SUBLIMATION G . Ross ROBERT~ON, UNIVERSITY OF CALIFORNIA AT LOSANOELES
The process of sublimation affords one of the neatest methods of purijying a crystalline chemical preparation. A few substances have vapor pressures as high as 760 mm. in the solid state, and are easily sublimed. A number of others must be specially treated, as by dilution of heated vapor with air, to permit sublimation m'thout distillation. To illustrate these cases hexachloroethane and naphthalene may be sublimed in beakers and Aasks for lecture demonstration. Anthracene i s sublimed in a student experiment involving suction of diluted vapors through a funnel. Benzoic acid i s sublimed from a retort into a cloth bag with the aid of refrigerated air.
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One of the most elegant methods of purification is that of sublimation. Unfortunately, its elegance is not always attended with practical laboratory efficiency. In the laboratory one usually does little more than play with sublimation-perhaps with a watch glass as apparatus. Nobody expects a yield of any consequence. Orthodox Sublimation Strictly interpreted, purification by sublimation first calls for the direct vaporization of a solid without intervention of the liquid state. Next follows a direct return from vapof- to solid, again without liquid. In many cases, however, it is convenient to melt a solid, boil the resulting liquid, and then return the vapor to the solid form wjthout passage through the liquid state. This modification often expedites matters, and for practical purposes gives the sort of purification desired. A sublimed crystalline preparation is normally purer than a distilled product because there is no liquid to extract impurities from the vapors in the cooling chamber. The newly deposited crystals do not dissolve the gaseous impurities to a significant extent. The partial vapor pressures of these gaseous impurities, while sufficient to cause them to dissolve in a liquid condensate, are insufficient to permit the formation of a new independent phase, either liquid or solid. Accordingly the undesired impurities escape from the field of the subliming crystals and are eliminated. Only a second solid of high volatility and large amount would come over and stay with the sublimate being collected. The chances of such an occurrence are slight, since but few substances may be sublimed anyway. Simple lecture demonstrations serve to illustrate the theory of sublimation. Hexachloroethaneand naphthalene. two inexpensive substances, may be used. Hexachloroethane, C&ls This unusual compound' attains a vapor pressure of about 760 mm. at 1 STABDBL, Ber 11, 1736 (1878).
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185", two degrees below its melting point. For an organic compound this is remarkable behavior. Naturally i t affords an ideal set-up for sublimation, as represented in the phase diagram of Figure 1. In this diagram, as well as Figure 2, scales are somewhat distorted to bring out the principles involved. LIQUID A few grams of pure hexachloroethane a r e placed in a large conical flask and strongly heated over a flame. As the temSOLID 7 perature reaches 185", the VAPOR cryst5.1~appear to be restless and stir about. Thev shrink in size but do not 185 187" TEMP melt. The escapingvapor, SUBLIMATION OF HEXACHLOROBTHANE FIGURE reaching the cool walls of the flask, deposits a white crystalline sublimate which sticks to the vessel. The original pile of crystals on the bottom vanishes. In this case the vapor, slightly superheated but a t the fixed pressure of one atmosphere, is cooled along the line AX. At X the substance passes largely to the solid state, missing the liquid field entirely. Finally, after complete sublimation the temperature will have fallen to B, the final temperature of the receiver, virtually room temperature. Naphthalene The sublimation of this hydrocarbon presents more difficulties, but is more typical of common practice. N a p h t h a l e n e has a vapor pressure of but 7 mm. a t 80°, its B melting point. Naturally, 'P. its boiling point is high M M. (218O) and distillation is easier than sublimation. Dired cooling of the pure vapor, prepared a t 760 7. mm. pressure and 2N0, causes most of the material 80' 2W condense to liquid, as Frcuns ~ . S U B L ~ T IOPONAPHTHALENE N suggested by the traversal of the continuous line BM, Figure 2. At 80° sublimation begins, and a small, unsatisfactory yield of sublimate is obtained over the path MY.
