R w 4 tde N e w England Association of Chem
John H. Wolfenden
Dartmouth College Honover, New Hampshire
I
I
chers
The Role of Chance in Chemical Investigation
I
once heard an exasperated experimenter define "the scientific method" as the way social scientists think regular scientists go about their business. This unpardonable observation was merely a melodramatic way of emphasizing a conviction that the simplicity with which scientific method is described increases in direct proportion to the remoteness of the describer from active laboratory investigation. The exasperated experimenter certainly exaggerated, but it is surely true that the process of scientific discovery is sometimes much less tidy and much more human, much less inexorable and much more chancey, much less logical and much more fun than one might glean from textbooks and journals. Examples of the intervention of chance or serendipity in chemical research, apart from their intrinsic interest, can sometimes convey more of the excitement and atmosphere of exploration than is usually disclosed in the rationalized and cosmeticized reports of which our journals are normally made up. I do not propose to discuss examples of the "merely" unexpected since some such element is present in every important discovery. Thus, for example, I would exclude Gomberg's discovery of triphenylmethyl on the ground that sooner or later some one would seek to synthesize hexaphenylethane by the eminently reasonable method that led to the discovery of the free radical. Similarly the analytical anomalies that led to the discovery of many elements must be ruled out on the same grounds. I want to restrict myself rather loosely to what we might call "accident or surprise of the second order" where some truly fortuitous circumstance' has contributed to the advance of chemical knowledge. This Based on a lecture delivered to the Twenty-Eighth Summer Conference of the New England Association of Chemistry Teachers a t Darbmonth College, August 1966. Picking the wrong bottle off the shelf must have made its own eontribut,ion to seient,ific progress, but the only example with which I am familiar is not strictly chemical. A key observation in the development of the preservation of fowl sperm by freezing would not have been made if an investigator a t the National Institute of Medical Research in Londnn had picked the right bottle out of the refrigerator. (PARKEG, A. S., Scientific America*, 194, 106 (June, 1956).)
brings us up against a diiculty immediately. On the one hand there is a certain myth-making tendency to invent imaginary episodes whereby discoveries might have been made. More frequent and more important are the cases where a genuine accident or change circumstance is quietly suppressed. The situation has a hush-hush quality. Scientific literature frowns on the fortuitous. The editors of the journals are convinced, not without reason, that the accident or circumstance is irrelevant to the scientific findings of the paper. It is perhaps not too cynical to suggest that potential editorship is often rendered unnecessary in advance by a certain reticence on the part of the author who is seldom anxious to cloud the relentless logic of his paper by reference to the lucky break. I t was not always so. I n the second volume of his 'LExperimeutsand Observations on Different Iiinds of Air" Priestley writes: I cannot, a t this distance of time, recollect what it was that I had in view in making this experiment; but I know I had no enpect,abion of the real issue of it. Having acquired a. cansiderable degree of readiness in making experiments of this kind, a very slight and evanescent motive would be sutiicient to induce me to do it. If, however, I had not happened, for some other purpose, to have had a lighted candle before me, I should probably never have made the trial.
Our change of habits and editorial custom have robbed chemical literature of something of its romance and even perhaps a little of its essential truth. The search for examples of serendipity can be a rather tantalizing chase-a little like trying to discover the wild oats sown in youth by a pillar of society. The analogy is not a happy one because there is nothing reprehensible about the lucky break, and we must not forget Pasteur's observation: "In experiment luck only favors the prepared mind." Daguerre, Dumas, and the Photographic Image
There is no inevitable logical arrangement for these intrusions of illogicality in our science, but the examples that follow are in more or less chronological order, and we may begin with the discovery of the daguerreotype. Daguerre, an artist, had become famous throughout Europe for his "Diorama," a fanVolume 44, Number 5, Moy 1967
/
299
tastic version of painted panorama which sometimes incorporated living people, goats, etc., in the foregound. He met Niepce who had developed a primitive form of photography depending on the light sensitiveness of bitumen and requiring an exposure of several hours. They drew up in 1829 an elaborate contract for the joint investigation of photographic methods. Shortly afterwards Daguerre, who brought no knowledge of photography to the partnership, rediscovered the light-sensitiveness of silver iodide, but no method was available for making the image permanent. Daguerre struggled to find a way, and it was somewhere about this time that he received encouragement from an eminent chemical source. Madame Daguerre called on Dumas, as he told the story some thirty years later. She said My husband is about t.o lose his reason. He bas given up his art and carries on fruitless chemical experiments. At present he has the obsession to ret.ainimages fixed on metal plates. He bas
sold our possessions to buy chemicsls and to build an apparatus.
