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Ninety Years of Polarography

2012; Wiley; Volume: 12; Issue: 1 Linguagem: Inglês

10.1002/tcr.201200001

1527-8999

Michael Heyrovský,

History and Developments in Astronomy

Polarography came to existence first as "electrolysis with dropping mercury cathode" in 1922.[1,2] The current-potential curves measured with that electrode, which was until then unusual in electrolysis, came out well reproducible, much better than with any other electrodes. The described measurements of electrolytic current-potential curves with mirror galvanometer were done manually, point by point. The name "polarography" signifying "recording of electrochemical polarization", was introduced three years later, after recording of the electrolytic curves became photographic, automated by the purposely constructed instrument named the "polarograph".[3] Some years of practical experience with polarography at the Department of Physical Chemistry of the Prague University, when it was shown that the dropping mercury electrode can be applied as cathode as well as anode,[4] made the original definition of the method more general: "electrolysis with dropping mercury electrode". The method was coming soon into use internationally, in electrochemical research and in analytical chemistry. In 1938, before the outbreak of the second World War, the total number of polarographic publications[5] was more than 600, and their number increased[5] to more than 1600 by the end of the war in 1945, mainly because of utilization of polarographic analysis in war industry. The success of polarography has been due to its simple principle, and especially to the unique properties of its basic component, mercury. The element mercury is in a wide range of temperatures a liquid of high surface tension, high luster and high electric conductivity. It can be prepared very purely by distillation, if necessary even repeated. Chemically it is a stable noble metallic liquid which dissolves various metals forming amalgams, thus giving the chance to study amalgams polarographically.[6,7] The surface of mercury is smooth, homogeneous and isotropic on the atomic scale. Because of that, in electrochemistry mercury has the highest hydrogen overpotential of all metals, which provides a wide potenial range in the negative direction for electrode reactions on the one hand, and many possibilities for carrying out electrocatalytic processes of hydrogen evolution on the other hand. When the electrode is realized by dropping mercury, it is regularly renewed with constant size and with a clean surface, which results in perfect reproducibility of measurements. The spherical shape of the mercury drop allows to experimentally quantitatively verify various theoretical calculations. Thanks to the above-mentioned advantages it was possible to gradually build up a basic theoretical background of the "classical" polarography, i.e., of simple potential-controlled electrolysis with dropping mercury electrode (DME). Basic equations were derived, above all, for the diffusion-controlled step-shaped polarographic currents or "waves"[8] with their limiting current directly proportional to concentration of the electroactive substance, for the "half-wave potentials"[9] characterizing quality of the electroactive substance, and for the currents controlled or affected by rates of included chemical reactions, the so-called "kinetic currents" and "catalytic currents".[10-13] In polarographic analytical chemistry it became useful to measure the concentration of electroactive substances by titration at constant potential of their limiting current, by the "polarometric"[14] or "amperometric"[15] titrations. Sometimes it was advantageous in place of the primary polarograms to record derivative or differential polarographic curves.[16] Thus the fundamental period of the existence of polarography was more or less accomplished, as was witnessed by the appearance of the first textbooks on polarography.[17-22] Encouraging results of electrochemical research with polarography inspired many scientists to modify the original simple method. When on the d.c. potential applied to the dropping electrode an alternating voltage of small amplitude was superimposed and the resulting alternating current was measured as a function of direct potential, in the thus introduced "a.c. polarography"[23] instead of polarographic waves appeared maxima of alternating current in place of half-wave potentials, and the curves became sensitive to adsorption occurring at the electrode surface. (When applied specifically to study adsorption phenomena the a.c. polarography was called "tensammetry".[24] With advanced electronics the a.c. voltage in a.c. polarography was replaced by square-wave alternating voltage, which resulted in highly sensitive "square-wave polarography".[25] Then to each individual mercury drop from the dropping electrode a single right-angle voltage pulse was applied and the current and measured towards the end of the pulse and of drop-life. That further increased the sensitivity of another polarographic method, called "pulse polarography".