Ollin J. Drennan
Western Michigan University Kalomazoo
Faraday's Contribution to Electrolytic Solution Theory
Michael Faraday (1791-1867) performed his chemical experiments between 1833 and 1843. This period occurred before the laws of conservation of energy had been established, and before any concepts of thermodynamics appeared. It was before the atomic theory of matter was unqualifiedly accepted and before chemists had agreed on a consistent table of atomic weights for the elements. It was before any clear concept of electricity had been established. About the only successful developments which affected electrolytic solution theory were the developments of the voltaic cell by Alexander Volta in 1800 and the mathematical formulations of Ampere and Poisson concerning electrical currents and resulting forces. Against this background Faraday made his contribution. He was a son of a poor man and was forced to leave school a t the age of 14. This lack of formal education, especially in current mathematical developments, affected the nature of his scientificwork throughout his life. At the age of 21 he was appointed assistant to Humphry Davy in the Royal Institution in London and there he remained for the rest of his life, as laboratory assistant, then lecturer in chemistry, and finally as director of the laboratory after 1825. When Faraday began his experiments in electrochemistry certain results were already known. Experiments had established that chemical decomposition was concomitant with the passage of electricity through certain liquids and solutions. Evidence of this chemical decomposition was known to be observed only at the poles of the cell where the electric current entered and left the solution. How these occurrences were to be explained was not clear. There were several different theories. The generally accepted concept of the structure of a solution was that it was static. The modern view of molecules incessantly in motion had not occurred to Presented at the annual meeting of the Midwest Junto of the History of Science Society at the Univenrity of Oklahoma, Norman, in April, 1961.
those who were busy forming chemical theory. Dalton had pictured matter as made up of atoms surrounded by caloric, much as apples in a barrel might be surrounded by water. Molecules of complex substances were thought by Berzelius and Davy to be composed of two parts, one charged negatively and the other charged positively, but this belief was not universally accepted by chemists. The mathematical theories of electricity were based almost exclusively on the inverse-square law of attraction, or action at a distance, following a pattern set by the mathematical developments derived from the theory of gravitation. Consequently, one of the theories that was proposed to explain the chemical decomposition which occurred when a current traversed a solution involved forces exerted by the two diierent metals which made up the two poles of a decomposition cell. The forces were to act on the electrically charged molecules, split them, and draw the charged portions to the proper pole for liberation as decomposition products. The forces which accomplished this were to vary inversely as the square of the distance between the poles and the molecules affected. I n effect, this constituted action at a distance. Perhaps the most important basic belief which motivated Faraday in his electrochemical research was the belief that action a t a distance was not the cause of chemical decomposition in a cell. This may have been the result of his lack of mathematical training, or it may have been due to his habit of trying to visualize physical processes. Whatever the reason, the first series of experiments carried out by Faraday in this field were to show that chemical decomposition would take place when a current was conducted through a conducting solution, even when there were no opposing metallic poles in the solution. He permitted the electricity to enter by way of a spark, or he interrupted the external metallic connection of the poles by another conducting solution. His results led him to write in 1833: I hope I have now distinctly stated, although in general terns, the view I entertain of the cause of electroohemical decomposition,
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as far as that cause can at present be 11,acedand understood. I conceive the effects to arise from forces which are i n l e m l , relative to the matter under decomposition-and not eztemal, as they might be considered, if directly dependent upon the poles. I suppose that the effects are due to a modification, by the electric current, of the chemical affinity of the particles through or by which current is passing, giving them the power of acting more forcibly in one direction than in another, and consequently making them travel by a series of successive decompositions and recompositions in opposite direction, and finally causing their expulsion or exclusion at the boundaries of the body under decomposition, in the direction of the current, and that in larger or smaller quantities, according as the current is more or less powerful. (6524, 1833)'
