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Julius Tafel was an organic chemist and physical chemist who studied the electrochemistry of organic compounds and the relation between rates of electrochemical reactions and applied overpotentials. He is the best known for the Tafel equation - one of the most important equations of electrochemical kinetics. Julius Tafel was born in Choindez, Canton Berne, Switzerland, on June 2, 1862. He attended the Realgymnasium (a secondary school with scientific direction) at Stuttgart and Nurnberg.
From 1880 he studied in Zurich, Munchen and Erlangen. In Erlangen he started to work as an assistant of Emil Fischer who had obtained a professorship there in 1882, and who recognized early the young talented chemist.
In 1882, Tafel received his Ph.D. degree under the guidance of Fischer, presenting a thesis on the isomerisation of indazole (that had just been discovered by Fischer and Kuzel) and the hypothetical isindazole. Together with Paal (another Fischer’s assistant), Tafel worked briefly on the synthesis of heterocyclic compounds, a field in which Ludwig Knorr became famous at that time.
Hermann Emil Fischer - Tafel’s supervisor
Educated at the universities of Bonn and Strasbourg (Ph.D., 1874), Fischer held several posts before becoming professor of chemistry at the University of Berlin in 1892. Under his direction, the chemical laboratory at Berlin became one of the most important in the world and attracted to itself a constant stream of brilliant pupils. During World War I Fischer was responsible for organizing the production of chemicals in Germany. He committed suicide in 1919, after two of his sons had been killed in the war.
Fischer’s research on the purines was instituted in 1881. He determined the structures of uric acid, xanthine, caffeine, theobromine, and other related compounds, and he showed that they are all derivatives of a single compound, a nitrogenous base that he named purine. His researches into the sugar group, begun in 1883, were of unparalleled importance to organic chemistry.
In 1875 he had published his discovery of the compound phenylhydrazine, a substance that in 1884 he found reacts with simple sugars to form derivatives called osazones. Despite major complications because of stereochemical relations, Fischer was able to use these derivatives to determine the molecular structures of fructose, glucose, and many other sugars, and he was able to verify his results by synthesizing those compounds. He also showed how to distinguish the formulas of the 16 stereoisomeric glucoses. In the course of his stereochemical research, Fischer discovered that there are two series of sugars, the D sugars and the L sugars, that are mirror images of each other.
His work on sugars changed the understanding of these compounds and he was awarded the Nobel Prize for Chemistry in 1902. His study of sugars led him to investigate the reactions and substances involved in fermentation, and, in his investigations of how enzymes break down sugars, Fischer laid the foundations for enzyme chemistry. Fischer’s researches on the purines, begun in 1894, culminated in his pioneering efforts to determine the structure of proteins.
It was already known that proteins were composed of amino acids, but Fischer found new ways of purifying amino acids and determining how they are combined together within the protein molecule. He then found ways to link amino acids to each other and began synthesizing proteinlike substances; in 1907 he was able to combine 18 amino acids into a polypeptide, which he then broke down by enzymes in the same manner as would occur in a natural protein.
Two photos showing Hermann Emil Fischer (1852-1919) in his lab
Ludwig Knorr - another Fischer’s assistant
Ludwig Knorr (1859-1921) studied chemistry at the university in his home town, Munich, and at Heidelberg. He became a close colleague of Emil Fischer and followed him to the Universities of Erlangen and Wurzburg, before being appointed Professor of Chemistry at Jena in 1888. Knorr gave his name to syntheses of pyrazoles (1883), pyrroles (1884) and quinolines (1886). He was the brother-in-law of Oskar Piloty who was also an assistant to Fischer.
In fall of 1885, Fischer moved to Wurzburg (less than 100 km west of Erlangen) and Tafel together with Knorr followed him as assistants. In 1886, Tafel worked on his Habilitation (study performed in Germany after Ph.D. in order to get further scientific promotion) in the field of organic chemistry, developing simple routes to amines by the reduction of phenylhydrazones of aldehydes and ketones. Tafel suggested sodium amalgam as a reducing agent that was novel for that time.
However, Tafel had to assist Fischer in his synthetic work that detracted him from his own research. Later Fischer described how Tafel had assisted him in the synthesis of bulk amounts of acrolein (2-propenal, more often called by the trivial name acrolein) and acrolein dibromide (both of them extremely irritate the respiratory tract if inhaled). The synthesis was performed outdoor, exclusively on windy days, but once Tafel accidentally was surrounded by a toxic cloud that resulted in severe nose bleeding. When Fischer reported his results before the Presidium of Chemische Gesellschaft (the German Chemical Society) on 23 July 1890, Tafel carried out the experimental demonstrations.
