Electrochemistry, Past and Present - ACS Publications - American

Chapter 1

Electrochemistry in Retrospect An Overview John T. Stock

Downloaded by 92.25.72.221 on November 16, 2015 | http://pubs.acs.org Publication Date: January 1, 1989 | doi: 10.1021/bk-1989-0390.ch001

Department of Chemistry, University of Connecticut, Storrs, C T 06268 The foundations of electrochemistry are to be found in the late 18th-century investigations by Galvani and Volta. Galvani believed that electricity arose within the frog's leg that he was examining. He could not possibly have foreseen the great advances in bioelectrochemistry that have occurred during the present century. Volta, who saw the frog's leg as a mere indicator of electricity, produced the first source of direct current, a battery. Immediately, this led to the beginning of "physical" electrochemistry. Electrochemical techniques and theories developed quite rapidly, especially in the hands of Ostwald and his associates. Towards the end of the 19th century, continued progress in these areas was accompanied by the development of applications, both in the laboratory and in industry. Progress in measurement, essential in most areas of physical and analytical chemistry, fostered the development of instrumentation. In turn, the availability of instruments fertilized the further development of electrochemistry. When engaged in any field of science or technology, it is not a bad idea to pause occasionally and to look backwards a little. In the case of areas such as astronomy, surveying, weighing and other measurements, the view may extend to the horizon, i.e., to the earliest human records. However, not all history is "ancient". Sometimes development in a particular area is so fast that we have to confine ourselves to the past decade, or even less. Rapid advances have one supreme advantage from the historian's point of view; sometimes we can persuade those who have made some of the history to write about it! The reading of a little history is an excellent backup to the usual literature searches that form part of any projected experimental investigation. An old idea or observation can sometimes trigger a new line of thought. 0097-6156/89/0390-0001$06.00/0 © 1989 American Chemical Society

In Electrochemistry, Past and Present; Stock, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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ELECTROCHEMISTRY, PAST AND PRESENT

Electrochemistry appears in many guises that are often interconnected. An overall history of the subject would run to many volumes. Although by no means all-embracing, the present work is at least representative. Two major areas that receive no attention are corrosion and electroplating. The first of these causes billion-dollar losses, while the other is a billion-dollar industry. To treat even a few aspects of either would require a plethora of chapters.


Early Electrochemistry Galvani's observations of the twitching of the detached leg of a frog are usually mentioned in texts that refer even briefly to the history of science. Galvani, an anatomist, believed that the twitching was due to electricity that arose in the muscle of the leg. The electric eel was a known producer of "animal electricity". Alessandro Volta (1745-1827), a physicist, concluded that the electricity arose from the junction of the copper hook that impaled the leg and the iron support upon which the hook hung. The twitching was seen as a mere indication of the externally-generated electricity. To augment the electrical effect, Volta built a "pile" consisting of alternate plates, such as of silver and zinc, separated by paper or cloth that was soaked in brine. Figure 1 shows the arrangement, described in 1800 (1). Volta may have felt that the wet separators merely acted as conductors. With Volta's invention, the intensive study of areas such as electrochemistry that require a continuous source of electricity could begin. Galvani's ideas, temporarily put aside, contain more than an element of truth. Living systems are not only affected by electricity, but can also generate it. A common step in a medical checkup is the recording of an EKG. Bioelectrochemical studies may be said to take in all creatures (or parts of them!) great and small--even down to the bacterial level. William Nicholson (1753-1815) read Volta's account before it appeared in print. Immediately, he and Anthony Carlisle (1768-1840) used a "pile" to electrolyze water and various solutions. They noted the liberation of hydrogen at one pole and of oxygen at the other (2). Systematic electrochemistry had begun with a process that would eventually grow to the kiloampere scale! Similar experiments were carried out by others. In 1803, Jons Berzelius (1779-1848) and William Hisinger (1766-1852) observed that, in the electrolysis of salts, the solution becomes alkaline near the negative pole and acidic near the positive pole (3). This simple observation had at least two major consequences. It may have led Berzelius to develop his electrochemical theory of affinity. A practical consequence was the eventual development of the now massive chlor-alkali industry.



