Measurement. In carrying out the rotation measurement, the plate is placed in the air bath of the circular-scale polariscope described on page 46, and the optical rotation carefully measured at 20° C for spectrally purified mercury light of wave length 5461 A. In order to eliminate accidental errors as far as practible, at least three sets of readings are made on different days. Moreover, all readings are compared to, and corrected by readings on a set of standard plates which this Bureau maintains as its primary standards. Because of the refinements in the method of checking quartz control plates, it is believed that the sugar values certified are not in error by more than 0. 01°S, or at most 0.02°S. Repeated determinations made several years apart seldom differ by more than this amount. (b) NATIONAL BUREAU OF STANDARDS SPECIFICATIONS In line with the requirements cited above, this Bureau has drawn up the following specifications for quartz control plates: (1) PURITY.-The plate shall be made of quartz which is optically homogeneous, i.e., it shall be free from twinning, striae, strain, and other optical defects. (2) PLANENESS, PARALLELISM. The faces of the plate shall be both plane and parallel within the following limits: There shall be no departure from true flatness by more than that corresponding to one-half wave length in air of sodium light anywhere within the free aperture of the plate, nor shall there be a radius of curvature of less than 100 m. The plate shall not differ in thickness between any points comprised within the free aperture by more than 0.00015 mm. This thickness corresponds to 0.01°S. (3) AXIS ERROR. The faces of the plate shall be accurately orientated at right angles with respect to the crystallographic axis. The axis error, i.e., the angle between the normal to the faces and the crystallographic axis, should be as small as possible and shall not exceed 10 or 12 minutes of arc. (4) MOUNTING. The plate shall be mounted loosely in a metal frame, the axis of which forms an angle of 90° with the faces of the plate. The amount of play between the plate and its surrounding frame shall be as small as possible in the direction parallel to the axis. of the plate, but the metal shall exert no pressure upon the plate. (5) DIMENSIONS.-The plates should be 15 to 17 mm in diameter and, after mounting, should have a free aperture not less than 10 mm. in diameter. Plates having low sugar values may be, and it is recommended that they be, composed of two thicker plates, one rightrotating and one left-rotating, mounted in separate mounts of the same type, preferably one on each end of the tube. (c) SPECIFICATIONS OF THE INTERNATIONAL COMMISSION FOR UNIFORM METHODS OF SUGAR ANALYSIS FOR QUARTZ CONTROL PLATES The following resolutions were adopted by the International Commission for Uniform Methods of Sugar Analysis at its Eighth Session at Amsterdam in 1932: 1. Quality tests of saccharimeter quartz control plates. (a) Optical purity.-At least the central 9 mm of the plate must be sufficiently optically homogeneous. Especially plates from 90° to 102°S must not show any 323414° -42- 6 striae. During the purity test, the plate should be rotated in its own plane at least 360° (about its own axis). (b) Plane parallelism. In general the wedge angle between the faces may at most amount to not more than 20 seconds. For plates between 90° and 102°S The radius of curvature of each of the faces shall this limit must be 10 seconds. be not less than 50 meters. (c) The axis error, that is, the angle between the optical axis and the normal to the plate, may at the highest amount to 10 minutes of arc. 2. Dimensions of the plates. The plates must be between 15.0 and 17.0 mm in diameter. After mounting they must have a free aperture at least 10 mm in diameter. Plates having values below 24°S should be composed of two thicker plates, one plus and one minus, the combined thickness of which, however, must be less than 1.6 mm. The edges of the plates shall be slightly beveled. 