DME circuit (19).
elimination of high resistonce
in some detail by Peizker (18) who designed an ingenious circuit, similar to that of Figure 10, which permits a pen-and-ink recording without the need for an extra resistance in the main circuit (19). Notice that the main circuit, on the left, is the basic polarographic circuit, with potentiometer PI, dropping mercury electrode, D, and a calomel layer anode, C, as well as a galvanometer, G. However, in a second circuit which also contains the anode, C , and galvanometer, G, one finds a third electrode, E and the high resistance, R, besides a second source of variable volbage, Bpand P2.
The latter is controlled photoelectrically by the galvanometer in the main circuit which is used as a nullpoint instrument. Thus the current of the main circuit is exactly compensated by current with opposite direction of flow in the second circuit. The drop of potential across the resistance, R, is used for plotting current on the Y axis of the recorder, while the potential across the cell is recorded on its X axis. The resistance, R, can be very high since it does not affect the main circuit so that very small currents can be recorded. Of the numerous polarographs with pen-and-ink recorders that are on the market today, several are manufactured in this country; but because of trademark restrictions, only one manufacturer can use the name "polarograph." The instruments vary in design, in the size of the records, and in the method used for obtaining them. Depending on the precision of the parts, these instruments differ also in stability and reproducibility of the curves. They also vary in the number and kind of additional circuits built into them. For instance, many of them have facilities for current compensation, either to balance out the linearly increasing condenser current or to balance out a preceding large diffusion current so that a subsequent wave may be measured at higher sensitivity. In the latter case, there may also be provision for damping of the large current oscillations by means of condensers. Lately some instruments have been further equipped with circuits to permit the automatic recording of derivative curves. The latter often help to distinguish overlapping waves because small changes in the slope of the current-voltage curves are more easily noticed in the derivative curves. Another recent development is the speeding up of the recording of current-voltage curves by the use of mercury electrodes that are made to drop rapidly by means of mechanical knockers. The recording apparatus can then be speeded up to draw a complete curve encompassing a two-volt span in less than one minute. Because of the fast dropping rate, the current oscillations are very much reduced. One design that, to my knowledge, has not yet been made commercially, hut which could well he made as an attachment to existing polarographs is the Kalousek circuit (80) for testing the reversibility of polarographic reactions. Kalousek used a commutator method which made the DME alternately cathode and anode, as illustrated in Figure 11. The slidewire of the polarograph, PI,and a variable resistance of similar magnitude, P,,are connected in parallel to a battery. The calomel layer anode a t the bottom of the electrolytic cell, E C is connected to the positive terminal. By means of switch, Sz, the DME can be connected either directly to the galvanometer, G, to provide the ordinary polarographic circuit, or to the commutator, C. The commutator can be a mechanical device or an electronic switch. The switch S , is usually in position 1, but is placed in position 2 when one wishes to find the proper setting for P2. With the commutator method and a switching rate in the applied voltage of 5-8 per sec, Kalousek obtained anodic waves of the freshly reduced product with identical half-wave potentials as the material which was reduced, as long as the reaction was reversible. If the reaction was irreversible, either no corresponding anodic wave, or an anodic wave with a different half-wave potential was obtained.
The tendency to speed up polarographic analyses has had its most promising results in the development of oscillographic and alternating current polarography. Electronic instrumentation has, indeed, advanced so far that it has made important contributions to polarography and increased its sensitivity. I n conventional polarography the rate of change of the applied voltage from zero to -2 volts is very slow compared to the dropping rate of the electrode. It is usually so slow that for any given drop the applied voltage may be considered constant (this is certainly the case in manual polarography). I n oscillographic pohrography the reverse is true. The same voltage sweep may be repeated many times during the growth of a single drop of mercury, or it may be synchronized to the drop time. Furthermore the sweep may be linear (saw-tooth) or in the form of a sine, triangular, or square wave. I n ac polarography only a small alternating potential increment of a few millivolts is superimposed on the conventional increase in the voltage applied from a potentiometer.
