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 an 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