Since the work with Fischer took quite a lot of his time, Tafel slowly progressed in his own research and his very promising synthetic approach (the reduction of phenylhydrazones with sodium amalgam in order to produce aldehyde and ketones) was captured by another scientist Goldschmidt who used it for the reduction of oximes that was an obvious modification of Tafel’s approach. Thus, Tafel had to find another subject for his research. He performed a study of some alkaloids (strychnine and brucine) trying to elucidate their structures, the task that was not completed during his lifetime. Between 1888 and 1893 Tafel was interested in the chemistry of dyes, alkaloids, heterocyclic and isocyclic compounds. But from 1893 he lectured mainly on general and physical chemistry.
In the fall of 1892, Fischer moved to Berlin and in his words: “a professor of chemistry moves not merely with his scholarship and books, but also with preparations, apparatus, and assistants” (note the sequence!). However, there were no available positions for Fischer’s assistants W. Wislicenus and J. Tafel (L. Knorr had moved to Jena already in 1889), thus they had to stay in Wurzburg. That university was a great center of scientific activity and many famous scientists, including E. Buchner (Nobel Prize in 1907), F. Kohlrausch, W.K. Rontgen (Nobel Prize 1901), W. Wien (Nobel prize 1911), A. Fick, were actively working there.
Group photo, ca. 1910, showing: (1) Tafel, (2) Manchot, (3) Emmert, (4) Reitzenstein, (5) Hermann Pauly, (6) v. Halban, (7) Schubel, (8) Herterich, (9) Houseman (USA), (10) Schepss, (11) Will, (12) Palmberg (Finland), (13) Lockermann, (14) Haas, (15) Father Fitzgerald (Ireland).
Tafel’s first publication in physical chemistry appeared in 1896 (J. Tafel, Zeitschrift fur physikalische Chemie 1896, 19, 592-598), after he spent some time with Wilhelm Ostwald in Leipzig. During this visit Tafel became informed about the novel electrochemical experiments with the use of lead cathodes.
Tafel also applied lead cathodes to the electrochemical reduction of strychnine (J. Tafel, O. Rosenheim, F. During, G. Fenner, Liebigs Annalen der Chemie 1898, 301, 285-348). This was Tafel’s first paper on electrochemistry. The discovery of the electrochemical reduction of organic compounds by lead cathodes was a seminal contribution to the new electrochemical science. In 1898 Tafel obtained the first professor position.
Although between 1892 and 1898 Tafel had some periods of illness and apparent inactivity, a great period of activity after the successful electrochemical reduction of strychnine in 1898 was culminated in 1902/03 when Tafel was able to present summaries and experimental illustrations of his work on the electrochemistry of various organic compounds to the 9th Meeting of the German Electrochemical Society (Wurzburg, 9/10 May 1902) and to the Physical-Medical Society (Wurzburg, 22 January 1903). Tafel discovered that it would be possible to reduce at electrodes substances for which he had not found other reduction methods.
In 1902 Tafel was promoted to extraordinarius and in 1903 to ordinarius (equivalent to associate professor and full professor, respectively). In 1903 he also became the director of the Chemical Institute. Julius Tafel was married in 1903 and lived in a very happy personal environment.
Very soon Tafel became a leading specialist in organic electrochemistry. Already in 1900 his main ideas were published (J. Tafel, O. Schwab, A. Veit, K. Schmitz, Zeitschrift fur physikalische Chemie 1900, 34, 187-228) and reported on scientific meetings (Naturforscherversammlung in Munich, 1899). When Tafel summarized his work again in 1906 in the invited review paper (J. Tafel, Zeitschrift fur Eleckrochemie 1906, 12, 112-122) nothing principally new had to be added.
Julius Tafel, ca. 1905
In order to follow the rate of the electrochemical reactions Tafel introduced a hydrogen coulometer in his experimental setup. It should be noted that such application of a hydrogen coulometer was already suggested before by Elbs, but his paper was rejected by Chemische Gesellschaft for publication in its Berichte, reflecting underestimation of the importance of kinetic measurements typical for that time.
Tafel also discovered that traces of various metals in a solution or on a surface of a lead cathode result in significant inhibition of the electrochemical reduction of the organic compounds studied. In order to overcome this problem, Tafel suggested to exchange the lead cathode after a few minutes of electrolysis with a new one that is not poisoned with traces of the inhibiting metals electrochemically deposited on the first cathode surface in the first minutes of the electrolysis.