[26] Several other techniques were developed from original polarography, maintaining its potential-controlled electrolytic regime and its main part, the dropping mercury electrode. However, the situation changed when the stable hanging mercury drop electrode (HMDE) was introduced[27-29] in polarographic measurements. With HMDE the polarographic waves were replaced by current peaks. For voltage-controlled electrolytic methods in general the name "voltammetry" had been suggested,[30] which was later officially accepted by the terminology committee of IUPAC. From that time polarographic studies in which a hanging mercury drop electrode was used have been called voltammetric, and in technical literature we can now read that "polarography is a subclass of voltammetry where the working electrode is dropping mercury electrode". The HMDE, stable over a wide potential range, could be polarized either by a single linear or by a cyclic potential scan. This offered a further increase of sensitivity by accumulation of a reaction product under constant potential at the electrode for some time and then by dissolving it ("stripping") during the following potential scan.[31] Because in voltammetric measurements with HMDE an identical apparatus can be used as in polarography (but for the indicator electrode), and because the theoretical background of both methods is closely related, voltammetry with HMDE may be considered as part of polarography in wider sense. To accelerate recording of polarographic curves, instead of photorecording with a mirror galvanometer, the cathode-ray oscilloscope had been used with a dropping mercury electrode several decades ago.[32-34] Because of a difference in time-scale the oscilloscopic curves differed from the polarographic ones. They were nearer to the voltammetric curves with current peaks in place of waves; at any rate the method was originally known as "cathode-ray polarography".[32] A more fundamental change was introduced in polarography when instead of potential-controlled electrolysis the current-controlled electrolysis was applied.[35,36] Such method should be called chronopotentiometry with dropping mercury electrode, instead of current-voltage curves from which the potential-current, potential-time derived curves are followed. As the physical conditions of electrolysis are here different from polarography, also the results are in principle different, above all due to the high rate of potential change, depending on intensity of the used polarizing current. These new conditions proved particularly useful in the study of compounds which catalyze hydrogen evolution like peptides, proteins and other biologically important macromolecules. In chronopotentiometry they produce a well developed catalytic "peak H" at negative potentials already in nanomolar and lower concentrations.[37] In analytically highly sensitive applications, like in sensors, the HMDE can be also replaced by some solid amalgam electrodes.[38] Polarization of dropping mercury electrode by alternating current had been used already a couple of decades ago in "a.c. oscillographic polarography".[39] At the time when that method was introduced it was not called chronopotentiometry; it was considered to belong to polarography because of the use of dropping mercury electrode. In the a.c. oscillographic polarography next to dropping electrode also the "mercury jet" or "streaming mercury" electrode[40] was successfully applied. Nowadays the term "polarography" is not usually understood as a single electrochemical method, but a rather wide area of electrochemistry and electroanalytical chemistry based on the unique properties of mercury electrodes. The above outlined complex situation of polarography in science has been expressed in 1959 in the form of awarding the Nobel Prize to Jaroslav Heyrovský "for his discovery and development of the polarographic methods of analysis". To that Heyrovský only privately commented that polarography belongs primarily to physical chemistry. As each of the modifications derived from "classical" polarography is based on a more or less different principle, it has proved useful in experimental research to mutually compare their results in order to understand deeper the electrochemical processes involved. That has become nowadays easily possible with a single computer-controlled instrument when its software program is provided by due principles of the compared methods. In this way the ninety-year old polarography manages again to cope with technological progress, and so instead of going to retirement it has the chance to survive further more years in its useful service to the progress of mankind. Michael Heyrovský J.Heyrovský Institute of Physical Chemistry Academy of Sciences of the Czech Republic Dolejškova 3, 182 23 Prague 8 (Czech Republic) E-mail: heyrovsk@jh-inst.cas.cz This article could be written thanks to financial support by the Czech Science Foundation (grant P206/11/1638), by the Grant Agency of the Academy of Sciences of the Czech Republic (grant IAA400400806) and by the Ministry of Education, Youth and Sports of the Czech Republic (project LC06063).