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Electrochemistry in Retrospect

Figure 1. Volta's "pile". (Reproduced from Phil. Trans., 1800.)


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ELECTROCHEMISTRY, PAST AND PRESENT Davy and Faraday. Humphry Davy (1778-1819) and Michael Faraday (1791-1867) were two of the scientists who brought great prestige to the Royal Institution. Davy made extensive electrolytic studies, sometimes using very large batteries. His aim was to isolate the components of alkali metal compounds. When all attempts using solutions failed, Davy turned to solid, slightly damp, potassium hydroxide. Mercury-like globules appeared, bursting into flame. Sodium hydroxide underwent a similar decomposition. Davy's 1807 account (JO marks the beginning of "fused salt" electrolysis, later to become important both in the laboratory and in industry. The literature concerning Faraday is extensive. A most convincing account of his many investigations is contained in his own diary. The original volumes rest in the Royal Institution, where he lived and worked for nearly half a century. However, their contents are readily available (5). Professor Ronald King, who organized the Institution's Faraday Museum, has written an excellent short account of Faraday's life and activities (6). It would be surprising to find a freshman chemistry text that fails to refer to Faraday's laws of electrolysis, or to terms such as "ion" and "electrode" that he introduced. The electrochemist remembers him every time the symbol "F" appears in a numerical expression. Although he did not commercialize his discoveries of the principles of the electric motor, the dynamo, and the transformer, these discoveries led to enormous changes in our daily lives. With the appearance of practical forms of these devices the era of "electricity for everyone" began. Batteries were not banished; today they are needed more than ever for standby supplies and for the vast amount of portable electrical equipment. Electrolysis and Electrolytic Conductance. In 1805, Christian Grotthus (1785-1822) put forward a theory that attempted to explain why electrolysis products appear only at the electrodes (7). Although shown later to be open to grave objections, no doubt this theory caused others to consider the problem. On the experimental side, areas such as quantitative electrolytic conductance and the determination of transference numbers were beginning to open up. In fact, Henry Cavendish (1731—1810) had examined the relative conductances of water and salt solutions as early as 1776 (8,9). Figure 2 is an attempt to illustrate his method. This involves the comparison of electric shocks when Leyden jars are discharged through the solutions and through the observer. Remarkably, Cavendish's results are not far from modern values. After direct current became available, attempts were made to use it in electrolytic conductance measurements. In 1847, Eben Horsford (1818-1893) tried to allow for the effects of electrode polarization (10). He varied the length of the liquid path, then adjusted a calibrated series-connected resistor to restore the current to its original value. However, precise measurements of electrolytic conductance by d.c. methods had to await the perfection of the 4-electrode system by Gordon and his co-workers in the 1940's.



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Figure 2. Cavendish's method for comparison of electrolytic conductances. A,B, tubes containing solutions; X,Y, sliding wires to adjust length of liquid path; 1-6, Leyden jars carrying equal charges. The experiment involved grasping X with one hand and the knob of jar 1 with the other. Next, jar 2 was discharged while grasping Y, and so forth. (Reproduced from Ref. 11. Copyright 1984, American Chemical Society.)


