M. C. Chaco University of Ghana Legon, Ghana
Microboiling Point Determination at Atmospheric Pressure
Several methods are described in the literature for determination of boiling points, using microquantities of liquids.' A useful modification makes use of a 10-cm-long X 1-mm-bore melting-point capillary (A in the figure) that is well dried and closed at one end. Capillary B, also closed a t one end, is 3 mm long, 0.5-0.55 mm bore, and 0.8 mm outside diameter. A column of liquid 5-7 mm long is introduced into A and forced to the bottom along with tube B by centrifuging or shaking. Tube A is mounted on a thermometer and introduced into a heating bath (in a beaker with annular stirrer) so that the lower 30 mm of A extend below the surface of the bath. As the bath temperature is raised, the air bubble inside tube B aaduallv exoands and moves the liuuid column upward along w h B. When the upward motion starts, the rate of heating is decreased. The temperature a t which the small capillary reaches the bath level gives the approximate boiling point2 of the liquid. An accurate value is obtained by cooling the bath to a few degrees below the observed boiling point and reheating a t about 1°/min or slower rate. The 'See CHERONIS,N. D., "Micro and Semimiero Methods," in "Techniques of Organic Chemistry," Vol. VI, (Editor:WEISSBERGER, A,), Interscience, (a division of J. Wiley & Sons, Inc.), New York, 1954, p. 180. 'The first determinations generally gave low (uwslly by 1-3') values and hence wch results are not used.
474
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Journal of Chemicol Education
liquid and the tube B must be brought down again to the bottom of tube A by shaking or centrifuging. So long as a bubble remains in B the process may be repeated on the same sample until constant readings are obtained. If no bubble remains, it may be reintroduced as follows: keeping the open end of A loosely covered with a piece of paper, A is shaken carefully (the liquid inside being kept hot if viscous) to bring B to the middle tmm of the tube. B is heated until the liquid inside comes out, keeping the open end of A still covered. A is then shaken or centrifuged to bring the contents to the bottom. The melting poiot and boiling A point of a stable solid may be determined The in small the same capillary operation. required for
1, -
3mm
0.5%mm
elt tin^ point tube for boiling point determinotion. A, Large copillary ,moll copillory tube.
the experiment may be easily prepared by drawing out Cmm-bore glass tubing. A small piece of carborundum fused into the end of a glass rod serves as a convenient file. If the length of B exceeds 3 mm it may give low values for the boiling point. For 10 compounds in the boiling range 30'-30G°C the maximum deviation was j ~ 0 . 6 ~ C .