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Most of the product is in the form of a compact cake of frozen distiiate in which volatile impurities may easily be occluded. Suppose, however, that we insist on subliming the whole available supply of naphthalene with complete avoidance of distillation. I n graphic terms, we must steer the lower dotted course BX, which lies wholly within the vapor field. As the point X is reached, sublimation occurs. It only remains t o be shown how practically to follow BX rather than BM. Plainly the reduction in pressure must be specially promoted. I t must outrun reduction in temperature in order that the line BM may be escaped. This is simply illustrated on the lecture table by boiling a few grams of naphthalene in a large beaker. A round-bottomed flask, containing cold water, is allowed to rest over the mouth of the beaker. The temperature at the bottom of the beaker is of course 21g0, and the vapor pressure 760 mm. As would be expected, more or less reflux distillation will occur in the lower half of the beaker; with this we are not concerned. A part of the vapor, escaping the distillation, rises, and is both diluted and cooled with air. Most important is the fact that it is diluted rapidly until its partial pressure falls below 7 mm. The vapor now strikes the cold surface of the flask, where it deposits beautiful snowlike crystals. Limited Field for Sublimation Comparatively few substances exert sufficientvapor pressure below the melting point to permit effective sublimation. Striking contrasts are noted in the accompanying table. TABLE I E V o m Pressure
Suhiloncr
Carbon Dioxide Acetylene Hexachloroethane Phosphoorus pentachloride Camphor Nitrogen Iodine Ammonia Benzene Naphthalene Benzoie Acid Water Bromine Zinc Sulfur p-Nitrobenzaldehyde Toluene Mercury
ot Mclling Poi",
5 . 1 atm. 1 . 2 atm. 800 mm. ca. 1 atm. 370 mm. 96 mm. 90 mm. 45 mm. 36 mm. 7 mm. 6 mm. 4.58 mm. 0.44 mm. 0.15 mm. 0.03 mm. 0.01 mm. 0.001 mm. 0.000001 mm.
Dlrlling Poiel
- 57O - 81.5" 187" 166" 179" -210" 114' - 78" 5.5" 80" 121" 0" - 7" 419" 119" 106' - 95" - 39"
The high position of carbon dioxide in the above list is illustrated in the convenient dryness of "dry-ice." The vapor pressure of solid carbon di-
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oxide is so great that the substance never gets warm enough to melt, except in storage under pressure. In the open evaporation keeps it solid until the supply vanishes. Subliming Point For the unusual substance like carbon dioxide or hexachloroethane one may record a "subliming point" in likeness to the familiar boiling point of a liquid. Naturally such would be the temperature a t which the vapor pressure reaches 760 mm. As in simple boiling, the vapor threatens to push the atmosphere away. For other cases one finds questionable statements in the literature. For example, a certain substance "sublimes a t 200 degrees," according to printed authority, when it is well known that the .vapor pressure of the solid phase never approaches 760 mm. The citation probably means that one must raise the temperature of the substance to 200' before enough sublimation occurs to be noticeable. There is of course no fixed point of sublimation in such a case. Rapid Crystallization The ability to form crystals rapidly is intimately connected with effective sublimation. Apparently simple symmetrical molecules such as benzene, naphthalene, anthracene, and even water are thus enabled to form beautiful crystalline sublimates even at their rather low vapor pressures in the solid state. E Although common ice has a maximum vapor pressure under normal conditions of but 4.58 mm., Nature practices its sublimation readily in the familiar form of a snowstorm. When the temperature falls too fast, and the partial pressure is too great (humid air) the line BM as of Figure 2 is encountered, and hail eventually reaches the earth. The product is of course largely frozen distillate. Other phases of sublimation are discussed a t lengtb by Kempfa and Lassar-Cohn.3 These authorities describe ingenious apparatus employed in sublimation, including research equipment for vacuum sublimation. Student Laboratory Experiment Anthracene is one of the best substances for student practice, preferably conducted in the elementary organic laboratory. The student is provided with a cheap, impure grade of the hydrocarbon readily obtainable in the market. Purified anthracene is prepared first, and i t is then con-
* KEMW,"Die Methoden der Organischen Chemie," by HOWEN,Thieme, Leipzig, 3rd edition, Vol. 1, p. 663. a L~ssnn- con^, "Organic Laboratory Methods," qiilliams and Wilkins Co., Baltimore, 1928, p. 373.
