A Precision Melting Point Apparatus EMANUEL B. HERSHBERG, Converse Memorial Laboratory, Harvard University, Cambridge, Mass.
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the determination; nevertheless it is perfectly satisfactory when used with external heating.
HE determination of the melting point of organic compounds by the capillary method is at present complicated by an abundance of different types of apparatus, involving errors of varying magnitude and varying widely in sensitivity and ease of manipulation. For purposes of research involving new compounds it is imperative t h a t t h e data obtained shall be of a higher degree of accuracy than t h a t required for identification and this in turn demands an apparatus itself free from error. When this is accomplished i t follows t h a t manipulation will be easier and sensitivity correspondingly greater. From a consideration of types of apparatus and of h e a t i n g , m e d i u m of heat transfer, and thermometers, which have been discussed a t length in the literature (5), i t was apparent that a m e c h a n i c a l l y stirred and e l e c t r i c a l l y heated cyclic liquid bath was best. The Thiele melting point apparatus (8), one of the earliest cyclic d e s i g n s , has been modified b y Avery (I), Markley (Q), and Conte (W), t o permit mechanical stirring, and electrical heating has been recommended by Pratt (6),Dennis (S), Sando (7), a n d o t h e r s . No one apparatus, however, completely fills the demands of r e s e a r c h and this explains FIGURE 1. DIAGRAM OF some of the existing discrepAPPARATUS ancies in melting points. A . 28-mm. outside diameter and I n the ideal a p p a r a t u s 25-mm. inside diameter there should be a zone adiaB . 17 cm. C. 8.5 cm. batic relative to the external D. Sleeve 19-mm. outside diam&er. 17-mm. inside disurroundings and it should ameter; 9 om. long. Loops be possible to immerse the No. 26 B. and S. gage platinum wire m e r c u r y c o l u m n of the 18-mm. outside diameter E. cap, TherF . Thermometer t h e r m o m e t e r completely mometer tube 7-mm. inside without requiring excessively diameter U . Stirrer cap long capillary tubes. This is H . Stirrer 5-mm. outside diamet'er glass tubing. Ball closely approached by the b e a r i n i s with 6.61-om. apparatus described below, in (0.25-inch hole and 2 2 om (0.875-1nch) outside diame: which a t e q u i l i b r i u m the ter, unground I . Knobs to center sleeve. lateral or vertical temperaJ . Liu and wedge to urevent ro. ture deviation in the zone tation of cap e n c l o s e d by the sleeve is within *0.025" C. At the same time a bail-bearing glass stirrer ensures rapid circulation, so that with increasing temperature the rate of rise of this zone is essentially constant. The sleeve further serves to guide both thermometer and melting point tubes and in conjunction with the cap which acts as a support permits insertion of new tubes without removing the thermometer. Internal electrical heating is by far the niosksensitive and most readily controlled, but no organic liquid has been found that will withstand elevated temperatures without rapidly darkening. Sulfuric acid is undesirable in this connection because it is electrolyzed and the gas bubbles obscure 312
Apparatus The melting point apparatus (Figure 1) consists of a Pyrex glass cyclic bath, A , with a glass stirrer, HI running in ball bearings and externally heated by electricity. The portion in which the determination is made carries the soft glass sleeve, D, with sealed-on platinum loops which guide both thermometer and tubes. The sleeve forms an annular space which isolates the cooler wall liquid from that coming in contact with the instrument, and is centered by small knobs, I , near the top and bottom. Caps F and G fit snugly into the tubes, F having two small holes a t the base of the thermometer tube for the insertion of the melting point capillaries. It also has a small wedge which fits into a lip, J , in the vertical tube and prevents rotation due to the twisting action of the circulating liquid on the thermometer. Stirrer H is of glass tubing and has a two-bladed propeller driven by a small synchronous motor. It just clears tube A but rotates with ample clearance at the enlargement. It runs in two ball bearings which are slipped on over short lengths of thin-walled rubber tubing well moistened with glycerol. The liquid circulates in a clockwise direction. The heating element is wound on the curved portion, E, and consists of 300 cm. (10 feet) of No. 28 B. and S. gage wire with a resistance of 0.14 ohm per cm. (4.1 ohms per foot), using about 325 watts a t 115 volts. First a layer of 0.078-cm. (0.031-inch) asbestos paper is wound on the glass in segments, preferably while wet, and the surface is rubbed gently to make a perfect coating without cracks. .4fter this first layer has dried an appropriate length of cotton string (240 cm. or 8 feet) is wound on and evenly spaced. The wire is anchored to a brass terminal and wound on parallel t o the string, which is then removed. A
FIGURE 2. PHOTOGRAPH OF APPARATUS
JULY 15, 1936
ANALYTICAL EDITIOK
second layer of asbestos is put on in a similar manner and the remaining 60 em. (2 feet) of wire are wound back with wide spacing so that both leads come out on the same side. Finally a top layer of asbestos is put on. Connections are made to brass clamps shown in Figure 2. The current is controlled by a 200ohm rheostat of 3-ampere capacity or by an auto-transformer. Capillary melting point tubes are of conventional size and sufficiently long t o extend at least 0.93 em. (0.375 inch) below the top of the sleeve. They are supported by small steel clips of a type commonly used for display purposes, clamped t o short lengths of 0.3-em. (0.125-inch) thin-wall rubber tubing. The tube is slipped in one of the holes in the cap and guided through a platinum loop. The thermometer is supported by a narrow rubber band fastened near the tor, and rests on the central tube of cap F . INTERNAL HEATING.For this Dumose 180 em. (6 feet) of No. 32 B. and S. gage wire with a resihaice of 0.33 ohm per'cm. (10 ohms per foot) are wound into a spiral, annealed at red heat, and inserted in the curved portion so that one lead comes out the tube at the back (shown in Figure 2, but not in Figure 1) and the other comes out at the lip under cap F. In this case about 200 watts are ample for heating the tube and the same controls may be employed as above.
