Electroanalytical instrumentation—how it all started: history of electrochemical instrumentation

Abstract

This review is a historic collection of old electrochemical and electroanalytical instruments, mostly concentrated on polarographs, potentiostats, pH meters, and titrators designed and commercially available from the 1920s to 1970s. The review briefly explains their operation and shows their photos. It is addressed to electrochemists who want to know what kind of instruments was used by previous generations of scientists and how the progress in electronics brought the instruments from very primitive to more sophisticated devices, still being far from the presently used computerized electrochemical analyzers. This review has a more educational aim rather than scientific in the present definition of being “scientific.” The collected references allow interested readers to find more scientific information on the electronic circuits used in the highlighted instruments, on the instruments operation and applications.

Graphical Abstract

Introduction

While the present/modern state-of-the-art of electrochemical theory, applications, and instrumentation is well known by researchers, including students, the history of science and particularly history of the used instrumentation are not broadly known. Partially, it is because of the lack of interest, especially among young scientists, but mostly due to the lack of available information and its absence in the electrochemistry courses offered to students. Very few review articles covering the history of electrochemical progress, including progress in the electrochemical instrumentation, are available [1,2,3,4,5,6]. It should be emphasized that the knowledge of science history, in other words, how it all appeared and progressed, is really important for the full understanding of the present state and future perspectives. There are good reasons to justify the historic overview, particularly for electrochemistry [7]. This is also the strong opinion of the authors of the present historic review and the motivation driving this article. The aim of the present review is to show how the electrochemical instrumentation has been progressing in parallel with the scientific advances and applications. Indeed, the improvements in the available instrumentation allowed progress in science and, on the other hand, the requirements of the scientific studies resulted in novel instrumental abilities to satisfy the experimental needs. In other words, the progress in science and the development of the instrumentation are interconnected and interdependent. There is a strong influence of progress in electronics on the development of the electrochemical instruments. Moving the electronics from vacuum tube amplifiers and then switching tubes to transistors, operational amplifiers, and finally to very sophisticated modern electronics resulted in tremendous progress in the instrumentation. Since the electroanalytical chemistry received a major boost with the invention of polarography by Jaroslav Heyrovský (Nobel Prize winner in 1959) in the mid-1920s, presently, after about 100 years of the progress, it is a good time to look back and overview how the progress was achieved with the improved instruments available.

It should be noted that the term “polarography” was introduced in 1924 by M. Shikata and J. Heyrovský for the electroanalytical measurements performed with a dropping mercury electrode (DME). Much later, in about 1940, the term “voltammetry” was coined by H. A. Laitinen and I. M. Kolthoff to describe measurements of current as a function of potential applied on small electrodes, mostly meaning DME used in polarographic measurements, but later, it has grown to encompass many other types of electrochemical/electroanalytical techniques. The origin of polarography, the importance of polarographic measurements performed with a DME, and the advantages of the DME compared to many other electrode materials are briefly explained in the excellent review of Bard [1].

The present review concentrates on the instruments produced for electroanalytical chemistry (mostly polarographs, potentiostats, pH meters, and titrators, briefly including other instruments) from the mid-1920s till approximately 1960s-1970s, then leaving aside more recent instruments which are better known by researchers performing electroanalytical studies (Fig. 1). The electrochemical instrumentation overview from 1970s till the present time deserves a separate review article, which may become available in the future.

Fig. 1

Progress in electrochemical instrumentation. The research team, photo in the middle: PhD student Evgeny Katz (standing, left), Dr. Yuriy Kozlov (standing, right), and Prof. Boris Kiselev (sitting). Photos adopted from http://www.heyrovsky.cz/data/dokument/soubor/76-83_heyrovsky_final.pdf, left; and [2] center and right with permission

Electrogravimetry—forerunner of electroanalytical methods

Electroanalytical studies were performed as early as in the mid-nineteenth-century. Particularly, the analysis of metals was performed by their electrochemical deposition on an electrode followed by their gravimetric analysis, then resulting in the development of the electrogravimetry technique [8]. The very beginning of this method originated from the work of Oliver Wolcott Gibbs, an American chemist [9, 10] (Fig. 2A), who analyzed copper and nickel with their electrochemical deposition. In 1880, Carl Luckow, a German chemist, wrote a highly important paper about the use of the electric current in analytical chemistry [11] (Fig. 2B). Later, the electrogravimetry technique was successfully applied to the analysis of nickel [12,13,14,15], copper [12, 13, 15] cobalt [12, 13, 15], lead [14], zinc [14], manganese [14], cadmium [16], and other metals [8]. Until the emergence of all the modern varieties of electrochemical analysis techniques, starting from polarography and later extended to various voltammetry methods, this technique was simply named electroanalysis, because electrogravimetry was the only known electroanalytical method at that time. Alexander Classen (Fig. 3A) and Edgar Fahs Smith (Fig. 3B) greatly contributed to the development of electrogravimetry method in Germany and the USA, respectively. Edgar Fahs Smith has written a monograph on electrogravimetry, which provided a review on the method, summarizing the results obtained by 1890 [17]. This book was translated and published in different languages, being important for popularizing this kind of electroanalysis. Smith used rotating electrodes and demonstrated that the convection facilitated the mass transport and significantly shortened the time required for the analysis. The later development of the electrogravimetry was overviewed in comprehensive review articles [18, 19].

Fig. 2

Adopted from https://en.wikipedia.org/wiki/Oliver_Wolcott_Gibbs#/media/File:Gibbs_Oliver_Wolcott.jpg; public domain. B Luckow’s electrode arrangement for the electrodeposition of metals followed by their gravimetric analysis. Adopted from [8] with permission

A Oliver Wolcott Gibbs (1822–1908), American chemist.

Fig. 3

A Alexander Classen (1843–1934); the lithography with original signature. B Edgar Fahs Smith (1854–1928). Adopted from [7] with permission

Potentiometric titration also appeared in the nineteenth century [20], and then became a broadly used electroanalytical method [21] (see special sections on the titration instruments below). Many other electroanalytical methods, e.g., coulometry, have been extensively studied in the second part of nineteenth and the beginning of twentieth centuries, then providing a solid background for further progress in electroanalytical chemistry, particularly boosted by Jaroslav Heyrovský with invented by him polarography.

The first polarograph made by Heyrovský and Shikata

While electrolysis of organic and inorganic compounds with a goal to produce new chemicals was already a popular research direction, the use of electrochemistry for analytical applications was limited by electrogravimetry and potentiometric titration. A real boost in electroanalytical applications started in the beginning of 1920s, when Jaroslav Heyrovský initiated his research leading him to the discovery of polarography—a novel analytical technique based on measurements of current with a variable electric potential applied on a mercury electrode [22]. However, the investigation of the electrochemical reactions with the dropping mercury electrode (DME) by Jaroslav Heyrovský and his research group in the early 1920s was limited by the fact that the point-by-point measurements and plotting of current–voltage curves were tedious and time-consuming. Considerable improvement was achieved in 1924, when the cooperation of J. Heyrovský and M. Shikata (a research assistant in the Heyrovský group), Fig. 4A, led to the construction of an apparatus which registered current–voltage curves automatically (Fig. 4B). The new instrument recorded photographically such a curve in several minutes, whereas manual recording took an hour or longer. The development of an automatic method of recording, a short time after the discovery of the new technique, played an important role in the development and dissemination of the method based on electrolysis with a dropping mercury electrode.

Fig. 4

A Jaroslav Heyrovský (right) and Masuzo Shikata (left), 1923 [22]. B The very first original polarograph of J. Heyrovský and M. Shikata (1924) [2]. Adopted from [22] with permission and from [2], courtesy of Heyrovský Institute of Physical Chemistry, Prague

Commercially produced photo-recording polarographs

The first electroanalytical instrument (polarograph) developed by Heyrovský and Shikata was a photographically recording instrument. Hard to believe, but it recorded current–voltage curves (polarograms) in the form of photo pictures (note that the photos were not digital like now, but required chemical processing).

