SEMIMICRO CRYOSCOPIC MOLECULARWEIGHT DETERMINATIONS A Simple Modijkation of the Rast Techniques Using Capillary Tubes A. G. PINKUS and 8 . G. BARRON Baylor University, Waco, Texas
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present suggestions eliminate two sources of 4 shows another method of shaping the capillary in difficulty encountered by the student in carrying out order to accommodate the thermometer. A side cryoscopic (Rast) molecular-weight determinations as view of this is shown in Figure 5. Figure 6 similarly usually described (1-4). The first of these difficulties shows how the hook-shaped capillary (Figure 2) after is the thick glass lump usually obtained in sealing the being filled and sealed off can be beut and attached to end of the capillary which is in contact with the sample the thermometer. These designs also permit a thorough mixing of solbeing determined. It is, of course, possible to blow out the end, but this is difficult for the average beginner. vent and solute in the melt by proper manipulation. This lump causes a temperature differential between The modifications described have been tried in a quanthe melting-point bath and the part of the sample in titative organic course and by research students and contact with the lump. Since the crystals are usually found to yield consistent and satisfactory results. Smaller bore capillary tubes of the same desigu are heavier than the melt, they sink to the bottom and a t this point they are in contact with the lump. This also suitable for accurate routine melting point deterresults in the recorded temperature being high, depend- minations. ing on the thickness of the glass lump. The second dif- ACKNOWLEDGMENT ficulty is that in trying to make the lump as small m The authors wish to acknowledge helpful discussions possible, an opening is often inadvertently left which with Dr. William B. Cook of Baylor University. allows the bath liquid to creep in (especially at higher temperature where the viscosity is less) and ruin the LITERATURE CITED determination. (1) CHERONIS,N. D., "Micro and Semimiera Methods," Interscience Publishers, Inc., New York, 1954, pp. 211-2. The designs found to be the most convenient are J. B., AND V. NIEDERL,"Micromethods of Quantithose shown. Figures 1 an$ 2 show two satisfactory (2) NIEDE~L, tative Organic Analysis," 2nd ed., John Wiley Ri Sons, capillary designs before being filled with solvent and Inc., New York, 1942, p. 218. sample. The latter can be introduced by using the (3) PREGII, F., A N D J. GRANT,"Quantitative Organic Microanalysis," 5th ed., Blakikiston Company, Philadelphia, conventional inner capillary tube and plunger (1-4) 1951, pp. 239-40. or by means of a micro-pellet. A,, "Quantitative Organic Microanalysis." Figure 3 shows the U-shaped capillary after being (4) STEYERMARK, Blakiston Company, New York, 1951, pp. 287-9. filled, sealed, and attached to the thermometer. Figure
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