She went on to beg Dumas to convince her husband of the futility of his experiments. His unchivalrous response was to call on Daguerre the next day and to encourage him to continue his experiments! Daguerre certainly needed encouragement because the search went on for another six years without success. Then one day he was startled to find that some iodized silver plates exposed and left in a cupboard for several weeks showed clear permanent positive pictures. The cupboard was one filled with chemicals, and Daguerre systematically removed them one at a time but the magical effect persisted. The distraction of Madame Daguerre must have beggared description. Finally it occurred to Daguerre that he had overlooked a basin of mercury a t the bottom of the cupboard; as soon as this was removed the cupboard lost its magical powers. Experiment soon showed that mercury condensed preferentially on those portions of the AgI surface that had been exposed to light. Hence the daguerreotype. Parisian Weather (1896) and Radioactivity
The principle that interesting things may ensue when a French investigator leaves something in a cupboard is even more portentously illustrated by the discovery of radioactivity. The perusal of Volume 122 (January-June 1896) of Comptes Rendus is a little startling. If one expects to find Becquerel's report of radiations from uranium as some sort of dazzling comet in a quiet intellectual sky, he will be surprised to discover that it is a modest little note hiding unobtrusively among longer papers dealing with "black light" (which had nothing to do with uv or ir), with the penetrating radiation from phosphorescent zinc sulfide, and with many other fantasies. The discovery of X-rays at the end of 1895 was responsible for all this. It provoked a scientific gold rush that is probably without parallel. This same volume of Comptes Rendus, restricted to papers submitted to the French Academy of Sciences, contained one hundred and twenty-three papers bearing on "Rontgen-rays" (out of a total of some fifteen hundred dealing with all branches of science). I n France the emphasis in these feverish investigations was deeply influenced by a suggestion of Henri Poincar6, the 300 / journal o f Chemical Education
distinguished mathematician; Poincar6 suggesteda that, since the Rontgen rays seemed to originate from the fluorescent area of the Crookes tube, it might well be that all fluorescence, whatever its origin, is accompanied by penetrating radiation. This was the snggestion that prompted a number of investigators to "discover" that phosphorescent sulfides emitted penetrating radiation, that led Troost8 to recommend a form of zinc sulfide made phosphorescent by exposure to burning magnesium as a very handy substitute for a Crookes tube in medical radiography, and that led Becquerel to seek penetrating radiation from uranium salts (whose fluorescence had been studied by his father) when exposed to sunlight. The experimental arrangement was simple but entirely adequate. A photographic plate wrapped in two layers of thick black paper was covered with a thin layer of crystals of potassium uranyl sulfate and exposed to sunshine. When the plate was developed, images of the crystals appeared, and thesilhouettes of interposed coins were also observed. Becquerel shrewdly eliminated the possibility of direct chemical action by showing that the interposition of a thin glass plate between crystals and black paper did not eliminate the effect. These observations, interpreted as further confirmation of the association of fluorescence (or phosphorescence) with Rontgen rays, were reported to the Academy in Paris on February 24, 1896. A week later, March 2, Becquerel reported very simply the observations for which he is famous. Experimental arrangements similar to those he had previously reported were prepared on February 26 and 27, but, as there was only occasional sunshine on those two days, he put the preparations away in a drawer. Furthermore, as the sun did not appear on February 28 and 29, Becquerel developed the plates on March 1. There is a little unfounded folklore to the effect that by some misunderstanding a laborat,ory assistant developed these plates that had not been exposed to sunshine. The original paper does not support this and implies that they were developed quite deliberately, perhaps as a sort of control, in the expectation of finding a very feeble image. It is well known that the silhouettes were found to be of an unparalleled intensity, and thus radioactivity was discovered. The relative contributions of Poincar6 and of Parisian weather to this epoch-making discovery may be matters for debate; one can argue plausibly that the weather played a double role-the sunlessness on February 26 and 27 led Becquerel to put his plates away and that on February 28 and 29 prompted him to develop them on March 1; both were probably indispensable to the chain of events. Nernst's Rudeness-Haber's
Persistence
Passing to the present century we shall find that chance is still operating and that the chemist has not yet achieved the unengaging status of total infallibility. Let me begin with two episodes from the work of Haber, of which the first concerns the synthetic ammonia process known by his name and shows how a dirty crack PO IN CAR^, H., R#J. gen. %i.,7, 52 (1896). TROOST, L., Compt. rend., 122, 564 (1896).
