A Semimicro Boiling-point Test Tube ARTHUR FURST and JOHN W . BOHNER' The University of San Francisco, Sun Francisco, California
T
HE semimicro boiling-point test tube described in this paper has proved very useful in our elementary and qualitative organic chemistry courses. It is of interest to note that although many designs of apparatus for the determination of melting points appear periodically in the literature, practically no new devices for~ the determination of boilina - ~. o i n t son a semimicro scale have been published. At the present time the boiling point of an unknown liquid can be estimated either by distilling a small quantity, or by the capillary tube method of Siwoloboff.2 This latter method is described in most textbooks. For example, see footnote 3, page 93. For a quick, though rough estimation of the boiling point, compounds have been boiled in open test tubes. The loss of liquid by evaporation is so great that the thermometer readings are rendered unreliable. Shriner and Fusona have added a percolator cup to the test tube but have not cut down the path of the escaping vapors. To minimize the loss by evaporation a test tube can be drawn out as shown in the figure. The bulb should have a capacity of about 10 ml. (A Folin sugar tube may also be used.) A slotted cork is used to hold the thermometer in place. The position of the thermometer is important and should be as shown; other positions do not give good results. The thermometer should not touch the sides of the tube. Note the level of the bath liquid in relation to the thermometer bulb. The ring of the refluxing liquid should be maintained about an inch above the upper part of the constriction, no higher. The advantages of this method are many. The ~~
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Present address: 1
tubes are easy to make. No complicated setups are necessary; once assembled no special attention need be given the apparatus. No stirring of the liquid bath is required. The boiling point is obtained in a few minutes with between 10 drops and 1 ml. of the unknown liquid. When a constant value is obtained on
U. S . Navy.
' Smo~oBoa~. A., Ber.. 19,795 (1886).
a SHRINBR. R. L.. AND R. C. FUSON, "The Systematic Identification of Organic Compounds," 2nd Ed.. John Wiley and Sons. Inc.. New Yirk, 1940, p. 93.
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the thermometer this value is recorded as the nncorrected boiling point. A rising temperature indicates an impure liquid. No more liquid is used with this method than with the capillary tube. And finally, the tubes can be used over and over again. Table 1 shows a comparison between this and the usual capillary tube method. The values are given nncorrected for atmospheric pressure, but do include stem correction. The greatest danger from superheating is overcome by using an oil bath. At all times the temperature of the bath is only a few degrees above that of the boiling
the boiling stones a t times no actual boiling was observed; in both cases the ring of the refluxing liquid was definitely noted. nAcd* BE"Bufyl m h 9 - BmmaNilroMost students found that wrapping the bulb with arnc Watn alcohol dride brnscnc Anilina bcnsma cotton did not improve the results, but added a few 210.6 155.5 184.5 Drawn testtube 70.5 99.7 118.1 140 obstacles. The cotton would either prevent the bulb 185.5 212 156.7 140 99.8 119 Cnpillarytvbe 80.5 184.4 211 139.6 156 118 100 Handbookvalue 80 from going through the constriction, or would remain stuck in the bulb of the test tube when the thermometer Liquid. When boiling stones were used no change in was removed. As a find check on this method eight elementary the temperature was noted in the boiling point of aniline which was used as the reference fluid. The average of organic students were shown the figure of the elongated eight determinations, four with boiling stones, four test tube, given directions for making it, and then issued without, was 184.3'C. The range was 184' to 184.Q°C. unknown liquids. The tubes were made with various The only difference was that actual boiling was ob- degrees of success. The unknowns were reported served when the boiling stones were used. Without with stem corrections.' Table 2 gives the results. TABLE I
Cola~uusonoa M s r m o s
TABLE 2. Slrdcnl No.
Unknown Reported Handbook
I
2
~ ~ i l i Water n ~ 184.3 99.7 184.4 100
3 i-Amylalcohol 100.6 102
4
~ e s a r 01 s UNENOWNS
5
7
6
~ f h y l ~ r r t a t e ~ A m y l a l ~ o h ~~l ~ ~ ~ 77 100.02 82.8 82.5 77.1 102
~
~ Carbontetrachloride ~ ~ l ~ l ~ o 61.2 76.8
8 h n-Propylaleohol o l 97.6 97.8
