W. R. Wilcox
Aerospace Corporation El Segundo, California
Melting Point Techniques for Volatile
I t is often necessary to determine the melting and freezing points of milligram quantities of mixtures as, for example, in the determination of phase diagrams or in the analyses of fractional crystallization results. Many satisfactory techniques for taking melting points of nonvolatile mixtures are available. However, when one or both of the components are volatile, considerable difficulty is encountered. I n the usual melting point techniques, the container used for the mixture either is open or has a vapor space with a temperature gradient. Consequently, when these techniques are used for volatile mixtures, sublimation occurs. This causes a change in the composition of the mixture and can yield regions of widely diiering concentration. Erroneous melting point data are therefore obtained. Two techniques for avoiding volatilization while taking melting points of small quantities of a mixture are described here. Hot-Stage Technique
A useful method for taking melting points and for observing melting and freezing phenomena utilizes the microscope hot stage.'J The sample is placed on a microscope slide and covered with a cover glass. Volatilization during measurements can be avoided by use of the following p r ~ c e d u r e : ~The slide with sample and cover glass in position is placed briefly on a very hot hotplate. When melting begins, the slide is immediately removed and placed on a wooden table at room temperature. Melting is normally complete by this time, and a gentle, but firm, pressure is applied to the cover glass with a metal rod or tweezers to force excess melt from under the cover glass. When properly done, the melt at this point nucleates and rapidly freezes. By this procedure, the vapor space is effectively removed from under the cover glass. At this point, excess solid is removed from the surfaces of the slide and cover glass with a razor blade, and nonporous cement is applied in a moderately thick layer around the edge of the cover glass to prevent volatilization from under the cover glass. The cement used for this sealing must be thermally stable and must This work was supported by the US. Air Force under Contract No. A F 04(695)-469. ' MCCRONE,W. C., JR., "Fusion Methods in Chemical Microscopy" Interscience Publishers, Inc., New York, 1957. Microscope hot stage available from Arthur H. Thomas Co., Philadelphia, Pennsylvania. WILCOX, W. R., "Fractions1 Crystallization from Melts," Ph. D. Thesis, University of California, Berkeley, 1960; also Report No. UCRL-9213, Lawrence Radiation Laboratory, Berkeley, California, 1960. ' Mmufact,ured by Thermon LManufacturing Co., Houston, Texas.
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Journal o f Chemical Education
be inert with regard to the sample. Both Thermon' and fast setting epoxies have been found suitable for use with organic mixtures. After sealing and thorough hardening of the cement, the melting point of the sample is taken in the usual manner. Polarized light, with a crossed analyzer, has been found very useful for distinguishing solid and liquid phases of many organics. Melting points are reproducible to about 0.2'C. Melting Point Block Technique
A new melting point technique suitable for volatile mixtures has been developed. The apparatus is shown in Figures 1 and 2. The sample is contained in a 1mm or smaller id capillary tube. The capillary is sealed with a torch to a length of about 3 cm. The sample is melted and rapidly frozen in the bottom of the tube to consolidate it. To take the melting point, the capillary is placed in a slot in the specially designed melting point block. The slot is machined for a close fit with the capillary tube. A thermometer (or thermocouple) well is located just below and a t right angles to the slot. Melting points obtained at heating rates
k-
1
4 in, diom
4
SLOT FOR CAPILLARY,
Figure 1.
Front view of m e l d block.
Figure 2.
Assembled melting point block.
of O.l0C/min or less are reproducible to 0.1"C with ASTM 0.2°-division thermometers. The melting point block is constructed of brass or other highly conducting metal. Nichrome heating wire is wrapped in two layers around the sides and bottom so as to give roughly uniform heating. Total resistance is about 35 ohms. Electrical insulation of the heater wires is provided by alumina cement. Alumina insulating bricks are cut to fit around the body of the block to provide thermal insulation. The outside dimensions of the insulated block are 23 X 23 X 13 cm. Power input control is provided by an ordinary
variable auto-transformer. After placing the capillary tube in the slot, the capillary is adjusted to center the sample, usually with the aid of reference lines on the block. Insulation on the upper surface of the block is provided by glass plates of a total thickness of about 11 mrn. Even with these precautions, difficulty may be encountered with sublimation and condensation on the upper surface of the capillary. This final difficulty can be avoided by proper choice of the wattage and location of the viewing lamp or lamps. A 7X magnifying glass permits easy and comfortable viewing of the sample.
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