Figure 1 1 .
Kolousek circuit for testing reverribility of reaction, at the
DME 120).
Use of Orcilloscopes
Matheson and Nichols, in 1938, were the first to use a cathode ray oscillograph in studies with the DME (91). They succeeded in synchronizing a linear sweep of about 2.4 v and a frequency of 60 c/sec with a capillary that produced mercury drops a t the same rate. Thus they obtained a steady picture on the screen of the oscilloscope with waves similar to those of an ordinary polamgram. Unfortunately the waves had peaks that were not true maxima but depended much on circuit resistance; also the charging current was too high. As a consequence, their technique found no adherents until 10 years later when Randles ($2) designed a more suitable apparatus. He minimized the influence of the charging current by applying a linear sweep only once during the last portion of the life of a drop when the rate of increase of the surface area of the drop is very small. This is shown schematically in Figure 12. At the top we see the change in surface area as a function of time. In the middle is indicated the linear sweep near the end of drop life, and a t the bottom, we see the resulting oscillographic record in the circled area which represents a current-voltage curve. Some typical results obtained in this way are reproduced in Figure 13. They show peaked curves, whether obtained a t a stationary platinum electrode (curve A), or at DME Volume 47, Number 6, June 1964
/
325
(curve B). Sotice that several ions can be analyzed simultaueously. Randles studied the various factors that can influence this peak height and concluded that it was strictly proportional to the concentration of the reducible material, that it was independent of drop time but proportional to the surface area of the drop, and that it was a function of the square root of the rate of change of potential. He also developed a graphical solution of the diffusion equation which fitted the experimental data. Recent modifications of this method, which make it suitable for the accurate analysis of depolarizers in concentrations as low as lo-' d l , have been described by Vogel(23).
Figure 12. Changer in electrode area [Al, applied voltage IEI and current lil as o function of time in lineor sweep oscillopolorogrophy. The part of the curve observed on the orcillorcope ween is indicated by o circle 1231.
Figure 13. Typical orcillopoiaiogrophiccurveri221. Left, ' i , X 1 0 - 8 M ,a X 1 0 . " M Pbtt. Cdi'- in 1 M KCI, rtotionory Pt electrode; right, in 1 M KC). DME. Cdi4, Znit
While these developments were still unknown, Heyrovskj. and Forejt ($4) had pursued another type of oscillographic polarography, which they published in 1943. I n this a sine or square-wave potential change of several volts is applied to the cell, and the potential changes a t the microelectrodes are recorded on the screen of the oscilloscope against tim?. I n the absence of depolarizers, the potential will then oscillate between the most positive potential caused by the oxidation of mercury and the most negative potential produced by the reduction of the "indifferent" cation. As shown in the left upper curve of Figure 14, these two limiting potentials are connected by straight lines, one of which represents the abrupt change from the positive to the negative potential (the reduction curve), the other the reverse (the oxidation curve). If now a depolarizer is added to the indifferent electrolyte, the change in potential will be momentarily halted a t a value corresponding to the oxidation-reduction potential of the depolariaer and the delay will be a function of its con326 1 lournol of Chemkol Educofion
centration. The oscillographic potential-time curve will therefore show a %nickwhich will be a t the same potential of the two arms of the square wave if the reaction is reversible (see left lower curve of Figure 14). If not, the knick on the reduction curve will he at a different potential as the knick on the oxidation curve, or the latter may be absent altogether.
Figure 14. Orcillogrom$ of the functions E = fit), dE/df = f'itl, and dE/dt = fUIE). Upper ~ u r v e r : 1 M KOH; lower curver: 1 M KOH containing 1 0 - 3 M Pb++ 1251.
