Modified Hershberg Melting Point Apparatus Helmut
E.
Drechsel, Smith, Kline & French Laboratories, Philadelphia, Pa.
methods and types of apparatus are available for determining melting points of organic compounds. One of the most popular is the capillary tube method employing an apparatus designed by Hershberg [IND. EKG.CHCM.,A K ~ LED. . 8, 312 (1936)], or some modification of it. In most cases the bath liquid is heated with a small gas flame or an insulated electric heater applied to the outer surface of the apparatus. Not only is an open flame a hazard in the laboratory, but by using it or an external electric heater, it is difficult to attain a desirably slow and constant rise in bath temperature when approaching the melting point. The factors influencing the reproducibility of a melting point are well known (Weissberger, A., ed., "Technique of Organic Chemistry," Vol. I, "Physical Methods of Organic Chemistry," Pt. I, 2nd ed., pp. 49 ff., Interscience, New York, 1949). ,4 modified Hershberg apparatus reduces these factors to a minimum, and allows the corrected melting point of a substance to be read directly from the thermometer calibrated in the apparatus. UMEROUS
The apparatus (Figure 1) consists of a U-shaped tube with a connecting tube between the two arms. The larger arm is twice the diameter of the smaller and is filled with approximately 175 ml. of silicone oil (silicone fluid, General Electric and Dow Corning). The smaller arm contains an agitator of the spiral or corkscrew propeller type with a clearance of 1 mm. between the blades and the walls of the tube. This permits thorough agitation without a vortex or bubbles in the oil, which decrease visibility when the melting point is being read. An unsheathed heating coil of Xichrome wire is located in the bottom of the U. Acceleration of heating is constant, depending on the voltage input supplied through a variable transformer. Figure 2 shows the voltage and time needed to obtain various temperatures. In normal use a setting of 10- t o 55-volt input will give a temperature range of 4 5 " t o 320' C. A holder for melting point capillaries is held in the larger arm of the U by a ground joint. A small glass sleeve or bushing is used in place of the conventional glass rings for keeping the melting-point capillaries in place. These tube-guides are sturdy and prevent the tubes from vibrating while the agitator is running. One thermometer covers the range 0" to 350" C. The thermometer is made from top quality material and is completely annealed to relieve all
strains, thus ensuring repeated accurate temperature readings. The immersion and emergent stem corrections are eliminated by calibration of the thermometer blank every 50" of the scale from 0" to 300" in the apparatus and under actual operating conditions. For convenience, two immersion rings engraved on the thermometer stem designate maximum and minimum immersion attained through expansion of the silicone oil during heating. Calibration points on the thermometer are established with a series of National Bureau of Standards certified registered thermometers, and are checked a t various points on the scale with certified U. S. Pharmaconoeia reference melting point compdunds (Table I). In the initial temperature tests four
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PART-D THfRMOMETf? 0 TO 350' 4 STEM
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.i PART A TUBE HOLD>,
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thermometers were made to cover the ranges 30" to 110", 110' to 190", 190" to 270", and 2'70" to 350" in 0.5" divisions. For calibration the blank thermometer was placed in the thermometer holder and the standard was fixed in the larger arm of the U-tube, so that the depth of immersion r a s thc same in each instance. The position of the immersion ring was marked on the blank thermometer. The bath was then heated to the desired calibration point-e.g., 30°-as determined by the corrected temperature of the standard thermometer and the height of the mercury column in the blank, measured in millimeters. Four reference points were thus established on each blank, from which the scale for each thermometer was engraved. The finished thermometers were checked against USP reference melting point compounds (Table I). These four thermometers were then used as standards, with the temperature
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-PART-0 ADAPTER
CHROMEL WIRE SlZf * 3 1 0 2 8 " . 110 v. CHROMEL WIRE SOLDERED TO A 04O'COPPER L E A D WIRE
2 KOVAR METAL TUBE TO GLAS? O D 075 WALL THICKNESS
Melting point apparatus
Table 1.
IO
14
18
22
26
30
IN M I N U T E S
Figure 2. Chart o f voltage input from transformer, in relation to temperature rise per minute fcr Drechsel melting point apparatus
Check o f Calibrated Thermometers
US: RI. P., Substance Vanillin Acetanilide Acetophenetidin Sulfanilamide Sulfapyridine Caffeine Oxanilide Anthraquinone
6
TIME
010"
Figure 1.
,
20-,
Thermometer
C. 81 - 83 114 -116 134 -136 164.5-166 5 190 -193 235 - 2 3 7 . 5 250 286 subl.
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KO.
M.P_Found, C.
A
1 2 2 2 3 3 3 4
81 - 83 114 -116 133.5-136 163.5-165 5 190 -192 235 -236 219.5-250 284 -284.5
0 0 0 +l 0 0 $0.5 +1.5
VOL. 2 9 , NO. 5, M A Y 1957 e
859
correction applied as found in Table I, to calibrate all additional melting point apparatus thermometers. A thermometer with a range of 0" to 350' C. in 1" divisions calibrated every 50" of the range was then used in the actual apparatus, Constructional Details. Figure 1 shows the assembled melting point apparatus with all parts in place.