The Heyrovský-Shikata instrument (Fig. 4B) consisted of a spring-driven phonograph motor, a potentiometric wheel, and a device geared to this wheel in a fixed ratio for recording galvanometer deflections (Fig. 5). The potentiometric wheel was rotated by means of a friction drive, and thus, the voltage applied to a dropping mercury electrode was increased. Simultaneously, but at a much lower rate, a sheet of photographic paper, fixed on a drum, was moved past a slit in an otherwise light-tight box. Light reflected from the mirror galvanometer was focused on this photographic paper. Changes in current obtained during the change in voltage would then deflect the galvanometer and be recorded as a curve with the voltage abscissa and the current as the ordinate. This rather simple device operated as a new electrochemical device named “polarograph.”

Fig. 5

Adopted from https://knowledge.electrochem.org/encycl/art-p03-polarography.htm; public domain

Schematics of the first Heyrovský-Shikata apparatus for measuring current–voltage curves in electrolysis with dropping mercury electrode for manual measurements (1922): A, battery; K, potentiometric wheel; V, voltmeter; G, mirror galvanometer; Z, electrochemical cell with a dropping mercury electrode and a counter electrode; L, source of light; S, scale (photographic paper); R, variable resistance; I, inlet and outlet for Ar or N₂ needed for deaeration. Note that the term “polarograph” was introduced only in 1925.

The photo-recording polarographs were the only available automatic instruments for electrochemical (polarographic) measurements for quite a long time. This instrument was the very first automatic recording analytical instrument described, decades before electronic recorders were available. Although Heyrovský and Shikata did not patent the instrument, they obtained a copyright on the name they coined for the technique and the instrument: “polarography” and “polarograph,” respectively.

Following the conceptual design of the first polarograph of Heyrovský and Shikata, many other models of similar instruments received further development and engineering resulting in new commercial models on the market. The first commercially available polarograph based on Heyrovský-Shikata design was produced by the company of Drs. V. and J. Nejedly, Prague, Czechoslovakia (currently Czech Republic) (Fig. 6). The modified Heyrovský’s polarograph, Model XI, was manufactured by E.H. Sargent and Co., Chicago (Fig. 7A). This instrument was self-contained, simple to operate, and in general was quite satisfactory. Further development of the Heyrovský’s polarograph resulted in a new Model LP-55 (Laboratorni pristroje, Prague) which was used in Heyrovský’s lab (Fig. 7B, C). The name “Polarograph” was a registered trademark of E. H. Sargent & Company, which manufactured manual (Model III) and photographic (Model XI) versions of the device. Other companies also produced similar instruments, but because of the copyright issues, had to call them by other, rather imaginative, names, such as the “Elecdropode” (Fisher Scientific Company) or the “Polarecord” (Metrohm, Ltd.)—see these examples in the following section of this review.

Fig. 6

Adopted from https://www.muv.uio.no/uios-historie/fag/matematikk-naturvitenskap/kjemi/polarograf-for-dosent-prytz.html and https://nasregion.cz/byl-prvnim-a-bohuzel-dosud-i-poslednim-ceskym-vedcem-ktery-obdrzel-nobelovu-cenu-profesor-heyrovsky-a-kapka-rtuti-142115/ with permission

Early commercial model (V-301) of the Heyrovský-Shikata’s photographic recording polarograph manufactured by the company of Drs. V. and J. Nejedly, Prague, Czechoslovakia.

Fig. 7

A Sargent-Heyrovský photographic recording polarograph (Model XI) (adopted from [1] with permission). B Polarograph LP-55 in Heyrovský’s laboratory. C Prof. J. Heyrovský with his polarograph LP-55, slightly different version (photos B and C are reproduced with permission; courtesy of Heyrovský Institute of Physical Chemistry, Prague)

The construction of photographically recording polarographs of the Heyrovský-Shikata type has been described by Furman et al. [23], and then by Abichandani and Jatkar [24] and Lingane [25]. Notably, James J. Lingane (Fig. 8) greatly contributed to the construction of polarographs based on the Heyrovský-Shikata concept, but with improved operation. Philbrook and Grubb described an improved wiring diagram for the Sargent-Heyrovský polarograph and a vibration-free mounting for this instrument [26]. Modifications of the circuit of the Sargent-Heyrovský polarograph have also been described by Baumberger and Bardwell [27]. The photographically recording polarographs manufactured by the German companies Leybold, Köln, and Hellige, Freiburg i. Br., are described in details in von Stackelberg’s monograph [28].

Fig. 8

Adopted from Bard AJ, Inzelt G, Scholz F (Eds.) (2008), Electrochemical Dictionary, Springer, with permission

James J. Lingane (1909–1994), American electrochemist.

It is very interesting to reproduce a description of developing of polarograms recorded with these polarographs [29] (the English and spelling correspond to the original text):

“Operation which is common to all types of photographically recording polarographs is the processing of the photographic paper. The paper carrying the recorded curves is taken out of the photographic box in a darkroom (this can be a small dark box on the laboratory desk) into which only red or orange light is allowed. The following solution is recommended for the developing process (it may be modified according to the type of photographic paper used); 1.5 g of metol, 30 g of anhydrous (or 60 g of crystalline) sodium sulphite, 6 g of hydroquinone, 45 g of anhydrous sodium carbonate (or 120 g of the crystalline salt), and, especially during the summer months, 0.3 g of potassium bromide, all dissolved in 1 L of distilled water. The developing process is usually followed visually and is halted once the abscissae are clearly shown. At 20 °C the developing time is 0.5–2 min. A longer time often results in a dark shadow covering the paper. Instead of pouring the developer from the photographic dish back into a flask, or leaving it in the dish where the air has access to a large surface area, it has been found useful to store the developer in a tall cylinder. The developing can also be carried out in this vessel and, under these conditions, autoxidation is considerably reduced.

Once the developing process has been completed, the paper is immersed in a stop-bath, which usually consists of 1–10% acetic acid solution. The polarogram is then transferred to a fixing bath. This is a solution containing 200 g of sodium thiosulphate, 10 g anhydrous or 20 g crystalline sodium sulphite and 25 g potassium pyrosulphite dissolved in 1 L of distilled water. The photographic paper is left in the fixing bath for about 10–15 min, and then transferred into a bath of running water for a further 15–30 min. The polarogram is finally dried electrically or between two sheets of filter paper.

Well processed polarograms are documents of the care taken with the experimental work. Insufficient developing of a part of the polarogram can be caused by the photographic paper sticking to the walls of the vessel in which the developing is carried out, by two parts of photographic paper sticking together or by inefficient immersion of the paper in the processing solutions. Black or coloured spots can be caused by impurities (mainly grease) that have been transferred from the hands to the paper when it was inserted into the polarographic box or when the recorded polarogram was processed. Harm can also be caused by carelessly opening the photographic box in the light or by incorrectly illuminating the darkroom. Photographic tweezers are recommended for transferring the polarograms between the processing baths”.

Let us think if now there is any young electrochemist/student patient enough to work with photographically recording instruments like the one described above! The present generation of electrochemists has never used this type of polarographs in the lab, but in 1970s, some old photographic polarograms could be still found in old archives and seen by students.

Manual polarographs

The first polarograph designed by Heyrovský-Shikata and the family of related electrochemical devices allowed automatic potential sweep and photographic recording of current–potential (I-E) curves. The further development of electrochemical instruments had two different directions: (i) further improvement of the automatic system for the potential sweep and recording abilities that resulted in visual (pen-recording) polarographs; (ii) simplification of the instruments that resulted in manual polarographs. The manual polarographs required manual change of the voltage applied to an electrochemical cell and point-by-point readout of the obtained current. The I-E curves were plotted also manually (note that computers for plotting the data were not available at that time yet). The aim of this simplification of the electrochemical instrument was to make the instrument cheap and affordable in any laboratory. Actually, the manual polarographs represented a step back from the automatic device designed by Heyrovský and Shikata, but it was justified by reducing the cost of the instruments.