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ELECTROCHEMISTRY, PAST AND PRESENT The introduction of a.c. methodology by Friedrich Kohlrausch (1840-1910) in 1869 and his extensive work in the field of electrolytic conductance led to the high-precision measurements by Washburn and others around 1920 (11). Even traces of dissolved electrolytes considerably increase the very small conductance of water. This effect is often used to continuously monitor the purity of distilled or demineralized water. The fundamental importance of conductance measurements became obvious when they were used by Svante Arrhenius (1859-1927) to develop his theory of electrolytic dissociation 02). From experiments with dilute solutions of strong electrolytes, Kohlrausch developed a relationship that allowed the maximum (or "zero concentration") value of the conductance, Ao, to be estimated from measurements made at finite concentrations. He also showed that Λo is the sum of the conductances of the anion and the cation. The additional information needed to find the separate ionic conductances from a knowledge of Λo is the ratio of the mobilities of the two ionic species. The faster-moving ion will have the higher ionic conductance. In the period 1853 to 1859 Johann Hittorf (1824-1914) published the results of his experiments. These involved the analysis, after electrolysis, of the solutions around one or both of the electrodes. The "transference numbers" thus obtained are related to the required ratio. It is interesting to note that the simple relationship used by Kohlrausch to obtain Λo values has been subjected to considerable theoretical study and correction, notably by Peter Debye (1884-1966) and Erich Hückel (1896-1980) and by Lars Onsager (1903-1976). Electrical Energy. The quantitative relationships between heat, electrical energy, and mechanical energy are cornerstones of any branch of electrical theory and practice. So basic were the investigations of these relationships carried out by James Joule (1818-1889) that his name is given to a unit (usual symbol, J ) . Although Joule's major achievements were the accurate determinations of the mechanical and electrical equivalents of heat, his interests were wide. From electrolysis experiments he began to formulate theories of the heating power of an electric current. He developed various electromagnetic engines, noting that the force of the engine when driven by a given battery decreases as the speed increases. This is, of course, due to the progressive development of back-emf. He was forced to conclude that the "duty" of his engines, with zinc as "fuel" could not compete with that of the steam engine, using an equal weight of coal (13). Ostwald, Father of Physical Chemistry Nowadays, we would describe much of the early work in electrochemistry as physical chemistry, which was established as



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a significant discipline by Wilhelm Ostwald (1853-1932). This came about through his own studies, writings, founding of periodicals such as Zeitschrift für physikalische Chemie, and the work of others that he inspired (14). In 1884, while Ostwald was Professor of Chemistry at Riga Polytechnicum, he received, and was impressed by, Arrhenius's dissertation on the conductance of electrolytes. Ostwald traveled to Sweden to meet Arrhenius; the two became lifelong friends and associates. The development of Arrhenius's theory and of Kohlrausch's conductance techniques allowed Ostwald to enunciate his "Dilution Law", relating the degree of ionization of a weak electrolyte to its concentration. He became interested in the emfs of cells, and hence in the problem of the potential of a single electrode system. He thought that it should be possible to employ a dropping mercury electrode as the basis of a system of absolute potentials. In 1887 Ostwald became Professor of Physical Chemistry at Leipzig, remaining there until his retirement in 1906. He was fortunate in his choice of assistants; one, recommended by Arrhenius, was Walther Nernst (1864-1941). The work on the factors that govern the emf of cells, begun at Riga, pushed ahead. One result was Nernst's announcement in 1889 of the relationship that is the basis of electrolytic potentiometry. This relationship soon acquired the simple title "The Nernst Equation". Ostwald rapidly built up a famous school of Physical Chemistry at Leipzig, often with a preponderance of English-speaking students. At that time, organic chemistry was the dominant area in Germany. Theodore Richards (1868-1928), the first American recipient of the Nobel Prize for Chemistry, studied with Ostwald in 1894. Richards, best known for his work on the determination of atomic weights, made major contributions to fields such as precise coulometry. Two more of Ostwald's many associates may be mentioned. In 1891 Max Le Blanc (1865-1943) studied the decomposition voltages of solutions at platinum electrodes. Compounds as dissimilar as sulfuric acid and sodium hydroxide gave essentially the same results; the overall reaction in both cases is merely the decomposition of water. Hydrochloric acid and the other hydrogen halides gave lower values; here halogen can replace oxygen as an anodic product. By use of a third, or reference electrode, Le Blanc was able to separately determine the anodic and the cathodic potentials. A few years later, Wilhelm Böttger (1871-1949) used the hydrogen electrode to study the potentiometric titration curves of acids and bases (15). Standardization of Potentials. The increasing interest in potentiometry stressed the need for standard values of electrode potentials. In 1890, Ostwald introduced the calomel electrode, calibrating this against a dropping mercury electrode. Nernst chose the normal hydrogen electrode, assigning to it a potential of zero. The upshot was a confrontation between the two scientists as the 19th century ended.