By recording the derivative of such curves, dE/dt versus time, the knicks appear as inscisuras on peaked curves which are much more easily measurable and hence can be used for quantitative analysis (see the middle curves of Figure 14). Difficulties in these analyses resulting from the continually changing size of the DME were avoided by Heyrovskj. and Porejt by use of a streaming mercury electrode. I n this electrode, which shoots a very fine stream of mercury from beneath the solution through its surface, a fresh electrode surface of constant area is contiuually exposed. Another way of overcoming the above difficulty is to illuminate the cathode-ray screen a t exactly the same moment during the growth of a drop of mercury. With a persistent screen and a mechanically regulated drop time, a good standing wave can thus be obtained also with the DME. I n a further impr07-ement, Heyrovskq has introduced the plotting of the derivative, dE/dt, versus the potential instead of time. The resulting curves have lost all resemblance to conventional polarograms but are nevertheless analogous, because voltage is plotted on the horizontal axis and d E / d l (which corresponds to the current) on the vertical axis (see the last group of curves of Figure 14). Such curves are essentially the drawn-out portions of the middle group of curves from which the uninformative period, during which the potentials remain a t the two extremes, has been eliminated. (The uninformative period has been transferred to the two pronounced dots a t either end of the curve.) I n reading the curves, one has to remember, however, that the oxidative curve in the middle set of curves is written from left to right, while it is written from right to left in the lower portion of the derivativepotential curves. The latter procedure has the advantage that one sees a t once if the inscisuras of the oxidative and reductive curves occur at the same or a t different potentials. The depth of the inscisura is
unfortunately not a linear function of the concentration of the depolarizer so that calibration curves are essential. The measurement of the inscisura has been simplified in an instrument, called the "polaroscope," in which a horizontal sweep is projected on the screen of the cathode ray tube and its displacement from top to bottom of the inscisura is evaluated with a potentiometer. Of considerable theoretical and practical interest is the fact that such inscisuras are produced not only during oxidations and reductions at the DME but also through capacitative effects brought about by surface-active material. Many substances that are electroinactive, i.e., which cannot be reduced or oxidized electrochemically, have turned out to be surface-active and are thus subject to oscillopolarographic analysis. This new application of polarography has already progressed to a stage that Heyrovskj. and Kalvoda could publish a special monograph (25) in which many of the details of its application are described. Somewhat similar applicatious have been developed by Bieber and Trampler (26) and Sevrik (27) who used periodical triangular voltage sweeps, but they have not found many applications to date.
substances could also be similarly used for analytical purposes. To distinguish these peaked waves from the ac polarographic waves, they called them "tensammetric" waves. Figure 15 shows both kinds of waves and the corresponding conventional polarograms. Note that the presence of the surface-active cyclohexanol only changes the slope of the cadmium wave in the conventional polarogram, while it completely wipes out the pseudo-capacity wava of the cadmium in the alternating-current polarogram.
Alternating-current Polarography
Alternating-current polarography, i.e., the study of the effects of a small alternating current superimposed on the conventional polarographic current-voltage curve, also had its beginning in 1938. In that year, Miiller, Garman, Droz, and Petras (28) found that a smooth sine wave is obtained on the oscilloscope screen when such a small alternating current is added while the applied voltage has reached the half-wave potential of some reducible substance in solution. This principle was applied by Boeke and van Suchtelen (29) in the construction of an apparatus in which the polarographic cell forms one arm of an alternating-current bridge. This permitted both quantitative and qualitative analysis of the depolarizers in solution. In more recent studies, investigators have concentrated on the capacitance effects brought out by such superimposed alternating currents. The differential capacity of the DME, which reaches a minimum a t the electrocapillary zero, will become exaggerated near the polarographic wave of a reversible system and reaches a maximum at the half-wave potential. Grahame (SO), who studied these effects intensively, has called this effect a "pseudo-capacity," because it is produced by the electroreduction and reoxidation of the reacting material during cathodic and anodic cycles of the alternating voltage. No such pseudo-capacity is observed in irreversible reactions, but equally marked changes in the differential capacity are found when surface-active substances are adsorbed, although in that case no electrolysis is occurring (Sf). Breyer, Gutman, and Hacobian ($2) have constructed a relatively simple apparatus which makes use of the pseudo-capacity for qualitative and quantitative analysis. Their method is essentially a differential alternating-current polarography, and their ac polarograms show peaks at the half-wave potential of reversibly reduced substances which, under proper conditions, are proportional to concentration. Breyer and Hacobian ($8) further demonstrated that the differential capacity changes produced by surface-active
