h C O S T ' E S I E S T FOR11 OF APPARATUS FOR T H E DETERPIIISATION O F MELTIXG TEMPERATCRE* BY GEORGE LYKN
Careful observations of the melting of some very pure organic substances' by the usual capillary tube method led to values of the melting temperature higher, frequently by more than IO, and occasionally by as much as 3 O , than determinations by means of the heating o r cooling curve. The latter however yields the correct result, as shown by the fact that the values obtained by different observers working with different set-ups checked within less than 0.1'; it is therefore to be preferred to the capillary tube method, and can be carried out with equal convenience by use of the simple form of apparatus t o be described. This is a modification of the apparatus described by Washburn,* by addition of an electric heating coil and of removable sample tubes, which increases its convenience and extends its range of usefulness; for it may be used to temperatures as high as 400°, for heating curves and boiling temperatures3 as well as for the more usual cooling curves. The range below oo can also be covered by immersion of the apparatus in a suitable low temperature bath. The amount of material required is of the order of I cc., but it is recoverable; but if temperature is measured by a thermoelement (which is preferable) a smaller tube may be used, and a much smaller sample suffices. The apparatus, sketched in the figure, consists of a small unsilvered vacuum flask B, a heating coil H wound FIG.I upon a glass tube, and the sample
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* Contribution
from the Department of Chemistry, Yale University. Cf. Andrews, Lynn and Johnston: J. Am. Chem. Soc., 48, 1274 (1926). * E . TI-. Washburn: J. Ind. Eng. Chem., 15, 275 (1924). In this case the sample tube would be longer and project beyond the flaskfar enough to serve as a condenser.
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GEORGELYNN
tube which rests upon some glass wool; all glass parts are preferably made of Pyrex. Tbeir exact size is immaterial; those suggested are: flask I S mm. inside diameter, 150 mm. deep; heater tube, 13 mm. outside diameter, 90 mm. long; sample tube 9 mm. inside diameter, 90 mm. long. In addition there is, of course, the rheostat necessary to control the current in the heating coil. The only part which requires further description is the heater coil, of $36 nichrome wire wound upon a glass tube in which three holes have been punched, as shown, by means of heated wire (of tungsten or nichrome). The fine wire is threaded several times through hole 3 in such a way as to hold in position the free end; approximately forty turns, about 2 mm. apart, are wound about the tube, and the other end threaded through hole 2 in such a way as to hold in place a copper wire which serves as one lead; the first free end is now brought up inside the tube and is used to fasten the second copper lead at hole I . To carry out a determination, material sufficient, when melted, to cover the bulb of the thermometer is put in the sample tube, which is then set in place, cotton being stuffed between the thermometer and vacuum flask; and an appropriate current is passed through the coil, so as to melt the substance. This current is now either cut off entirely-if the melting temperature is not too high-or set at a lower value, so as to give a satisfactory rate of cooling: the temperature readings taken at regular intervals, are plotted, supercooling being relieved, if necessary, by inoculation with a tiny crystal upon a glass thread. The flat on the curve is the melting temperature; its length, relative to the total period of freezing, is at the same time an excellent criterion of the purity of the substance.' I
Cf. W.P. White: J. Phys. Chem., 24, 392
29, 914, I048 (1925).
(1920);Andrews,
Kohman, and Johnston:
