A. L. TH. MOESVELD Van't Hoff-Laboratorium Utrecht, Holland
T H E scientific work of Ernst Cohen was not restricted t o a single field, but one particular subject was foremost in his thoughts: the study of physical isomerism. The research which gave him his doctor's degree was on the allotropy of tin and after his retirement from the chair of general chemistry he continued his work on the transformation of white into gray tin with undiminished vigor, now as an honored guest in the laboratory of which he had been the Director for a third of a century. Two other domains of physical chemistry had a major interest for him: electrochemistry, often in close contact with studies of physical isomerism, hut also in connection with the Weston standard cell; and piezochemistry, in which field he was the first to make accurate measurements up t o rather high pressures (1500 atm.). The scientific work of Cohen has been surveyed on more than one occasion by his pupils and coworkers,l while he himself publisheda in book form his "George Fisher Baker Lectures," in which he reviewed in a very lucid way his most important contributions to science. Coben was an experimenter first and foremost, and he loved to make experiments himself. Discussions of the results obtained and of ways for new attacks on the problem in hand were usually held in the workrooms or in his private laboratory, not in his own room before the desk on which he wrote his papers. Theories might be all right, hut be would not fit his facts t o anybody's pet hypothesis. It seems to be much easier to have visions of new theories in a study or conference room than when surrounded by instruments and apparatus.
TIN
on tin the phemencohen his nomenon of allotropy or polymorphism of a chemical element was well known, although it was considered as something exceptional and not a common property of the elements. ~h~ curious behavior of tin in bars or as an object in daily use, especially its partial transformation into a gray powder, was an. unsolved riddle. It seemed to depend on age, as old specimens of tin ware were seldom found without blemishes. But even if they had comDletelv fallen to Dieces or disinte=ated into Chen. Weekblad, 15, 1418; 1426, 1438 (1918); 24,481(1927). '
For a complete bibliography up to 1918, see ibid., 15, 1452 (1918); up to 1927, ibid., 24,489 (1927); and up to 1939, &idid., 36,519 (1939). a "Phyaico-chemical Metamorphosis and Some Problems in Piezoohemistry," McGraw-Hill Co., New York. 1926. Also the German edition: "Phwikalisch-chemische Metamor~hose!'
powder, chemical analysis did not indicate impurity to any marked degree. Cohen and van Eyk showed that tin was a clear example of allotropy and that the two allotropic forms, white and gray tin, had very ditTerent physical properties. Moreover, white tin was stable above 18'C.,' gray tin below that temperature; exactly 8s in the case of rhombic and monoclinic sulfur a transition point separates the two domains of stability and tin is another example of "enantiotropy." The behavior of the metal tin under different conditions was now obvious. Only when tin is exposed to temperatures lower than the transition point-which in this case is not far from the mean temperature in the temperate zone of the earth-there is a possibility of slow transformation. The difference in specific volume of white and gray tin is very marked (about 25 per cent) and gray tin has the greater value, so even a small transformation is easily seen by the formation of "warts." The success of Cohen and van Eyk in demonstrating the reversible reaction, white tin Ft gray tin,which goes to the right below 1 3 T . and to the left above 13'C., was the result of a new and very sensitive method of measuring the direction of a reaction, namely, by measuring the e. m. f. of a galvanic combination. This combination is simply a solution of a tin salt and two electrodes, one of white and one of gray tin. On passing the transition point this e. m. f. changes its sign, in accordance with the fact that the current-generating reaction also is reversed. The transition point can also be found by taking into account the difference in physical properties of the two modifications, for example, in the specific volume. The transformation of white tin into gray is accompanied by a change in volume of 25 per cent; so, when using a dilatometer a t a fixed temperature, a positive change of volume means the formation of gray tin from white. Two temperatures can easily be found between which no visible change occurs. Above the higher, a contraction is the sure indication that we are above the transition point. Below the lower, an expansion means that we are below this point. The upper and the lower limit can be brought nearer to each other by observing for longer times or by using means to accelerate the velocity of transformation. I n his first series of studies of tin, Cohen found that this velocity is not constant or linearly dependent on the differeke between the temperature of t h e experiment and that of the transformation point. Above it a Later investications showed that the transition ~ o i u was t s bitlower, at 13'"C7
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the velocity rises quicker, as might be expected; below the transition point the velocity first becomes greater, then passes through a maximum, and finally becomes very small. This also is to be expected because of two opposing factors. A greater distance from the equilibrium temperature gives rise to a greater speed, but the general effect of a lowering of the temperature is a slowing down of a reaction. However, the transformation is a heterogeneous one and is consequently dependent on the manner of occurrence of white and gray tin in the mass of the metal. The part already transformed can act as a nucleus for the change of untransformed material, and at the same time new nuclei can he formed spontaneously. So it is evident that repeated transformation of a sample will have the effect of making the transformation easier; the "previous history" of a metal has an important influence on its tendency to transform. During the whole period of Cohen's scientific life this problem had his attention for practical and theoretical purposes: how can we catalyze the polymorphic transformation; which factors determine the velocity of this type of reaction? Much of this has been made clearer by his studies. POLYMORPHISM IN GENERAL
of transformation. Cases were also known, and described in the literature, ofelements existing in different modifications, hut in which a reversible transformation could not he realized. Yellow phosphorus can he transformed into red, hut red cannot be changed back into yellow except by leaving the solid state altogether and passing t h r ~ u g hthe vapor or a solvent to produce the yellow form. The case is even more common than enantiotropy; monotropic forms are very plentiful if care is taken to avoid stabilization. During the first ten years of experimental work in the van't Hoff Laboratory many elements were tested in dilatometers, and a long series of papers on the enantiotropy of several metals and a few metalloids, like tellurium, were published. Soon a complication showed itself; it was found that the temperature of the transition point was not independent of the previous history of the sample under investigation. This complication evidently arose from the fact that more than two modifications were present in the metal, which could-and often did-transform at the same time. Practical difficulties prevented the obvious remedy of first reducing the complex metal to a mixture of two modifications only, keeping it for a long time a t a temperature a t which the least stable forms would be transformed. As a rule, the velocity of stabilization was too slow, and there was also a fair chance that a high temperature would produce another modification, not present beforehand, but stable at that temperature. This again showed the importance of finding ways to accelerate the transformation a t lower temperatures. In one case it was possible to unravel the complica-
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tions due to the presence of more than two modifications in a metal, by making use of a sensitive electrical method. When cadmium is deposited by a current i t usually is first formed in the least stable form, Cd,, which sometimes passes into Cde, in other cases into Cd.. All three show a differentpotential against a comparison electrode, e. g., a Cd amalgam, which in each case is strictly reproducible, however. In this way it was possible to find two transformation points, as well as the heat of transformation, from the temperature coefficientof the e. m. f. Cadmium prepared by melting and cooling will as a rule consist of a mixture of the three forms. The result of a long series of researches on the physical isomerism of metals and chemical compounds brought home the fact that it really is not an exceptional phenomenon, but rather the usual thing, for a substance to have more than one form. Bridgman, who studied polymorphism extensively at high pressures found an example.of enantiotropy fdr every three or four cases tested. Monotropy is even more abundant and less easy to demonstrate, since a monotropic form has no proper domain of stability. It is not surprising that Cohen stated that every suhstance can be shown to be polymorphic. At the same time, he pointed out that the cases much more divergent than could he expected from the accuracy of the experiments and the chemical purity of the specimens. This sometimes was the starting point of an investigation by Cohen and his coworkers leading to a new case of physical isomerism. Often, when the enantiotropy or monotropy of a substance was established beyond doubt a diligent search in the older literature showed curious discrepancies in physical measurements, obviously due to polymorphism. CRITICISMS
However, a prophet often has no honor in his own country, and many doubts have been expressed by others as to the occurrence of polymorphism either generally or in particular cases studied by Cohen. There are several reasons for the unwillingness to accept the views of Cohen. First, doubt is thrown on the reality of a find if it cannot be reproduced by another experimenter, although he follows closely the directions given. This inability to obtain the same result is no fault of the experimenter, but inherent in the behavior of metastable forms or in the lag of transformation in the solid phase. The formation of a monotropic form is dependent first on the appearance of a nucleus from a solution, a melt, or a vapor, then on its growth-two factors which we cannot yet control fully. Furthermore, this monotropic particle is liable to stabilization at any time, thus changing over into the well-known form and making it impossible to detect theunstableone. If it is a case of enantiotropy we can try to get our result by changing the temperature t o one at which the new form is stable. But this does not guarantee in the
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least that the transformation will really take place. plete transformation can rarely be achieved in a relaMost reaction velocities are notoriously slow in the solid tively short time. state. Now the treatment of the product of a crystallizaFrom the work of Cohen and his school it is evident tion usually ends with the removal of the solvent by that traces of impurities can play an important part in evaporation, often a t a higher temperature. For a preparing metastable forms. Thallium picrate is a short time one might expect a beneficial effect from this case in point. Two enantiotropic modifications are higher temperature on the stabilization, but soon anknown, with a transition point at 47°C. The stahle other factor enters the picture. The removal of the form at room temperature is red, the one stable above solvent generally stops the stabilization completely, so 47°C. is yellow. Normally, the yellow form appears the result is a mixture of two or more allotropic forms always from the supersaturated