The Kovar metal to glass seal is made with the following Corning glasses: S o . 774 (Pyrex) to 3320 (Uranium) to 772 (Nonex) to 7052 (Kovar sealing glass). The Chrome1 wire is silver-soldered to a 0.40-inch copper lead, and the copper lead wire is soldered into the Kovar metal tube, also with silver solder. The leads connecting the appa-
ratus and the variable transformer are made from a braided 0.40-inch copper wire covered with plastic insulation tubing to give flexibility, and a suitable electric plug is attached to fit the variable transformer. A is the melting point capillary tube holder with four small guides to support the capillary tubes. The 19-mm. disk a t the bottom was cut from a section of borosilicate glass rod 19 mm. in diameter. The two air holes in the tube holder are important, as they relieve the pressure in the apparatus as the silicon oil expands. ' B is the adapter for the spiral stirrer; this serves as a guide for the stirrer. Enough clearance should be present between the adapter and stirrer shaft to permit a pressure release as the silicone oil is heated.
C is the stirrer with the spiral blade, The blade is made from a section of I/ginch thick borosilicate plate glass cut 20 mm. in width. When the spiral is fabricated it will narrow about 2 mni. to a finished product of 18 mm. in width. It is important that the spiral be fabricated so as to circulate the silicone oil in a counterclockwise direction, so that the silicone oil is pulled through the agitator and down through the test chamber. D is the test thermometer. Figure 2, a voltage input chart, shows voltage required in relation to a temperature rise per minute. A patent on this apparatus has been applied for; the apparatus is manufactured by Kontes Glass Co., Vineland, ? J i, 1
Device for Orienting Small Crystals under the Microscope Betty J. Steinbach' and Thomas R.
P. Gibb,
optical characterization of small I crystals is greatly expedited by device which permits a single crystal or THE
a
fragment to be oriented in various nays under the polarizing microscope, particularly when the crystal has a preferred habit or cleavage-for example, boric acid-or only a single small crystal is available. The novice finds it much easier to visualize the optic directions and to measure optical properties with the aid of such a device, as the crystal may easily be turned to show centered or a t least recognizable interference figures. The axial rotation stage (available from Kenneth A. Dawson Co., Belmont, Mass.) was developed in the hope of satisfying the need for a crystal-orienting device intermediate between makeshifts of limited applicability (9) and the intricate universal stage of Federov ( I ) . The device described permits orientation on three axes of rotation, allowing continuous examination of interference figures and usually measurement of all indices of refraction. Rotation about one of these axes is limited to 20" or 30°, depending on the objective used. Nevertheless, this limited rotation is a desirable feature, as it frequently eliminates the need for precise alignment of the crystal with the shaft. It is particularly desirable as a means of obtaining centered figures. In the construction of the stage (Figure 1) the following specifications are essential. The center of rotation of the yoke, F , must be slightly more than the thickness of a microscope slide above the top of the supporting plate A , as the 1 Present address, Massachusetts Institute of Technology, Cambridge, Mass.
860
ANALYTICAL CHEMISTRY
Jr., Tufts University, Medford, Mass.
crystal must be a t the center of rotation of the yoke and must also be immersed in an index of refraction liquid. The liquid is contained by capillary attraction in a cell which consists of two small pieces of microscope slide about 0.5 X 0.3 em. cemented to a slide about 2 mm. apart and covered by a fragment of a cover glass. If the center of rotation of the yoke is too high, the crystals may not be close enough to the condenser to give a good interference figure. A segment, G, of a protractor is mounted on the base beside the yoke
I
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which carries a pointer, H . A second segment, J , is mounted on an extension of the yoke. The pointer, I , on the shaft is mounted on a sleeve and may be positioned by means of the setscrew. J need cover only 180" because, if the pointer is properly set, both melatopes and both bisectrices of an interference figure can be brought to the cross hair without going off the scale. If the 180" scale is used, the side extension of the yoke need not be as long as illustrated, as it need not extend beyond the edge of the microscope stage. Brass stock inch thick was used for the base, B , and yoke; the axle mas made of '/Isinch drill rod, machined down to 0.030 inch a t the tip.
A similar instrument, lacbing the third axis of rotation, has been described by Wood and Ayliffe (S), who also describe a method for mounting crystals in a preferred orientation by means of a goniometer. This is seldom necessary with the device described here. Small crystals are simply "picked up'' from a flat surface by the tip of the shaft, which is previously touched to a tiny drop of Pliobond cement. The authors acknowledge the assistance of Andrew Levesque, who constructed the device and contributed materially to its design. I
Figure 1. Top and side views of rotation device and glass cell A . Baseplate B. Base C . Crystal mounted on shaft above cir-
cular aperture in base plate D. Pivots (setscrews with conical end) E. Shaft F. Yoke G. Segment of protractor H and I . Pointers J . Protractor K . Sleeve with set screw attached to pointer I L. Slots to accommodate stage clips M. Cell
LITERATURE CITED
(1) Federov, E. S., 2. Krist. 27, 337 ( 1897 ). (2) Gibb, T. R. P., Jr., "Optical Methods of Chemical Analysis," Chap. VI, McGraw-Hill, New York, 1942. ( 3 ) Wood, R. G., Ayliffe, S. H., J . Sci. In&. 12, 194 (1935).
EXCERPTED from a thesis submitted in partial fulfillment of the requirements for the B.S. degree at Tufts University. Contribution 237 from ,Department of Chemistry, Tufts University.
