Historical and Critical Notes ELWOOD R. SHAW' and EARLE R. CALEY The Ohio State University, Columbus
J o u ~ m u x(1) was apparently the first to observe the unusually high cryoscopic constant of camphor and the first to employ it as a solvent for the determination of the molecular weight of organic compounds. But the name of Rast is usually associated with the use of camphor for this purpose, though his first publication (3) on the subject appeared ten years later than that of Jouniaux. Rast evidently developed his well-known method without being aware of the prior work of Jouniaux and believed at first that the principle of his method was entirely novel. The circumstances under which Rast made what he believed to be an original discovery are somewhat unusual. His inspiration appears to have come from his study of a copy of the Physikalisch-Chemische Tabellen of Landolt-Bornstein-Roth which he persuaded his father t o buy for him, and he worked out his very useful method in his father's drug store, not in an academic laboratory, though a t the time he was a student at the University of Wiirzburg. As Rast (3) now recalls it: On one day of the Christmas holidayys of 1922, while s t home reading, I came upon the camphor-salal diagram of Caille. This was indeed no carefully plotted curve, but it seemed to me that camphor must h w e an incredible Beckmsnn constant. I calculated it out and came on the value 40. I had all that was needed for thr e x p e n m m ~b t the drug P I O W : ~m~lyti(vd I ~ t l a n v t - rndr~llg , point R w k , cwtmphw, xnd r \ w . r l purr orcmic c u l ~ mw+ r ~
~
His first experiment was with acetanilide, for which he found the correct molecular weight when he used the constant 40. On the following day he determined the molecular weights of phenolphthalein, picric acid, and resorcinol, and a little later the molecular weights of caffein and sulfonal. With these also he obtained correct results. On his return to the university, he imparted the results of the experiments to his teacher, Von Halban, who advised him to publish his findings. After a conference with Dimroth, the head of the Chemical Institute, and after a check of the camphor-salol diagram, thiswas done. Withm a very short time after publication, Houben Present address: Department of Chemistry, Antioch College, Yellow Springs, Ohio.
VOLUME 35, NO. 7, JULY, 1958
(4) reported that the method of Rast was already widely used, and predicted that it would soon be adopted in all laboratories. A comparison of the original procedure of Rast with that published by him later in Abderhalden's Handbook (5), and with that in a recent textbook (6), shows that there has been little change except for improvements in the apparatus. I n spite of the quick acceptance and success of this method for the determination of molecular weights, there was some adverse criticism, some of it directed to the question of priority. For example, Le FBvre (7) remarked in a critical note on the Rast Method that, In view of the increasing use of an excellent and convenient procedure for the determination of molecular weights cryoscapically in camphor solution, the observation should be made that the usnsl method of reference to the method in our chemical journals scarcely hestows credit where it is most due.
After listing by title the two prior publications of Jouniaux, Le FBvre goes on to say, In 1922 Rast, without refereice to previous work, suggested that the determination of melting points of solutions in camphor could conveniently be made in of about 10 percent strength capillary tubes in the ordinary way. This was his only essential innovation.
. ..
Though this might seem t o be true, it does not adequately account for the fact that the publications of Jouniaux on the use of camphor as a solvent for the determination of molecular weights received little notice, whereas the publications of Rast on the same subject received immediate and wide attention. One reason for the success of the publications of Rast is that he included an explicit procedure for the determination of molecular weights. Jouniaux, on the other hand, merely described some experiments that he had made. Another reason is that the procedure of Rast required very small weights of organic compounds, whereas the weights used in the experiments of Jouniaux ranged from a half to one gram, a differencethat was of considerable importance from the standpoint of practical research in organic chemistry, especially a t a period when micromethods in organic analysis were generally coming into exclusive use.
Some of this adverse criticism was also directed to the value of the cryoscopic constant used by Rast or to the validity of the data on which it was based. Le FBvre, in the same critical note, says, Rast further (without discussion) adopted 400 as the molecular freezing point for camphor, calculitting this value from the melting points of salol-camphor mixtures given in Landolt-BarnsteinRoth (4th ed., p. 556) which themselves are taken from a paper by Caille (8). Jouniaux had previously deduced a higher value, namely, 498. The latter figure seems preferable.