Operation The apparatus described is suitable for the conventional short Anschdtz enclosed-scale thermometers, but the over-all dimensions are not critical and may be varied over considerable limits. Sulfuric acid is added to a point level with the top of the upper horizontal connecting tube and the stirrer operates a t a speed just below the point a t which cavitation occurs. Using two identical thermometers and a special sleeve without platinum loops, the temperature difference between points 0.62 cm. (0.25 inch) from the top and bottom of the sleeve was measured a t equilibrium conditions and was found t o be approximately +0.025" C., or within the limits of accuracy of the thermometer reading. There was no observable lateral deviation from the center of the tube to the sleeve wall. Without the sleeve, however, the vertical deviation was about 0.2" and the lateral deviation from the center of the tube to the wall was considerably greater, depending on the closeness of the outer thermometer to the wall of tube
A. With external heating the bath rose from 40" to 200' C. in 7 minutes and t o 280" in 14 minutes. If the tube was externally heated a t the maximum rate and the current shut off atj loo", the temperature rose 7", while a t 250" it rose about 1". With internal heating a much smaller amount of power raised the temperature over the same interval in about 12 minutes and the maximum overshoot was only about 1' or 2". The highest temperature reached with concentrated sulfuric acid was about 295" C. and a t this point smooth, steady ebullition took place without bumping. With Nujol paraffin oil a s the bath fluid and heating internally with a 200-watt element, the temperature rose to about 310" C. Unfortunately the oil darkened rapidly and other organic fluids behaved similarly. The rheostat was calibrated for the equilibrium temperatures reached after about 0.5 hour a t a setting and this greatly facilitates subsequent determinations. The rate of rise may be readily controlled by setting the rheostat a t a pwdetermined number of degrees above the thermometer temperature, while maintaining this difference by continually decreasing the resistance. Gas heating also gave satisfactory results. I n this case the tube was heated a t the curved portion, E , after first winding on a single layer of No. 22 B. and S. gage copper wire.
Aclrnowledgment The author is indebted t o L. F. Fieser and M. S. Newman for helpful suggestions and criticisms.
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Literature Cited (1) Awry, IND.ENQ.CHEM.,20, 570 (1928). (2) Conte, Ibid., Anal. Ed., 2, 200 (1930). (3) Dennis, J. IND.ENQ.CHEM.,12, 366 (1920). (4) Markley, Ibid., Anal. Ed., 6, 475 (1934). (5) Meyer, Hans, "Analyse und Konstitutions-Ermittlung organischer Verbindungen," 5th ed., pp. 41-60, Berlin, Julius Springer, 1931. (6) Pratt, J. IXD.ENG.CHEM., 4, 47 (1912). (7) Sando, Ibid., Anal. Ed., 3, 65 (1931). (8) Thiele, B e y . , 40, 996 (1907). RECEIVED Apiil 15, 1936
A New Type of Laboratory Stirrer EMANUEL B. HERSHBERG Converse Memorial Laboratory, Harvard University, Cambridge, Mass.
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HE stirrers shown in the photograph have proved very
satisfactory for stirring mixtures thick with solid where the propeller type of stirrer is ordinarily useless. I n such instances as condensation reactions with sodium, the preparation of benzalacetophenone, the Blanc reaction, and some Friedel-Crafts reactions the stirrers shown have performed good service where other types failed. Unlike propellers, these stirrers can be made much larger than the mouth of the flask and the wires swung sufficiently to permit insertion as shown in the upper left-hand example. Once inside, they follow the contours of the flask without s c r a t c hi n g and are even more easily removed. Various alloy wires of No. 18 B. and S. gage may be used, depending on the type of reaction. I n particular, NEW TYPE OF LABORATORY STIRRER c 1-1 r o m i u m - n i c k e 1 a n d copper-silver alloys h a v e been found to be very resistant to attack and have the requisite stiffness and resilience. Stirrers of all types run more smoothly and for longer periods of time with a minimum of attention if ball bearings are used as guides. I n general two sets spaced a few inches apart are necessary and are best fastened by inserting a piece of rubber tubing with the inside well moistened with glycerol and then slipping the whole on the glass tubing as shown in the lower center of the photograph. Ordinary commercial unground ball-bearing assemblies 2.2 cm. (0.875 inch) in outside diameter and having a 0.94-cm. (0.375-inch) hole used in conjunction with 6.5- to 7-mm. glass tubing have proved a very satisfactory and inexpensive combination for general use. An occasional drop of oil is all the lubricationnecessary. RECEIYED April 9, 1936.