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verted into anthraquinone by a standard method much like that of F i ~ h e r . ~ The anthraqui'none in turn is sublimed. The sublimation of anthracene, ordinarily a tedious process, is simply accomplished by the method of T. W. Deakers, student in this laboratory: Three to 4 grams of anthracene are placed in a 400-cc. beaker which stands over a wire gauze. The beaker is covered with a piece of stiff card, or asbestos paper, about 15 cm. square, which has a central circular opening about 2 to 3 cm. in diameter. A piece of filter paper, slightly oversize, is wedged tightly against the perforated plate of a common Biichner funnel. The funnel is inverted over the beaker and card, and is connected by rubber hose to the suction service or aspirator. It should be held securely in position with the aid of a clamp. Gentle suction now draws air from the beaker through the filter paper. When the anthracene is strongly heated, its vapors, highly diluted withinrushing air, pass freely into the funnel used as subliming chamber. A beautiful cream-white mass of sublimate is collected ready for the oxidation experiment normally following. Preparation of Sublimates in Quantity Genuine sublimation of an ordinary organic compound, whose vapor pressure in the solid state seldom reaches 10 mm., is bound to be slow. We may then resort to "pseudo-sublimation," using the apparatus of Figure 3. A small current of ordinary compressed air passes into the retort, blowing upon the surface of the melted substance undergoing purification. A second current, of larger volume, is iirst chilled by passage through a copper FIGURE 3 tube coil immersed in ice water. Both supplies of air pass into a wide-mouthed bottle, the bottom of which has been cut off. A common flour or sugar bag is tied over the opening. Cotton batting or cheesecloth is stuffed into the spaces beside the two entering tubes to prevent leakage backward. The substance in the retort never boils, but is kept a t a temperature so high that it may have a vapor pressure of perhaps 100 mm., varied as practice warrants. The aerated vapor, blown out into the bag, tends to condense into a mist. The mist of course is finely divided liquid, and the process so far is really distillation. Before the liquid has time to aggre"FISHER, ''Laboratory Manual of Organic Chemistry," John Wiley and Sons. Inc., New York City, 1931, p. 220.
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gate, i t mixes with the turbulent stream of chilled air. Each droplet of mist freezes promptly. As a result a fluffy crystalline product collects in the bag. It is retrieved much like dust from a vacuum cleaner. Persons working during severe winter weather should be able to modify the above procedure to advantage by pumping air from outdoors. Despite the formation of liquid mist, i t is probable that pseudo-sublimation gives nearly as pure a product as genuine sublimation. To be sure, the individual liquid droplet, in rough conformity with Henry's law, would absorb for the moment any volatile impurity which accompanied the preparation from the retort. As the droplet freezes, however, i t forms so small a crystal, or so simple a cluster of crystals, that the volatile impurity is quickly squeezed out and blows away. The loose structure of the cotton bag permits a few crystals to get away a t first, but the pores in the fabric are soon choked. Such losses are sufficient,however, to necessitate the use of a hood in the process. Naphthalene is readily "sublimed" in quantity by this process, the material in the retort being heated to about 140'. Benzoic acid is handled with like facility a t about 170". With iodine--probably because of its high vapor pressure a t room temperatures-serious losses occur. It is suspected that some of the so-called sublimates of commerce are really products of the frozen mist process. Substances as zinc, v. p. 0.15 mm., and sulfur, v. p. 0.03 mm., could hardly be produced in quantity if a vapor must be reduced to such rarity before deposition of product were allowed. Although zinc dust and flowers,of sulfur are often called sublimates, they may actually deserve another designation. Substances such as iodine and arsenic, however, may readily come into commercial form as genuine sublimates in conformity with their catalog designations. Steam Sublimation When the melting point of a substance exceeds 100°, but where the vapor pressure is appreciable at 100" (e. g., iodine and quinone), sublimation in steam may be employed. When steam is passed through liquid iodine, a mixture of the two substances passes to a condenser, where practically the entire quantity of iodine must sublime, since the entire quantity of steam condenses. Thus losses caused by the escape of a non-condensable gas like air are avoided. Unfortunately, the resulting product not only does not have a beautiful appearance, but moreover has to be dried. The drying process of course raises problems with a volatile solid. In semitropical states an interesting project for high-school students is available in the recovery of camphor from camphor leaves by steam sublimation. In California the camphor is a common street tree. Steam is passed through a five-gallon can full of leaves, the fresh young spring growth being taken. A beautiful, glistening, highly aromatic sublimate is collected in an ordinary Liebig condenser.
