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The data in table 6 were obtained with uncolored sugar solutions. It will be observed that with a white-light source the presence of the bichromate absorbing solution makes a difference of 0.12° in the saccharimeter reading. It thus becomes important that the instrument be used under the same conditions as prevailed at the time it was standardized. If, as is usually the case, a quartz control plate is used, the plate should be read in the instrument under the same conditions as prevailed when the plate was standardized.

(d) TYPES OF LAMPS

(1) MONOCHROMATIC.-Many different types of lamps designed for use with the saccharimeter are available. They may be divided into two classes those giving a monochromatic or nearly monochromatic light and those giving white light. In the first class only the yellow sodium lines and the yellow-green mercury line (λ=5461 Å) have been utilized to any extent and these usually for special purposes. Sources of this type must be used for circular-scale polarimeters; they are in general not satisfactory with saccharimeters which were designed for the more intense white-light source.

(2) WHITE LIGHT.

Gas. A considerable variety of lamps suitable for white-light sources is available both for gas and for electricity. Most of the gas lamps utilize the Welsbach mantle, which is the source formerly most generally used in saccharimetry. The light is convenient, has considerable intensity, and the radiating surface has a nearly uniform intensity over a sufficiently large area. A ground-glass screen may be used close to the mantle if desired, and is necessary if the lamp cannot be so placed as to eliminate a mottled appearance of the field when the telescope is in focus for the analyzer diaphragm.

Electric.-The available electric lamps are of several types. A ground-glass disk, which becomes the new source of radiation, must, with few exceptions, be used with all types, and is preferably located as near the radiating surface as the temperature will permit. In figure 21 is shown an electric lamp developed at this Bureau, which has recently been modified to carry the bichromate filter. The concentrated-filament incandescent stereopticon lamp for 110 volts is used. The area illuminated is ample for the purpose, and the intensity sufficient. Convenience has been the chief consideration in the design. The base B is heavy. The ground-glass disk, R, 38 mm in diameter, is easily removable and is adjustable vertically with respect to the body, J. Thus the filament and the disk may be kept

The maker's identifying specifications for this bulb are G. E. Mazda, clear spotlight, 100 watts, 110 volts, C5 filament, P25 bulb, medium screw base.

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FIGURE 21.-Saccharimeter lamp with bichromate cell holder (NBS design). A, base perforated for ventilation; B, cast-iron stand; C, hollow post; D, pinion for adjusting height of lamp; E. set screw; F, solid post fitted with rack; G, glass cell; H, porcelain lamp socket; I, removable disk light screen; J, lamp housing fitting into flange, K; L, adjustable slide; M, removable cover held by springs, N: O, side tube carrying ground glass and bichromate cell; P, binding posts; Q, insulating blocks; R, flange to hold ground-glass plate; S, slot opening for ventilation.

centered, giving uniform illumination of the disk. The cap, M. permits the heat to pass off but no light to escape into the room, The height of the lamp is regulated by the rack and pinion, D. The center of the disk can thus be accurately set in the axis of the optical system of the saccharimeter. The electric connection is made at the binding posts, P. This lamp has proved very satisfactory in the laboratories both of this Bureau and of the United States Customs Service and is the type most highly recommended for general use. The firm of Schmidt & Haensch has taken advantage of the small size of the 6-volt lamp to mount it in an attachment which fits the metal housing containing the polarizing system of their saccharimeter.

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Left, tungsten arc in vacuum (30-ampere arc); right, ribbon-filament lamp (6 volts, 108 watts).

The heat developed so near the optical parts is objectionable. The illumination is only fairly satisfactory when the lamp is new, and the efficiency in most cases decreases rapidly with use.

In special cases where the 100-watt lamp does not yield sufficient light, other sources have been utilized.

The Nernst glower was useful for some years but is now of little more than historical interest. It has been largely displaced by incandescent stereopticon bulbs of higher wattage than that described above. They are obtainable in 250-, 500-, and 1,000-watt ratings.

Other high-intensity sources are (1) (fig. 22 right) the 6-volt, 108watt ribbon filament lamp, which is operated from a small transformer; and (2) (fig. 22 left) the tungsten arc in vacuo developed by

the General Electric Co. as a source for photomicrographic work. The latter is a 30-ampere arc in a small glass bulb, the incandescent tungsten electrode serving as the light source. It is operated by a special high-reactance transformer operating from the 110-volt alternating-current line. Both of these types can, by the use of proper optical systems, be made to give uniform polariscopic fields without the use of the ground glass, thereby greatly increasing the intensity available. Such sources, however, find only limited uses and for very special purposes. The 100-watt lamp described above in detail is adequate for all ordinary work.

5. CERTIFICATION OF QUARTZ CONTROL PLATES

Quartz control plates for use in checking saccharimeters will be accepted by this Bureau for standardization with reference to the sugar value. The conditions as to mounting, purity of quartz, correctness of fabrication, etc., are given on page 57. This Bureau reserves the right to reject any plate showing defects which may render it unreliable or otherwise unsatisfactory in service.

Certificates are issued showing the optical rotation of the plate for pure monochromatic light of two wave lengths, X=5461 A and 5892.5 A, as well as the sugar value of the plate at 20° C in International Sugar Degrees. When desired, an accompanying table of temperature corrections will be furnished covering the range 15° to 30° C, by means of which saccharimeter readings made at any temperature within that range may be corrected to the reading that would have been obtained had the readings been made at 20° Č.