a t a scientific meeting- led to his development of the process. Haber's researches on the ammonia equilibrium benan in 1904 in the most conventional of wavs " bv " association with Austrian industrialists. Using a flow method with iron as catalyst, he approached the equilibrium from both sides a t atmospheric pressure and found an equilibrium concentration between 0.005 and 0.012'3&at lO0OoC in a stoichiometric mixture of nitrogen and hydrogen. Haber (wrongly as it happens) favored the upper figure but concluded with some justice that
-
. . .from dull red heat upwards no catalyst can prodnee more than traces of ammonia at ordinary pressure; and even a t greatly increased pressures the position of the eqnilibrium must remain very unfmwable. To attain practical success wibh a catalyst a t ordinary pressure its temperature must not be higher than 300°C. Haber thought the method hopeless and so did his industrial associates. He dropped the subject and there the matter would have rested, if Nernst had not been first critical and later tactless. Reviewing existing equilibrium data, Nernst in 1906 discovered that the ammonia equilibrium was the only one where the experimental results were seriously at variance with the Nernst heat theorem. Nernst therefore carried out his own measurements under 50 atm pressure to reduce the experimental error by increasing the equilibrium ammonia concentration. He obtained much less ammonia than Haber but in fair agreement with that expected from his theorem. Recalculated for a stoichiometric mixture a t lO0OoC and a total pressure of 1atm the figures were: Theory: Haber: Nernst:
0.0045% ammonia 0.012yo ', 0.0032% "
Nernst wrote a letter pointing out this discrepancy to Haher who, with Le Rossignol, proceeded to repeat his measurements, still a t atmospheric pressure. The new ammonia percentage was 0.0048% a t 1000°C and 1 atm-in excellent agreement with the Nernst Heat Theorem but a good deal lower than the value that Haher had reported earlier. Before Haber's new figures could be published there occurred the 1907 meeting of the Bnnsen Gesellschaft where Nernst read a paper on his own work. Haber took this opportunity to correct his old figures and announced his own new ones. Correcting Nernst's results to 1 atm pressure the Nernst and Haber figures for ammonia percentage ran something like this: Nernst-Jost Haher-Le Rossignol
700°C 0.0174 0.0221
800°C 0.0087 0.0108
1000°C 0.0032 0.0048
I n the ensuing discussion Nernst refused to accept Haber's redetermination because of the low pressure and consequent low ammonia concentration. He went so far as to recommend Haber to work a t higher pressures. Haber maintained his figures were accurate (they are). Nernst wound up the discussion and laid t,he foundation stone of the industrial ammonia synthesis by being rather rude. His words were: May I perhaps make one more observation, which is of general technical interest.. I t is very unfortunate that the equilibrium is mare displaced towards the side of very low ammonia forma-
tion than the extremely inaccurate figures (stark unrzchtigen Zahlen) of Hither had formerly led us to assume, since one had inferred from them that it might be possible to synthesize ammoma from nitrogen and hydrogen. Now however the eonditions are much less favorable, the yields being about three times smaller than was thought.
No one can deny that this was tactless, especially as Haher had publicly corrected his own figures a t that very meeting. Indeed, since Haber's new figures were in better agreement with the heat theorem than Nernst's own, the discourtesy was all the more puzzling. At any rate Haber felt himself personally slighted; and, although his new figures gave less justification than the old ones for believing any synthesis would be technically feasible, he went to work immediately with Le Rossignol to reexamine the equilibrium a t 30 atm pressure. These experiments confirmed the accuracy of his data obtained a t 1 atm and gave him sufficient experience in high pressure technique to start the further explorations of better catalysts and higher pressures, which led to the Haber process as we know it now. Looking back on the story there is a certain humorous inconsequence abont it. On the basis of erroneously high ammonia percentages Haber decides the process is not worth exploring further. Nernst corrects him, hut the correction is so rudely administered that in confirming the even less favorable percentages Haber goes on to lay the foundation for a historic industrial process. One might add as a footnote a rather bizarre incident in connection with the development of ammonia catalysts. The scientific equilibrium studies a t high temperatures had employed iron or manganese, presumably pure; Haber managed to extend experiments to much lower temperatures by using osmium and uranium. When the Badische Anilin and Soda Fabrik took up the large scale development of the process they sought a catalyst more readily available in quantity. Rosch, the engineer, is said to have recommended to Mittasch a trial of iron from various ore sources on the ground that iron had a complicated spectrum. Alchemy Refuted; Techniques Refined
The second episode from Haber's work is not strictly serendipitous, but I welcome any excuse for telling what must surely be one of the tallest true4 stories in science. At the end of World War I, Haher, whose discoveries had done much to strengthen German military power, sought to solve the German reparations problem by extracting from sea water a small fraction of the fifty billion tons of gold that Arrhenius estimated it to contain. There is no room here to tell the long but fascinating story of how, with improved analytical techniques and rigorous exclusion of traces of gold from apparatus and reagents, Haber was able to show that the true gold content of sea water was less than one thousandth of the 5 pg/l that Arrhenius had reported. Although the reparations problem was not solved by Haber's work, the techniques for measuring minute amounts of gold were incidentally greatly improved and 'Sceptics are referred to HABER,F.,N ~ ~ u T w ~14,410 ~ s . , (1926).