Actually, a polarograph was a simple instrument at that time with some essential requirements: (i) It should have a means of applying a variable voltage from 0 to at least 3 V to the cell. (ii) It needs a potentiometer for accurately measuring the applied voltage. (iii) A means of measuring the microampere current resulting from the electrode reaction is needed. Lingane (Fig. 8) has designed a simple electric circuitry to realize a polarographic instrument which was capable of higher precision and accuracy than most commercial instruments of that time [30]. This was one of the simplest circuits which satisfied all requirements.

Many manual polarographs were commercially produced in 1940s–1950s (Fig. 9). A manually operated polarograph with the commercial name “Elecdropode” was manufactured by Fisher Scientific Co., Pittsburgh, PA (Fig. 9A). This polarograph was completely self-contained and easily portable. Mounting the cell and dropping mercury electrode assembly directly on the instrument cabinet did not allow a temperature control; however, this problem was easy to solve by dismounting the cell assembly and its placement in a thermostat. Direct-reading manual polarograph developed by Standard Oil Company, Indiana, USA, is shown in Fig. 9B [31]. A few more examples of polarographs producing I-E curves point-by-point and then plotted manually are collected in Fig. 9C–E [32, 33].

Fig. 9

A Manual polarograph “Elecdropode” manufactured by Fisher Scientific Co., Pittsburgh, PA, USA [1]. B Direct-reading polarograph developed by Standard Oil Company, Indiana, USA [31]. C Manual polarograph (Model III) developed by E.H. Sargent and Co., Chicago, USA [1]. D Manual polarograph “Voltamoscope” developed by Cambridge Instrument Co., Grand Central Terminal, New York, USA [32]. E Manual polarograph developed by Copeland and Griffith [33]. Adopted from [1, 31,32,33] with permission

Visible recording polarographs

The development of electronics after World War II and the tendency to increase the speed and simplicity of analyses resulted in an increasing interest in instruments that were able to record polarographic curves directly with a pen on paper. It was very important to eliminate processing a photographic record onto a photo paper. Instruments of this type were usually more easily handled by less skilled technical personnel.

The first European instruments of this type, e.g., the Danish Radiometer PO-3 polarograph, and some of the instruments produced by Cambridge Instruments, Tinsley and Evershed & Vignoles, used moving coil milliammeters. The movement of the coil was usually transmitted to the pen writing on the paper. The record obtained was no broader (in the direction of the current axis) than that obtained on the usual photographically recorded polarogram. As the sensitivity of such milliammeters was usually low, it is necessary first to amplify the polarographic current. The amplifiers used for this purpose were based on various principles. For example, in the PO-3 polarograph manufactured by Radiometer, the original DC signal was changed into an AC signal by a vibration changer at the input to the amplifier. The signal produced by the amplifier was again rectified by a copper oxide rectifier. This arrangement made it impossible to distinguish between cathodic and anodic currents, which was a large inconvenience. In more advanced types of such instruments, the potential drop across a resistance, parallel to the polarographic cell, was amplified and recorded by electronic devices based on the compensation principle. Among the numerous instruments of this type, the following can be mentioned: Sargent Mark XXI and XV polarographs, Leeds and Northrup “Electrochemograph,” Evershed & Vignoles instruments, polarographs produced by Japanese companies Yanagimoto and Shimadzu, the Swedish Polarolyzer LKB, etc. The pen carriage can be driven by an electronically controlled two-phase balance motor. This system was used in the Radiometer PO-4 polarograph and in the Swiss Polarecord E-261 produced by Metrohm. In all pen-recording instruments, either the polarization circuit (sometimes including the amplifiers) and the recording instrument were in two separate units or are combined as one component.

Some of these instruments did not fulfill the principal requirement that the current-measuring system has the same characteristic properties as an aperiodically damped galvanometer. As a result, the polarographic curves obtained with these instruments were suitable only for qualitative analytical purposes (because the value of the limiting current was influenced by the electrical properties of the recorder and amplifier). Most of the requirements were satisfactorily fulfilled in the Czechoslovak LP-60 polarograph which was actually one of the best polarographs of that time.

Visible recording tast-polarograph “Selector-D” produced by Atlas-Werke, Bremen, Germany, is shown in Fig. 10F. Tast-polarographs were equipped with a small hammer for controlling the mercury drop lifetime [34]. The capillary was shaken with the hammer at the time when the mercury drop was already large enough and almost stopped growing. The current was sampled just a moment before the mercury drop disconnection from the capillary. In this case, the diffusional current had its maximum value, but the capacitance current was very small because the electrode surface was not increasing. This provided a smaller ratio between the capacitance current of the electrode/solution interface and the diffusional current of the analyte, thus increasing the sensitivity. Another approach to the control of the mercury droplet was based on a needle plugging a capillary. This allowed cyclic voltammetry measurements on a mercury hanging drop with the fixed steady-state surface area.

Fig. 10

A Visible recording polarograph “Voltamograph,” Cambridge Instrument Co., UK [32]. B Visible polarograph manufactured by Radiometer, Copenhagen, Denmark [32]. C Visible recording polarograph “Polarecord” (Model E-261) with the polarographic stand (Model E-354) Metrohm, Ltd. Herisau, Switzerland [35]. D Polarograph “Electrochemograph” (type E) using a strip-chart recorder manufactured by Leeds & Northrup Co., North Wales, PA, USA [1]. E Visible recording polarograph Model XV, E.H. Sargent and Co., Chicago, USA (https://digitalworks.union.edu/theses/2251; open access). F Visible recording tast-polarograph “Selector-D,” Atlas-Werke, Bremen, Germany [2]. Adopted from [1, 2, 32, 35] with permission

The most representative examples of the visible pen-recording polarographs are collected in Figs. 10 and 11 [1, 2, 30, 32, 35]. Some of these instruments were used till the end of 1970s; particularly, one of the authors (E. Katz) performed his PhD research using a pen-recording polarograph OH-102 in the 1970s (Fig. 1, photo in the middle).

Fig. 11

A Visible recording polarograph (Model XXI) manufactured by E.H. Sargent and Co., Chicago, USA [30]. B Polaro-Analyzer designed by Schulman-Battey-Jelatis and manufactured by the Rutherford Instrument Co., Alexandria, VA, USA [30]. C Polarograph Model OH-102 produced by Radelkis, Hungary [2]. Adopted from [2, 30] with permission

Some of the recording polarographs were able to operate in the mode of the derivative polarography which allowed a significant increase in sensitivity (much lower detection limits). Originally derivative polarography was designed for the use of two synchronized mercury dropping electrodes [36, 37]. Lévêque [38] employed an electronic circuit which has the advantage of yielding derivative polarograms with only a single mercury dropping electrode. The fundamental operational principle of this type of circuit was originally discussed by Paul Delahay, who was a scientist greatly contributed to the experimental and even more to the theoretical electrochemistry (Fig. 12) [39].

Fig. 12

Adopted from https://cpb-us-e1.wpmucdn.com/blogs.uoregon.edu/dist/6/6681/files/2014/03/news13-2bj6605.pdf; public domain

Paul Delahay (1921–2012), American chemistry professor.

Oscillopolarographs

In the 1920s–1930s, the fundamental work on electrochemical kinetics and electrolyte conductivity by Tibor Erdey-Grúz (Fig. 13A) and Max Volmer (Fig. 13B) was performed using alternative currents analyzed with oscilloscopes. The technical aspects of this work later resulted in introducing oscilloscopes to polarographic analysis.

Fig. 13

Adopted from Bard AJ, Inzelt G, Scholz F (Eds.) (2008), Electrochemical Dictionary, Springer, with permission

A Tibor Erdey-Grúz (1902–1976), Hungarian scientist. B Max Volmer (1885–1965), German scientist.

Various attempts were made to modify and improve classical polarography. The aim has been to increase the sensitivity and resolution of the method or to speed up the recording of polarograms. Heyrovský and Forejt invented a new oscillographic polarographic method in the early 1940s (Fig. 14). The use of an oscilloscope introduced into polarography allowed the higher sensitivity as well as a much higher speed of the analysis, which was particularly important for the detection of unstable redox species produced electrochemically.