Actually the numerical value of the constant recommended by Rast is 40. The figure 400 is the corresponding value based on 100 grams of solvent instead of 1000 grams. On the basis of this latter weight the value of the cryoscopic constant recommended by Jouniaux is 49.8 instead of 498. This last figure was obtained by Jouniaux as the average of five experiments in which the value of the constant ranged from 492 to 505. I n the following discussion of the different constants recommended by Rast and by Jouniaux, the more usual basis of 1000 grams of solvent will be used. I n a paper which followed the above-mentioned critical note, Le FBvre and Webb (9) reported the results of a redetermination of the melting point curve of mixtures of salol and camphor. They found considerable divergence from the curve based on the data of Caille. From this new curve they derived cousiderably higher values for the cryoscopic constant for camphor than that recommended by Rast. However, in a second paper, which quickly followed the first, Le FBvre and Tidemau (10) reported that the cryoscopic constant was found to range from 37.9 to 41.0 for solutions of twelve other organic compounds in camphor. They concluded, The mean value for K is thus found to be 396 li.e., 39.61 which differs greatly from that obtained by Jouniaux. It is difficult to reconcile our figure with that of this worker. The apparently convincing msthemstieal verification advanced by him contains arithmetical errors, and has been shown elsewhere to he useless as evidence in support of his experimental work.
The mathematical verification to which allusion is made was published by Jouniaux (11) shortly after his experimental paper appeared, and the refutation of it by Le FBvre and Tideman (12) was published as a note about the same time as their paper. In this note they say, In conclusion, it is evident that neither as a verification of his own work nor as a means of discriminationbetween the two values of K for camphor so widely different as 400 and 498 are such applicstions of thermodynamic formulae of much practical use.
The work of Le FBvre and co-workers did not settle the issue of the widely different constants recommended by Rast and by Jouniaux. Durand (IS) contended that the constant of Jouniaux was essentially correct. He criticized Rast's method as involving two principal errors, one, the use of solutions too concentrated for the sound application of Raoult's law, and the other, the use of a constant that was too low. Durand concluded that these two errors approximately compensated each other. This could explain the satisfactory results usually obtained by Rast's method. Durand advocated the use of much more dilute solutions and a constant of 50.0. Unfortunately, much more dilute solutions require more exact weighing of the organic substance and the use of a more delicate thermometer. Hence the procedure rec-
ommended by Durand involved the loss of some of the practical advantages of Rast's procedure. A little later, Bohme and Schneider (14) pointed out that not only were the two very different constants of Rast and of Jouniaux supported by the experimental evidence of various workers but that still other constants were recommended by other morkers, also based on experimental evidence. Bohme and Schneider even doubt the validity of molecular weight determinations by the Rast method and its modifications, for they say, The surprise is, that even with the most varied "constants," most authors always find almost theoretical results, which makes the value of such molecular weight determinations rather questionable.
They also point out that in view of the unavoidable error of a few per cent in weighing out the organic substance and in reading the thermometer, a range of experimental values will be the usual result of determiuations by this method. They concluded that the various investigators selected the most appropriate from a number of measurements and that this explains why the theoretical and experimental values for their molecular weight determinations agree so well even when different constants are used. Some of the data in the very critical review by Bohme and Schneider suggested to Meldrum that the constant might vary with the concentration of the organic substance in the camphor solution. By careful experiments Meldrum and co-workers (15) showed that it did vary from a minimum of 37.7 a t about 0.2 molal to a maximum of 50.0 at about 0.01 molal. This explains why the constant 40 gave satisfactory results with the concentrated solutions recommended by Rast and why the much higher constant 50 still gave satisfactory results with the dilute solutions recommended by Durand and by others. Though the work of Meldrum and co-workers ended most of the controversy over the discrepancy between the constants of Rast and of Jouniaux, it still did not explain why the latter investigator, who worked mtth concentrated solutions, found such a high constant. Rast (16) suggested that this high constant was caused by failure to stir when he determined his melting points, and Bohme and Schneider suggested that it was caused by his use of relatively large weights of camphor and organic substances, but neither of these suggestions leads to any plausible explanation. It is curious that in all this controversy about the discrepancy between the constant of Rast and that of Jouniaux, none of the participants called attention to the unusual equation that Jouniaux used to calculate his constant from his experimental data. Though Rast gave no equations for his calculations, it is obvious that they were based on the simple expression: M = -K P C
(1)
where M is the unknown molecular weight, K is the cryoscopic constant, P i s the number of grams of solute per 1000 grams of camphor, and C is the depression of the freezing point in degrees. The symbol C corresponds to the AT now commonly used to denote the depression of freezing point temperature. For the determination of the constant the equation therefore is:
JOURNAL OF CHEMICAL EDUCATION
But Jouniaux used the following equation for the determination of the constant: K = M
C"C'(P" - P ' ) PPP'(C" - C ' )
where C', C" and P', P" are measurements of the depression of the freezing point and the amounts of solute per 100 grams of camphor, respectively, in two independent experiments in which the concentrations of solute are considerably different. He derived this equation from an exprem;on usually attributed to Raoult (1 7) :
where (C/P)o is the ratio of the depression of the freezing point to the amount of solute as the concentration approaches zero. Jouniaux found the value of this ratio from two pairs of observations by means of the following equation:
Value of the Cryoscopie Constant K for Camphor Calculated f ~ o m the Data of Jouniaux by Means of the Formula Used bv Rast Concentration
nanva..