Thus, Tafel suggested what is called now pre-electrolysis for the electrochemical purification of solutions. Tafel also introduced electrochemical pre-treatment of lead cathodes by their polarization to positive potentials to generate on the surface a lead oxide layer that resulted in the enhanced electrochemical reduction of organic materials.
The inhibition of the electrochemical reduction of organic compounds originated from the concomitant reduction of water with hydrogen evolution, which is catalyzed by metals other than lead. Electrochemical purification of lead cathodes resulted in a large overpotential for electrochemical hydrogen evolution, thus allowing the desired electrochemical reduction of the organic compounds. The electrochemical oxidation of the electrode surface yielding a thin oxide layer on the surface resulted in further inhibition of the hydrogen evolution, thus resulting in more efficient reduction of organic compounds.
Chemical Coulometers
Cell for H2 - O2
coulometric analysis
H2 - O2
coulometer
Very high precision of charge measurement is possible with a chemical coulometer. However, chemical coulometers are always less convenient to use than electronic integration instrumentation. As a consequence chemical coulometers are no longer commonly used in the electrochemical laboratory. The types of chemical coulometer which are most precise are: the silver coulometer; the iodine coulometer; the hydrogen-oxygen gas coulometer; the hydrogen-nitrogen gas coulometer.
The gas coulometers were based on volumetric analysis of electrochemically generated gases (e.g. cathodically generated hydrogen). The schemes of a hydrogen-oxygen coulometer shown here were developed by Lingane (J.J. Lingane, J. Amer. Chem. Soc. 1945, 67, 1916-1922). A hydrogen coulometer used by Tafel was probably simpler, but it was based on the same principle.
Based on his kinetic measurements, Tafel proposed the catalytic mechanism for hydrogen evolution (rate-determining step: the chemical combination of hydrogen atoms) which bear his name. Most importantly, by combining the measurements of current with an analysis of the overpotentials for electrochemical reactions, Tafel empirically discovered the first formulation of the electrochemical kinetics law, Tafel’s law, which showed the exponential relation between the electrochemical reaction rate and the overpotential:
Jahn and Schonrock had in fact derived a similar equation from thermodynamic considerations (H. Jahn, O. Schonrock, Z. Physik. Chem. 1895, 16, 45), and Jahn had presented the experimental proof for this equation (H. Jahn, Z. Physik. Chem. 1898, 26, 385). F. Haber later argued that this equation could be derived from the Nernst equation.
However, Tafel noted that his observations are particularly studied on irreversible electrochemical reactions where the thermodynamics cannot be applied. Thus, Tafel’s studies were the first to separate the electrochemical kinetics from thermodynamics, allowing irreversible reactions to be studied systematically.
Additional discovery made by Tafel showed that hydrocarbons with isomerized structures could be generated upon electrochemical reduction of the respective acetoacetic esters. This unexpected anomaly is known now as the Tafel re-arrangement.
Tafel’s life was rather short. He retired at the age of 48, due to poor health and spent his last years in spas attempting to regain his vigor, frequently attended by pupils at his bedside, even during fever spells. In his last years, between 1911 and 1918, he wrote as many as 60 book reviews covering practically every field of chemistry. When his health was temporary improved, he even started to write a textbook of organic chemistry, which he wanted to put on an entirely new basis.
However, his conditions were deteriorated soon, and he was not able to finish this textbook. Finally, he was suffering from insomnia (chronic sleeplessness) resulting in his complete nervous breakdown. Julius Tafel committed suicide on September 2, 1918, in Munich, at the age of 56.
Tafel was one of those scientists whose life is not well known, although the equation that bears his name is certainly as important to the overall understanding of kinetics in nature as is the corresponding equation due to Arrhenius.
This text is mainly based on Tafel’s biography written by Klaus Muller (K. Muller, J. Res. Inst. Catalysis, Hokkaido Univ. 1969, 17, 54-75). Dr. Klaus Muller kindly provided a copy of his paper and the original photo of Tafel for this web page. Other sources of information about Tafel also were used: J.O’M. Bockris, A.K.N. Reddy, M.Gamboa-Aldeco, Modern Electrochemistry, Second Edition, Fundamentals of Electrodics, Vol. 2A, Kluwer Acad., 2000, p. 1106; B. Emmert, Julius Tafel. Berichte der Deutschen Chemischen Gesellschaft 1918, 51, 1686-1687; in the Internet: ( 1 ).
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