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ELECTROCHEMISTRY, PAST AND PRESENT It was realized that chemical equilibria were governed not by stoichiometric concentrations, but by "active masses". The contributions to the theories made by various workers culminated in the concept of ionic activity by Gilbert Lewis (1875-1946) in 1907. Later on, Lewis used this concept to set up a listing of electrode potentials based on the standard hydrogen electrode. This was generally adopted, with one source of difficulty, that of sign. We had the so-called 'American' and 'European' sign conventions! In 1953, international agreement confirmed the hydrogen scale and settled the question of sign, specifying that half-cell reactions shall be written as reductions. Although this agreement regularizes the values of the electrode potentials that we use, the problem of measuring or calculating the absolute values of these potentials, or of the activities of single ionic species still remains. Preparative Electrochemistry Electrochemistry was a precocious infant, enabling Humphry Davy to discover (i.e., to prepare) potassium, sodium, and other reactive metals in 1807. As already indicated, the now-massive chlor-alkali industry that began towards the end of the last century owes its origin to early observations during the electrolysis of salt solutions. Although mineralogically abundant, aluminum was quite expensive when it had to be produced by purely chemical means. Nowadays electrolytically-produced aluminum allows the container for a "TV dinner" to be treated as a throwaway item. The almost simultaneous but independent reporting of the cryolite-alumina electrolytic process in 1886 by C.M. Hall in the U.S.A. and by P. Herault in France is an amazing coincidence. An excellent account of the events that led Hall to his discovery is available 06). Although aluminum production is the outstanding example of fused-salt electrochemistry, the laboratory-scale applications of this field are quite wide (17). A recent example is the electrolytic preparation of the spinel Xv2O4 (X = Zn, Mg, or Cd) (18). A recent bibliography of the synthesis of inorganic compounds by electrochemical means contains more than 4000 references (19). The development of any industrial chemical process inevitably involves chemical engineering, process control, and a close study of any means to improve overall efficiency. An illustration is the winning of bromine from brine that contains less than 0.2 per cent of bromine as bromide (2_0). When the process was started in 1889, bleaching powder was added to large batches of brine, then the liberated bromine was carried off in a current of air. The introduction of electrolytic oxidation allowed this batch process to be changed to a more economic continuous one. Over-oxidation, leading to contamination of the bromine with chlorine, was minimized by sampling, chemical analysis, and subsequent adjustment either of the brine flow rate or of the electrolysis current. During the next few years,


