Approximate Boiling Points of Submilligram Samples Many direct procedures, some using capillary tubes, have been described by which the boiling point of a liquid sample no larger than a few milligrams can be determined. With more expensive apparatus and sophisticated techniques, such as differential thermal analysis or gas chromatography, the boiling point of an even smaller sample can be ascertained. The lower limit of the inexpensive capillary tube method can be extended by using a U-shaped capillary, open a t both ends and having an inside diameter of 0.17-0.20 mm. Such a size can be easily pulled out from 5 mm a d . soft glass tubing; 8-in. sections of the resulting capillary are cut sway and carefully heated in a small yellow flame with the farce of gravity being utiliaed for the formation of the "U" shape. Comnmmd
1-Butanol Acetic Acid n-Xvlene
Experimental Ranee
Range Midonint
99.4-100.5 116.6119.3 115:6118.6 137.9-139.4
100.0 118.0 117.1 138.7
Literature Value 100.0 117.5 118.1 138.4
P oi the. 1.\ smnplr lmgth 1,rte.ce~.5 and 10 IIIII, &AII 0.1-0.lr ntg is rskct~8~1'i r . w C ~end tuhc by r a p h g ncrion and :tll~wtdt u flow t u tilt t:utvcbf t l ~ c . " 'The I I ~ Pis nt~nchrrlJirectl\ to a thermometer with tape or small section of rubber tubing so that the liquid sample is near the mid-point of the thermometer bulb. Any reliable melting point apparatus accepting capillaries can be used for the beat source: a heatine: - rate of no mare than one degree - Der . minute is recommended. I t is essential that the open ends of the U-tube extend above the heating medium so that condensation of vapor will occur thus limiting the rate of sample loss. As the boiling point is approached the st~mplewill usually oscillate vigorously between the sides of the ''U." Slightly above the true boiling point the liquid will shoot up one or bath of the sides of the tube and be eliminated from the heated area. Some of the sample will recondense in the upper part of the sides. A sin or seven power hand lens is quite convenient for observing the behavior of the sample a t the critical moments. Typical results of this procedure are given in the table. The range starts with the vigorous oscillation and terminates a t the temperature a t which the liquid is expelled from the hot area. All experimental values have been corrected; reagent grade chemicals were used for all observations and literature values were taken fmm the "Handbook of Chemistry and Physics" (48th ed). I n all cases studied the accepted value of the bailing point fell within the experimental range, often close to the midpoint of the range. We thank the National Science Foundation for the support given under grant GY-2982.
THOMAS MCCULLOUGH, C.S.C. WERNERH. BRAUN PHILLIPA. REIDINGBR
Volume 47, Number
I , January 1970
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57