Faraday was led, by his desire for clarity of expression, to introduce an entirely new set of terms for the process that took place when a current passed through a conducting solution. He believed that an open-minded study could not be made of the matter if the terms which must he used in its description were already accepted to have certain preconceived meanings. For instance, he believed the word "pole" contained. the connotation of action a t a distance so clearly through its use in describing magnetic poles that it could not be used for the metallic wire placed in a conducting solution without introducing action at a distance as the explanation of how the metallic wire affected the solution. So with the help and advice of the classical scholar, William Whewell, Faraday introduced the words electrolyte, electrode, ion, cathode, anode, and cations and anions. The electrodes were the wires in the solution from which and to which the current flowed. The ions were those charged portions of molecules which were attracted in the direction of the current. Faraday carefully differentiated between the boundary of the solution toward which the positive ion moved and the negative metallic wire. The boundary he called the cathode, and the positive ion he called the cation. He called the negai tive metallic wire the negative electrode, and described it with the positive electrode as "merely the surfaces or doors by which the electricity enters into or passes out of the substances suffering decomposition" (#558, 1833).' The care with which Faraday separated the negative electrode from the cathode in h is discussion accented his desire to rely on chemical causes for the decomposition. Having developed a terminology which he felt could safely be used to describe experimental results, Faraday proceeded to establish the relationships which are today called "Faraday's laws of electrolysis." He showed first that when a current traverses a body of water into which a small amount of acid has been added, the amount of water which is decomposed by the current is exactly proportional to the total amount of electricity which has passed through the water. Faraday varied the strength of the current, the size of the electrodes, the amount of acid added to the water, and the shape of
the container holding the water. In all cases the proportionality was maintained. As a result of these experiments, Faraday was led to state, again in 1833: that the chemical power of a current of electricity is in direct proportion to the absolute quantity of electricity which passes. (6821,1833)'
But this was not all. Using the water cell as a measuring device for amount of electricity which had passed, Faraday placed other cells in series with the water cellother elements were liberated at the electrodes. I n this kind of experiment Faraday was able to determine the relative amounts of different elements which were decomposed with the same amount of electricity. He found that the ratios of the amounts decomposed were always the same, and in most cayes they were the same as the ratios obtained when the ratios of chemical equivalents were taken. Thus he concluded: I have proposed to call these bodies generally ions,. . . and the numbers representing the proportions in which they are evolved electrochemical equivalents. (6824, 1833)'
With the statement of these two relationships, Faraday solved the quantitative aspect of electrolysis. There still remained the problem of how the process of electrolysis occurred. The postulation of ions in a solution was an effort to make use of a word which held no connot,ation of previous physical meaning. For this reason Faraday was forced to accept the static picture of a solution when such a picture was needed. He approached the question of the nature of ions in a more concrete manner, however, when he began to investigate the action of a voltaic cell. He was able to show that two dissimilar metals could be formed into a battery if they were connected together a t both ends: one pair of ends connected by a conducting wire, and the other pair of ends connected by plunging them into an electrolyte. While a cnrrent flowed in the external wire, chemical decomposition took place in the solution. Faraday tried different solutions and found that each caused a different amount of current. He then replaced the external connecting wire with a second electrolyte. He found that this situation-two metals, joined a t each end with a different electrolyte--indicated that the electrolyte which wonld cause the largest current by itself would overpower the second electrolyte and cause a current to traverse the second electrolyte in opposition to its natural inclination. From this series of experiments, Faraday deduced that the cause of the current was the chemical affinity of the electrolyte. He held that each electrolyte had an affinity characteristic of its composition. If two electrolytes were opposed in the above experimental arrangement, the electrolyte with the greater affinity wonld overpower the electrolyte with the lesser affinity. He drew the conclusion: .. . the forces termed chemical affinity and electricity are one
and the same. (918, 1834)l
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FARDAY,MICHAEL, "Experimental Researches in Electricity," London, 3 volumes, 1839, 1844, and 1855. Faraday numbered the paragr~phsin his "Experimental Researches" consecutively from his first paper to his last. Therefore, the easiest reference to his work is by paragraph number. But because this kind of reference does not indicate the date of the reference, after each number which refers to the appropriate paragraph will be placed the date of the paper which contains that paragraph.
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With this conclusion Faraday believed that he had found the force which caused, or drove, the process of electrolysis. The research described so far was carried out by Faraday in the years 1833 and 1834. The intermittent periods devoted to electrochemical experiments in the following years found him turning to a closer study of affinity between ions and the structure of a solution which must be dictated by the results of experiment.
