ANALYTICAL EDITION
386
Vol. 3, No. 4
Solid Carbon Dioxide ifi Laboratory Technic' D. H.Killeffer DRYICE CORPORATION OF AMERICA, 52
UMEROUS conditions of crystallization and vapor pressure not easily obtained a t the lowest temperatures produced by customary freezing mixtures can be reached in laboratory practice by the use of solid carbon dioxide, now available throughout the United States under the trade-mark "Dry-Ice." The introduction of solid carbon dioxide for commercial refrigerating purposes and its manufacture on a large scale a t many points has justified a careful analysis of its possibilities for laboratory use in producing cold. Not only is the commercial material, which is pure carbon dioxide, more convenient to use than small amounts of
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Figure 1-Condensing System Using Solid Carbon Dioxide
snow laboriously solidified in the laboratory, but its cost is actually lower even than that of the liquid carbon dioxide used as the raw material for its small-scale preparation. It is easily possible with solid carbon dioxide to produce temperatures as low as its normal sublimation point, -78.5" C., and by proper manipulation considerably lower temperatures, down to -100" C., can be reached without -great difficulty. Although this range is not as low as that obtainable with liquid air, it does reach a point where practically all . common solvents, and many not so common, are either below their freezing points or so close to them that their vapor pressures are negligible. The influence of such reduced temperatures particularly affects solubilities and vapor pressures. Unlike water, 'alcohol, ether, and many other solvents pass 0' C. without a break in either solubility or vapor pressure curves, and what1 Received
June 5 , 1931.
'VANDERBILT
AYE., N E W YORK,N.
Y.
ever water may be present in them is still held in solution to much lower temperatures. Many crystallizations of delicate or low-melting materials, when conducted at the very low temperatures producible with solid carbon dioxide, can be effected easily and much more completely than at higher points. The wider spread of temperature possible in this way greatly increases the yield of crystals per crop, reducing the residue in the mother liquor without requiring evaporation of the solvent. Not only can the crop be increased, but by proper precautions the rate of cooling can be so adjusted as to produce crystals of the most desirable size, a result much less easily obtained by evaporation of the solvent. Perhaps the most spectacular application of this technic, and one easily carried out for demonstration purposes, is the crystallization of water itself from such solvents as ether or acetone. At solid carbon dioxide temperatures, -78.5" C., the solubility of water in ether is so low that the ether poured off from water crystals will no longer give a positive test for water with anhydrous copper sulfate. Although no accurate determination has been made of residual moisture, it is apparent from boiling-point curves of ether thus treated that a result is attained which will answer many of the practical purposes served by dehydration with metallic sodium and subsequent distillation. A similar, but less complete, dehydration is effected with acetone. Numerous crystallizations and separations of delicate materials and resinous mixtures may be similarly accomplished with greater facility at low temperature. Low-temperature distillations under vacuum are of limited application under ordinary circumstances because of the relatively high vapor pressure of many organic materials a t ice-bath temperatures. By greatly increasing the temperature range through the use of "Dry-Ice" (either directly or indirectly) to cool condensers and receivers, this limitation can be materially modified. Evans, Cornish, and Atkinson (1)have recently discussed a method used by them in evaporating water from biological solutions at low temperatures. For their purposes the evaporation was best accomplished a t the lowest possible temperature, The rate is limited by the vacuum obtainable, which in turn is limited by the vapor pressure of the condensed water. By using solid carbon dioxide to cool the receiver and condenser, the effective vapor pressure of the condensate can be made so low that only a negligible a m o u n t of water Figure 2 - s e t - u ~ vapor will have t o be handled through of Condenser to Pro-. the vacuum pump. duce Rapid Cooling Similarlv. more volatile solvents than water can bk distilled in vacuum and recovered from a solution at or below 0" C., if the condenser and receiver temperature is kept low enough to prevent their evaporation. The recovery of volatile solvents which may be valuable themselves or which may be hazardous if allowed to escape (ether, carbon bisulfide, etc.), is easily accomplished by the low condenser temperatures thus produced.