Volume 44, Number 5, May 1967
/
301
available for a rather intriguing purpose. I n 1924, Miethe in Germany reported the transmutation of mercury t o gold by an electric discharge through the vapor. Using the analytical refinements and precautions against accidental contamination that had emerged from the sea water work, Haber was able to show consistently negative results with one exception. It is this one exception that provides the tall true story. One young member of Haber's research staff always found gold in the mercury. This worried everyone, especially the young man, and it was arranged for his procedure to be carefully watched by colleagues. The only clue that emerged was the fact that a t one stage in the analytical procedure the young man always removed a pair of gold-rimmed glasses and laid them down on the bench; with the same hand he then picked up a thin strip of very pure lead and added it to the analytical vessel. He was asked to repeat the analytical procedure using steel-rimmed glasses and the formerly mysterious traces of gold were no longer found! The transfer of analytically detectable amounts of gold from spectacles to the mixture being analyzed in this way may seem a little less incredible when one is told5 that V. M. Goldschmidt, whose researches sometimes involved trace metals, would admit no visitor to his laboratory until they had removed all jewelry lest his gold and silver analyses be contaminated. True Serendipity-Platinum
Catalysts
I owe my next example to the circumstance that in July 1949, Dr. Roger Adams was, like myself, eastbound on the S.S. Il4auretania. We found we shared a common interest in serendipity, and he was kind enough to tell me of a fine example from his own experience. Adams had a research student seeking to improve platinum black as a hydrogenation catalyst. A method due to Willstatter was tedious and found to lead to products of variable catalytic activity. The research student, after about a year and a half, had succeeded in effecting significant improvement, and it seemed justifiable to work on a larger scale, involving 20-30 g of platinum. Before the procedure was completed a porcelain casserole containing the preparation cracked, broke, and distributed the whole platinum suspension over the antiquated and corroded wooden table and the floor. The student hurried to scrape up from table and floor all of the expensive material that he could and hurried to report this desolating mishap to Adams. The salvaged material was contaminated by wooden splinters and by the organic pigment coating the table. Adams suggested treating the murky mixture with aqua regia. This dissolved the platinum hut did not oxidize all the organic matter. Sodium nitrate was then added to the mixture in the hope that a t the higher temperature oxidation might be completed. The brew was evaporated and melted. During the fusion a heavy brownish powder began to separate from the melt. Further investigation showed that this entirely unexpected product was a hitherto unreported hydrated platinum dioxide and that it was readily reduced by hydrogen to give a platinum black of unprec6 B ~J. D., ~ J . ~Chem. ~ Soc., ~ 2108 , (1949). 'VOORHEES, V., and ADAMS,ROGER,J . Am. Chem. Sor., 44, 1397 (1922).
302
/
Journol o f Chemical Educotion
edentedly high and reproducible activity. Subsequent preparations were made very simply by fusing sodium nitrate with pure chloroplatinic acid. The happy catastrophe on the laboratory bench led to what is still, I am informed, the best platinum black catalyst available to the organic chemist for hydrogenations. The full flavor of the &air can only be savored by turning to the published report.6 Without reference to the accident hut invoking a W'ilstatter-Wieland controversy of the period (which suggested the value of the presence of some oxygen in any efficient platinum black catalyst) the authors write:
..
.The most appropriate method conceivable for the formation of an oxide of platinum is the one used so often for the oxides of other metals, namely the fusion of the nitrate. In the case of platinum, t,his may be carried out by fusing chloroplatiniinic acid with sodium nitrate. Since sodium nitrste fuses at 3 1 2 T ideal conditions are obt,ainsble. It is actuslly found that an extremely active oxide of platinum is produced.