-VOLT*
Y. S.C.E.
Figure 15. Effect d cyclohexanol on the alternating-current and conventional pdorogromr of 10-s N Cd++ in 0.1 N KCI. lo1 and (bl, alternating-current polarogrornr; ( 4 and (dl mventionol polarogramr (01 and (4 without, (b) and (dl with 0.1 N cyclohexmol 1331.
Baker (34) has described a square-wave polarograph which employs a square wave with a frequency of 225 c/sec and a strobiug circuit which delays the application of this square wave for about 2 sec. I n addition, a pulse filter is used to separate the alternating component of the cell current from any slowly changing diffusion current. This permits minor constituents to be detected in the presence of the major constituent when the concentration ratio is as small as 1 :50,000. Figure 16 shows a typical square-wave polarogram obtained with this instrument. It looks very much like derivative curves obtained in conventional polarography, but it is much more sensitive, permitting the detection of reversibly reduced ions at concentratione M . A further improvement in this as low as 4 X technique seems to be the use of higher frequency currents, such as radio frequency of about 200 kc/sec, instead of the square wave. A change in the opposite direction, i.e., a reduction in frequency of the square wave has resulted in the development of "pulse polarography" (34). Rectangular pulses of about 0.04sec duration are applied to the cell, but only once during Volume 4 1 , Number 6, June 1964
/
327
the lifetime of the drop of mercury, usually 2 sec after the fall of the previous drop. These pulses are small (about 30 mv) and are superimposed on a slowly changing voltage. The polarogram look like oonventional polarogram but are more sensitive, permitting the detection of reversibly reduced and irreversibly reduced depolarizers a t concentrations as small as lo-' M.
(3) MICHAELIS,L., in ~ D E R E A L D E N ,E., "Handbuch der Biochemisehen Arbeitsmethoden," Vol. V., Urban and Schwarzenberg, Berlin, 1911, p. 511. (4) KufEm, G., Ann. Physik, 316, 529, 698 (1903). l , Trans. Faraday Sac., 19, 692 (1924). (5) H ~ w o v s ~J., P , A N D SHIKATA,M., Rec. trav. ehim. Pays(6) H ~ w o v s ~ J., Bas, 44, 496 (1925). D., Collection Czechoslov. Chem. Cmmuns., 6, 498 (7) ILKOVI~, (1934). J., AND ILKOYI~, D., CoUedim Ciechoslou. (8) HEYROVSKP, Chem. Communs., 7 , 198 (1935). , H., A N D BAUMRERQER, J. P., Tran8. Electro(9) M ~ L E R0. chem. Sac., 71, 181 (1937). C. P., AND NOLLER,C. R., J. Am. Chem. Soc., 58, (10) JURS, 1254 (1936). I. ,M., AND LINGANE, J. J., Chem. Rev., 24, 1 (11) K o ~ ~ n o m (19R91 - - - - ,. (12) MWLLER,0. H., Chem. Rev., 2 4 , 9 5 (1939). I. M., AND LINQANE,J. J., "Polarogrephy," (13) KOLTHOPP, Interscience, New York, 1941, 1st ed., 1952,Znd ed. (14) MDLLER,0. H., "The Polarographic Method of Analysis," Chem. Education Publishing Co., Easton, Penna., 1st ed. 1941, 2nd ed. 1951. J., 'rP~larographie,"Springer, Vienna, 1941. (15) HEYROVSKP, D. J., AND