October 15, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
I n distillations, cooling of condensers with solid carbon dioxide must be done in such a way as to prevent or minimize freezing of the distillate on the condensing surface. Water, mercury, carbon tetrachloride, and even anhydrous ammonia freeze a t “Dry-Ice” temperatures, and consequently the design of condensers for handling them must be such that freezing will not clog condensate passages or unduly reduce the heat-transfer rate a t the cold surface. An arrangement, which allows the liquid to flow readily off the condensing surface into a receiver to be further cooled, is advantageous where there is danger of freezing the distillate. A number of solvents, especially ether, alcohol, acetone, carbon disulfide, and others are still liquid a t -78.5” C. and can be easily handled through such a low-temperature condensing system. Figure 1 shows a suggested condensing system, in which a temperature of -20” C. was reached in the condenser jacket. That shown in Figure 2 produces very low temperatures, down to -78.5” C. The dehydration of low-freezing solvents by cold and the crystallization of other materials from them has already been suggested above. The technic of doing this is very simple and will readily occur to the investigator. To produce rapid cooling, a slush of solid carbon dioxide in alcohol or ether is placed in a convenient receptacle, preferably a vacuum jar (Pyrex glass), and the solution to be cooled in a convenient container is placed in the mixture. Where slower cooling is required, “Dry-Ice” in small pieces without a solvent may be used and a double-walled vessel for the solution to be cooled still further prolongs the cooling period if this is desired. The application of extreme cold in the separation of mixtures of materials of widely different freezing points in this way offers many possibilities of value. The removal of fat8
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from alcohol solutions of perfume concretes has been commercially practiced for some time. The separation of benzene from a mixture with ether and many other similar operations can be readily accomplished by crystallization a t very low temperatures. The use of solid carbon dioxide a8 a condensing or carboncooling agent in the production of high vacua with mercurydiffusion pumps is more convenient and cheaper than the use of liquid air and is being widely adopted. Dehydration of gases to an extent attainable only by the most careful use of chemical dehydrating agents, and the separation of many low-boiling hydrocarbons and other compounds from gases is conveniently accomplished by passing the gas through a glass or metal condenser cooled by solid carbon dioxide either alone or mixed into a slush with alcohol or ether. Safe storage of products sensitive to oxidation but not injured by carbon dioxide can sometimes be best accomplished by carbonation with “Dry-Ice.” Thus, some years ago samples of cracked petroleum distillate very sensitive to light and oxygen were stored in carbonated-beverage bottles, dropping into each 6-ounce sample approximately 3 grams of “Dry-Ice” before capping with an ordinary carbonated beverage crown. So far as the changes in color and gum formation would indicate, oxygen elimination was probably complete. These possibilities are outlined here to point out the convenience in laboratory practice of this commercial product and to suggest ways in which it can be usefully applied. Literature Cited (1) Evans, Cornish, and Atkinson, J . A m . Chem. SOC.,62, 4334 (1930).
Determination of Butyl and Ethyl Alcohols in Mixtures’ C. H. Werkman and 0. L. Osburn DEPARTMENT OF BACTERIOLOGY, IOWA STATE COLLEGE, AMES,IOWA
HE partition method Ethyl and butyl alcohols are readily and accurately acetaldehyde, acetoin, or ace(3-7) for the determidetermined by oxidation by potassium dichromate in tone be present in the distilnation of fatty acids in acid solution to the corresponding acids and quantitalate to interfere with the final a mixture may be applied to tive determination of the acids by the partition method. results due to oxidation to the determination of ethyl and The method is designed for quantitative determinaacetic acid, they should be rebutyl alcohols in a mixture tions of the two alcohols in fermentation liquors. moved bv DreciDitation with wit6 good results. The prosome suitatle sibstance such. cedure is simple, rapid, and accurate. The alcohols are oxi- as 2,4-dinitrophenylhydrazine as prepared by van Niel (9). dized to fatty acids with potassium dichromate in the presence For the oxidation, 50 cc. of the alcoholic distillate are placed of orthophosphoric acid. The acids are distilled and the dis- in a 200-cc. balloon flask containing 10 grams of c. P. potassium tillate partitioned between isopropyl ether and water as de- dichromate and 25 cc. of 85 per cent phosphoric acid (ortho). scribed in the partition method. From a calculation of the per- The flask must be fitted with an efficient reflux condenser, centage of acid distributed to the aqueous phase, the percent- preferably with a spiral condensing tube. A few pieces of ages of each of ethyl and butyl alcohols can be read directly from porcelain are added to prevent bumping, and the mixture the nomogram in Figure 2. This method is especially useful heated a t such a rate that it boils in 1.5 minutes. Gentle in the quantitative determination of ethyl and butyl alcohols boiling is then continued for 3 minutes. The condenser tube in fermentation liquors when either one or both are present. is washed down two or three times during the boiling with Van der Lek (1) has determined these alcohols by oxidizing 5-cc. portions of water. If the process is carefully carried out with dichromate and sulfuric acid, steam-distilling the volatile and if the condenser is efficient, no volatile constituents will acids, and then determining the acids by the method of be lost. The flask is cooled rapidly by immersing it, with the Duclaux. condenser still attached, in cold water. As soon as the is cooled to avoid loss of volatile acid, the condenser flask Procedure is again washed down with 15 cc. of water. The flask is reThe alcohols are distilled from the neutral fermentation moved and connected to a Liebig condenser, and the mixture medium by direct distillation. Should substances such as distilled until it begins to foam. There is no tendency to bump, and distillation proceeds smoothly. When the foam 1 Received March 9, 1921.
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