Experimental Error: Catalyst to Discovery
The role of an experimental error in promoting the discovery of deuterium (or conceivably retarding the discovery of the oxygen isotopes) seems not to he generally known. I n 1927 the "physical" atomic weight of hydrogen (on the 0 = 16 scale) was determined to he 1.00778 =t 0.00015 by Aston's mass spectrograph, while the chemical atomic weight was believed to he 1.00777 =t 0.00002. Unless one dissented sharply from the inscription over Einstein's fireplace-"God is subtle hut not malicious"-the inference, from the all-hut-identity of the two figures, that both elements must be isotopically simple was just about irresistible. I n 1929, Giauque and Johnson concluded from the absorption spectra of oxygen that the element must have three isotopes. This discovery could only he reconciled with the all-but-identity of the physical and chemical atomic weights of hydrogen by the existence of one or more heavy isotopes of hydrogen. The relative abundance of the oxygen isotopes suggested that, if there was only one kind of heavy hydrogen and if its mass was twice that of ordinary hydrogen, it must he present to the extent of one in four or five thousand. This was the situation that triggered Urey's search for what we now call deuterium, which he and his collaborators discovered in 1932. Its abundance was of the order of magnitude expected and now-for a brief spell-everything seemed to fit into a satisfactory pattern. However, the experiments on nuclear reactions that started about this time and, more specifically, the inferences about the masses of the "reactant" and "resultant" nuclides that could be drawn by applying the mass-energy equivalence law to the energy liberated in the nuclear reactions pointed more and more to the conclusion that Aston's physical atomic weight of hydrogen was an underestimate and suggested that a value of something like 1.0081 was more likely. I n 1936 Aston redetermined this quantity and reported a new value of 1.00812 0.0001. I t is intriguing to speculate about what might have happened if this more nearly correct value had been known in 1927 or in the period 1929-32. This can be an amusing parlor game, especially if it is noted that some of the methods used for purifying hydrogen for atomic weight purposes (such as electrolysis of diffusion through palladium) cut down the deuterium content rather sharply.
*
Lucky Choice: Reagent and Method
My final example is the discovery of the geometrical isomers of azobenzene. An English investigator became interested in the fact that very dilute aqueous solutions of paraffin-chain salts dissolve nonpolar organic substances very much more effectivelythan pure water. This he explained by aggregation of the paraffinchain ions into micelles consisting of a hydrophilic outer layer of ionic groups and an essentially p a d n o i d interior; on this basis the solvent power for nonpolar molecules is somewhat like that of an emulsion of paraffin in water with the solvent powers of the paraffin added to that of the water. To carry out quantitative studies, he selected cetylpyridinium chloride as the paraffin-chain salt and decided that it would be laborsaving and neat to measure the solubility of the nonpolar organic solute by choosing a colored substance and measuring the light absorption of the solutions photoelectrically. By a happy accident he chose azobenzene as the colored nonpolar substance. The investigator was G. S. Hartley7; and he noticed a lack of reproducibility in the photometric analyses; this was traced to an increase in the light absorption of azobenzene solutions when exposed to light. This in turn was found to be due to the formation of the cis-isomer. This is a rather pure example of the "lucky break." The discovery of ?HARTLEY, G. S., J . Chem. Sac., 633, 1968 (1938).
cis-azobenzene depended not only on the choice of azobenzene among a wide range of colored nonpolar compounds but also on the selection of a photometric method of analysis. The Mind of the Investigator
It is not easy to draw any obvious lesson from this anthology of serendipity; certainly chance can not he incorporated into the strategy of an investigation! The examples we have considered may, however, do something to reassure young chemists that research is often a lot more fun than scientific journals usually suggest. Something of the interest of chemical discovery lies in the interplay between how nature works and how the mind and temperament of the investigator work. Let me end with a quotation from Samuel Butler's Alps and Sanctuaries. Although written in connection with how one might learn to paint, it is the most perceptive observation about scientific research that I know:
..
. I t does not matter whet a man does; so long as he does i t with the attention which affection engenders, he will come to see his way to something else. After long waiting he will certainly find one door open and go through it. He will my to himself that he e m never find another. He has found this, mare by luck than cunning, but now he is done. Yet by and by he will see that there is one more small unimportant door which he had overThen after looked, and he will proceed through this too.. years-hut not probably till after a great m a n y 4 o o r s will open up all round, so many and so wide that the difficulty will not be to find a door, but rather to obtain the means of even hurriedly surveying a port,ion of those that stand invitingly open.
.
Volume 44, Number 5, May 1967
/ 303