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sampling and analysis were supplanted by a continuously indicating galvanometer system. Eventually, the electrochemical indication was extended to provide automatic control of the current. Organic Electrochemistry. Fichter's classic text names Faraday, Schoenbein, and Kolbe as the founders of organic electrochemistry (21). Although published nearly half a century ago, this text and its massive bibliography are valuable aids to the study both of reactions and of the history of electrochemistry. The electroreduction of nitrobenzene received early attention, probably because of the need for dyestuffs precursors. A patent for making aniline by this procedure was granted over a century ago. Depending upon the conditions, nitrobenzene can yield a variety of reduction products. By his introduction of controlled-potential electrolysis near the end of last century, Fritz Haber (1868-1934) was able to elucidate the electrochemical and chemical steps that give rise to these products. The Luggin capillary, named for Haber's friend and colleague, dates from this period (22). Interest in organic electrochemistry, never at a standstill has increased greatly during the past few decades. Apart from the carrying out of small-scale syntheses, much attention has been paid to the elucidation of reaction mechanisms leading, for example, towards an understanding of how various alkaloids are formed in living systems. Surprisingly, large-scale organic electrosynthesis was not commercialized until around 1965. The most obvious example is the manufacture of the nylon intermediate adiponitrile. Some Other Aspects of Industrial Electrochemistry Impurities in copper seriously decrease its value as a conductor of electricity. Accordingly, the purity of copper needed for cables became a matter of great concern as long-distance telegraphy developed. Electrorefining, first introduced over a century ago, provides a means for the production of high-purity copper. Not much "off the shelf" equipment was available when early industrial processes were being developed. Nowadays the control engineer is called during the early stages of plant design. Before canning, tomatoes, peaches, and the like are "caustic peeled" in a continuous process that involves treatment with hot, quite concentrated NaOH solution, followed, of course, by a thorough water wash. A typical conveyer system is diagrammed in Figure 3. Temperature control, long practiced in industry, presents no special problems; the maintenance of the desired alkalinity is another matter (11). An effective approach makes use of the oldest electrochemical measurement, conductance. However, the modern electrodeless sensing system uses totally-enclosed coupled coils and is not fouled by suspended matter in the electrolyte.


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Figure 3. Control of temperature and of NaOH concentration in a fruit-peeling system. TIC, temperature indicating controller; CIC, conductance-indicating controller; I/P, interface between conductance monitor and CIC. (Reproduced from Ref. 11. Copyright 1984, American Chemical Society.)



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Electrochemical Machining. "Destructive electrolysis" conjures up grave visions of losses by corrosion. There is, however, at least one area in which such destruction is put to constructive, i.e., valuable, uses. This is the technique of electrochemical machining (23). The shaping of hard, brittle, or tough metals by conventional means has always been an engineering problem. Machining involves the controlled removal of metal; in many cases this can be done electrolytically. The flow of electrons ignores the hardness of a workpiece that can blunt the conventional cutting tool. All that is needed is susceptibility to anodic attack when flooded with a suitable electrolyte solution. Properly applied, electrochemical machining can produce objects of quite complex shapes and with a smooth finish. Quite small holes can be drilled by a modification of the technique. Instrumentation As amply demonstrated by the early workers, it is possible to carry out a preparation electrochemically with little other than a suitable electrolysis cell and a source of power such as a battery. The situation is quite different in fields such as analytical or physical chemistry; instrumentation becomes important, and may be a controlling factor (24). A basic requirement is the ability to measure electrical quantities such as potential, current, and resistance or conductance. An early method for assessing the "strength" of a battery was to find the length of thin wire that the battery could cause to be heated to redness. Oersted's 1820 discovery of the deflection of a magnetic needle by an adjacent current-carrying wire provided a new approach to "strength" measurements. Within a year, Ampere had enunciated the basic principles of electromagnetism, while three independent workers showed that a multi-turn coil could greatly increase the sensitivity of Oersted's device (.25). Here is the ancestry of the sensitive, and eventually accurate, ammeters and voltmeters of later times. The laying of the Transatlantic cable in 1858 accentuated the need for sensitive and reliable instruments. A galvanometer designed by William Thomson (later Lord Kelvin) deflected when energized through 3700 miles of cable; the sole source of energy was a slip of zinc dipped in dilute acid contained in a silver thimble! In Thomson's telegraphic "siphon recorder", patented in 1867, pen-to-paper friction was eliminated by ink-jet printing (26). The same general idea is used in modern computer printers. The null-point method introduced by Poggendorf in 1841 paved the way for precise potentiometry. Although the essentials of "Ohm's Law" were published in 1826, they received little attention until they were publicized in 1843 by Wheatstone, whose name is associated with the "bridge" method for comparing electrical resistances. The Move Towards the Electronic Age. Progress during the past 100 years has been very rapid. Coulometry, the measurement of quantities of electricity, was initiated by Faraday. Eventually


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coulometry became so p r e c i s e t h a t , u n t i l 1948, t h e ampere was defined i n t e r n a t i o n a l l y in terms of t h e r a t e of d e p o s i t i o n of s i l v e r under c o n s t a n t - c u r r e n t c o n d i t i o n s . Galvanometric pen r e c o r d e r s were commercialized in t h e l a t e


1890'S (26).

