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polarizations at temperatures not far removed from 20°C is the simplest, but, unfortunately, has received least consideration. Its design. should have careful study, even to the slightest detail. This is essential owing to its widespread and constant use for both scientific and commercial purposes.

An all-metal tube of small bore (fig. 28, No. 4), being one type that is in more or less general use, has the same diameter for the entire length. These tubes frequently are constructed with walls so thin that they are easily bent or distorted, resulting in a change of length and ruining the tube. In many cases the bore is too small. The diaphragming in the modern saccharimeter is designed to give the highest possible illumination of the field. To utilize this as well as to eliminate the undesirable "halo" in the field of the instrument, it is necessary that the bore have a diameter of not less than 9 mm. Considerable time is lost in filling owing to the fact that the tube must be so completely filled that no air bubble remains. The weight of the tube is carried upon the caps which hold the cover glasses. If the tube is rotated in the trough of the instrument, the caps may be tightened, and the additional pressure may cause double refraction in the cover glasses, which has the effect of changing the rotation. of the plane of polarization.

Glass tubes must be used when the solutions contain acid or other corrosive chemicals. Metal collars threaded to fit the screw caps are cemented on. Wax is sometimes used for this cementing, but this is objectionable on the ground that the wax sometimes softens and permits the collar to be displaced until it extends over the end of the tube. Thus the length of the column of liquid being polarized is increased and an error introduced in the observation. A mixture of glycerine and litharge or a similar cement is more satisfactory, and the ends of the glass tube should not extend more than 1 mm beyond the threaded collar.

(2) BATES.-In the laboratories of the U. S. Customs Service, as well as in other laboratories, there is required a tube which is both rugged and as free as possible of defects. The Bates tube (fig. 28, No. 1) designed at this Bureau, has proved to be entirely satisfactory in meeting these requirements. It will be observed that the weight is carried upon two shoulders, which are integral parts of the tube, and not upon the caps, thereby eliminating all danger of tightening when the tube is rotated in the trough of the instrument. The bore is 9 mm, permitting the utilization of the full aperture of the polarizing system. This also reduces to a minimum the light depolarized by reflection from the walls of the tube. The field of the instrument appears as a bright circle with no overlying haziness, and permits readings of increased accuracy. Both ends are enlarged with all the attendant advantages; hence but one size cover glass and washer is required. The walls are unusually heavy, eliminating all danger of bending. These tubes are available in 400-, 200-, and 100-mm lengths. Glass tubes of the same design are also in general use.

(3) SPECIAL.-There are a number of tubes available for special purposes. For polarizations, where the temperature must be controlled or measured, a water-jacketed tube is recommended (fig. 28, No. 2). This consists of an inner tube of either metal or glass having a tubulature midway between the ends to permit filling and inserting a thermometer. A watertight jacket of metal surrounds

the inner tube provided with a nipple at each end to allow the circulation of hot or cold water. Landolt has designed a glass tube with one end enlarged and having sliding caps which fit over metal mounts, this construction being intended to eliminate the possibility of excessive pressure on the cover glasses. This tube has been modified by having the caps held in place by bayonet catches. However, the Landolt tube has not been generally used in the United States. Tubes provided with screw caps are preferred by most chemists, and if care is taken not to tighten the caps too much, they are entirely satisfactory.

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The continuous-flow tube of Pellet (fig. 28, No. 3) is widely used in factory control work. It is provided with a tubulature at each end, permitting filling and emptying without removing the tube from the instrument. The tube is filled by pouring the solution through a small funnel into one of the tubulatures. After the polarization, the succeeding solution is poured into the tube, displacing the first solution. These tubes effect a saving of time and are satisfactory for use with solutions of approximately the same polarization. Yoder devised a volumetric tube having the graduation mark on the connec

tion joined on the middle of the tube. The usual size is 10 ml, but tubes of various volumes may be constructed by varying the length and bore.

When the temperature of the solution is below the dew point, moisture condenses on the cover glasses. Wiley has overcome this by an ingenious desiccating cap (fig. 28, No. 6), which carries calcium chloride or other desiccant, and screws to the end of the polariscope tube.

A special glass-lined Bates tube is shown in figure 28, No. 5. It is made in 50- and 25-mm lengths and is useful in cases where a limited volume of liquid is available or where high rotating liquids are to be measured.

(4) WATER-JACKETED FOR HIGH-TEMPERATURE POLARIZATION. It is frequently necessary to make polariscopic observations at high temperatures. Difficulty has been experienced in such measurements, using the ordinary water-jacketed tubes, due to leakage and distortion of the field caused by uneven heating. There has recently been developed at this Bureau a tube designed to eliminate the existing defects. The new tube, figure 29, is constructed entirely of an ironnickel alloy having practically zero expansion for temperatures from

FIGURE 29. High-temperature polariscope tube (NBS type).

0° to 100° C. The tube is welded into a water jacket which extends well beyond the end of the inner tube, thus maintaining an even temperature throughout the length of the liquid-column under observation. The tube is provided with a tubulature for the insertion of a thermometer.