3. Identification marks. Near the edge of the plate shall be engraved "IP" (International Plate), the number of the plate, and the year. 4. Plate mountings. It is proposed to accept the form of mounting prescribed by the Physikalisch-Technische-Reichsanstalt wherein the plate is free from pressure, but the clearance is a minimum. For plates below 24°S, the two plates are to be mounted in separate mounts of the same type, one on each end of the holder tube. 5. Rotation measurement. The rotation in circular degrees shall be made at 20°C, using spectrally purified light either of wave length 5461 or 5892.5 A, obtained, respectively, from a mercury vapor arc of suitable design, or from a sodium light source, such as the Pirani or Osram sodium arc lamp. 6. The plates shall be tested in one or more of the four national physical laboratories, viz., National Bureau of Standards, Washington, D. C.; National Physical Laboratory, Teddington, London; the Physikalisch-Technische Reichsanstalt, Charlottenburg, Berlin; and the Laboratoire National, Paris. All details of the tests and measurements shall be left to these four institutions. At the Ninth Session at London in 1936, the Referee on Quartz Control Plates made the following recommendations which were unanimously adopted by the Commission: 1. The resolution of the Eighth Session shall be amended to read as follows: (a) Optical purity. The central 9 mm at least of the plate must be sufficiently optically homogeneous. It is essential that plates from 90° to 102° S should not show any striae. During the purity test, the plate should be rotated in its own plane at least 360° (about its own axis). It is recommended that the four National Physical Laboratories should investigate tests for fixing a quantitative limit to permissible defects in homogeneity. (b) Identification marks. Near the edge of the plate shall be engraved “I. P.” (International Plate), the number of the plate, the year, and the sign of the testing National Physical Laboratory. (c) Plate mountings.-The form of mounting to be acceptable for testing shall be such that the plate shall be free from compression and the clearance a minimum. For plates below 24° S, the two plates are to be mounted in separate mounts of the same type, one on each end of the holder tube. 2. The four National Physical Laboratories, viz., National Bureau of Standards, Washington, D. C.; National Physical Laboratory, Teddington, London; the Physikalisch-Technische Reichsanstalt, Charlottenburg, Berlin; and the Laboratoire National, Paris, be requested to collaborate and determine: (a) The rotation of the 100° S plate in circular degrees for the wave length (optical center of gravity) produced by the Osram sodium vapor arc lamp. (b) The rotation in circular degrees for the wave length (optical center of gravity) produced by the Osram sodium vapor arc lamp for plates of the following approximate values, 25, 50, 75, and 100° S. It is suggested that sets of such plates be interchanged between the above laboratories for comparative tests to be made. (d) CERTIFICATION OF QUARTZ CONTROL PLATES The usual procedure at this Bureau in testing quartz control plates is: 1. Examination of the mounting to see if the plate is satisfactorily mounted. 2. Examination between crossed nicols as a test of purity. 4. Measurement of the axis error. If a plate satisfactorily passes these tests, its optical rotation is measured, at 20° C, using spectrally purified light of wave length 5461 A obtained from a mercury arc. The rotation for λ=5892.5 A may be obtained by direct measurement, or by multiplying the rotation for the Hg light by 0.85085. The sugar value of the plate is then assigned, based upon either of the two rotations given above. The sugar value is discussed more fully below under "Saccharimeters." This Bureau reserves the right to reject any plate showing faults which tend to make it unreliable. (e) SPECIAL TESTS Additional data upon quartz control plates submitted for test may be had by special arrangement. Standardization for wave lengths other than λ=5461 A and λ=5892.5 A will be made, provided the order of accuracy desired is consistent with the intensity and purity of the source available. 