B. L.
Johnson
Montana State University Bozeman, Montana
Useful Techniques for Micro and Macro Sublimation
Some simple and easy to assemble devices for semi-micro collection of pure substances and micro extraction were given in a previous article ( I ) . Here are presented some further designs, for microand macro sublimation apparatus, which also depend on internal distillation-condensation. Sublimation, when it can be used, is one of the finest and easiest ways of purifying an organic compound. To illustrate how it may be done on a macro scale, sublimation of phthalic anhydride is a classical example. Into a tall lipless beaker is placed 5-10 g of commercial phthalic anhydride. A balloon flask of appropriate size filled partly with cold water is put onto the open beaker. Heat is applied to the beaker. Shortly it will be found that fine crystals of pure phthalic anhydride appears adherent t o the bottom of the balloon flask. The crystals grow quickly in size. Sublimation a t the micro level is sheer artistry and ingenuity. I n toxicological examinations of cadavers only a few milligrams of the toxic material may come to hand. Though spot reactions are frequently sufficient for legal purposes, conventional chemical identification-melting point, plus derivatization-give such high assurance of validity that the analyst always attempts such investigation. Because of the small amount usually isolated micro methods find application, and sublimation may be performed. Two watch glasses of the same diameter, approximately a/a in., but of unequal depth are obtained. Into the glass of the greater depth (No. 2) are put a few milligrams of sample. The glass of lesser depth (No. 1) is put on glass No. 2 and 2-3 drops of water as a coolant is added (see Fig. 1). Heat from a micro burner is now applied cautiously. An asbestos pad with an appropriate size hole in it will serve admirably as a supSedimenfed Sample Here
, Figure. 1. Wotch gla.. micro *ubiim.tioh
Figure 2.
Capillary tube micro sublimotion.
608 / Journal of Chemical Education
port for the watch glasses and also serve well to localize heating. An electric melting point apparatus may substitute for the preceding technique. A sublimate will condense on the underside of glass No. 1 and when substantial enough can be scraped off with a clean sharp knife blade and its melting point determined and derivatives made in conventional micro ways. Figure 2 depicts the essential feature of this type of sublimation. A long melting point capillary is drawn out from a soft glass test tube, about 8 in. in length. I t is bent in gentle heat about 2 in. from one end to make an angle (see Fig. 2). The short end is then filled with the sample in the conventional way, and the small plug is broken apart by tapping, and its dust is allowed to move down in the tube to the bend. This operation can be repeated until enough sample has gathered at the bend to warrant success. The short end is now cut off as close to the bend as possible without creating the possibility of the sample melting when this end is heat-sealed in a micro flame. Gentle heat is now applied at the bend. Sublimate will form in the upper reaches of this tube. Sublimate forming near the bent end can be moved farther up the tube by judicious heat. When visible sublimate appears, heat should be removed, and the bend cut off; any sublimate deposited at the cut off end should be volatilized out of that part of the tube by gentle heat. This end should then be sealed. A melting point or sublimation point can then be taken using a liquid bath. I n many instances, hut not all, the sublimate may be derivatized and purified and the melting point of the derivative determined, all without removing it from the capillary tube, once the melting point of the sublimate indicates the probable identity. Literature Cited (1) JOHNSON, B. L.,
J. CHEM. EDUC.,4 4 , 4 6 5 (1967). , STEVENS, Chemist Analyst, 20, 1 22 (1931). ( 2 ) G r ~ s sR. ( 3 ) KEMPP,R., "Suhlimieren, Houhen, Die Methoden der Organivchen Chemie," (3vd ed.), G. Thiene, Liepeig, 1925, Part 1, pp. 662-707. (4) MAHER, J. R., Science, 17 529 (1935). VON E., dngew. Chem., 54 161 (1941). ( 5 ) PFELL, ( 6 ) "Thorpe's Dictionary of Applied Chemistry" (4th ed.), Vol. IV, 1940, pp. 6 7 9 4 9 0 .
