Two Improved Pressure-Regulation Devices - Analytical Chemistry

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Two Improved Pressure-Regulation Devices JACK H. THELIN', Rutgers University, New Brunswick, N. J.

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HE regulators described in this article resulted from a need for a sensitive and positive vacuum control in ex-

periments on vapor pressures of binary mixtures. The first instrument (Figure 1)possessed certain features and characteristics which may be useful for other purposes. KO rubber tubing is necessary, for the regulator may be attached directly to the system t o be controlled. Articles ordinarily found in the laboratory were used in its construction. It may be used either with weights or with the mechanism shown in Figure 2.

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Using weights, a n y pressure in a range from 1 to 200 mm. is quickly obtained and kept, b u t above 400 mm. the pressure fluctuates and the instrument "spills", letting in large amounts of air. Control in the range from 200 t o 400 mm. requires some care. The precision attainable with both regulators is 0.1 mm. in the pressure range above 100 mm. Below 100 mm. the variation in pressure is inappreciable. The operation is shown by Figure 1. A decrease in pressure in the system decreases the force exerted on the left balanve pan, indicated by the rise of the height of mercury, and permits the right-hand balance pan to lower and let air in through the capillary. The dynamic equilibrium set up in this way may be altered to raise or lower the pressure in the system by the simple addition or subtraction of weights on the right-hand pan. A table of weights corresponding to various pressures m a y be made, but these are subject to slight changes in the atmospheric pressure. 1 Present address, Calco Chemical Division, American Cyanamid Go., Bound Brook, N. J.

FIGURE 3

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ANALYTICAL EDITION

December 15, 1941

Using the mechanism shown in Pi ure 2, any pressure from atmospheric down to 1 mm., with delnite control a t any point, may be obtained. This mechanism eliminates the disadvantages of the weights. When pressure on the balance pan is increased by turning the screw down on the perforated rubber stopper, R, more air is admitted t o the system until the mcrease of air pressure in the system closes valve B against the pressure of R. Heavy sponge rubber can be used in R. For pressures in the system close to 760 nun. it is best not t o have the source of m e uum running at full capacity. The balance wits bolted to a ring stand, which in turn was bolted to a wooden base, so that the regulator could be carried from place to place as a unit. To ensure smooth operation a ballast flask was included in the system aud the bottom of the tube dipping into the mercury was constrrcted. About 70 ml. of mercury were used, but this could he reduced by using a smaller

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flask. The glass tubing was 6 mm. and the capillary was 1.5 mm. in inside diameter. The rubber stopper used as,& valve seat was an ordinary stopper boiled in sodium hydroxide solution for approximately 0.5 hour. The capillary mas ground down on an emery wheel and the flat portion which presses against the rubber stopper was ground down, using first a c o m e and then a h e grade of emery powder in glycerol as a grinding medium. The grinding was facilitated by clamping a cork borer in a vertical position and using it as a bearing for the capillary tube as it was rotated. I n this way the plane end of the capillary is made perpendicular t o the axis of the capillary. In order to have a more compac6 unit and to avoid exposure of the mercury to the air, a lever arm was substituted for the balance and a dosed manometer was used. An excellent instrument has been designed by Sebierboltz (1). Figures 3 and 4 show the construction in some detail. The screw which constitutes fulcrum 1is not tightened up completely so that the entire glass portion moves with it as a center. As the mercury falls in the small glass arm and rises in the large portion, owing t o lowering of pressure in the system, less weight is s u p ported by the clamp pressing on the lever a m a t B and more by fulcrum 1. The more firmly the m e w mechanism presses on rubber stopper 2, the higher will the pressure rise in the sy8tem and the higher will the mercury rise in the small side arm to force + h e r stopper 1 against the crtpillary and shut off the supply of air. About 0.75 ml. of air left in tube Ciqcreases the range of pressure over which the instrument is sensitive. With the air pocket,

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INDUSTRIAL AND ENGINEERING CHEMISTRY

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the range of control due to the movement of mercury extended from 1 to 350 mm.; without the air pocket the range was from 1 to 220 mm. Besides increasing the effective range of the regulator 100 mm., the air pocket helps to prevent the breaking of the glass arm by the mercury if the pressure in the system suddenly increases. No rubber tubing is used in the regulator itself. Since connection to the system is made close to the fulcrum, heavy pressure tubing may be used without interfering with the action of the regulator. If gases other than air are desired in the system during a distillation, the apparatus shown in Figures 5 and 6 may be added. This addition is facilitated by the relatively little movement between the capillary and the rubber stopper. If side arms D and E are left open to the air, air will be drawn into the system, but if nitrogen or carbon dioxide is passed through D and out E, these gases will be drawn into the system instead. Before starting a distillation the system should be flushed out with the gas to be used by applying a vacuum and having the capillary open as much as possible. The glass collar is fastened to the capillary by a short length of rubber tubing, G. A glass-to-glass seal is better, since no flexibility is desired at this point. Rubber stopper H ,

Vol. 13, No. 12

forming the valve seat, was cut halfway through with a cork borer and then the outside portion was cut away with a razor as shown in Figure 5 . Two No. 8 rubber stoppers were bored to fit G and H , respectively, and a 4-mm. section was taken from the bottom of each. They were fastened together with a short length of Gooch rubber tubing t o provide free movement between the capillary and rubber valve seat. In Figure 7 the same effect is accomplished using a mercury seal. This is much more rugged and easily handled, but a sudden increase of nitrogen or carbon dioxide pressure might blow out the mercury.

Acknowledgment The author wishes to acknowledge the helpful encouragement of P. A. van der Meulen and D. L. Cottle. Appreciation is also due F. G. Horstmann, Bellerille, N. J., for the unlimited use of his machine shop

Literature Cited (1) Schierholtz, 0. J., IND.ENG.CHEU.,\SAL. ED.,7, 284 (1935)

Disposal of Acid Fumes in Wet Assaying EDGAR J. POTH AND GEORGE A . ELLIOTT Surgical Hunterian Laboratory, Department of Surgery, The Johns Hopkins School of >Iedicine, Baltimore, Md.

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ORKERS in old or improvised laboratories are frequently confronted with the problem of disposal of acid fumes from wet-assay digestions. If the fume-laden gases are passed through zeolite sand covered with water, the fumes are effectively removed. The surface afforded by the grains of sodium aluminum silicate adsorbs and reacts with the fumes rapidly and completely and allows relatively large volumes of inert gas to be passed without the passage of the acid fumes. The fumes are removed so completely that any type of evacuating pump can be employed with the exhaust open to the laboratory. The commercial grade of synthetic zeolite sand gradually

dissolves and passes off with the condensed overflow. The absorbent is most economical. The application of this procedure t'o micro-Kjeldahl digestion is illustrated in Figure 1. Bottle B contains zeolite sand covered with water. A acts as a trap for sand grains from B. A spray trap, C, employing the indentations featured in the Vigreux type of fractionating head, drains well and efficiently breaks up the spray. The tip, D, drains the side arm and prevents the formation of fluid locks in the system, which would cause fumes to escape at the open connect,ions. A trap, C, fits into the mouth of each digestion tube or flask and serves t o prevent loss of sample in the spray and t o connect the digestion vessel to the fume-absorbing system.

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DETERMINATIONS