Sublimation Apparatus-Electrically
Heated
Sublimation as a purification technique ean be demonstrated with a hot plate, beaker, and watch glass containing ice or Dry Ice. However, this crude technique is restricted to a few highly crystalline organic molecules with adequate vapor pressure. Far molecules with lower vapor pressure, and particularly those sensitive t o heating without protection from oxygen, a more sophisticated apparatus involving control over vacuum, sample heating, and cooling to obtain condensation of sublimate is needed. Earlier sublimation device designs have been d e s ~ r i b e d and ' ~ ~ one of these devices is commercially available. Details of construction of the apparatus are shown in the drawing. The sample is introducedS below the indentation of sublimation tube a. Tube a is then passed into cooling jacket b and then into aluminum heater4 c until the identation is inside heater c. The open end of heater e may be packed with glass wool or filled with an aluminum plug to avoid heat lass. The heater is attached to a ring stand a t an upright angle or vertically using the stainless steel handle5 d. Sublimation is achieved by connecting vacuum stopcock e t o tube a, cautiously evacuating the system to remove any solventVrrom the sample, " & n ~~~>~ . and gradually increasing the voltage applied to the heating cartridge7 contained in the intermediate sized hole of heating block e. A thermometer is inserted in the smallest hole of e to measure the temperature. After sublimation, the vacuum tube is disconnected and the pressure is allowed to rise to ambient by cautiously opening high-vacuum stopcock e . Solid samples are then scraped from the interior walls of the test tube or removed from smaller tubes by scoring with a file, wetting the scored mark, and breaking, which exposes the sample for removal with a microspatula. If the sample is valuable or becomes electrically charged, it may be advantageous to use the modified Pasteur pipet f to remove sample by washing with solvent. This is accomplished by mounting the sublimation tube, mouth inclined downward and over a funnel, which in turn is inserted into a stripping flask or storage container. Pipet f , attached with flexible tubing t o a wash bottle, is then inserted into tube a and is used to direct a stream of solvent t o wash material from tube a. Tube n may be rotated during the procedure. If smaller samples are to be sublimed, aluminum insert adapter g,drilled with smaller hales ("/a%a, 13/la in.) to accommodate different sizes of tubes, may he used. We have found tubing sizes of 9 mm, 14 mm, and 20 mm to be useful. For a given diameter of glass tubing, three adapters are required, one for the cooling jacket and two for the heater. The heater is equipped with two insert adapters rather than a single long one to facilitate removal and to reduce the chance of seizing. These adapters are interchangeable and are held in place with Allen head screws. Some organic compounds on heating liquefy rather than sublime and thus Are generally purified by distillation. We have found the apparatus useful for evaporative distillation. For this purpme, the indented sublimation tube a is used. Since temoerature control is precise and easily achieved, the apparatus may be preferable to Kugelrohr distillations for some samples. Liauid samdes are convenientlv removed from the distillation tubes with p b e t f or, for small samples, by cutting the t,uhe. also he sublimed 0; distilled into drawn-out bulbs and then seaied fo; later analwis or storaee.1n summarv. .. . Snmnles mav ~~,~ rng to s r r ~ r ngram;, l is e.isil\ and rnpldly ns.rrmhled, the drvrtbrd oppar;,ttrs mas, he used for n urnplt rnnw trim a and through rcpcated use haspruvm t o hen relinlrlt.sy,lvar.rtusfor por~i~rntmn uiwusnir ,wnprund; rhm~~ghst~hl~mari.,n or evaporative distillation, For instructional purpose, most courses can afford a t least one complete apparatus for demonstration, or a few sets to be shared in moups. Thus the apparatus shown in the drawing should be within budgetary limits in undergraduate teaching or for purifica&n of organic compounds for research purposes. Safetv Precaution: T h e a n. o. a r a t u s should be well insulated9 before use t o prevent accidental injury t o persnnnel. \Ye have found t h ~ npparstui s t~iclulin in~tructionand rssmtial in the purilicntiun ot h>drorartnmr for use it, rhrrmudynamic r t u d m We thank the Uartles\ illr Knrrfi. Rrcenrrh Cpntrr Iw iuppcrt thrc (ugh tunds supplied bv the Air Fwcr Office of Scientific Research.
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Mallow. F. B.. J. CHEM. EDUC., 39,261 (1962). Aglass apparatus of the described design may be purchased from the Ace Glass CO., Vineland, NJ. Walter, R. I., J. CHEM. EDUC., 45,538 (1968'). Introdudion of samples is simplified by their prior enclmure in small test tubes, or screw-cap vials. This reduces accidental contamination of the walls of the sublimation vessel. 'The heater is a 3 X 7-in. cylindrical aluminum block drilled with three full-length holes (9132, 5/s, and 1.25-in.). 5 Stainless steel is used since the thermal conductivity is low. 6 If solvent is removed rapidly, scattering of sample can result. 7 Watlow Firerod, 5/s X 7-in., 115-V, 250-W, LlA33A. Graeve, R. and Wahl, G. H., J. CHEM. EDUC., 41,279 (1964). 9 Several layers of asbestos or glass tape is suggested. Oklahoma State University Stillwater. 74074
806 1 Journal of Chemical Education