The e a r l y 1 8 0 0 ' S a n c e s t o r of the

"string

galvanometer" gave r i s e t o t h e e l e c t r o c a r d i o g r a p h and t h e e l e c t r o m a g n e t i c o s c i l l o g r a p h , both making p o s s i b l e the observation or recording of r a p i d l y - v a r y i n g c u r r e n t s . From i t s b i r t h in a home workshop, the modern type of X-Y recorder became a commercial item in 1951 ( 2 7 ) . Photographic recording of c u r r e n t - v o l t a g e curves was a f e a t u r e of the polarograph, invented in 1925. Developments such as t h a t of t h e vacuum t r i o d e , cathode ray tube and, l a t e r , t h e t r a n s i s t o r and t h e s o l i d - s t a t e i n t e g r a t e d - c i r c u i t device, had an enormous influence on the design and c o n s t r u c t i o n of i n s t r u m e n t s . E l e c t r o n i c systems can respond with l i g h t n i n g speed to an almost-zero s i g n a l . The g l a s s i o n - s e n s i t i v e e l e c t r o d e was discovered in t h e e a r l y 1900's, but i t s appearance in the pH meter had t o wait for developments in electronics. Automation, both in the l a b o r a t o r y (28) and in industry (29), became n o t i c e a b l e around 1900. Most, but not a l l of t h e e a r l y devices were e n t i r e l y mechanical. Mention may be made of a w a t e r - s o f t e n i n g system, patented in 1906, t h a t claimed to e l e c t r o p h o t o m e t r i c a l l y "analyze" t h e feed water and t o a u t o m a t i c a l l y a d j u s t t h e dosages of lime and soda ( 3 0 ) . Nowadays computerization can control a s i n g l e instrument, a fully-automated l a b o r a t o r y , or an e n t i r e f a c t o r y . P r e s e r v a t i o n of H i s t o r i c I n s t r u m e n t s . Equipment t h a t has been superseded i s put a s i d e and then q u i t e often scrapped; t h i s i s t o be expected. However, if t h e item in question i s t h e l a s t of i t s kind, or i s unique, perhaps a p r o t o t y p e , t h e scrapping becomes an a c t of e x t i n c t i o n . Because of u n i v e r s a l r e c o g n i t i o n and value as a n t i q u e s , long-outdated microscopes, t e l e s c o p e s , timepieces and the l i k e have q u i t e good chances of s u r v i v a l . General l a b o r a t o r y equipment, recognizable by s c i e n t i s t s but not by many o t h e r s , has not fared so w e l l . H i s t o r i c i n d u s t r i a l equipment seems to have a q u i t e poor chance of s u r v i v a l . F a c t o r i e s have l i t t l e space for d i s c a r d e d equipment which, in any c a s e , lacks the " b r a s s , mahogany, and g l a s s " eye appeal of an old l a b o r a t o r y instrument. Only sheer luck saved the h i g h - p r e s s u r e flowmeter shown in Figure 4 . This instrument once formed part of B r i t a i n ' s f i r s t s y n t h e t i c ammonia p l a n t . D i l i g e n t and p e r s i s t e n t searching for a p a r t i c u l a r h i s t o r i c instrument sometimes r e s u l t s in s u c c e s s . Unfortunately, f a i l u r e i s much more common; apparently the item in question no longer e x i s t s ( 3 1 ) . The f a t e of instruments t h a t w i l l form p a r t of the h e r i t a g e of the h i s t o r y of science for those t h a t are t o follow us l i e s in our hands!



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Figure 4. Jasper Clark (2nd from right) presents an historic flowmeter to the Science Museum, London. (Reproduced from Ref. 31. Copyright 1980, American Chemical Society.)


