Thomas McCullough, CSC and Robert A. Baranek St. Edward's University Austin, Texas 78704
II
k m h k r 0 Techniques for Organic Chemistry
Before the advent of gas chromatography, tlc, paper chromatography, and relatively inexpensive spectrophotometers, laboratory work for students of elementary organic chemistry usually dealt with sample sizes of from 1 to 50 g while unknowns for qualitative organic analysis were of the order of 5 - 1 5 ~ . But nowthe emphasis has shifted to smaller and smaller amounts, for why bother with 10 g when 0.002 g is sufficient for analysis by gas chromatography or infrared spectroscopy, and who needs 5 ml of sample when 2 lambdas is sufficient for spotting a chromatogram? With even quite simple equipment it is possible to obtain useful data with liquid samples ranging in size from 0.01-0.03 ml. The heart of this technique lies in a semimicrocapillary which can be prepared from 5-mm soft glass tubing, a hot flame, and a sharp file. The tubing is heated until quite soft, removed from the flame, and firmly pulled to produce the desired capillary diameter of approximately 0.2 mm (Fig. 1A). When cool, the glass is cut at the arrows, heated gently in a yellow flame to give the desired curve, and carefully fire polished to yield the final product (Fig. 1B). The fine tip of the capillary will collect liquid samples by capillary action, the curve will prevent rolling about on a balance pan, and the large opening at the butt end will permit the insertion of the plastic tip from a squirt bottle to quantitatively flush out the sample into the desired vessel. With this capillary and an ordinary analytical balance, the specific gravity of a few milligrams of liquid sample can be determined to two or three significant figures. The sample is collected in the previously weighed capillary and the length of the sample measured to the nearest 0.1 mm with the help of a hand lens. If especially volatile, the sample can be retained by capping the capillary tip with a slightly larger piece of short straight capillary sealed a t one end. The curved capillary should be kept horizontal a t all time to dripping or seepage into the larger capillary. There is no need to cap the larger o~enineof the curved ca~illarvas lone as the s a m ~ l e does not extend into the curved portion. After bekg 'IDDLES,H. A,, Ed., 6,455(1934).
AND
JACKSON, C. E., I d . Eng. C h m . A d .
weighed, the sample is flushed out; the curved capillary is dried, reweighed, and filled to approximately the same mark with a standard liquid, water being quite satisfactory. The standard liquid is also measured for length, weighed, and the specific gravity of the sample determined from this formula Spec. grav. =
sample wt. sample length
water length water wt.
This curved microcapillary and a small filtering crucible can be used for a semimicro gravimetric determination of carbonyl compounds. The crucible is weighed and plugged at the bottom (outside) with a tightly fitting rubber stopper which just touches the bottom of the fritting. The crucible is then half filled with 2 M HC1 saturated with 2,4dinitrophenylhydrazine.' A weighed sample of aldehyde or ketone is flushed from the capillary with a suitable solvent, water or alcohol, into the crucible which is then sealed with a second rubber stopper. The entire assembly is kept a t O°C for almost an hour, after which the stoppers are carefully removed, the remaining solution removed by suction, and the precipitate washed right in place with 2 M HC1 and water. A final weighing of the dry cruoible and precipitate gives data for near quantitative results. The advantage of this procedure is the elimination of any transfer of solid material from a reaction vessel to a weighing vessel, for the crucible serves as both. This procedure can he used for other gravimetric reactions which occur at room temperature or below. We thank the National Science Foundation for the support given under grant GY-2982.
w
Semimicmcapiilory prepmed from glass tubing. copillmy; 8, flnbhed product.
A, Initial pulling of
Volume 48, Number 1 1, November 1971
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