Ralph J. Curtis and Althur Dolenga General Motors Reseorch Laboratories Warren, Michigan 48090
Low Temperature Melting Point Apparatus
During work with compounds and their mixtures which have melting points between -40.0 and +10.O0C, it became evident that it would be advantageous if the sample could he observed while in the temperature range of -65.0°C to room temperature. Because of extreme supercooling properties and tendencies to form metastable crystalline phases, the materials were extremely difficult to crystallize. As a result, much time was consumed in sample conditioning on the differential scanning calorimeter (Perkin-Elmer DSC-1B) only to find at the end of the test that the sample was not crystalline when the test was started. Differential scanning calorimetry and differential thermal analysis (DTA) are the only methods capable of measuring temperature-dependent transitions in the -100°C to room temperature range, hut, in both cases, the sample cannot he observed while the test is in progress. A recent puhlicationl describes an experimental instrument in which simultaneous DTA and microscopic examination under polarized light can he made, hut it is limited to applications above room temperature. A Nalge 25X melting point microscope with crossed polarizers2 or a Leitz-Wetzlar polarizing microscope with a Mettler microfurnace can be used with Dry Ice or liquid nitrogen cooling down to a temperature of about -2OSC, hut condensed moisture becomes a serious problem. None of the commercially available melting point instruments are usable below room temperature nor can they he modified for low temperature use. A melting point apparatus has been devised, using mostly standard laboratory equipment, which eliminates these shortcomings and permits direct observation of the sample over a temperature range of -65°C to room temperature. With back-lighting from a microscope illuminator and the use of a small magnifying lens (5-7X), the condition of the specimen can he observed a t all times. The apparatus shown in the figure consists of (1) a 300-ml capacity Dewar condenser3 with the top outlet closed off attached to (2) a 100-ml round-bottom flask containing a drying agent, (3) an L-shaped bracket for supporting a 10 x 75-mm test tuhe in a vertical position and (4) an alcohol-filled thermometer. With this arrangement, the interior surfaces of the condenser remain free of condensed moisture. The cold hath is prepared by adding Dry Ice in small quantities to about 175 ml of acetone in the condenser until the temperature drops to about -65°C. A second portion of acetone in a separate container is cooled with Dry Ice and added to the cold solvent in the condenser to raise the level to just below the sample holder for a total volume of 245 ml. When the Dry Ice has all sublimed, the hath is stabilized against sudden (and violent) foaming by adding boiling stones and stirring until a constant stream of bubbles rises from the bottom. During the test, the bubbles provide sufficient agitation to ensure uniform hath temperature. The sample container is a 10 x 75-mm test tuhe in which a small depression about 1.00 mm in depth has been made on the interior surface about 30 mm from the open end. Unmodified test tubes may he used for more viscous samples, while a 10 x 74-mm centrifuge tube4 is satisfactory for very fluid materials. The sample is prepared by clamping the flame-cleaned test tuhe
Dewor Condenser
IOOrnl Flask
With Drying Agent
LOWtemperature melting paint apparatus (side view),
(containing a few granules of drying agent) in a horizontal position and carefully placing a small drop of sample in the depression. When the centrifuge tuhe is used, the sample is placed in the bottom by means of a microsyringe. The test tuhe is then tightly stoppered, cooled with Dry Ice or liquid nitrogen to crystallize the sample, and transferred to the cold acetone bath. In an apparatus having the dimensions shown in the figure, the initial rate of temperature rise (from -62°C) is about O.7"Cjmin which decreases to about 0.3'Cjmin as the temperature approaches 10°C. At the melting point when the sample hegins to flow, the tube can he removed and placed in a borizontal position so repeat determinations can be made. Calibration runs using the centrifuge tuhe sample container have been made with benzene (mp, 5.5"C), n-decane (mp, -29.7'C), acetonitrile (mp, -45.7"C), water and carbon tetrachloride (mp, -23.0°C). In all cases, melting occurred at the expected temperature (&0.5"C) and, in addition, the solid-solid transition of carbon tetrachloride a t -47.4"C5 was observed as a change from a dense white solid to coarse transparent crystals.
'Kunihisa, K. S., Bull. Chem. Soc Jap., 46,2862 (1973).
Available from VWR Scientific, Cat. No. 36981-050. Constructed by the Glass Technology facility at the GM Research Laboratories from standard size Pyrex tubing. Condensers of this type are available from several sources but are of different sizes, and their suitability in this application has not been investigated. *Available from Kimble Division of Owens-Illinois Co., Plain Micro Centrifuge Tube, Cat. No. 45156. 3Stavely, L. A. K., and Gupta, A,, Trans. Faraday Soe., 45, 50 (1949).
Volume 52,Number 3, March 1975 j 201
