Boiling Points and Small Volumes An apparatus and a method J a c k E. Leonard University of Texas at El Paso. El Paso. TX 79968 Gary Behelfer, and Leonard E. Mohrmann Texas A&M University. College Station, TX 77843 The qualitative analysis of an organic unknown depends on accurate determination of the physical constants of the unknown. While meltine ooints can be determined easilv on the milligram wale, hoiling point determinations usually reauirc much lareer volumes. The inverted tuhe mcthod' rives relatively accurate results on a small volume, hut it requires some elnsshlowineskill which must students do not have. The alternative of ca&ng out a small scale distillation is slow and requires at least 10 ml of sample. We were pleased, therefore, to come across a recently published procedure by Smith and Sugdenz which works satisfactorily for 2-3 ml samples. We have modified their apparatus and used the method with over a thousand students, and we have found it to be quite simple and accurate.. Figure 1shows the apparatus required. In addition to this block, which is machined from stock aluminum rod to the dimensions shown, the student needs a clean 18 X 150 ml test tuhe3, a 76-mm immersion thermometer, ring stand and clamns. a heat source. and a small olue of cotton or elass wool. ~ i g & e2 shows the apparatus in uHe. The hlock is cramped (if necessarv) .. and the tube containine 1cm of liauid and one or two boiling chips is inserted. The thrrmomrtpr is inserted into the wst tube so that the hottnm of the hulb is at the same level as the top of the hlock. After clamping the thermometer so that it does not touch the walls o i the tube, the top is I~useI,v sealed with glass wool. The device is then heated. (Wr use electric hot plates, but Smith and Sugden report that a mirroburner o; hot water hath could alsibe used.) The heating is continued until the retluxing liquid has risen to thr height of the 76-mm immersion line. At this point the temperature on the thermometer should have stabilized at the boiling point of the liquid.
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Figure 2. Smith-Sugden bolling point apparatus in use.
In order to test this apparatus, we performed boiling point determinations on a series of five compounds with boiling points ranging frum 6 l 0 C to 239°C. ~ b & ta 5U-mi sample of the compound wasdistilled in an all.glahs simple diitillatim apparatus.'l'wodistillation ruts were titkenafter allowing the forerun to remove anv vulatilr impurities present. 4 diiiermt thermometer ifishrrbrand 0-26U°C whitehackedr %asused for mrasuring ;he temperature of each cut. Thc thermometer was then used with thr h i l i n r ooiat atmaratus iollowinc the method outlined above. The results ale shown in the table. As can he clearlv seen from the results in the table, two kinds of errors can be expected when using this method in coniunction with literature values in organic qualitative anaiysis. The first is due to using an uncalibrated thermometer. The distillation and the boiling point block both yield temperatures lower than the specifi%ions provided by the supplier. The second problem is of equal magnitude and harder to overcome: the chemicals as supplied do not have the literature boiling point, and there is no single "literature value" for the boiling point. This finding introduces a 1-2°C uncertainty a t all temperatures. Hence, under these representative conditions, the measured boiling point will be fully ~~
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Figure 1. SrnithSugden boiling paint apparatus (modifiedspecifications)
Roberts, Royston M.. Gilberl. John C., Rodewald. Lynn 8.. and Wingrove. Alan S., "Modern Experimental Organic Chemishy," 3rd Ed., Holt, Rinehart, and Winston, New York, 1979, pp. 19-20. 2Smith, Harry C., and Sugden, John K.. Chem. and lnd., 906 11976L .~ . Smith and Sugden give the dimens.ons for Jse with a 16-mm X 150-mmtest tube. We have found thar in practice t is J s e f ~lol try obl lhe test tubes beforeallowmg st-dents to use them in Ihe oioc*s, since the variation in diameter of commercially available tubes exceeds the tolerance allowed by the drill size. We would like to express our appreciation to Prof. P. S. Mariano for bringing this apparatus to our attention. Volume 58 Number 12 December 1981
1029
Boillng Point Cornparloons
... -,
.
Compaund
mnilinn --, , ,, ,Dnin+ -, \PC> CRC Tablesr
Supplier
Suppliera
Merck
Chloroform
Mallinckrodl#4440
59.5-61.5
61-62
61
4Melhyl-2-pentanone
Fisher W 2 1 3
116.2+0.1
117-118
116.8
3-Heplanone
MC/B #HX120
146-148
Diethyl Malanate
Aldrich D9, 775-4
199
Ouinoline
Fisher W
236.6-237.5
5
198-199 237.7
148 199.3 239
Di~lillation~ 60.0- 60.5 60.5- 61.0 114.5-115.0 115.5-116.5 145.5-146.0 146.0-146.0 196.5-197.0 197.0-197.0 234.0-234.5 234.5-235.0
Present Method 60.3 60.5 115.0 116.0 145.5 146.0 197.0 196.5 233.5 235.0
mis nmber Isthe sumlier'a boillw mint soeclfication primedon ms bonle.
accurate as that determined by traditional methods, hut students will need to allow about a 4 O uncertainty in the hoiline ooint if thev use the method correctlv. Common problems encountered in student uie have been too r a ~ i dheatine with low builine liouids leasilv detected. since the liquid hb;;i away without ;efl&ing), too siow heating
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1030
Journal of Chemical Education
with high boiling liquids (so that the reflux line does not rise high enough), failure to put an insulating plug at the top of the tube, failure to check for a stable thermometer reading, and having the thermometer placed so that it touches the boiling liquid. However, nuncof these problems isserioua and all ran hespotted easily and corrected by careful work4.