Benzoic acid (M.W. = 122.12)
200. 7 89.2
65.2 32.5
397(a) 44.5
49.2
Naphthalene (M.W. = 128.16) 2-Naphthylamine (M.W. = 143.18) 1-Nitronaphthalene (M.W. = 173.16)
210.5 93.6 233.6 103.8 485.0 125.7
65.0 32.5 67.2 33.2 99.2 33.2 Av. = Av. of ( a ) =
396(a) 44.5 41.2(a) 45.8 354(a) 45.7 42.3 39.3
49.4 49.9 50.5 49.8
who follow such instructions 1141 not obtain correct molecular weights except by accident. ACKNOWLEDGMENT
By substituting the right side of this last equation in equation (4), equation (3) is readily obtained. What all the critics of the work of Jouniaux appear to have overlooked is that if his experimental data are substituted in the simple cryoscopic equation (equation (2)) the constant thus obtained is very near to that obtained by Rast and by the others who worked with concentrated solutions. This is especially true for his data on the more concentrated solutions. The actual results of this substitution are shown in column I of the table. The figures published by Jouniaux, as obtained from the same data by the use of his equation, are shown in column 11. I n other words, the constant of Jouniaux is unusual because of the unusual equation he used. Moreover, if his equation is used both for the determination of the constant and for the determination of unknown molecular weights, the resulting values for the latter are as good as those obtained by theuse of the simple equation. The past confusion has largely resulted from attempting to use the constant of Jouniaux in the simple cryoscopic equation. Similarly, the equation recently recommended by Linstead (18) yieldsfrom the data of Jouniaux a constant that is very close to that determined by the equation of Jouniaux, and it would be equally wrong to use this constant in the simple cryoscopic equation. Unfortunately, this kind of error occurs in some laboratory manuals still in use. In one such manual, for example, the simple cryoscopic equation is given, the method of Rast is recommended, and 49.8, the constant of Jouniaux, is listed in the accompanying table of constants. Obviously, students
VOLUME 35, NO. 7 , lUL.Y, 1958
The above notes were take11 from a more detailed account (19) in which the Rast method and its various modifications are discussed in detail. LITERATURE CITED ( 1 ) JOUNIAUX, A,, Bull. m e . ehzm., 141 11, 722 (1912). ( 2 ) RAST,K., Be?., 55, 1051, 3727 (1922). ( 3 ) RAST,K., Personal communication, Leverkusen-Bagerrverk, Fitbrikkasino, Germany, Msrch 15 and April 9, 1956. (4) HOUREN, J., J. pmkt. Chem., 105, 27 (1922). ( 5 ) RAST, K., Article in E. Abderhalden, "Handbuch der Biologischen Arheitsmethoden," Abt. IIIA, Bd.1, XI.99, Urban and Schaartzenburg, Berlin-Vienna, 1928, p. 754. ( 6 ) FIESER,L., "Experiments in Organic Chemistry," 3rd ed., D. C. Heath and Co., Boston, 1955, p. 22. ( 7 ) LE FPVRE,R. J. W , Natwe, 126, 760 (1930). ( 8 ) CAILLE,E., Compt. rend., 148, 1461 (1909). ( 9 ) LE FBVRE,R. J. W., A N D W. H. WEBB,J . Chem. Soc., 1931,
-.
1714
( 1 0 ) LE FBVRE,R. J. W., AND C. G. TIDEMAN, J. Chem. Soe., 1931, 1729. A,, Compl. rend., 154, 1592 (1912). ( 1 1 ) JOWNIAUX, Nature,. 127.. 972 ( 1 2 ) LE FPVRE.R . J. W.. and C. G. TIDEMAN. (1931). 8 DURAND, J. F., Bull. soe..ehim., 151 4 , 67 (1937). BOXME,H., ANDE.SCHNEIDER, Angew. Chem., 52,58 (1939). MELDRUM, W. B., L. P. SAXER,A N D T. 0. JONES, J. Am. C h m . Soe., 65, %I23 (1943). RABT,K., C h m . Zlg., 59, 853 (1935). RAOULT, F. M., Ann. ehim. phys., 161 8 , 289 (1886). LINBTEAD, R. P., J. A. ELVIDGE,AND M. WHALLEY,"A Course in Modern Techniques of Organic Chemistry," Butterworth's Scientific Publications, London, 1955, p. 146.