Fig. 14

Adopted from https://english.radio.cz/february-marks-100-years-invention-polarography-8741642; courtesy of Czech Academy of Sciences, with permission

Jaroslav Heyrovský performing experiments with an oscillopolarograph.

In this method, the galvanometer with its relatively long period of oscillation was replaced by a cathode ray tube, and the potentiometer used as a source of continuously increasing voltage was replaced by a generator which yielded a saw-tooth voltage increasing from zero to − 2 V in the course of hundredth of a second [40]. The curves of the function I = f(E) obtained on the oscilloscope screen were, however, distorted by a large charging (capacitance) current as a result of the rapid change of the electrode potential. Moreover, the curve on the screen altered in size owing to the growth of the mercury drop area. John Randles who made important contributions to the theoretical background of polarography, cyclic voltammetry and electrochemical impedance spectroscopy (Fig. 15), improved this method [41, 42] and eliminated the latter problem by polarizing the electrode with a single pulse of linearly increasing voltage during which the droplet surface area did not change appreciably. At a rate of a potential change of 0.3–1 V s⁻¹, the current was obtained within the duration of a single mercury drop to complete the polarographic curve practically undistorted by the charging (capacitance) current.

Fig. 15

Adopted from https://www.rsc.org/images/Newsletter08_tcm18-117122.pdf; public domain

John Edward Brough Randles (1912–1998), English electrochemist.

One of the first commercial oscillopolarographs “Polaroscope P-576” was designed by J. Heyrovský and J. Forejt [30], constructed by V. Nessl, and manufactured by “Závody Průmyslové Automatizace,” Prague-Smichov (Fig. 16A). One more example of oscillopolarographs is a Polarographic Analyzer System “Chemtrix” that provided digital readout as well as oscilloscope images (Fig. 16B). The theory of the oscillopolarography was briefly discussed by Müller et al. [43]. A comprehensive overview on the oscillographic polarography is available in Kalvoda’s and Heyrovský’s books [44,45,46].

Fig. 16

A Polaroscope P-576, “Závody Průmyslové Automatizace,” Prague, Czechoslovakia [2]. B Polarographic Analyzer System “Chemtrix” [2]. Adopted from [2] with permission

Alternative current (AC) polarographs

Friedrich Kohlrausch, a German physicist (Fig. 17), was the first to apply in 1873 an alternating voltage to polarizable electrodes. He was interested in the measurement of the conductivity of electrolytes and found that the electrodes become polarized when a direct current flowed. To avoid these polarization effects, he decided to use alternating voltage. However, his alternating current (AC) measurements were limited to conductivity study only.

Fig. 17

Adopted from https://en.wikipedia.org/wiki/Friedrich_Kohlrausch_(physicist); public domain; Wikipedia

Friedrich Kohlrausch (1840–1910), a German physicist.

The AC polarography began in the late-1930s when a small sinusoidal alternating voltage was applied on mercury dropping and counter electrodes under polarographic conditions with a linearly increasing DC polarization [47]. Müller et al. were the first to demonstrate the possibility of carrying out polarographic measurements by superposing an alternating voltage of low amplitude to the DC voltage applied to a polarographic cell [43]. The AC polarography when used with a lock-in amplifier or frequency analyzer offers considerably increased sensitivity over the early described direct current (DC) techniques and can also reveal important mechanistic and kinetic information not easily available using more traditional DC voltammetric techniques. An AC voltammetric measurement was usually performed in an electrochemical cell where diffusion was the dominate mode of transport. The AC voltage is often combined with either a steady DC signal (chronoamperometric mode) or voltage sweep (linear sweep voltammetry). In the latter case, during AC voltammetry, an alternating potential was added to the DC potential ramp used for linear sweep voltammetry (LSV) or cyclic voltammetry (CV). Only the AC portion of the total current was measured and plotted as a function of the DC potential portion of the potential ramp. Because the flow of an alternating current requires the electrochemical reaction to occur in the forward and reverse directions, the AC voltammetry was particularly useful for studying the extent to which electrochemical reactions are reversible.

The first instrument for the AC polarographic analysis was described by MacAleavy in Belgian and French patents of 1941 and 1942 [48, 49]. Presumably due to the wartime conditions, these patents passed unnoticed for many years. Commercial AC polarographs were marketed much later in 1950s by the Yokogawa Electrical Works Ltd., Tokyo, Model POL-2A (Fig. 18A), and by the Yanagimoto Co. Ltd., Kyoto, Models PA-102 (Fig. 18B). Barker has developed a technique radio-frequency (RF) polarography using high-frequency voltage combined with the DC polarization, then resulting in an amplitude-modulated radio-frequency current [50]. A commercial RF polarograph was developed by Y. Yasumori and it was marketed by the Yanagimoto Co. Ltd., Kyoto (Fig. 18C). A comprehensive overview on the AC polarography/voltammetry is available in Breyer and Bauer’s book [51].

Fig. 18

Adopted from the Yokogawa Electrical Works, Ltd., catalog (1979), open access

A The Yokogawa AC polarograph POL-2A produced by Yokogawa Electrical Works, Ltd., Tokyo, Japan. B The Yanagimoto AC-DC polarograph PA-102. C The Yanagimoto high-frequency polarograph Model PF-1. This polarograph used amplitude-modulated radio-frequency current.

Square-wave polarographs

In square-wave polarographs using an alternating voltage of square-wave shape, suppression of the base current was achieved by using the fact that, after the application of a voltage pulse to the electrode, the base current decays more rapidly than the Faradaic current. Thus, measurements of the current a short time before each new pulse lead to elimination of the base current from the recorded AC polarogram.

Commercial square-wave polarographs were marketed by Mervyn Instruments, Woking, Surrey, Square-Wave Polarograph (Fig. 19A); by Yanagimoto Co. Ltd., Kyoto, Square-Wave Polarographs Model PA-201 (Fig. 19B); Modular Square-Wave polarography Mark III (Fig. 19C); and the excellent device OH-104, Radelkis, Budapest, Hungary [52]. A special Model PM-1 was an attachment which converted the AC bridge polarographs to the square-wave instruments. These instruments are claimed to be 200 times more sensitive as compared to conventional DC polarographs. A comprehensive overview on the square-wave polarography/voltammetry is available in Bond’s book [53].

Fig. 19

A The Mervyn modular square-wave polarograph [52]. B The Yanagimoto square-wave polarograph (Model PA-201). C The Mervyn-Harwell square-wave polarograph (Mark III) [52]. Adopted from [52] with permission

Multipurpose electrochemical analyzers and microprocessor-controlled electrochemical analyzers

While modern computerized electrochemical analyzers are available as computer-attached or even computer-included devices, sometimes of small size, but still very powerful for a broad range of various electrochemical methods, the first electrochemical analyzers were much more primitive and they still operated with pen-recording modules.

Princeton Applied Research Corporation, USA, was a leading company that put on a market multipurpose electrochemical analyzers allowing many electrochemical techniques (normal pulse voltammetry, differential pulse voltammetry, derivative polarography, stripping voltammetry, etc.). The new instruments had microprocessors providing automatic operations and data manipulation. Figure 20A shows Princeton Applied Research Corporation Electrochemical System Model 170 (PAR-170). The instrument enabled a wide range of polarographic techniques to be obtained from the one unit. Another device produced by Princeton Applied Research Corporation, Polarographic Analyzer System Model 374 (PAR-374) was a computerized (microprocessor-controlled) polarograph equipped with an automatic cell changer (Fig. 20B).

Fig. 20

Adopted from the Princeton Applied Research Corporation catalog (1982), open access

A Princeton Applied Research Corporation Electrochemical System Model 170 (PAR-170). B Princeton Applied Research Corporation Polarographic Analyzer System Model 374 (PAR-374). Computerized (microprocessor-controlled) polarograph with an automatic cell changer.

