An Electrically Heated Melting Point Apparatus EDWIN DOWZARD AND MICHAEL J. RUSSO, The New York Quinine & Chemical Works, Inc., Brooklyn, N. Y.
point thermometer and capillary), centered in the 80 X 180 mm. tube, and a fourth hole to insert a 0' to 250" C. thermometer midway between the tubes. This board is put in place, the leadin tubes are inserted, and it is then "tacked" down to the main portion of the cover in several spots with water glass. Number 14 gage rubber-covered wire is used throughout t'he electrical circuit, connection being made to the Nichrome ribbon by means of brass wire connectors. The electrical input to the heating coil is controlled by means of a 5-ampere Allen Bradley radiostat and measured by an ammeter having a range of 0 to 5 amperes in 0.2 ampere divisions. For safety and convenience, a toggle switch and a IO-ampere fuse are inserted in the circuit. In order to make the apparatus a complete unit, capable of being moved about the laboratory quickly and easily, a base 40 X 27.8 X 18.75 em. (16.375 X 11.125 X 7.5 inches) was constructed w i t h 1.25-cm. (0.5-inch) angle iron. This base served t o supp o r t not only the jar but all the other e l e c t r i c a l equipment as well. As neither the size nor shape of this base affects the o p e r a tion of the apparatus, it may be varied t o s u i t t h e indiv i d u a 1 ' s requirements. However, a very definite idea of its c o n s t r u c t i o n m a y b e obtained from Figure 2.
W H E melting
A
1point
apparatus h e r e i n d e scribed has been
found very satisfactory for routine laboratory testing. Its freedom from l i q u i d s and moving parts assures continuous service over e x t e n d e d p e r i o d s of t i m e w i t h o u t adjustments of any sort. R e s u l t s closely simulate those obtained with large, mechanically stirred, liquid baths. It is suitFIGURE 1. MELTINGPOINT APPARATUS able for the determination of melting points up to about 310" C., with a reproducibility of within 0.5' C.
Apparatus A round battery jar (Figure l), about 150 X 230 mm., is fitted with a cover composed of four disks of asbestos board, each 5 mm. thick and 150 mm. in diameter, cemented together by means of water glass. The diameter of the bottom disk is reduced so that it fits snugly into the jar. Two small holes, about 25 mm. apart, are drilled 12 mm. from the outer circumference of the cover to take two Pyrex tubes 3 mm. in inside diameter. In addition to these holes, a tapering hole is cut in the center of the cover to hold an 80 X 180 mm. Pyrex tube, closed at one end, firmly centered in the jar. This tube has two 3-mm. holes, 12 mm. apart, drilled 25 mm. from the top, and two similar holes just at the start of the curve at the bottom. The tube is set into the hole in the cover and then wound with five equally spaced turns of Nichrome ribbon 0.15 X 0.0125 cm. (0.06 X 0.005 inch), the ribbon being anchored by means of the small holes at the ends of the tube and then brought up through the glass tubes in t'he cover to serve as lead-in wires. The individual turns of ribbon about the tube are securely fastened in place by spotting each turn with a drop of paste made of short asbestos fiber and water glass. The holes in the 80 X 180 mm. tube and the ends of the lead-in tubes are also sealed with this paste. A fifth disk of asbestos board, similar to those already described, is drilled to take the two small lead-in tubes, a hole to support a Pyrex test tube, 18 X 150 mm. (to hold the melting
Operation T h e manipulation of the appaFIGURE 2. PHOTOGRAPH OF MELTING ratus not POINTAPPARATUS e n t a i l a n y difficulty after a few trial runs are made to familiarize the operator with its behavior. The capillary, containing the sample, is attached to an Anschutz thermometer of appropriate range and set into the inner test tube, the thermometer being held in place by means of a cork stopper with a very small slit up the side to allow for air expansion within the tube. The current is then turned on and the input regulated so as t o attain a rapid increase in temperature until a point approximately 25" below the supposed melting point of the substance is indicated on the auxiliary thermometer. The input is then reduced until a steady rise of about 3" per minute is attained in the inner tube. As the melting point
FOR GWEKIKPUT TABLE I. TEMPERATURE
Elapsed Time &%92,
0 5 10 15 20 25 30 35 40 45 50 55 60
65 70
1-Ampere Input Temp. Temp, rise C./min. C. 26.0
.. ..
.. 4 i :o 42.4 43.6 44.8 45.2 45.7 45.8 46.0 46.2
0:28 0.24 0.24 0.08 0.10 0.02 0.04 0 04
2-Ampere Input Temp. Temp. rise C./min. C. 26.0 4810 62.4 72.8 79.4 84.2 87.6 90.5 92.6 94.5 95.5 96.2
.. ..
.. .. 2:88 2.08 1.32 0.96 0.68 0.58 0.42 0.38 0.20 0.14
3-Ampere Input Temp. rise Temp. O
c.
C./min.
24.5 41.5 79.0 108.0 127.0 139.1 146.5 151.5 155.0 157.0 157.2 157.4
3:4 7.5 5.8 3.8 2.4 1.5 1.0 0.7 0.4 0.04 0.04
...