solution, even if the of variable composition, depending upon the conditions temperature drops to as low as 0%. before crystalliza- of concentration, temperature,.etc., during crystallization begins. Gradually, small red particles appear tion and evaporation. among the yellow needles, and in a few days a fair When preparing a chemically pure substance one amount of yellow material has changed into red. generally uses the same methods that are employed i n When a small amount of amyl alcohol is added to the the formation of polymorphic forms. There is always a warm, clear solution red crystals often appear together chance, therefore, that metastable forms will make their with the yellow, and the transformation on standing appearance during the purification of a substance, regoes much more quickly. However, amyl alcohol from sulting in a mixture of modifications, that is t o say, to another stock may totally lack this property of favoring a chemically pure but physically impure product. Cothe formation of the stable form. It has not been pos- hen defines a physically pure substance as one which sible to determine what impurity in the amyl alcohol is does not contain more than one polymorphic form. responsible for the effect. Naturally, when our aim is to prepare metastable A second reason for disbelief in the universal charac- forms we can choose conditions t o favor this purpose, ter of polymorphism has been the reluctance of scientific hut still in principle we follow the same course in puriworkers t o consider the recorded physical constants of fication. Those conditions are, for example, the addisolid substances as unreliable, although there are ex- tion of small amounts of L'catalysts," the use of particutraordinary discrepancies even in the data on specific lar solvents, and "chilling." The firsttwo are useful gravity of common substances. Worse still are the in obtaining a metastable form, the last is of advantage figures for electrical conductivities of pure metals. in preserving it. Nevertheless, there has been for many years a tendency All this applies also to enantiotropic forms. Here, t o look for other causes, and t o reserve physical isomer- too. e x c e ~ tat the transition noint, all forms but one ism for a few "classical" examples. arebnstab~e. There is no certainty that the form which crystallizes is the stable one at the temperature of the PHYSICAL VS. CHEMICAL PURITY experiment. The form which appears first must evenSmce it is not always easy to prepare pure allotropic tually he transformed into the stable one, if the product forms it is scarcely likely that a physical property is to he physically pure. Thallium picrate practically will be in error because the wrong modification always crystallizes as yellow needles from solutions has been studied. But a much more serious objection kept below the transition point, although the red form has been made by Cohen to the ensemble of physical is the stable one in that case. Physical impurity has a constants. He was the first to introduce the concep- more serious effect upon physical constants than the tion of physical purity as distinct from chemical purity. mere fact of polymorphism. Cohen has demonstrated Originally, Cohen studied the transformation of poly- that many physical constants have been determined of morphic substances with the view of obtaining the pure physically impure substances of indeterminate compoallotropic forms. During these researches a complica- sition. tion arose, namely, the general tendency to "partial" stabilization. It was found that a complete transform* HEATS OF SOLUTION tion was the exception, the velocity of transformation A case in point is the heat of solution of well-known slowing down to a very small value long before all the salts like KNO1. When chemically pnre KNOI (and unstable forms had been transformed. Let us consider many other salts) are melted and solidified, divergent what takes place when we try to prepare a monotropic values of the heat of solution are found, depending on modification. It may happen that from a solution only the metastable forms still present in the sample after the metastable form crystallizes, although it is quite heat treatment. I n this case confirmation came from possible that the stahle form will appear a t the same an unexpected side. Roth, after listening t o a lecture time. In any case, there is always a tendency for ameta- by Cohen on physical constants hefore the Bunsen stahle form to change into the stable one, so there are Geselkchaft, proposed in jest to advise the publishers of a t least two factors which make the permanent exist- the well-known "Landolt-Bornstein" to destroy all ence of a pure monotropic form doubtful. On the other the matter relating to physical constants of solids. hand, a pnre stahle product is only possible if the stahili- Shortly after that, Roth wrote to Cohen that in his zation is complete. Experience teaches us that a com- laboratory experiments on the heat of solution of salts
had shown greatly divergent values depending upon the previous treatment of the salts. However, these facts were known nearly a century ago. Person, and later Berthelot, found a change in the heat of solution of a number of salts with time. Berthelot explained this effect as a partially retained heat of melting, which was gradually given up during the time of storage. One has only to translate heat of melting into heat of transformation and there is complete agreement between Cohen's and Berthelot's views. There is of course no certainty that two experimenters will observe the same time-dependency of the heat of solution of any given substance, or, indeed, he able to observe any abnormal 'value of it. The "normal" value, of course, would he the one belonging to the fully stabilized, physically pure salt. Traces of impurities in the salt itself, or in the water used for recrystallization, may either cause the appearance of metastable forms from the molten salt or have a stabilizing influence on polymorphic forms present after solidifying.
preparing a sample of the monotropic form, using new chemicals and glassware. SOLUBILITIES
More progress was possible through the study of a curious phenomenon-the retarded crystallization of supersaturated solutions of cadmium iodide. The solubility of cadmium iodide in water has been accurately determined in Cohen's laboratory. I n the first stages of a new investigation on the influence of high pressure on it, control experiments on a new sample, prepared in the usual way, gaveunexpected, widely divergent results which were always higher than the true value. For the purpose of the investigation it was necessary to reach the equilibrium always from higher concentrations by shaking slightly supersaturated solutions with solid CdIz. As might be expected, our thoughts first turned t o the possibility of a polymorphic form being responsible for this phenomenon, which was provisionally christened 8-CdI2, to distinguish it from the normal or-CdL. A search in the literature suggested its existence, which stimulated us to make an experiMETASTABLE FORMS mental search for it. Sublimation of ordinary CdIzin a This brings us to the question: what is responsible for strong current of carbon dioxide and chilling the vapor the appearance of metastable forms and what is the with carbon dioxide snow gave us a very light, powdery mechanism? There is no doubt but that impurities substance which we called "flowers of cadmium iodide," are the chief cause of metastable forms and that they in accord with its similarity t o flowers of sulfur. The act by being adsorbed on the'newly formed nuclei, specific volume was not only greater than that of athereby modifying their velocity of growth. Mineral- CdIz, but in contact with xylene--the liquid used in the ogy has presented us with beautiful examples of physi- pycnometer-it gradually approached the normal one. cal isomerism-for example, calcium carbonate, which This contraction did not take place when the density exists both a calcite and aragoniteand long ago min- was measured in a volumenometer, where the substance eralogists called attention to the probability that small is only in contact with air or another gas. It was extraces of foreign material ("agents min6ralisateurs") pected that thk 8-CdIz would show an abnormally high play a part in their formation. On the other hand, solubility and thus furnish an explanation for the phethorough investigation in the van't Hoff Laboratory on nomena described above; but absolutely normal values the formation of polymorphic forms often showed the were found. This could only mean that water stabilizes increasing reluctance of metastable substances to ap- 8-CdIzinto or-Cdd during the shaking, before saturapear during the progressive purification of a suhstance. tion was reached, and that this metastable form could There are, however, two restrictions to be made here. not be responsible for the high solubility. In some cases, repeated purification seems t o be of It seemed unlikely that impurities were responsible, little effect, and secondly, one has to bear in mind that for CdIa is prepared by shaking very pure iodme and in consequence of this repeated purification the chances cadmium in distilled water. However, one "impurity" for the survival of an unstable modification, once is possible-a small amount of Cd(OH)amight be disformed, become less. The laboratory atmosphere be- solved in the salt, although this by-product of the reaccomes more and more charged with particles of dust of tion is supposedly filtered off,along with the excess cadthe stable form, which of course may act as a stabilizer mium, before crystallization. If this is responsible for when coming in contact with the unstable one. The the abnormal solubility addition of a trace of HI ought fact that in some cases even very pure material can still to restore the normal behavior, which proved to be the produce monotropic forms, when they are formed in case. sealed glass containers, shows the danger of contaminaThis shows that supersaturation still exists even after tion by stable particles floating in the atmosphere. shaking the solution for 48 hours in contact with solid Cadmium iodide was a case in point. A monotropic CdIz. The same phenomena were observed with lead form of cadmium iodide had been repeatedly prepared nitrate and zinc sulfate, after adding traces of the hyin the laboratory and its behavior studied. When, af- droxides. ter the lapse of several months, we tried again to preThis fact is of the utmost importance in the producpare the monotropic form, using the same method as tion of pure monotropic forms, as well as in the formabefore, it was impossible to reproduce our former ex- tion of mixtures of polymorphs of variable composition. periments. Only the pure stable form made its appear- For if we evaporate or cool a solution beyond thesatuance. In another laboratory we succeeded a t once in ration point nuclei of the solute will appear sooner or
