Microboiling and Melting Point Determination by Differential Thermal Analysis G . Kerr and P. S. Landis Mobil Research and Development Corp., Central Research Division, Princeton, N. J. THEEXTENSIVE USE of gas-liquid chromatography to obtain very small samples of pure compounds on which physical and spectral properties are measured prompts us to report a simple, rather obvious technique for obtaining boiling points on micro samples. Micro techniques for the determination of boiling points are described in the literature (1, 2). Indeed, the use of differential thermal analysis to study phase transitions has been previously described (3-5). However, these reported techniques require samples of at least 20 to 100 pl. It is apparently not widely recognized that reliable boiling point data can be obtained by differential thermal analysis of 2- to 5-111 samples. Samples of this size are trapped at the exit ports from gas-liquid chromatography columns and transferred with a 10-pl syringe to the standard capillary of a DuPont Model 900 differential thermal analyzer. The reference thermocouple was placed in a high boiling silicone oil. Typical results on a variety of organic compounds of diverse boiling points are given in Figure 1. The literature values for the boiling points are given under the names. In all cases the sample was heated from room temperature t o the boiling point at a rate of 10 “C per minute. The boiling point
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range is taken as the temperature span of the descending portion of the endotherm, which reflects the heat of vaporization of the liquid. The literature values for boiling points compare favorably with the observed points of maximum inflection. A typical analysis requires 10 to 20 minutes. The procedure is adaptable to the use of vacuum so that boiling points under reduced pressure can be determined. An example of melting point determinations on 2-pl samples is afforded by the m-xylene-p-xylene system. Figure 2 shows the experimental points determined by D T A on mixtures of known composition. These two hydrocarbons are eluted simultaneously from most gas-liquid chromatographic columns. Our results show that micromelting point can be used to determine whether the eluent is essentially pure (9Ox or better) m- or p- or a mixture of each. In addition, the melting point of the eutectic can be determined. Indicative of the sensitivity of the instrument is the detection of the melting point of the eutectic in the 90% p-xylene mixture: at this concentration, approximately 0.0002 ml of eutectic is present. Refinements in measurements are possible by careful temperature programming and instrument standardization. ACKNOWLEDGMENT
(1) N. D. Cheronis, “Micro and Semimicro Methods” in “Techniques of Organic Chemistry,” Vol. VI, A. Weissberger, Ed., Interscience, New York, 1954, p 180. ( 2 ) M. C. Chaco, J. Clzem. Educ., 44,474 (1967). (3) E. M. Barrall, R. S. Porter, and J. F. Johnson, ANAL.CHEM., 37, 1053 (1965). (4) D. A. Vassal1 and J. C. Harden, ibid., 34, 132 (1962). ( 5 ) P. D. Garn and G. D. Anthony, ibid., 39, 1445 (1967).
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The authors are indebted t o Arthur Julian for the determinations described here.
RECEIVED for review January 19, 1968. Accepted February 28, 1968.
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ICnDDnDANnt 1 1 1 1 -I.”-
-
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OIMETHYLACETIC ANHYDRIDE FORMAMIDE 136.4‘ 153’
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Figure 2. Melting points of rn- and p-xylene system
BENZALDEHYDE
178.
METHYL THIENYL
IOOOBENZENE 189. 0-TOLUIDINE 199.8.
KETONE 214’
PHENYLACETONITRILE 234’
ISOEUGENOL 267.5’
Figure 1. Differential thermographs for a variety of organic compounds 1 176
ANALYTICAL CHEMISTRY
Melting of eutectic Melting of p-xylene 0 Melting of m-xylene 0
0