, .
...
...
74
4-Ampere Input Temp. rise Temp. O C./min. O
c.
28.5 63.0 131.5 173.0 191 6 203 0 211 0 217 2 221 5 224 2 224 5
..
c.
6.9 13.7 8.3 3.7 2.3 1.6 1.24 0.86 0 54 0.08
...
...
...
...
...
. . 1
..
C./min.
...
...
..
5-Ampere Input Temp. rise Temp.
...
JANUARY 15, 1936
ANALYTICAL EDITION
is approached the input is further gradually reduced until, at a point about 3" below the supposed melting point of the substance, a temperature rise not exceeding 0.75" per minute is
obtained.
Table I, listing temperature for a given input a t various elapsed times, should be considered as being only approximate. The given temperatures are those indicated on an Anschutz thermometer placed in the inner test tube. KOaccount was taken of the lag of the apparatus a t any particular point. During the preliminary rapid rise of temperature, up to about 200' C., this lag is a noticeable and variable factor, depending mainly upon the rate of temperature rise and must be taken into account for accurate temperature control. A close approximation of its magnitude may be obtained by placing a long-range thermometer midway between the tubes, as is illustrated in Figure 1. As indicated in Table I, temperatures up to about 284" C., with the correct rate of temperature rise a t that point, may be obtained by using 5 amperes. However, the time required to reach this point may be considerably shortened and the useful range of the apparatus increased to approximately 310" C., a t the expense of a slight overload, by using 5.33 amperes.
Experimental It having been stated (1) that melting point apparatus employing a n air bath yields results that are consistently higher than those obtained in a liquid bath, it was deemed advisable to check the authors' apparatus against one employing a bath of phosphoric acid, previously heated as directed by Snell ( 2 ) to determine the magnitude of this variation under present conditions. The bath, having a volume of 1.5 liters, was electrically heated and mechanically stirred during each determination. The rate of temperature rise, in all cases, was between 0.5" and 0.75" C. per minute
75
a t the melting point. The temperature a t which the substance became a clear fluid throughout was defined as the melting point. The U. S. P. X defines this as the "end of melting." The melting points as determined in the authors' apparatus (first figure) and by the modified Snell method (second figure) were as follows: salol 42.6" and 42.4", acetanilide 115.O'and 114.5', phenobarbital 176.0" and 176.0", saccharin 225.0' and 224.5", phenolphthalein 261.0' and 260.8". The results indicate that the melting points obtained in the air bath are slightly higher than these obtained in the liquid bath. However, as the apparent melting point of a substance is a somewhat variable figure, depending not only upon the diameter and thickness of the capillary tube used and upon the slight variations in filling the tube but also upon the operator, a second series was obtained by means of the authors' apparatus to determine how closely triplicate determinations would check. All figures were obtained by one operator on successive days. The minimum and maximum values of the observed melting points were found to be as follows: salol 42.5" and 42.6", antipyrine 110.5"and 111.0", phenobarbital (The phenobarbital used in this series was drawn from a lot other than that from which the material used in the previous determination was taken.) 174.8" and 175.1", caffeine 235.0" and 235.4', phenolphthalein 260.7" and 261.0'. The maximum difference obtained was 0.5". If this figure is taken as the maximum experimental error, the differences in results obtained with the described apparatus and the phosphoric bath are insignificant.
Literature Cited (1) Markley, K. S., IND.ENG.CHEM.,Anal. Ed., 6, 475 (1934). (2) Snell, F. D., Ibid., 2, 287 (1930).
RECEIVED October 19, 1935.
Prevention of Foaming in Crude-Fiber Determinations H. W. GERRITZ, Division of Chemistry, Agricultural ExperimenL Station, Pullman, Wash.
T
H E mechanics of crude-fiber determinations is made tedious by a tendency of Inany materials t o foam during digestion. The custom of breaking the foam by blowing through an auxiliary tube or through the condenser requires the constant attention of the analyst. The author has found that a fine jet of air projected on the center of the boiling liquid is as effective in dispersing the foam
as a larger current of air. Furthermore, if proper precautions are taken, a fine jet of air niay be projected on the surface of the digesting material throughout the digestion period without increasing evaporation. The diagram illustrates a convenient arrangement of apparatus for routine crude-fiber determinations, in which compressed air is let into the digestion flasks by means of capillary tubes through the condensers. Tube A is a compressed air supply from a laboratory pump to the apparatus connected with a capillary tube, C, through stopcock B. Tube C may be of any suitable diameter, but is drawn out at the tip t o a diameter of about 1 mm. To prevent possible breakage, tube C is made just long enough to reach the bottom of condenser D. A stream of air is thus projected onto the surface of the digest in flask E. The author has used this apparatus for more than a hundred determinations consisting of mixed feeds and packing-house residues, Analyses were made by the official method (1). There was a great saving of time on the part of the analyst, and good results were obtained.
Literature Cited (1) .4ssoo. Official Agr. Chem., Official and ed. (1930).
RECEIVED October 28, 1935.
Tentative Methods, 3rd