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later. Now an unstable form has a greater solubility than the stable one and as a rule its saturation will not be reached once the stable form has made its appearance. But if impurities are present they will be adsorbed on the newly formed nuclei, and they may lower the velocity of growth so much that the saturation point of an unstable form is reached and exceeded. Consequently, a monotropic form m y crystallize and grow, since adsorption is very specific.' This gives a very clear picture of the part played by traces of impurities; through adsorption they may change enormously the velocity of formation of nuclei and the velocity of growth. Solvents can act in the same way; liquids like toluene, which have little solvent action on salts, still can exert a very great stabilizing effect on polymorphic mixtures. When toluene is able to dissolve an adsorbed "impurity," contact is a t once rebtablished between the different forms in the mixture and stabilization (by "inoculation") can start. Since "impurities" can seldom be reproduced at different times and places, it is not surprising that results are often difficult to duplicate by other investigators with different samples of the same chemical compound. Cohen always maintained that a failure to obtain a polymorphic form is no proof of its nonexistence, especially since the conclusion is often based on experiments which are quite unsuitable to induce transformation. It is sometimes impossible to verify the existence of known transition points by measuring the conductivity of metals, even though temperatures are maintained for long periods. Working in this manner, even the transition point of white and gray tin would remain hidden, except in very special circumstances. To make the transformation reaction start at all is often a major problem. Cohen was always very careful not to hold physical isomerism responsible for irregularities unless other explanations had been eliminated by careful experiment. During an investigation of the solubility of salicylic acid it was found that samples from different sources showed discrepancies. Although a monotropic form could have explained the observations, a painstaking study showed that traces of water, which salicylic acid keeps enclosed and does not lose with careful drying, were responsible for the observed facts. Solubility effects also directed Cohen's attention to the old question: do small particles have a greater solubility than matter in bulk? An elaborate investigation with J. J. H. Blekkinghs on the solubility of BaSOn was performed to settle it. No clear e'vidence was found of a real increase in solubility; spurious effects were noted, due to very fine suspended particles. THE TIN EQUILIBRIUM AGAIN
During the last years of his life Cohen again took up Adsorption can even be very different on different faces of a single crystal, varying the "crystal habit" of a compound. For example, sodium chloride crystallizes as octahedra or cubes, depending upon the presence or absence of urea in the solution during crystallization. 6 BLEKKINGH, J . J. H., 2.Ph&. Chem., A-186,257 (1940).
the problem of the transformation, white tin e gray tin. Tin of great purity can be obtained commercially in blocks. Sometimes exposure of filings of such a block to a temperature of -40°C. (liquid ammonia in Dewar vessels) starts the transformation; with other samples inoculation with gray tin is necessary to induce it, and in some cases even then the white tin remains unchanged. Two factors may accelerate the transformation: traces of impurities and deformation or strain in the material. Van Arkel and Koets6studied the influence of deformation on'the behavior of different metals and showed that places of disorder in a crystal lattice may be considered as "germs," which bring about recrystallization of the material as well as allotropic changes. When rearrangement leads to another lattice at such spots, different from the original one, we have the formation of the nucleus of another modification. Cohen and his collaborators applied these ideas to the case of tin and found a very marked influence of deformation on the velocity of transformation. Tin rods can be deformed by rolling, by drawing into wires, and by repeated bending and stretching. However, working a metal in this way generates a considerable amount of heat and precautions were necessary t o avoid it