4. REFERENCES [1] Erasmus Bartholinus, Experimenta Crystalli Islandici disdiaclustici quibus mira et insolita refractio detegitur (Amsterdam, 1670). [2] Christian Huygens, Traité de Lumière (Leyden, 1690). [3] l'Abbé Alexis Marie Rochon, Recueil de mémoires sur la mécanique et sur la physique (1783). Mém. [4] Etienne Louis Malus, Mém. Soc. d'Arcueil 2, 143, 149 (1808-1809). Savants étrangers 2, 303 (1810-1811). Théorie de la double refraction (Paris, 1810). [5] Dominique François Arago, Oeuvres complètes 10, 54. Mem. l'Institut, Class Math. Phys. 12, 115 (1811). [6] Jean Baptiste Biot, Mém. l'Institut, Class Math. Phys. 12, 135-280 (1811); 13, 1-371 (1812); 13, 1-18 (1813); 13, 18-38 (1814) [7] Jean Baptiste Biot, Bul. Sciences Soc. Philomatique, 190–195 (1815). [8] Jean Baptiste Biot, Mem. Acad. Sci. 2, 41-136. (1817.) [9] Jean Baptiste Biot, Sur la construction des appareils destinés a observer le pouvoir rotatoire des liquides, Ann. chim. phys. 74, 401-430 (1840). [10] William Nicol, Edinb. New Phil. J. 6, 83-94 (1828); 14, 372 (1831); 27, 332 (1839). [11] Eilhard Mitscherlich, Polarisation et Saccharimétrie, by D. Sidersky, 2d edition (1908), page 46. [12] Robiquet, Polarisation et Saccharimétrie, by D. Sidersky, 2d edition (1908), page 48. [13] K. Ventzke, Erdmann's J. prakt. Chem. 25, 65–84 (1842); 28, 111 (1843). [14] Henri Soileil, Compt. Rend. 20, 1805 (1845); 21, 426 (1845); 24, 973 (1847); 26, 163 (1848). [15] Joseph Johann Pohl, Wiener Bericht 22, 492 (1856). [16] H. Laurent, J. phys. 3, 183 (1874); 8, 164 (1879). [17] Rev. J. H. Jellet, Brit. Assn. Rep. 29, 13 (1860); Proc. Roy. Irish Acad. 7, 348 (1860); Proc. Roy. Irish Acad. 8, 279 (1863); Trans. Roy. Irish Acad. 25, 371-450 (1875); Z. ver. Rubenzuckerind. 15, 456 (1865). [18] Alfred Cornu, Bul. Soc. Chim. Paris 14, 140 (1870). [19] H. Landolt, Die Polarisationsapparate Bericht über die wissenschaftlichen Instrumente auf der Berliner Gewerbeausstellung in Johre, p. 357 (1879), (Berlin, 1880). [20] F. Lippich, Z. Instrumentenk. 2, 176 (1882); 14, 326 (1894); Weiner Ber. 91, 1059 (1885). [21] D. B. Brace, Phil. Mag. 161-170 (1903). [22] F. Lippich, Z. Instrumentenk. 12, 340 (1892). [23] H. Landolt, Ber. deut. chem. Ges. 27, 2872 (1894). [24] F. J. Bates, Bul. BS 2, 239 (1906) S34. [25] H. Ebert, Wied. Ann. 34, 39 (1888). [26] H._Landolt, Optical Rotation of Organic Substances, 2d edition, p. 404. Translated by J. H. Long (Chemical Publishing Co., Easton, Pa., 1902). [27] Pribram, Z. anal. Chem. 34, 166 (1895). [28] H. Landolt, Z. Instrumentenk. 4, 390 (1884). [29] General Electric Vapor Lamp Co., Bulletin 900. [30] Osram, Berlin 17, Germany. [31] Lines in the Arc Spectrum of the Elements, compiled and published by Adam Hilger, Ltd., 98 Kings Road, London, N. W. 1, England. [32] Ch. Fabry and A. Perot, Compt. Rend., p. 407 (1898). [33] R. A. Houstoun, Phil Mag. 7, 456 (1904). [34] General Electric Vapor Lamp Co. Bulletin 112; also Rev. Sci. Instr. 9, 325 (1938). [35] T. M. Lowry, Phil. Mag. 18, 320 (1909). [36] F. J. Bates, Sci. Pap. BS 16, 45 (1920) S371. [37] H. J. S. Sands, Proc. Phys. Soc., London, 26, 127 (1914). [38] E. Brodhun and O. Schönrock, Z. Instrumentenk. 22, 353 (1902). [39] E. Gumlich, Wissensch. Abhandl. Phys.-Tech. Reichsanstalt 2, 212 (1895). [40] O. Schönrock, Z. Instrumentenk. 22, 1 (1902). [41] V. Lang, Pogg. Ann. 156, 422 (1875). [42] Sohncke, Wied. Ann. 3, 516 (1878). [43] Le Chatelier, Compt. rend. p. 109 (1889). IV. MEASUREMENT OF ROTATION IN SUGAR DEGREES 1. DEVELOPMENT OF THE SACCHARIMETER (a) HISTORICAL INSTRUMENTS (FIG. 17) In the development of the saccharimeter there are two factors that have of necessity received most consideration. They are, first, sufficient illumination of the field for average work and, second, the highest obtainable sensitivity consistent with meeting necessary requirements. Because of the rotatory dispersion, or the different rotation. for different wave lengths, precise measurement of rotation ordinarily requires the use of a monochromatic light source. This is especially true if the precision attainable by the utilization of the photometric principle as realized in the halfshade is to be obtained. This condition holds in polariscopes designed primarily for absolute measurements of the rotation of the plane of polarization. Unfortunately a simple monochromatic source of sufficient intensity and otherwise suitable for all polarimetric work has never been realized. In order to obviate this difficulty, Soleil, a Parisian optician [1], as early as 1845 invented the first quartz-wedge compensator and applied it to the polariscope of Robiquet, permitting the use of white light illumination and obviating the necessity of using monochromatic light. He used a double quartz wedge so arranged that it was in