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Electroanalytical Chemistry For a concise overview of the influence of electrochemistry on the practice of analytical chemistry, the relevant section of the American Chemical Society monograph, A History of Analytical Chemistry, is hard to beat (32). The account deals both with techniques and with persons. One of the contributors is Emeritus Professor I. M. Kolthoff, very much a maker of history and also the scientific ancestor of generations of electroanalytical chemists. Through him we have, as a bonus, a direct linkage with Arrhenius, Haber, Nernst, and other scientific giants of the past. The applications of electrochemistry to analytical chemistry are numerous. A table in a modern text (33) lists more than a dozen interfacial techniques; other fields such as conductometry are dealt with in separate chapters. Accordingly the present comments are limited to a few of the basic areas of electroanalytical chemistry. Electrogravimetry. In his History of Analytical Chemistry, Szabadvary devotes an entire chapter to electrogravimetry (34). He points out that electrolytic deposition was recommended as a qualitative test for copper in 1800. The first electrogravimetric determinations were reported by Wolcott Gibbs in 1864. By a remarkable coincidence, the German industrial chemist C. Luckow reported similar investigations in 1865, indicating that he had practiced electrogravimetry since 1860. Szabadvary does not mention the major contributions of E. F. Smith in U.S.A., or of H. J. S. Sand in Britain. Once a major analytical technique, electrogravimetry has largely passed into history. Potentiometry. As is obvious from the vast and continuing outflow of literature concerning ion-selective electrodes, direct potentiometry needs no emphasis. The development of the glass electrode, the pH concept, and pH measurement is a fascinating and important branch of this aspect of potentiometry. According to Szabadvary (35), the first potentiometric titrations were reported by R. Behrend in 1893. The technique became very popular, and has remained so; the first monograph, by E. Mueller, appeared in 1923. Automatic potentiometric titration, first performed in 1914, has an extensive literature (36)• Conductometry. Because conductometric titration curves are easily linearized, precise results can often be obtained at low titrand concentrations. According to I. M. Kolthoff, who made major contributions to this field, the first analytical determination by this method occurred in 1895 (37). Because no liquid junctions are involved, conductometry can be useful in studies of nonaqueous systems, including solvents such as liquid ammonia or concentrated sulfuric acid. In fact, measurements can be made without metal-to-solution contact. The use of high-frequency a.c. permits the sensing system, sometimes


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merely a pair of metal rings, to be located outside of the solution container. The modern industrial electrodeless sensor is a completely-sealed immersible device. Coulometry. Faraday's laws of electrolysis, enunciated in 1834, form the basis of coulometric techniques. By the beginning of the present century the silver coulometer had been shown to provide an accurate means for the measurement of quantities of electricity. An excellent survey of various chemical and other coulometers is available (38). The electronic digital coulometer, first described in 1962 (39), was a major practical advance. Although the measurement of electricity through its chemical effects began quite early, the reverse of this process received little attention. The first major development occurred in 1938, when the Hungarian workers L. Szebelledy and Z. Somogyi described the titration of, for example, HCl with electrogenerated hydroxyl ion (40). The technique, coulometric titration, was rapidly extended, largely by American workers (41 ). Considerable selectivity can often be obtained by keeping the working electrode at a suitable fixed potential. Until the invention of the potentiostat in 1942, the method required the constant attention of the operator. Sometimes a combination of two quite different disciplines can yield more information than is given by either one alone. A good example of this approach is the technique of spectroelectrochemistry. Voltammetry. The discovery of polarography, the study of voltammetry at a dropping mercury electrode (DME) focused attention upon the analytical potentialities of voltammetric methods. Although excellent for cathodic processes, mercury is susceptible to anodic attack, especially in the presence of ions such as chloride. Platinum and other solid electrodes are useful as anodes, and can be used as cathodes if the potential is not too negative. A supreme advantage of the DME is the periodic renewal of the electrode surface; the surface condition of a solid electrode can greatly affect its performance. Attempts to electrochemically clean a platinum electrode led to the discovery of quantitative anodic stripping analysis during the then classified work on radionuclides. Later work, published in 1952, clearly demonstrated the value of stripping analysis as a means for trace determinations (42). The sensitivity and versatility of polarography has been greatly extended by the introduction of pulse techniques. Although explored in the early days of polarography, the theory and practice of pulse polarography did not develop until the early 1950's. The first commercial electronic instrument appeared around 1958. Cyclic voltammetry involves the scanning of the potential in one direction, followed by an immediate reverse scan to the starting potential. Often the cycle is repeated. The technique is now used extensively and is valuable for such purposes as the diagnosis of electrode reactions. A key paper of 1964 which


