Electrochemistry, Past and Present - ACS Publications - American

Chapter 9

The Choice of the Hydrogen Electrode as the Base for the Electromotive Series

Downloaded by UNIV OF SYDNEY on May 3, 2015 | http://pubs.acs.org Publication Date: January 1, 1989 | doi: 10.1021/bk-1989-0390.ch009

Alfred W . von Smolinski1, Carl E. Moore 2 , and Bruno Jaselskis2 1Department of Medicinal Chemistry and Pharmacognosy, University of Illinois at Chicago, Chicago, IL 60680 2Department of Chemistry, Loyola University of Chicago, Chicago, IL 60626 The electromotive series is a list of the elements in accordance with their electrode potentials. The measurement of what is commonly known as the "single electrode potential", the "half-reaction potential" or the "half-cell electromotive force" by means of a potentiometer requires a second electrode, a reference electrode, to complete the circuit. If the potential of the reference electrode is taken as zero, the measured E.M.F. will be equal to the potential of the unknown electrode on this scale. W. Ostwald prepared the first table of electrode potentials in 1887 with the dropping mercury electrode as a reference electrode. W. Nernst selected in 1889 the Normal Hydrogen Electrode as a reference electrode. G.N. Lewis and M. Randall published in 1923 their table of single electrode potentials with the Standard Hydrogen Electrode (SHE) as the reference electrode. The Commission of Electrochemistry of the I.U.P.A.C. meeting at Stockholm in 1953 defined the "electrode potential" of a half-cell with the SHE as the reference electrode. The Status of the Hydrogen Electrode. Probably no area of electrochemistry is more greatly neglected in current texts than the history of the choice of the hydrogen electrode as the reference standard for electromotive force measurements. Since all tables of potentials of oxidation-reduction half-reactions are based on the half-cell reaction1/2H 2 =H + +e-,it would seem that the selection of this reaction as the standard should warrant more attention. If the selection is treated at all, it is usually dismissed as an arbitrary choice, which it is, with no reference made to the people and events involved in establishing this fundamental reference point for the EMF scale. One possible exception may be noted (1). The referenced edition of this work is perhaps the best previously existing source on this topic. However, the subsequent edition omits the subject entirely.

0097-6156/89/0390-0127$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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Early History of the Electromotive Series. The roots of the choice of hydrogen as the standard for electromotive force measurement (2) may be traced to the decade of the 1790's and the discoveries of that period which were to change the whole state of science. Allessandro Volta, as a result of a series of experiments on what was later to be called galvanic electricity, published, as early as 1792, a list of substances (3,4) in an order such that "each is positive toward all which follow it and negative to all which precede." In a footnote to a report in Ostwald's Klassiker dated August 1796, Volta (4) reported as follows: In this table, which is not particularly different from that which Dr. Pfaff has made, zinc stands at the top; approximately in the middle are lead and tin, toward the end, silver; then finally graphite, carbon and copper sulfide. (Translated from German.) Thus almost at the inception of electrochemistry as a science, there was a recognition of the intrinsic electrochemical properties of substances and an attempt to arrange these substances in an order consistent with observed properties. In 1798 Ritter (2) linked the electrical and chemical properties by showing that the same series was obtained when comparing the ability of the metals to separate other metals from their salt solutions. These studies by Volta led to the discovery of the cell which now bears his name and which, for the first time, provided a source of electricity at low voltage and moderate current. This discovery opened the way for many other investigators, who immediately capitalized on it to do a wide variety of experiments. In 1807 J.J. Berzelius (2), with Baron Hisinger as coauthor, published a paper based on the electrolysis of salt solutions. As a result of his extensive studies, he formulated an electrochemical series. He arranged 54 elements beginning with the negative elements oxygen, sulphur and selenium and concluding with the positive elements sodium and potassium. Berzelius' version of the electrochemical series played an important and orientating role in the direction of the chemical research which was to follow. Almost one hundred years after Volta's report of the voltaic cell, electrochemistry had become an essentially quantitative science. In the 1830's Faraday had published his laws of electrolysis, and in the 1880's Nernst (5) had developed a mathematical treatment of cell potentials with respect to ion concentration. The outpouring of electrochemical data that were reported after the discovery of the voltaic cell proved again and again, as Volta had shown, that there were intrinsic differences in what we would now call redox properties of substances and that these immutable constants of nature needed to be anchored to a suitable reference point so that data originating in different laboratories would be interchangeable. In 1849 Beetz, in a paper entitled "The Electromotive Forces of Gases," (6) reported the use of the hydrogen electrode as a starting point for his EMF measurements. It should be noted that this paper by Beetz was published well before the birth of Nernst and Ostwald, who were to play determinative roles in the selection of the base of