His success in solving both problems was not complete by any means. I n fact he was able only to give several very fruitfulspeculative suggestions, but the suggestions were to become the inspiration for his immediate followers. Faraday attempted to determine the magnitude of the force of chemical affinity by observing that the amount of electricity which was necessary to decompose a given weight of a substance should be a measure of the affinity holding that substance together. By accepting the theory of Davy that the compounds were made up of ions charged with unlike charges, he was able to calculate that the result was enormous. I t was so high that he confessed "I am ahnost afraid to mention it." The results were that to decompose one grain of water 800,000 charges of a Leyden hattery which he had described would he required. He could only wonder a t the fact that so much chemical affinity could he overcome by the almost negligible power of two or three voltaic cells. This observation that small potential differences were ahle to effect such magnitudes of decompositiou was to lead him and later investigators to dissatisfaction with the static view of a solution. Further dissatisfaction was created when Faraday found that very small currents would pass through an electrolyte hut would produce no visible decomposition. The static view, first proposed by the German, Grotthuss, in 1805, held that molecules would not he separated into ions until some minimum potential would he impressed across the cell. Any lesser potential would produce some strain in the electrolyte in the form of aligning the molecules with the positive ion nearer the negative electrode and the negative ion nearer the positive electrode. When the minimum potential was reached, the theory forecast a surge of current as the molecules were all split apart simultaneously. Faraday's experiments showed that there was a minimum potential necessary to produce decomposition. However, when potentials less than the minimum were applied, a strong current resulted which very quickly decreased to a very low value. The low-valued residual current continued, contrary to the predictions of the theory. Faraday did not understand the strong initial current which resulted when the very small potential was applied. The term polarization was suggested a few years later and the explanation of the phenomenon came still later. But Faraday did describe it and speculate about it. The low minimum residual current was interpreted by Faraday to mean that there was a current passing through the electrolyte which was metallic in nature--that is, a current which did not cause chemical decomposition. This interpretation was inconsistent with the laws which Faraday had already established because it would represent a current which was not directly proportional to the chemical decomposition accomplished. This complex phenomena caused much confusion for following investigators until, in the year 1873, Helmholtz established a satisfactory explanation of the entire process of polarization and the currents which occur because of it. The explanation supplied by Helmholtz and, par-
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tially, by preceding workers, was founded on the hypothesis that ions were in motion and were present as ions in electrolytes, even before the current occurred in the electrolyte. Faraday was slowly working in that direction. He realized that it was peculiar for a small potential to overcome a large chemical affinity. He realized that currents were observable a t potentials which were supposed to he insufficient to effect decomposition according to the static picture of the electrolyte. He developed a third set of experimental results which needed some additional postulates for adequate explanation by the static theory of solution. Faraday examined the effects of changes of temperature on the electromotive force of a cell and on the chemical decomposition which was the result of currents. He found that a larger current flowed in a hot electrolyte than flowed in the same electrolyte when cold and when driven by the same hattery. But he also found that while the current was larger, the decomposition which resulted per total amount of electricity passed was the same. Faraday drew the conclusion that the increased current did not come from increasing the electromotive force, but that the movement of ions in the electrolyte encountered less resistance in a hot solution than in a cold solution. The static theory would have predicted just the opposite. Faraday did not appreciate the conclusive nature of this fact. He explained the result as a further proof that the process was the result of chemical action because a chemical change increases its action when the ingredients are heated. It must he concluded that Faraday's place in the development of electrolytic solution theory rests on his several contributions. The first, and undoubtedly his most positive contribution, is the statement of the two laws of electrolysis which are known by his name. However, it should he pointed out that his effecton later developments was perhaps just as great in those areas in which his contribution was less certain because it acted as a stimulus for other workers. Due to his background and his training, he brought certain capabilities to bear on the problems of solution theory. He was ahle to suggest avenues which might profitably he examined. Other men with different backgrounds and training were ahle to investigate those suggested avenues. The second contribution, then, upon which must rest Faraday's place in the development of electrolytic solution theory is that collection of insights, intriguingly presented, which provided the inspiration and the direction for later workers. Helmholtz was to express this sentiment as he closed the Faraday lecture which he was privileged to give before the Fellows of the Chemical Society of London in 1881. Helmholtz said: But I abstain from entering into further specialities; perhaps I have already gone too far. I would not have dared to do it, had I not felt myself sheltered by the authority of that great man who was guided by a never-erring instinct of trutk2 1 HELMHOLTZ, HERMANN, "On the Modem Development of Faraday's Conception of Electricity," C h m . Soc. Journ., 39,277 304 (1881). Also in Helmholts'a Wiasasehajtliehe Abhandlung a , Vol. 111, Leipzig, 1895, pp. 52-87.
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