Moving from two-electrode to three-electrode configuration—potentiostats for electroanalysis and constant-potential electrolysis

Many polarographic and later voltammetric (e.g., cyclic voltammetry) measurements were performed using two-electrode configuration composed of a DME or any other small working electrode and a counter electrode, also serving as a reference. The two-electrode configuration provided reasonable quality of the electrochemical measurements as long as a working electrode was small, the potential sweep was not fast, and the background solution included a high concentration of an electrolyte in an aqueous solution, then providing a small current over small resistance in a liquid phase. In other words, the potential drop in the electrolyte solution was not significant to affect the potential measurement precision, so, the applied voltage was close to the actual potential of the working electrode. However, when the analytical measurements started to be performed in non-aqueous solvents with much higher resistance or/and the potential scan rates were much faster (particularly in cyclic voltammetry), then resulting in larger currents, the potential drop in the liquid phase became significant, thus requiring its compensation. At this time, the standard electrochemical/electroanalytical configuration included three-electrode configuration with a reference electrode added. The three electrodes, working, counter, and reference, have operated with a potentiostat having more sophisticated electronic circuitry than previously used two-electrode polarographs [54, 55]. While sometimes the reference electrode is not really needed or difficult to be used, particularly in micro-sized electrochemical cells or in implantable electrochemical devices operating in biological fluids, the modern electroanalytical devices have always the circuit allowing their operation with three electrodes. The change of the electrochemical instruments operating with two electrodes to three electrodes, thus operating as potentiostats, has been in the approximately 1940s when numerous potentiostats became commercially available (Figs. 21 and 22) [56,57,58,59,60,61,62,63,64,65]. Some of these devices were used for control-potential electrolyzes rather than electroanalytical applications. Notably, the electrolysis was conducted with large area electrodes, thus operated with large currents that required very powerful electrical devices. It should be also mentioned that many polarographs produced in 1950 and later (shown in the sections above) have already included potentiostat electrical circuits. A good example of such devices was a Hungarian polarograph, Model OH-102, Radelkis (Fig. 11C). More sophisticated instruments, such as double or multichannel potentiostats, have been developed to operate with several working electrodes simultaneously.

Fig. 21

A Caldwell, Parker, and Diehl potentiostat [56]. B Penther-Pompeo potentiostat [57]. C Lamphere-Rogers potentiostat [58]. D Potentiostat PAS 134 [59]. Adopted from [56,57,58,59] with permission

Fig. 22

A Lingane multipurpose electrochemical servo instrument [60]. B Hicking potentiostat [61]. C Dolling Model II [62]. D Dolling Model III [62]. E Lingane-Jones potentiostat [65]. Adopted from [60,61,62,63,64,65] with permission

A multipurpose electroanalytical instrument constructed by Lingane [60], Fig. 22A, was operating with various functions, including the potentiostat function maintaining the potential of a working electrode constant during electrolysis, in electrogravimetric analysis of metals, electrolytic separations of metals prior to final determinations by other methods, coulometric analysis, and electrolytic preparations by the controlled-potential technique. Electrolysis at constant total applied voltage, or with constant current (i.e., operating in potentiostatic or galvanostatic regimes), was also possible.

Notably, before potentiostats became widespread, many electrochemical measurements were performed under a galvanostatic regime when the current was maintained nearly constant, while the variable potential applied on a working electrode was measured with the help of bridge methods (slide-wire bridge, rheostat) or later with a voltmeter. Since 1942, when the English electrochemist Archie Hickling built the first three-electrode potentiostat [61], substantial progress has been made to improve the instrument. It is interesting to note that the word “potentiostat” was coined by Hickling. A very important contribution to the electronic design of potentiostats was done by Hans Wenking in the 1950s. Hans Wenking (Fig. 23) was a German electrical engineer [62], physicist, and inventor who devoted his lifetime to the development of electronic equipment for chemistry and physics, particularly constructing the first potentiostats, which became major parts of modern electrochemical instruments. He was the first who described the basic principles of potentiostats.

Fig. 23

Hans Wenking (adopted from [62] with permission)

In 1952, Hans Wenking constructed an electronic amplifier for controlling an oscilloscope using a mirror galvanometer with the signals recorded on photographic paper. This amplifier was later used as a core of a new potentiostat with only a power supplier added. This potentiostat, being an important instrument for electrochemical investigations, was designed by Wenking during his work at the Max Plank Institute in Göttingen, Germany. At that time, Hans Wenking was working in a group of Professor Karl Friedrich Bonhoeffer where he was appointed to develop a potentiostat that was needed for electrochemical experiments, particularly, for studying corrosion.

Until 1957, Wenking’s potentiostat was manufactured only for the internal use of the Max Planck Institute in Göttingen. Later, Hans Wenking together with Gerhard Bank established “Elektronische Werkstatt Göttingen” to commercialize the potentiostats. From 1959, the company operated under the name “Gerhard Bank Electronik.” Wenking designed the instruments as a freelance, but the brand “Wenking potentiostat” (Fig. 24) soon became a famous trademark. The consequence of the potentiostat development was a rush in the development of electrochemical science. The phenomena of metal passivity could be better explained, mechanisms of oxide layer formation, and far beyond the materials science. The potentiostat became a standard instrument for most electrochemical investigations, particularly for electroanalytical measurements.

Fig. 24

Adopted from https://wie-tec.de/Bank-Elektrotechnik_1, public domain

Bank Elektrotechnik Wenking ST72 Standard Potentiostat ST 72.

Independent of Wenking’s work, similar instruments were designed by other companies. Tacussel was one of those companies which came to a similar design as Wenking’s potentiostats and started manufacturing potentiostats in France. In the USA, Wenking’s potentiostats were leading on the market and became standard instruments in electrochemical labs.

Wenking has never published his results in scientific papers. On the other hand, Wenking never concealed the technical details of his instruments. The circuits and layouts designed by him were included in the operation manuals for the instruments, and even in some manuals, a detailed theoretical treatise was given. Wenking’s instruments were always state-of-the-art and electrochemists of the 1950s-1970s used them for many different applications.

Electroanalyzers

The electrochemical analysis performed by these devices included the separation of inorganic materials by electrolysis resulting in the electrodeposition/precipitation of inorganic compounds or pure metals from their solution state. Then, the electrolytically separated materials were analyzed by weighting their samples. The equivalence of the amount of a material deposited/precipitated with the amount of the electricity consumed, according to Faraday’s law, provided another means for the quantitative estimation of the electrolyzed material. The developed method combined electrogravimetry and coulometry representing a new electroanalytical technique. This method was similar to coulometric titration used later. Simple DC-current sources were used for this purpose before potentiostats were developed. Several examples of these simple devices for electrolytic separation of mixtures (usually metals) prior to their analysis are shown in Fig. 25 [66]. Notably, these rather simple devices were not related to the presently used electrochemical analyzers operating with many electrochemical methods, such as cyclic voltammetry, chronoamperometry, and impedance spectroscopy.

Fig. 25

A Self-contained unit for electroanalysis produced by Arthur H. Thomas Co., Philadelphia, PA, USA. B Self-contained Cenco Electroanalyzer with air stirring produced by Central Scientific Co., Chicago, IL, USA. C High-speed heavy-duty water-cooled electrolytic analyzer with magnetic stirring produced by E.H. Sargent Co., Chicago, IL, USA. D Electrograph, produced by Fisher Scientific Co., Pittsburgh, PA, USA. Adopted from [66] with permission

pH meters

Presently, the pH scale, which is a logarithmic expression of the H⁺ ion concentration, is commonly and broadly used: pH =—log[H⁺] (note that the more correct definition uses the H⁺ ion activity according to the IUPAC document [67]). This convenient measure of the H⁺ concentration was introduced by Søren Peter Lauritz Sørensen (1868–1939), a Danish biochemist, in 1909 [68] (Fig. 26). In his original notation, it was p_H (presently pH). Because the pH value affects many chemical reactions and properties of materials, its measurement was highly important. Particularly, Sørensen came to the definition of pH upon studying the pH effect on proteins.

