Modified Hershberg Melting Point Apparatus

Helmut E. Drechsel, Smith, Kline & French Laboratories, Philadelphia, Pa.

UMEROUS methods and types of apparatus are available for deter-

mining melting points of organic com- pounds. One of the most popular is the capillary tube method employing an apparatus designed by Hershberg [IND. EKG. CHCM., A K ~ L . ED. 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.

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 Elec- tric and Dow Corning). The smaller arm contains an agitator of the spiral or corkscrew propeller type with a clear- ance 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 visi- bility when the melting point is being read.

An unsheathed heating coil of Xi- chrome wire is located in the bottom of the U. Acceleration of heating is con- stant, 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 conven- tional glass rings for keeping the melt- ing-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 elim- inated by calibration of the thermometer blank every 50" of the scale from 0" to 300" in the apparatus and under actual operating conditions. For con- venience, two immersion rings engraved on the thermometer stem designate max- imum and minimum immersion attained through expansion of the silicone oil dur- ing 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

PART-D

n THfRMOMETf? 0 TO 350' c 4 S T E M

.i P -PART-0 I ADAPTER

PART A - p TUBE HOLD,,> l

CHROMEL WIRE SlZf * 3 1 0 2 8 " . 110 v.

CHROMEL WIRE SOLDERED TO KOVAR METAL TUBE A 04O'COPPER TO GLAS? LEAD WIRE O D 075

010" WALL THICKNESS

Figure 1. Melting point apparatus

thermometers were made to cover the ranges 30" to 110", 110' to 190", 190" to 270", and 2'70" to 350" in 0.5" divi- sions. For calibration the blank ther- mometer 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 thermom- eter. The bath was then heated to the desired calibration point-e.g., 30°-as determined by the corrected tempera- ture 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

2 0 - , , 2 6 IO 14 18 22 26 30 T I M E IN MINUTES

Figure 2. Chart o f voltage input from transformer, in relation to temperature rise per minute fcr Drechsel melting point apparatus

Table 1. Check o f Calibrated Thermometers US: RI . P., Thermometer M.P_Found,

Substance C. KO. C. A Vanillin 81 - 83 1 81 - 83 0 Acetanilide 114 -116 2 114 -116 0 Acetophenetidin 134 -136 2 133.5-136 0

Sulfapyridine 190 -193 3 190 -192 0 Caffeine 235 -237 .5 ' 3 235 -236 0

Sulfanilamide 164.5-166 5 2 163.5-165 5 + l

Oxanilide 250 3 219.5-250 $0.5 Anthraquinone 286 subl. 4 284 -284.5 + 1 . 5

VOL. 29 , NO. 5, MAY 1957 e 859

correction applied as found in Table I, to calibrate all additional melting point apparatus thermometers. A thermom- eter 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-sold- ered 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 vari- able 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 diam- eter. 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 be- tween the adapter and stirrer shaft to permit a pressure release as the silicone oil is heated.

Device for Orienting Small Crystals under the Microscope

Betty J. Steinbach' and Thomas R. P. Gibb, Jr., Tufts University, Medford, Mass.

THE optical characterization of small I crystals is greatly expedited by a device which permits a single crystal or fragment to be oriented in various nays under the polarizing microscope, particu- larly 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., Bel- mont, Mass.) was developed in the hope of satisfying the need for a crystal-orienting device intermediate between makeshifts of limited applica- bility (9) and the intricate universal stage of Federov ( I ) . The device described permits orientation on three axes of rotation, allowing contin- uous examination of interference figures and usually measurement of all indices of refraction. Rotation about one of these axes is limited to 20" or 30°, de- pending on the objective used. Never- theless, 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 (Fig- ure 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 Insti- tute of Technology, Cambridge, Mass.

860 ANALYTICAL CHEMISTRY

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 attrac- tion 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 ( __- - - 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

C is the stirrer with the spiral blade, The blade is made from a section of I/g-

inch 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 fabri- cated so as to circulate the silicone oil in a counterclockwise direction, so that the silicone oil is pulled through the agi- tator and down through the test cham- ber.

D is the test thermometer. Figure 2, a voltage input chart, shows

voltage required in relation to a tem- perature rise per minute.

A patent on this apparatus has been applied for; the apparatus is manufac- tured by Kontes Glass Co., Vineland, ?i, J 1

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 assist- ance of Andrew Levesque, who con- structed the device and contributed materially to its design.

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.