effect a quartz plate of variable thickness, opposite in rotation to the sugar solution being measured. Since quartz has almost the same rotatory dispersion as sucrose, this device compensated or balanced out the rotation produced by the sugar solution, wave length by wave length, and returned the vibration planes of all the different wave lengths to the original vibration direction common to all before they entered the optically active substance. Since the calibrated wedge is driven across the field until conditions are as they were before the rotating substance was placed in the intrument, rotatory dispersion is practically eliminated and white light may be used. The absence of light sources of sufficient intensity has always been one of the most potent factors in influencing the design of saccharimeters. The higher limit of the sensitivity has been almost entirely determined by this factor. Even with the average halfshade angle of 6° to 8° the polarizing and analyzing nicols are practically crossed, so that only a mere fraction of the incident light ever reaches the eye of the observer. If the halfshade angle is decreased in order to increase the accuracy with which observations can be made, the intensity of the light transmitted is rapidly reduced. Thus monochromatic sources are inadequate in intensity when even fairly accurate settings are to be made, unless the active substance whose rotation is to be measured is quite transparent. Unfortunately this is not the case with many of the optically active liquids. This is especially true of the average raw sugar solution, and hence Soleil, as stated above, invented the quartz-wedge compensator which permits the use of white light with its relatively great intensity. (b) MODERN INSTRUMENTS (FIG. 17) The practical necessity for using white light has resulted in the quartz compensating instrument displacing practically all other types of saccharimeters, as is evidenced by the similarity in the perfected instruments of Fric; Bausch & Lomb; Schmidt & Haensch; Peters; Reichert; and others. This has been brought about despite the fact that the compensating wedge ordinarily prevents the full use of the adjustable halfshade angle of the Lippich polarizing system. Thus, while the best results in the designing of polariscopes for use with a white-light source have so far been obtained by using the Lippich polarizing system and a quartz-wedge compensation, all makes have had the great weakness of an unadjustable halfshade angle, and therefore a fixed sensitivity. Only one value of the halfshade angle can be used, and it must necessarily be large enough to give sufficient light to read, for example, the darkest-colored raw-sugar solutions. When polarizing substances having a small coefficient of light absorption, such as the better grades of sugars, are used, in which case the observer has more light than he needs, he still has available only the low sensitivity which corresponds to that value of the halfshade angle which gives sufficient light to polarize substances with a relatively large coefficient of absorption, such as very dark raw sugars. If then it were possible to retain the quartz compensation and at the same time have the halfshade angle adjustable, an advance in polariscope construction would be made comparable with the invention of the wedge. The defect due to the lack of adjustable sensitivity on a whitelight instrument has been in evidence not only in ordinary use but especially so when the saccharimeter was used for research work. The ordinary quartz compensating polariscope is utilized in practically every chemical laboratory. The highest available precision of the instrument is required in order to meet the demands of routine Yet the research investigator also has been compelled to depend upon it. The futility of taking a large number of observations on an instrument sensitive to 0.15 percent and using the average value as good to 0.015 percent is too well known to need discussion here. Nevertheless, the chemist has been compelled to do this because the majority of the research problems involving the use of the polariscope require the measuring of rotations with a precision greater than 0.1 percent. There can be no question that the present status of polarimetry would have been immeasurably advanced had there been a use. |