Fig. 26

Adopted from https://commons.wikimedia.org/wiki/Category:S%C3%B8ren_Peter_Lauritz_S%C3%B8rensen#/media/File:Soeren_Peter_Lauritz_Soerensen_1868-1939_2.jpg; public domain

Søren Peter Lauritz Sørensen (1868–1939), Danish biochemist.

While the pH was semi-quantitatively estimated using colored reactions of pH indicators, automatic instrumental analysis of pH was always a challenging goal. It was already addressed at the beginning of the twentieth century, when several instruments capable of automatic pH analysis, usually obtained by titration methods, were introduced. The most popular at that time was the automatic titrator designed by Dr. W. T. Bovie (Bovie hydrogen ion potentiometer) and produced by Leeds & Northrup Company (Fig. 27).

Fig. 27

Adopted from https://www.periodpaper.com/products/1922-ad-bovie-potentiometer-arthur-h-thomas-h-ion-concentration-scientific-102822-iec1-070; public domain

Bovie hydrogen ion potentiometer (Leeds & Northrup, 1921).

However, direct reading of the pH values without titration of a solution, using electronic measurements, has been recognized as a very important and challenging goal. The first invention to achieve this goal was done in 1906, when Max Cremer (Fig. 28A) [69, 70] discovered that an electrical potential develops when two liquids of different pH levels come into contact at opposite sides of a thin glass membrane. Importantly, the potential produced was proportional to the pH difference on both sides of the glass membrane. This discovery provided a background for developing a pH-sensitive glass electrode. In 1909, Fritz Haber (Fig. 28C) and Zygmunt Klemensiewicz (at that time Haber’s student) (Fig. 28B) used the principle described by Cremer in 1906 to create the first glass electrode that measured hydrogen activity. However, technical difficulties, including the large internal resistance of glass electrodes, prevented the large-scale potentiometric measurements of pH. Because of these difficulties, the use of a very sensitive, but expensive, galvanoscope was necessary to obtain reliable results. The technical problem was solved by Dr. Arnold Beckman (Fig. 29) in 1934. He proposed that the current obtained through Haber and Klemensiewicz’s electrode can be amplified, allowing it to be measured using a cheap milliamperometer. The principle point was to improve the glass electrode, and the requirements to electric measurements were surely dependent on glass material and electrode construction. There were important findings between Haber and Beckman, as related to glass materials with lower internal resistance. Beckman devised a simple, high-gain amplifier using two vacuum tubes for this purpose. This advance represents the development of the first pH meter, known at the time as an “acidometer.” Beckman tried to interest various companies in manufacturing the device, but they declined. As a result, he formed in 1935 National Technical Laboratories, based in Pasadena, CA, USA, which developed the first commercial pH meters.

Fig. 28

Adopted from https://www.chemistryworld.com/opinion/cremers-electrode/3008550.article; https://en.wikipedia.org/wiki/Zygmunt_Klemensiewicz; https://en.wikipedia.org/wiki/Fritz_Haber#/media/File:Fritz_Haber.png, respectively, public domains

A Max Cremer (1865–1935), a German physiologist. B Zygmunt Aleksander Klemensiewicz (1886–1963), a Polish physicist and physical chemist. C Fritz Haber (1868–1934), a German chemist.

Fig. 29

Adopted from https://artsandculture.google.com/asset/arnold-beckman-with-ph-meter/8QHVcn5f7EzvyA; public domain

Arnold Beckman (1900–2004), American chemist, inventor, investor, and philanthropist with a pH meter.

The first commercial pH meters were introduced in the USA by Beckman, the founder of the Beckman Instruments Company (now Beckman Coulter). The Beckman model was known as the Model G acidometer and was later renamed the Model G pH meter (Fig. 30). This device was revolutionary because it was the first to combine the whole apparatus (amplifier, electrochemical cell, electrode, calibration dials, batteries, and measuring gauge) into one unit. During its first year, Model G sales reached 444 units. The model introduced in 1935 continued to be sold until the mid-1950s, with an estimated 126,000 sold during its lifetime.

Fig. 30

Adopted from https://www.sciencehistory.org/files/beckmanmodelgchfjpg-2; with permission

Beckman pH meter Model G. A A general view with the open door of the electrode compartment. B Top view with the controls and readout. C The window displaying pH and voltage.

Many other improved models of pH meters were produced by various companies (Figs. 31, 32 and 33). Over the years, Beckman, other companies, and academic researchers made great improvements in pH meters as they developed into their modern form, with higher stability glass electrodes, microprocessor control, and light-emitting diode readouts. Beginning in the 1950s, electrodes were also developed for other ions, such as F⁻, Na⁺, K⁺, and Ag⁺. The pH meter and ion-selective electrode have now become indispensable scientific tools. Some of the pH meters were specifically designed for their industrial usage (Figs. 31C and 33C) [71].

Fig. 31

Adopted from https://en.wikipedia.org/wiki/PH_meter; public domain

Beckman pH meters. A Model N. B Model H-2. C Model W.

Fig. 32

Adopted from https://aphmuseum.org/record/coleman-model-3-ph-electrometer/; public domain

Coleman pH electrometer with a pH scale, Model 3. A General view with the open door of the electrode compartment. B Top view with the controls and readout. C Label.

Fig. 33

A Stabilized pH indicator (Leeds & Northrup). B Cambridge Bench pH meter. C Industrial type stabilized pH indicator (Leeds & Northrup). Adopted from [71] with permission

Potentiometric titrators

While at the end of nineteenth and the beginning of twentieth centuries, potentiometric titration was performed mostly manually with homemade setups, later, from the 1930s, the automatic electronic instruments for potentiometric titration were commonly introduced, frequently using pH sensing glass electrodes.

The theory, early applications, and respective instrumentation of the potentiometric titration were reviewed in papers written by Furman in 1930, 1942, and 1950 [72,73,74]. A large number of automatic potentiometric titrators were developed [75,76,77], and several instruments were commercially available. The titrant was delivered to the sample solution, and the titration was automatically halted either at the inflection point of the assumed S-shaped titration curve or when the potential of the indicator electrode reached the equivalent-point potential. For example, Lingane [78] devised an automatic titration apparatus based upon the motor-driven syringe buret and a potentiometer recorder equipped with a switch to stop the action at any predetermined potential. The apparatus was shown to be well adapted to titrations of various types and was utilized in titrations with chromous solutions. A very high degree of precision was attained with the instrument. Several examples of the instruments for potentiometric titration are shown in Fig. 34.

Fig. 34

A A potentiometric titrator with a Schmitt-type trigger [75]. B Potentiometric titrator with a condenser [79]. C An instrument for potentiometric titration according to Karl Fischer’s method [80]. D Dual alternating current titrator [82]. E Automatic potentiometric titrator (center) with an electrochemical generation of a titrating reagent (bromine) with a recorder (right) and auxiliary battery unit (left) [83]. Adopted from [75, 79, 80, 82, 83]

A potentiometric titrator with a Schmitt-type trigger in combination with a single-stage pentode preamplifier was described in the literature [75]. The applied electronic circuit allowed to switch abruptly and definitely at a preset level of an input potential. The trigger action was reversible, with a small dead zone. A stabilized line-operated instrument was suitable for automatic potentiometric titrations (also photometric titrations) with a sensitivity of ± 5 mV. The instrument was assembled as a portable unit in a radio-type cabinet (Fig. 34A). A potentiometric titrator with a condenser (a “capacitor” in the modern terminology) connected to the terminals of a potentiometric cell was constructed by Delahay [79] (Fig. 34B). This condenser provided a sharp maximum in near the end-point of a titration, thus increasing the sensitivity.

The instrument for potentiometric titration developed by van Lameon and Borsten [80] is shown in Fig. 34C. It was used for the determination of moisture in non-aqueous organic solvents according to the Karl Fischer method. Two platinum electrodes, one polished and one platinized, allowed potentiometric titrations using the Karl Fischer-Johansson procedure with an accuracy equal to the dead-stop technique. The instrument included a laboratory pH meter. The polished platinum electrode was used as a reference electrode and it was placed in a tube containing a sulfur dioxide solution. Electrical contact with the solution to be titrated was made through a thin asbestos fiber sealed through the tube wall. The platinized platinum indicator electrode was placed in a second glass tube, perforated at the bottom, to minimize stirring effects.