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essentially opened up the field refers to the limited earlier work (43). Amperometric Titration. This analytical process is performed by observing changes in the cell current as the titration proceeds. Titration at one indicator electrode was first reported by Heyrovsky and Berezicky in 1929. A few years later, Kolthoff and his co-workers began their extensive studies of the method which, like conductometric titration, yields linear curves. Biamperometric titration involves the application of a fixed, usually small, voltage across a pair of identical electrodes. First performed as early as 1897, the technique was rediscovered and applied in 1926. Although the titration curves are not linear, the end point is usually marked by a sharp change in the current. A text (44) and a succession of reviews (45) describe the history and subsequent development of these and related titration techniques. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.

10. 11. 12. 13. 14. 15. 16. 17.

18. 19. 20. 21. 22.

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23. McGeough, J. A. Principles of Electrochemical Machining; Wiley: New York, 1974. 24. Stock, J. T.; Orna, M. V. The History and Preservation of Chemical Instrumentation; Reidel: Dordrecht, 1986. 25. Stock, J. T. J. Chem. Educ. 1976, 53, 29. 26. Stock, J. T.; Vaughan, D. The Development of Instruments to Measure Electric Current; Science Museum: London, 1983. 27. Moseley, F. L. Symposium on the History of Chemical Instrumentation; A.C.S. Annual Meeting, Washington, D.C., 1979, Paper No. 11. 28. Stock, J. T. Educ. Chem. 1983, 20, 7. 29. Stock, J. T. Am. Lab. (Fairfield, CT), 1984, 16 (6), 14. 30. Stock, J. T. Trends Anal. Chem. 1983, 2, 211. 31. Stock, J. T. Anal. Chem. 1980, 52, 1518A. 32. Laitinen, H. A.; Ewing, G. W., Eds. A History of Analytical Chemistry; American Chemical Society: Washington, D.C., 1977, Chapter 4. 33. Kissinger, P. T.; Heineman, W. R. Laboratory Techniques in Electroanalytical Chemistry; Dekker: New York, 1984, p. 6. 34. Szabadvary, F. History of Analytical Chemistry; Pergamon: New York, 1966, Chapter 10. 35. Ref. 34, Chapter 13. 36. Lingane, J. J. Electroanalytical Chemistry, 2nd Ed.; Interscience: New York, 1958, Chapter 8. 37. Kolthoff, I. M. Konduktometrische Titration; Steinkopff: Dresden, 1923. 38. Ref. 36, pp. 340, 452. 39. Bard, A. J.; Solon, E. Anal. Chem. 1962, 34, 1181. 40. Ref. 34, p. 316. 41. Ref. 32, p. 269. 42. Lord, S. S.; O'Neill, R. C ; Rogers, L. B. Anal. Chem. 1952, 24, 209. 43. Nicholson, R. S.; Shain, I. Anal. Chem. 1964, 36, 706. 44. Stock, J. T. Amperometric Titration; Interscience: New York, 1965. 45. Stock, J. T. Anal. Chem. 1984, 56, 1R, and earlier parts cited. RECEIVED August 12, 1988


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