A Stillhead for Rapid Concentration in Vacuo HUBERT BRADFORD VICKERYAND GEORGEW. PUCHER Connecticut Agricultural Experiment Station, New Haven, Conn.
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IOCHEMICAL laboratories are frequently faced with the problem of concentrating large volumes of aqueous or alcoholic solutions. This is a time-consuming operation, especially when care must be taken with respect to the maximum temperature permissible, and devices which will promote a rapid rate of distillation are highly desirable adjuncts to the standing laboratory apparatus. Figure 1 shows an extremely convenient and efficient stillhead that has been used for general concentration purposes in this laboratory during the past,.y4 years. When this is
together with a cooling surface to reduce this volume and condense a part of the vapor, before it must pass through tubing of small size. The desirability of an arrangement constructed on this principle was first pointed out to the authors by H. A. Spoehr. A number of heads of variously modified dimensions have been constructed-for example, a longer coil has been fitted into a correspondingly longer cooling chamber, or has been sealed to tubes passing through the wall of the cooling chamber. The most serious disadvantage of a longer coil, aside from its inconvenience, is the danger of fracture just below the rubber stopper when the flask must be shaken. A fusedin coil is more difficult to construct, but is necessary if the presence of rubber must be avoided. The heads are suspended from an overhead bar by a length of brass chain which contains a spiral spring of the type used for bird cages; this flexible arrangement cares for the rise of the flask in the water bath as the load diminishes, and permits agitation of the flask by hand should this be necessary. The Graham condensers are likewise suspended from chains. These are placed a t the end of the bench, or pass through holes cut to accommodate them, and are protected by being wrapped with a single layer of stiff wire gauze. Large filter flasks are employed as receivers; stopcocks in the vacuum line to each permit the stills to be used singly when desired. The supply of water to the cooling coils is controlled independently from that to the condensers.
l l
60cm G m h m
condenser
FIGURE 1
attached to a &liter flask, a distillation rate of 2 liters of water an hour can be maintained from solutions that do not froth badly, while from alcohol solutions the rate can easily be made to exceed 3 liters an hour. A similar apparatus, on a scale somewhat more than twice as large, and provided with a 5-liter bulb on the vapor tube, when attached to a Pfaudler steam-jacketed laboratory still, permits from 10 to 11 liters of water an hour to be distilled. The cooler coil in this apparatus is constructed of block-tin pipe, and a baffle plate is hung inside the bulb so as to intercept spurts of liquid from the still. The dimensions given in the diagram are those of the stillheads set up in pairs on each workbench. The apparatus consists of a vapor tube constructed of 2-cm. tubing, furnished with a side arm of the same diameter, which is sealed into the wall of a cooling chamber 48 mm. in diameter. The cooling chamber contains a coil of about 14 turns of 8mm. tubing suspended from the rubber stopper in the top. The advantage of the apparatus is the provision of relatively large-bore tubing to accommodate the hot expanded vapor,
To begin a distillation, screw pinchclamp A on the rubber connection between stillhead and condenser is closed, and the receiver is evacuated. The ring-neck flask is then placed in position in the hot-water bath with the air-inlet tube reaching t o the bottom (this tube should not be constricted at the lower end), and the short length of rubber tube at the top is clamped. The neck of the flask is held firmly against the lower surface of the rubber stopper while pinohclamp A is opened. Water is admitted to the condenser, and a slow stream of air is allowed to flow through the air-inlet tube. After distillation has begun and any tendency of the solution to froth has subsided, water is admitted very gradually to the cooler coil. The rate of distillation immediately accelerates, and great care must be exercised with many solutions to prevent frothing over. Should this become imminent, the water supply t o the cooler is immediately shut off and clamp A is closed. New supplies of solution may be added to the flask as needed through the air-inlet tube; when this is done, however, clamp A must be closed and the water supply to the cooler shut off. Alternatively, a short length of 2-mm. tubing through the stopper of the flask may be used, with proper connections, to admit a continuous slow stream of fresh solution.
Although primarily designed for the rapid distillation of large volumes of solutions, the stillhead is also advantageous when only small volumes are to be concentrated. The reversed rubber stopper permits the use of ring-neck flasks as small as 500 cc., and the less convenient side-neck distillation flasks are not required. This has many advantages when concentrating a solution previous to crystallization. RBCEIIV~D June 18, 1934. The expenses incurred in the design of this apparatus were shared by the Connecticut Agricultural Experiment Station and the Carnegie Institution of Washington
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