(b) TEMPERATURE CORRECTIONS

It is customary to determine the exact length of polariscope observation tubes at the standard temperature of 20° C. The length of any tube at any temperature may be obtained by the following formula:

L=L20 [1+B (t−20° C)],

(35)

where L20 is the length at 20° C and B is the coefficient of linear expansion of the material of which the tube is made. For glass, B= 0.000008, and for brass, 8=0.000019. From eq 35 for L20=200, L30-200.016 for glass and 200.038 for brass. It is evident that the errors resulting from changes in length of tubes of either material are negligible in ordinary use, but it is customary to apply the correction in all precision measurements.

(c) ACCURACY

All types of polariscope tubes are accepted by the Bureau for test. The tolerances adopted by the National Bureau of Standards for polariscope tubes are given in table 8.

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Glass which is not free from strain is doubly refractive, and on this account should not be used between the polarizing and analyzing systems of a polariscope. It is therefore of great importance that cover glasses be made of optical glass and thoroughly annealed. A strain in the cover glass is in effect a rotation, a cause of many baffling discrepancies that occur in polarizations. There is no infallible method of detecting this trouble after the glass is in use, the only safe procedure being to use cover glasses which have been tested, and to put as little pressure as possible on the screw caps. After a setting has been made, it is advisable to rotate the tube in the trough of the instrument if the halves of the field show variations in intensity, strain exists in one or both of the glasses. Strains may be caused by poorly fitting rubber washers. These should be made from the best quality rubber, soft and flexible. They should be made of such size that they lie flatly and evenly in the cap with no marginal elevation. Once a glass has been strained, it should not be used for several days or until test shows the absence of strain. All cover glasses must have plane, parallel surfaces, free from scratches, and should never be less than 1 mm thick. A thickness of 1.5 to 2 mm is preferable. The necessity for optically perfect glasses has not received the attention its importance demands. The cover glasses used in the laboratories of the National Bureau of Standards for Bates-type polariscope tubes conform to the following specifications:

"Cover glasses shall be made from clear, colorless optical glass, thoroughly annealed, free from strain, and shall show no optical rotation or double refraction when observed in a precision polarimeter. The surfaces shall be plane and parallel and be free from scratches. The edges shall be slightly rounded to prevent chipping. Plane parallelism shall be within 5'; thickness 1.85 ±0.15 mm; diameter 23.2 0.2 mm." The National Bureau of Standards will accept polariscope tube cover glasses for test.

3. VOLUMETRIC FLASKS**

(a) SPECIFICATIONS

(1) MATERIAL AND ANNEALING.-The material should be the best quality of glass, transparent, and free from bubbles and striae. It should have small thermal hysteresis and should adequately resist chemical action. All flasks should be thoroughly annealed before being graduated.

(2) DESIGN. The cross section of the neck must be circular, and the shape of the flask must be such as to admit of complete emptying

See test fee schedule 423, p. 553. **See test fee schedule 241, p. 558.

and drainage from the whole interior surface at the same time. The bottom of the flask should be slightly concave upward and should be of sufficient size to enable the flask to stand on a surface inclined at an angle of 15° to the horizontal. The neck must be cylindrical for at least 1 cm on each side of every graduation mark, but may be enlarged in the form of a bulb between graduation marks (for example, Giles flasks). At the graduation mark the inside diameters of the neck of the flask must be within the limits given in table 9.

TABLE 9.-Diameters of necks and tolerances for volumetric flasks

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(3) GRADUATION MARKS.-The graduation marks must be of uniform width, finely but distinctly etched, must be perpendicular to the axis of the flask, and must extend completely around the neck. On flasks having a capacity of 100 ml, or less, the graduation mark shall be not less than 3 cm from the upper end, nor less than 1 cm from the lower end of the neck, and on flasks having a capacity of more than 100 ml, the graduation mark shall be not less than 6 cm from the upper end, nor less than 2 cm from the lower end of the neck.

(4) UNIT OF VOLUME.-The unit of volume employed is the liter, which is defined as the volume occupied at the temperature of its maximum density (4° C) by a quantity of pure water having a mass of 1 kg. The water is under a pressure of 760 mm of mercury, and the weighings are reduced to vacuo. The one-thousandth part of the liter, called the milliliter (ml), is also employed as a unit of volume.

(5) STANDARD TEMPERATURE.-Twenty degrees centigrade has been adopted by the Bureau as the standard temperature for volumetric apparatus, and an extra charge is made for testing apparatus for use at other temperatures.

(6) INSCRIPTIONS.-Each flask must bear, in permanent and legible characters, the capacity in liters or milliliters, the temperature at which it is to be used, the method of use, i. e., whether to contain or to deliver, and an identification number. In the case of flasks with stoppers, the stopper must bear the same number as the flask, or, if standard interchangeable grindings are used, they must bear the proper identification marks. A suitable arrangement of the inscription is as follows:

No. 134
Contains

100 ml

at 20° C.

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