Origin of the Clerget M e t h o d C. A. BROWNE
U . S. Department of Agriculture, Washington, D. C.
T
HE earliest contribution by Clerget relating to the method of double or invert polarization, which bears his name, is found in a short notice in the weekly Comptes Rendus of the French Academy of Sciences for May 8, 1843 2 , entitled "The Analysis of Sugars by Polarized Light". The article is short, and I have made the following translation:
The beautiful observations of Monsieur Biot upon the phenomena of polarized light indicate the utility in qualitative and quantitative analysis which can be derived from the optical properties of solutions of many substances. This utility would be especially very great for industry when a practical determination of the nature of the sugars or of the richness of saccharine liquids is desired. T h e formulas which Monsieur Biot has given in his memoirs furnish the complete solution of the problem which, however, does not seem to have been generally applied to its full advantage. In my opinion this is due to the fact that these formulas, because of their precision and need of the use of logarithms, demand an expenditure of time that cannot ordinarily be given, principally for the reason that they require for one of their elements an exact determination of the density of the liquids under examination by means of the balance. This consideration has caused me to seek means of simplification, and I have developed a process of fixed weights which eliminates density determinations. It is based upon the numbers, obtained by the single addition of two figures or by simple subtraction, which permit the weighable quantity of crystallized sugar contained in mixtures to be read off directly in hundredths upon the divided circle of the observation apparatus. This statement clearly indicates that Clerget's method, so-called, is simply a practical modification of an analytical polariscopic procedure previously elaborated by Biot who was the actual originator of the process. The story of Biot's part in the development of the method of double polarization is now almost forgotten. In 1834 Dr. Pallas, head physician of the military hospital of Saint Omer and owner of a maize plantation in French Africa, presented to the French Academy of Sciences an interesting observation on the apparent changes in sugar content of cornstalks during their period of growth. The question was referred by the academy to a joint committee of chemists and physicists who after consultation recommended that Pallas be encouraged to con-
tinue the investigation which gave promise of leading to conclusions of scientific and industrial importance. In a later report Pallas stated that, if the ears of corn were removed at the beginning of their development, the sugar content of the stalks seemed subsequently to undergo a considerable increase. This important observation was referred by the academy to another committee of distinguished scientists, consisting of the agricultural chemist Boussingault, the physical chemist Regnault, the industrial chemist Payen, and the physicist Biot, who reported that the conclusions were of such importance to plant physiology and industry that the problem would be taken over by Professor Biot for accurate analytical verification. Biot's brilliant solution of this problem, contained in the Comptes rendus of the academy for 1842, verified Pallas' statement as to the increase in sugar content of cornstalks as a result of removing the ears. But it also marked the establishment of a new polariscopic method, using the principle of inversion, for determining cane sugar in mixtures of other sugars—a now universally employed method which has been of inestimable value to pure science, to agriculture, and to industry in ail parts of the world. Only the main steps in Biot's development of the double polarization formulas that he employed in this work will be given. For details reference is made to the original article by Soubeiran and Biot, entitled "Recherches exp6rimentales sur les produits sucr£s du mals" 3 , of which the year 1942 is the centenary. « Compt. rend., 15, 523-42 (1842).
Biot found that when a solution of pure cane sugar was inverted in the cold for 4 to 5 hours with 0.1 its volume of hydrochloric acid, a direct reading of 1 circular degree on his instrument was changed to —0.38 circular degree or a difference of 1.38 circular degrees when correction was made for dilution. For sulfuric acid the factor under the conditions of his experiment was found to be 1.3867 for a solution of pure sucrose. For a similar volume of clarified cornstalk juice under the same conditions an average invert reading of —0.3411 circular degree was found on five samples for each degree of direct reading which gives a difference of 1.3411 circular Then -L—-— = 0.967116 which 1.3867 is the sucrose equivalent for each circular degree of direct polarization of the juice. We have here for the first time in scientific literature the introduction of what we now term the Clerget factor, but which in justice to the man who originated it should be renamed the Biot factor. Direct polarization of the juice from the cornstalks from which the ears had been plucked, calculated to a tube length of 500 mm., was found by Biot to be 42.02 circular degrees which, multiplied by 0.967116, gives 40.64 circular degrees as the polarizing power of the sucrose in the j*uice. For converting circular degrees OS) obtained in a tube of L mm. length into grams of sugar per 1 cc. Biot employed degrees. &
a
the formula 1.4 — which, divided by the Li
density of the juice, gives the weight of sugar in 1 gram of juice. In this way Biot found the weight of sucrose in 1 gram of juice from the cornstalks on which the ears had been removed to be 0.1066 grams or 10.66 per cent and from cornstalks on which the ears had been left to be 0.0815 grams or 8.15 per cent, equivalent to an increase of 2.51 per cent sucrose as a result of removing the ears. This is the first application of a classic method of sugar analysis now common in
i Paper presented before the Division of Sugar Chemistry and Technology at the Dallas meeting of
the
AMERICAN CHEMICAL SOCIETY, April
1938. * Compt. rend., 16, 1000-1 (1843). 322
19,
ROBERT Q L E N K
Photograph of an original set of Clerget apparatus purchased by Valcour A i m e of Louisiana about 1 8 5 0 and now preserved in the Louisiana State Museum, N e w Orleans C H E M I C A L
A N D
E N G I N E E R I N G
N E W S
Column 1 Column 2 (5) Column 3 ( 0
D a t e s of juice extraction Densities of the juice compared with t h a t of distilled water a t 4 ° C . Lengths in millimeters of the observation tubes in which t h e direct polarizations were made Direct angular polarizations using the bluish v i o l e t transition tint of the field as the end point (a Soleil double quartz end-point d e v i c e was used, a s the half-shadow polariscope had n o t y e t been invented) Lengths of the tubes, variable a s in the previous case, in w h i c h the invert polarizations were made Original direct readings recalculated t o tubes of length V Nature of the inverting acid which is sulfuric Degree of dilution of the original juice produced b y addition of the inverting acid which is 8 /»ths Recalculation of the revised direct polarization ( « ' ) t o correct for this dilution Observed invert reading in t u b e s of length V
Column 4 (a) C o l u m n 5 (*') Column 6 (a') Column 7 Column 8 Column 9 ( a " ) C o l u m n 10 ( a ' " ) C o l u m n 11
Calculated value of the invert polarization for 1 circular degree direct polarization of the juice S a m e value for pure sucrose which, w i t h sulfuric acid a s t h e inverting agent, was found t o be — 0.3867, temperature a n d other conditions being the same as in the juice experiments
C o l u m n 12
Jean Baptiste Biot (1774-1862) thousands of research and industrial laboratories in all parts of the world. A glance at t h e original record of Biot's brilliant polariscopic determination of the sucrose content of cornstalk juice helps in evaluating the part which Clerget played later in applying his teacher's discovery to practical sugar laboratory conditions. Page 536 from the Comptes rendus of September 12, 1842, gives the observational data of Biot's analyses. The upper half of the table relates to the juice of the cornstalks with ears removed and the lower half t o cornstalks with ears unremoved.
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