6. REFERENCES

[1] H. Soleil, Compt. rend. 20, 1085 (1845); 21, 426 (1845); 24, 973 (1847); 26, 163 (1848).

[2] F. J. Bates, Bul. BS 4, 461 (1908) S86; 5, 193 (1908) $98.

[3] K. Ventzke, Erdmann's J. prakt. Chem. 25, 65-84 (1842); 28, 111 (1843).

[4] Jean Baptiste Biot, Mém. acad. sci. 15, 101 (1838).

[5] O. J. Broch, Dove Rep. Phys. 7, 113 (1846). Ann. chim. phys. 34, 119 (1852).

[6] A. Girard and V. de Luynes, Compt. rend. 80, 1354 (1875).

[7] D. Sidersky, Bul. assn. chim. sucr. dist. 3-4, 255 (1885-1886).

[8] D. Sidersky, Bul. assn. chim. sucr. dist. 50, 355 (1933).

[9] E. Mascart and H. Benard, Ann. chim. phys. 17, 125 (1899).

[10] H. Pellet, Ann. chim. phys. 23, 289 (1901).

[11] F. J. Bates and F. P. Phelps, J. Research NBS 17, 347 (1936) RP916.

[12] O. Schönrock, Geiger's Handbuch Physik 19, 705 (1928).

[13] E. Roux, Bul. assn. chim. sucr. dist. 55, 404 (1938).

[14] Proc. Third Session, International Commission for Uniform Methods of Sugar Analysis, Paris, (1900).

[15] A. Herzfeld, Z. Ver. deut. Zucker-Ind. 50, 826 (1900).

[16] O. Schönrock, Z. Ver. deut. Zucker-Ind. 54, 521 (1904).

[17] Proc. Seventh Session, International Commission for Uniform Methods of Sugar Analysis, New York, 1912.

[18] F. J. Bates and R. F. Jackson, Bul. BS 13, 67 (1916) S268.

[19] Proc. Eighth Session, International Commission for Uniform Methods of Sugar Analysis, Amsterdam, 1932, Int. Sugar J. 35, 17, 62 (1933).

[20] E. Gumlich, Wiss. Abhandl. physik.-tech. Reichsanstalt 2 212 (1895).

[21] T. M. Lowry, Phil. Trans. 212, 288 (1912–1913).

[22] T. M. Lowry and W. R. C. Coode-Adams, Phil. Trans. 226, 391 (1926–27) [23] B. Tollens, Ber. deut. chem. Ges. 10, 1403 (1877).

[24] Raffaello Nasini and Vittorio Villavecchia, Public di Lab. chim. delle gabelle, Rome, p. 47 (1891). Roma, R. Acc. Lincei Rend. 7, 285–290 (1891); Gazz. chim. ital. 22, 97-104 (1892).

[25] Hans Landolt, Das optische Drehungsvermogen, p. 420 (Friedrick vieweg und Sohn, Braunschweig, Germany; 1898).

[26] R. F. Jackson, Bul. BS 13, 633 (1916) S293.

V. TEMPERATURE CORRECTIONS AND CONTROL

1. QUARTZ-WEDGE SACCHARIMETER

The question of temperature corrections for polarimetric apparatus, as well as for the optically active substances, is a difficult one. The literature on this subject is extensive, but it has not always been a simple matter to select the proper correction owing to the different results secured by various investigators. For the ordinary polarimeter used for measuring absolute rotations no instrument correction is necessary. It may be used at any temperature if care is taken to allow for any zero-point shift that may occur. However, a correction is unavoidable for saccharimeters, in which a quartz wedge is used to neutralize the rotation of the substance being tested if temperatures other than the standard are used. The rotation for a plate of quartz in the neighborhood of 20° C is given by the following:

a=a+a20 0.000143 (t—20).

(27)

The linear coefficient of expansion of quartz perpendicular to the optic axis is 0.000013, and of the scale is 0.000018 if it is of nickelin, and 0.000008 if it is of glass. Thus the total temperature coefficient [1] for the ordinary quartz-wedge saccharimeter is

or

0.000143-0.000013+0.000018

0.000148 (metal scale) (28a)

0.000143-0.000013+0.000008 0.000138 (glass scale). (28b)

If the scales were etched on the wedges, the scale coefficient would become zero. Since the effect of the expansion coefficient 0.000148 is to lower the reading of the scale with an increase of temperature, the apparent polarization of any substance is lower than it should be and the reading at 20° (S) is given by the following:

S20=S1+St 0.000148 (t-20).

(29) When a quartz control plate is read in a saccharimeter, this effect is not completely compensated. Since the temperature coefficient of the plate is 0.000143, the reading (S20) of the plate is then

S20 SS, 0.000005 (t-20).

(30)

The correction given by this equation changes sign if the scale is of glass, and is so small at all times as to be negligible in ordinary polarizations.

2. SUCROSE

The influence of temperature on the specific rotation of sucrose has been studied by numerous investigators, of whom Dubrunfaut [2] was the first to discover that the constant decreased with increase in temperature. Unfortunately, subsequent determinations of this variation have not shown satisfactory agreement.

Schönrock [3] at the Reichsanstalt carried out an elaborate investigation and found that for the normal sugar solution (p=23.701) the temperature coefficient (8) was independent of the wave length of the light used, but that it decreased with increase in temperature as follows:

10° C, 0.000242; 20° C, 0.000184; 30° C, 0.000121.

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