Delahay developed a vacuum tube voltmeter for use with electrodes of high resistance [79, 81]. The developed instrument [82] was incorporated into a dual titration device (Fig. 34D) operating with two sets of electrodes. Titrations of the same or different types were proceeding simultaneously with intermittent voltage readings.

Another instrument for automatic continuous potentiometric titration is shown in Fig. 34E. During the WWII, the need arose for instruments capable of measuring and recording automatically and continuously the concentrations of toxic gases in chemical warfare. As shown in Fig. 34E, automatic continuous titrating instrument was originally developed for the determination of mustard gas in the air [83]. The unknown gas sample was continuously aspirated through a titration cell in which it was absorbed in solution. Titration was performed by electrolytic generation of the titrating agent (bromine) in the cell; the electrolysis was so controlled that a very small excess of the titrating agent was maintained in the cell. The control was achieved by means of negative feedback, characteristic of servomechanism applications; the feedback loop involved the chemical reaction. The electrodes responded to the consumption of bromine with a potential change which, applied to the amplifier, produced a relatively large increase of the current in the titrator circuit. The titration end-point was detected potentiometrically. Figure 34E shows the titrator with a recorder and auxiliary battery unit. The cell included four electrodes: two of them for the electrolytic production of bromine and two other electrodes for the potentiometric control of the titration process.

Coulometers, coulometric titrators

Coulometry can be performed as a constant-potential (potentiostatic) or a constant-current (galvanostatic) technique. Constant-potential coulometry can be used both as a technique of chemical analysis and as a means for determining the value n (number of electrons per molecule) in the Ilkovic equation. By Faraday’s law, the total quantity of chemical change proceeding at an electrode is directly proportional to the quantity of electricity (charge) passed. The fundamental requirement for the coulometric analysis is that the electrode reaction under investigation proceeds with a 100% current efficiency. If this condition is satisfied, the amount of electricity consumed can be related to the concentration by Faraday’s law. In the galvanostatic regime, the constant-current integration in order to obtain the consumed charge is straight-forward. On the other hand, the charge evaluation in the potentiostatic regime (when the current is variable) was not so trivial.

Since the current value is supposed to be changed upon the constant-potential electrolysis, the current integration is important to yield the charge value associated with the electrochemical process. Very high precision of charge measurement was possible with chemical coulometers. However, chemical coulometers are always less convenient to use than electronic integration instrumentation. As a consequence, chemical coulometers are no longer commonly used in electrochemical laboratories. The types of chemical coulometer which are most precise are (i) the silver coulometer [84, 85], (ii) the iodine coulometer [86], (iii) the hydrogen–oxygen gas coulometer [87, 88], and (iv) the hydrogen–nitrogen gas coulometer [89]. In the time when electronic current integration was not available yet, the silver coulometer and the iodine coulometer, while being superior to all others in precision, were inconvenient to use in analytical procedures. The hydrogen–oxygen gas coulometer was capable of an acceptable 0.1% or better precision, if proper precautions were taken, and it was considerably more convenient. For current densities below 0.05 A⋅cm⁻², the hydrogen–oxygen gas coulometer reads significantly low and the hydrogen–nitrogen gas coulometer was used instead. Electromechanical integrators were used in the 1950s and 1960s, but must now be considered obsolete. The best precision reported was 0.1%, which was acceptable at that time. These devices have now been replaced by electronic integrators.

In the constant-potential coulometry, the electrode potential should be maintained constant using a potentiostat. In addition to general potentiostats used for various applications, special potentiostats for constant-potential coulometry (particularly, for constant-potential coulometric titrations) have been designed. For example, the potentiostat designed by Kelley, Jones, and Fisher [90] was marketed by the Numec Instruments and Controls Corp., Apollo, PA, USA, as the electronic controlled-potential coulometric titrator (Fig. 35A).

Fig. 35

A The Electronic Controlled-Potential Coulometric Titrator marketed by the Numec Instruments and Controls Corp., Apollo, PA, USA [92]. B Coulometric Current Source, Model IV, E.H. Sargent and Co., Chicago, USA [92]. C Coulometer E211 manufactured by Metrohm, Herisau, Switzerland. D The coulometer “Coulachron” produced by the Canal Industrial Corp. E Precision coulometric titrator [91]. Adopted from [91, 92] with permission

Figure 35 shows a few examples of the coulometers [91, 92], some of them were used for coulometric titration. Coulometric Source, Model IV, manufactured by E.H. Sargent and Co., Chicago, USA, is shown in Fig. 35B. The Sargent Coulometric Source incorporates a timer which reads out directly in micro-equivalents of titrant generated. Coupled with the Sargent-Malmstadt Automatic Titrator, this instrument can be used as an automatic coulometric titrator (Fig. 35B). Coulometer E211 manufactured by Metrohm (Herisau, Switzerland) is shown in Fig. 35C. This instrument was used for coulometric titrations with constant current (in a galvanostatic regime). The coulometer “Coulachron” produced by the Canal Industrial Corp. (Fig. 35D) was designed for the automatic determination of a wide variety of materials normally found in body fluids. The instrument was made for both amperometric and potentiometric detection of the end-point of the coulometric titration. Precision constant-current coulometric titrator developed by Gerhardt, Lawrence, and Parsons [91] in the Research Division of American Cyanamid Co., N.J., USA, is shown in Fig. 35E.

Conductometric titrators

Conductometric titration is an analytical method used to identify the concentration of a given analyte in a mixture. It involves the continuous addition of a reactant to a reaction mixture and the analysis of the corresponding change in the electrolytic conductivity of the reaction mixture. It should be noted that the electrical conductivity of an electrolytic solution is dependent on the number of free ions in the solution and the charge corresponding to each of these ions. The theory of conductometric titration was discussed in details in Kolthoff’s paper published in 1930 [93]. An example of a conductometric titrator designed by Delahay [94] is shown in Fig. 36.

Fig. 36

The conductometric titrator designed by Delahay. A Outside view of the indicator current device for conductometric titration. B Inside view of the indicator current device. Adopted from [94] with permission

High-frequency titrators

High-frequency oscillators for the detection of end-points in volumetric chemical reactions were first used as early as 1938 in the laboratories of the Agricultural and Mechanical College of Texas. When a high-frequency (radio-frequency) electromagnetic field is applied to an electrolyte solution, there is an absorption of energy and a change in dielectric constant of the solution, as predicted by Peter Debye (1884–1966, Dutch-American physicist and physical chemist) and Hans Falkenhagen (1895–1971, German physicist and electrochemist) and as shown experimentally by several groups [95]. The absorption of energy and the dielectric constant are also functions of the concentration at a particular frequency. When the electrolytic solution loads many types of high-frequency oscillators, the change in dielectric constant causes a change in the frequency of the oscillator. The change of frequency of an oscillator due to the change of the electrolyte solution composition could be used to follow the titration process. For example, the frequency change plotted vs. volume of the added titration agent represents a titration curve. Maximum changes of the measured frequency occurred near the titration end-point, thus allowing to detect the end-point of the titration. Acid–base reactions, precipitation reactions, complex formation reactions, and redox reactions were studied using high-frequency titration techniques [96].

Examples of the high-frequency titrators [95, 97] are shown in Fig. 37. The high-frequency oscillator-type titrometer [97] is shown in Fig. 37A. The 30-MHz Titrometer [95] is shown in Fig. 37B–D.