9. VON SMOLINSKI ET AL.

Hydrogen Electrode and Electromotive Series

the EMF system. Thus the use of hydrogen as a reference antedates Nernst's proposal of its use by about fifty years. The use of cadmium and zinc bars as reference electrodes (7) had become an established practice in the storage battery industry by the time that Nernst was concerning himself with the problem of a standard. Ostwald (8) also mentions that in earlier times amalgamated zinc in a concentrated zinc sulfate solution was used as a constant electrode and occasionally as a reference electrode but that its use posed some serious problems. The need for a common reference point for the electromotive series was evident, and serious but ineffective attempts had been made to supply this need. There were philosophical as well as practical questions involved in the choice to be made. At this point in the history of electrochemistry, no recourse appears to have been made to committee selection of an appropriate reference electrode. Modern Foundation of the Electromotive Series Ostwald's Use of the Dropping Mercury Electrode. The modern foundation of the electromotive series is closely related to the effort of selecting a proper reference electrode for EMF measurements. At the turn of the century, W. Ostwald and W. Nernst did pioneering work in the field of reference electrodes, each approaching the problem from a different angle. For a better insight into the disagreement on the choice of a reference point, it might be helpful to look a little more closely at the two principals. Ostwald (9) was a man of tremendous energy and enthusiasm. In 1883 he founded the Zeitschrift fur physikalische Chemie (10). Then in 1902, at a time when philosophy and science were not considered compatible by most scientists, he established the Annalen der Naturphilosophie. In his mid-thirties he succeeded to the chair of physical chemistry at Leipzig. He was in his prime when he started recruiting for his laboratory at Leipzig. Ostwald recognized Nernst as a young man of great promise and invited Nernst to join him as an assistant when he had completed his doctorate with Ettinghausen at Graz. Both Nernst and Arrhenius moved to Leipzig, where, under Ostwald's support and tutelage, each developed into the kind of forceful young scientist whose work leads to fame. Later, by age fifty, with thirty-four of his pupils now professors, Ostwald's interest had shifted. He resigned his position and devoted his time to philosophical problems. Some gauge of the confusion that existed at this time may be obtained from the critical review (11) of the field of oxidation potentials prepared by Abegg, Luther, and Auerbach in 1911. Ostwald antedated Nernst by several years in his proposal of a reference electrode for the electromotive series. He was, of course, aware of the early attempts to prepare reference electrodes. He specifically mentions that in earlier times amalgamated zinc in concentrated zinc sulfate solution had been used as a reference electrode, that this combination when used as a reference electrode (8) gave liquid junction potentials with other electrolytes which could not be calculated, and that the electrode also contaminated the experiments.


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Ostwald had what appeared to be a very elegant concept. It involved the measurement of a single electrode potential. The method of measurement was in good accordance with his philosophical views and with the chemistry of the times, and it would, in his opinion, yield an absolute potential. An absolute potential was a sharp contrast to the relative potential obtained by referring a measured half-cell to another single electrode reaction arbitrarily set at zero. Ostwald's measurements of half-cell potentials could be directly related to heats of ionization (12). In his opinion, an absolute half-cell redox potential would allow the establishment of an electromotive series which would be analogous to the absolute temperature scale. In 1887 Ostwald published a paper in which he showed that with the help of a dropping mercury electrode it is possible to measure the potential difference between a metal and a solution of electrolyte (13) . He describes his use of the dropping mercury electrode and the Lippmann capillary electrometer, giving some experimental details and numerous tables of data. He clearly points out the low accuracy of the method based on the measurement of the surface tension of mercury, noting that it appears to be fundamentally sound but that near the maximum value of the surface tension there is only a very small change of surface tension for a relatively large change of electromotive force. This measurement problem inherent in the Ostwald electrode potential was one of the points attacked by Nernst. Ostwald also discusses the background of the Null point method (14) in his text and quotes extensively from von Helmholtz, Wied. (1882), 16, 35. One von Helmholz quotation is the following: From this I conclude that if a rapidly dripping and otherwise insulated mass of mercury is in contact with an electrolyte through the dripping tip, then the mercury and the electrolyte cannot have different potentials. Had they different potentials - for example, if the mercury were to be positive - then each falling drop would form a double layer on its surface, which would take away +E from the mercury and render its positive potential smaller and smaller until it was the same as that of the liquid. Thus a dripping mass of mercury is an electrode with the help of which one can connect liquids with the electrometer without any change of potential. In 1890 Ostwald introduced the normal calomel electrode (15) as a reference electrode of fixed potential in equilibrium with aqueous potassium chloride solution. Ostwald calibrated a normal calomel electrode against a dropping mercury electrode and obtained a mean value of 0.560 volt. He referred to this value as the absolute value of the electrode potential of the normal calomel electrode. Ostwald recommended the use of the normal calomel electrode as the null electrode, or the standard electrode, in the measurement of the potential difference at a metal-solution junction. He suggested that the electromotive potential measurement with the normal calomel




electrode as reference electrode should be considered to be on an absolute scale. A table of standard potentials, according to Ostwald, appears in Arrhenius' textbook (2) of electrochemistry. The potentials are as follows:

Potential Difference Between Metals and Their Salts in Normal Solution

Volts


Volts

Magnesium Zinc Aluminum Cadmium Iron Nickel

+1.22 +0.51 +0.22 +0.19 +0.06 -0.02

Lead Hydrogen Copper Mercury Silver

-0.10 -0.25 -0.60 -0.99 -1.01

It is interesting to note that Arrhenius does not mention the hydrogen electrode in his textbook on electrochemistry which was published in 1902. Nernst's Introduction of the Hydrogen Electrode. After two years at Leipzig, Nernst, using the theory of the galvanic cell as his thesis, completed a successful defense and was made a lecturer; this appointment opened the road to a professorship. He then left, as Mendelssohn (9) colorfully describes it, the large and benevolent shadow of Ostwald and proceeded to Göttingen by way of Heidelberg. Nernst (10), whose father was a judge, had a broad range of interests. Fond of the theater, he wrote a play which was produced on the Berlin stage. At one time he even considered a career as an actor. As a child he developed a love for chemistry and was allowed to develop a basement laboratory. He finished the Gymnasium at the top of his class but with the ambition of becoming a poet, not a chemist. This strange, pudgy, little man projected a trusting and credulous image of innocent astonishment which never changed, even on the many occasions when he engaged in withering sarcasm. The following story is told in a biography of Nernst by Mendelssohn (9): "A story became current among his colleagues that one day God had decided to create a superman. He began his work on the brain and formed the most perfect and subtle mind, but then was unfortunately called away. The archangel Gabriel saw the unique brain and could not resist the temptation to shape the body, but unfortunately, due to his inexperience, he only succeeded in fashioning a rather unimpressive looking little man. Dissatisfied with his efforts, he left his work. Finally, the devil came along and saw this inanimate thing,


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ELECTROCHEMISTRY, PAST AND PRESENT

and he blew the breath of life into it. That was Walther Nernst."


Nernst rose rapidly up the academic ladder to the professorship. Professor Nernst was an excellent business man and became quite rich. Nernst appears to have first introduced the normal hydrogen electrode, arbitrarily set at zero volts, in a lecture given before the German Chemical Society on May 24, 1897 (16). This lecture was published in the Berichte the same year. The following quotations are from pages 1556 and 1557 of this publication. Each is followed by an English translation; the numerical values of the chart are not repeated in the translation. In der nachstehenden kleinen Tabelle-ihre Ferstellung und ihre Erweiterung für wichtigsten bekannten Ionen scheint eine Aufgabe von der grössten Bedeutung zu sein - sind einige Zahlenangaben zusammengestellt, an die ich einige Bemerkungen knüpfen möchte. Zersetzungsspannungen E1 (Kationen)

für

normale

Concentrationen

E2

(Anionen)

Ag+

-0.78

I-

0.52

Cu++

-0.34

Br-

0.94

H+

0=

0.0

1.08*

Pb++

+0.17

Cl-

1.31

Cd++

+0.38

OH-

1.68*

+0.74

SO4=

++

Zn

1.9 HSO44

2.6

In the following small table — its ascertainment and its extension to the most important known ions seems to be a task of utmost importance — some numerical values are compiled, to which I would like to tie some remarks. Decomposition Potentials for Normal Concentrations E1 (cations) E2 (anions) Diese Zahlen beziehen sich auf Normalconcentration der Ionen; eine Verminderung der Concentration urn eine Zehnerpotenz erhöht nach unseren früheren Betracbtungen die Werthe um 0.058 volt, (n - Zahl der Ladungen oder chemischer Werth des Ions). Die Lösungstension des Wasserstoffs ist null gesetzt; da wir immer Anode und Kathode haben, so kann zu allen obigen Zahlen ein beliebiges, aber gleiches additives Glied hinzugefügt werden, d.h. ü b e r seinen Werth durfen wir willkürlich verfügen.




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These values refer to a normal concentration of ions; a tenfold reduction of the concentration increases, according to our former considerations, the values by 0.058 volts (n = number of charges or the chemical values of ion). The solubility tension of hydrogen has been set at zero; since we always have an anode and a cathode, we can add to all the above values some constant quantity — i.e., a value which can be arbitrarily established. Three years later at the Seventh Congress of the German Electrochemical Society, which met on August 6-8, 1900, in Zurich in the Chemiegebäude, Nernst again presented his choice of the hydrogen electrode as well as some criticism of Ostwald's calomel electrode (7). He gave this presentation at the second session on August 6 "at two hours past lunch". As an interesting sidelight, Van't Hoff chaired this session. Nernst questioned the validity of the theory of von Helmholtz which held that the polarized mercury electrode at its maximum surface tension has the same potential as the solution. He presented his own opposing view, which was to the effect that there is a surface layer of ions and that one could scarcely assume them to be without effect on the capillary voltage. In addition, he pointed out that the introduction of other ions into the layer would change the potential and deform and shift the maximum in an unknown way and that the use of absolute potentials was of no particular significance. He explained that for the last several years in his work and lectures he had used the normal hydrogen electrode, set at zero, as a reference point. He gave the following values based on hydrogen set at zero.

Elektrodenpotentiale K Na Ba Sr Ca Mg Al Mn Zn Cd Fe Tl Co Ni Sn Pb

+3.20 +2.82 +2.75 +2.54 +2.21 +1.85 +1.276(?) +1.075 +0.0770 +0.420 +0.340 +0.322 +0.232 +0.228

Electrochemistry, Past and Present - ACS Publications - American

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