WEAVER,J. R., Ind. Eng. (16) LYKKEN,L., POMPEO, Chem., Anal. Ed., 17, 725 (1945). D. T., PECSOK,R. L., A N D JENSEN, K. K., Anal. (17) SAWYER, Chem., 30, 481 (1958). (18) PEISKER,J., CollectiDn Czechoslm. Chem. Communs., 24, 2122 (1959). (19) PEIZKER,J., CoNeclion Crechoslou. Chem. Cmmuns., 24, 2416 (1959). M., Collectin Czeehoslou. Chem. Communs., 13, (20) KALOUSEK, ins --- ( i u m (21) MATEESON, L. A,, AND NICHOS, N., Trans. Electrochem. Soe., 73, 193 (1938). J . E. B., Trans. Faraday Sac., 44, 322 (1948). (22) RANDLES, P., AND KOLTHOPF, I. M., "Progress (23) VOGEL,J., in ZUMAN, in Polaroma~hv." - . Vol. 2, Interscience. New York. 1962. p p 429-48. J., AND FOREJT,J., Z. ph~sik.Chem., 193, 77 (24) HEYROVSK*, \
Figure 16.
Square-wove polarogram for 1 M KC1 containing 2
Cuf+,Pb++ ,TI+ , Cd++,Zn++,and
X 10-6M
4 X 10-@MIna+1341.
\----,-
Conclusion
Obviously the instruments described in this brief discussion cannot include all the numerous designs that have been published or are commercially available. Much additional information may be found in the excellent series of articles by S. Z. Lewin that have appeared recently in the section on Chemical Instrumentation in THIS JOURNAL (55). This author not only describes the features of the various polarographs but also mentions their manufacturer or distributor in this country as well as their price. I have purposely refrained from mentioning development of polarography into other specialties, such as polarometric or amperometric titrations, coulometry, chrono-potentiometry, chromato-polarography, and polarography with the hanging mercury drop or with stationary and rotating electrodes of mercury or other metals. What has been presented I consider to be the most essential features of the newer instrumental developments as well as the history of the early Czech models. Literature Cited
(1) HEYROVSKP, J., Chem. Listy VZdu P~rzZmysl,16, 256 (1922). G., POQQ.Ann., 149, 547 (1873). (2) LIPPMANN,
328
/
l o u r n d o f Chemical Education
.
llR4R)~ \----,.
(25) HEYROVSKP, J., A N D KALVODA,R., "Oszillographische Polarographie mit Wachselstrom," Akademie Verlag, Berlin, 1960. (26) BIEBER,R., A N D TRWMPLER, G., Helu. Chim. Aetn, 30, 971 (1947). (27) S E V ~ ~A,, K ,Collection Czeehoslm. Chem. Commus., 13, 349 (1948). R., H., GARMAN, R . L., DROZ,M. E., AND PETRAS, (28) M ~ L L E R J., Ind. Eng. Chem., A n d . Ed., 10, 339 (1938). H., Z. Elektmchem., 45, (29) BOEKE,J., AND VAN SUCHTELEN, 753 (1939). D. C., J. Am. Chem. Soc., 63, 1207 (1941). (30) GRAHAME, D. C., Chem. Rm., 41, 441 (1947). (31) GRARAME, (32) BREYER, B., GUTMAN, F., AND HACOBIAN, S., Australian J. Sci. Resea~ch.A3. 558. 567 (1950). . . (33) BREYER, B., AND H~COBIAN,S., A1~6tmlian J . Sei. Research, A5, 500 (1952). (34) BARKER,G. C., in ZUMAN,P., AND KOLTBOPF,I. M., "Progress in Polnrography," Vol. 2, Interscience, New Yark, 1962, pp. 411-27. (35) LEWIN,S. Z.,J. CHEM.EDUC.,39, A261, A355, A445, A519