Fig. 37

A High-frequency oscillator-type titrometer [97]. B 30-MHz titrometer general view [95]. C 30-MHz titrometer condenser unit and titration cell. D 30-MHz titrometer coaxial junction on condenser unit for titration cell. Adopted from [95, 97] with permission

From homemade instruments to commercial devices

It is interesting to note that the majority of electrochemical instruments in nineteenth and the beginning of the twentieth centuries were first homemade before they were properly engineered and commercially produced. It is interesting to note that the homemade instruments, e.g., the first polarograph made by Shikata and Heyrovsky (Fig. 4B), provided accuracy of the current and potential measurements comparable or even better than the presently available modern instruments based on sophisticated electronics. For example, mirror galvanometers allowed measurements down to micro- or even nano-amps. The old potentiometers were able to register at least 10 µV and the accuracy was mostly limited by the quality of a reference electrode. Calomel reference electrodes, Hg/Hg₂Cl₂, were actually better in the terms of stability and reproducibility than Ag/AgCl-reference electrodes widely used at present. The users of the modern computerized electrochemical instruments might be misled by the artificial precision of the numbers generated by a computer, while the real experimental precision is much lower (rarely better than ± 1 mV in the potential measurements in voltammetry experiments). Overall, the commercially available instruments provide better stability, easy operation, faster measurements, a friendly interface, etc., but not necessarily much better results. The difference is the same kind like comparing an electronic calculator vs. logarithmic ruler (who remembers what is that?).

Examples of experimental setups with polarographs as major parts

It should be noted that electrochemical instruments (e.g., polarographs) do not operate stand-alone, but require many auxiliary components, such as electrochemical cells with electrodes, tubing for supplying solutions in the titration function, capillaries for Ar or N₂ used in deaeration, and a jacket for thermostated water supply. In other words, the whole experimental setup includes many different components attached to the electrochemical devices, being selected based on the specific experimental needs. While the electrochemical devices, particularly their electronic components, progressed a lot, the mandatory auxiliary components may not be changed significantly, probably except for the electrochemical cells, which have a tendency to miniaturization, especially in bioelectrochemical experiments.

Figure 38 shows two examples of complete electrochemical setups operated in the 1940s. Figure 38A shows an example of an experimental setup (1948) for polarographic determination of a free monomer in a heteropolymerization reaction mixture using “Electrochemograph” [98]. Figure 38B shows an example of another experimental setup (1949) for the polarographic determination of tin using the Model XXI Sargent polarograph [99]. Note how many additional components are included! A few more experimental setups with included pH meters are shown in Fig. 39. One more example of a modern electrochemical setup is shown in the “Conclusion and Outlook” section (Fig. 41).

Fig. 38

A Assembled “Electrochemograph” (1948) [98]: (A) Shunt box. (B) Polarizing unit. (C) Thermionic amplifier. (D) Constant-temperature bath. (E) Tank nitrogen. (F) Relay box for a constant-temperature bath. (G) Micromax recorder. (H) Polarographic cells. (I, J) 5-gallon bottles containing supporting electrolyte. B Setup of equipment for tin determination (1949) [99] includes (i) a Model XXI Sargent polarograph with a dropping mercury electrode adjusted for a drop time of approximately 4 s per drop; (ii) a H-type polarographic cells with a saturated calomel electrode; (iii) a nitrogen tank, after passage over copper turnings at 450 °C; (iv) a Fisher unitized constant-temperature bath, used at 30.0 ± 0.5 °C; (v) a general electric reflector drying 250-W type infrared lamp for fuming the samples; and (vi) a dropping mercury electrode held with a holder. Adopted from [98, 99] with permission

Fig. 39

Examples of experimental setups with included pH meters. A Constant-temperature baths and measuring instruments including a pH meter, 1957. B Autoclave and measuring instruments including a pH meter, 1957. C pH meter based on a vibrating reed electrometer, 1950. D pH measurements using the L&N Hemotron constant-temperature electrode assembly, 1969. Adopted from [100] with permission

Fig. 40

Adopted from http://waywiser.fas.harvard.edu/objects/15671/kohlrausch-drumtype-slide-wire-potentiometer;jsessionid=A7EF782726FC63EBF886A8B10DAC18A3?ctx=bc0a6ad5-a89f-4093-a81e-087e43d048a3&idx=36; public domain. B Helical potentiometer (Helipot) invented by Arnold O. Beckman in 1940 and used in his famous pH meters. Adopted from https://commons.wikimedia.org/wiki/File:Beckman_Helipot_potentiometer_SA1400A_2007.075.002.jpg; public domain

A Kohlrausch drum-type slide-wire potentiometer (helical potentiometer) invented by Friedrich Wilhelm Georg Kohlrausch and used in many early polarographs. The shown instrument was manufactured in ca. 1920s.

Conclusions and outlook

Looking back for more than 50 years, one can see a tremendous change in the way of how electrochemical experiments are done (Fig. 1) [101]. There have been myriad of small, but important inventions: e.g., IR compensation, Kohlrausch drum-type slide-wire potentiometer (Fig. 40A), high-quality helical potentiometer (Helipot; Fig. 40B) invented by Beckmann in 1940. The electrochemical instrumentation has been progressed a lot due to novel electronics and modern computers, which allowed very sophisticated analytical methods with computer-controlled experimental setups and with the experimental data processed and presented via user-friendly software [102]. The modern computerized electrochemical instrumentation allows much easier and faster experiments compare to the old manual, photographic recording, etc. instruments. Unfortunately, the seeming simplicity may sometimes result in poor planning of the experiments. It should be noted that the digital instruments are significantly different from analog instruments. Indeed, the digital instruments are always sampling currents at specific times, while the analog devices can measure the current continuously (if the technique is not specifically designed for the current sampling). In some cases, the digital vs. analog instruments may result in the different data collected.

It should be noted that the electroanalytical methods spread to many other research areas of electrochemistry, including, but not limited by, a study of mechanisms of redox reactions, observation of intermediate redox species (possibly with a very short lifetime), measuring the kinetics of interfacial electron transfer, and characterization of immobilized redox species, catalysts, biomolecules, polymers, etc. Therefore, while the original goal of the electroanalytical chemistry in its narrow definition was qualitative and quantitative analysis of soluble redox species, in a broader modern definition, the electroanalytical chemistry is merged with physical chemistry, material science, (bio)catalysis, bioelectrochemistry, and many other related subtopics. The processes and compositions studied with various electroanalytical methods are not necessarily related to the redox transformation of the target molecules, but may be based on the analysis of interfacial capacitance or impedance at the electrode surfaces functionalized with different materials which are not redox active. Overall, the “electroanalytical” chemistry is a broad collection of methods and research goals, much broader than the original pure analytical application in terms of classical analytical chemistry. A good example of the electroanalytical chemistry in a broad definition was a study of the surface of conducting solids named “electrography” [103]. The electrographic images provided information on the distribution of elements on the surface of solid materials and their electrochemical activity. Electrography represented the first electroanalytical technique for solid materials.

Modern electrochemistry has moved to biological materials and systems resulting in numerous electrochemical biosensors [104, 105], biofuel cells [106], and other bio-related systems (bioelectrochemistry) [107]. These subtopics of modern electrochemistry are clearly related to electroanalytical methods used for characterization, optimization, etc. of the bio-related electrochemical systems. The computerized electrochemical analyzers have been integrated with other physical analytical systems, then allowing various analytical methods under electrochemical control (Fig. 41) [108]. Further progress in electroanalytical methods and instruments proceeds in the direction of their miniaturization (down to micro- or even nano-scale) [109], super-high sensitivity (up to single-electron transfer quantum processes) [110], operation in biological systems (implantable [111] or wearable [112] electrochemical/electronic systems), and integration with other methods (e.g., electrochemical quartz crystal microbalance [113], optoelectrochemical (spectroelectrochemistry) systems [114], and electron spin resonance (ESR) combined with electrochemistry [115]). While modern applications require the knowledge of the presently available instruments, systems, and methods, the fundamental knowledge in electrochemistry needs to remember the way of its progress and this goal was the motivation for writing this historic review.

Fig. 41

The experimental setup included the confocal fluorescent microscope and the electrochemical instrument: an example of an integrated analytical system composed of an electrochemical device and an optical system. The photo shows the setup in the Katz-Smutok lab at Clarkson University