An Improved Automatic Gas Effusiometer LEONARD C. DRAKE, %cony-Vacuum Oil Company, Inc., Pauleboro, N! J.
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HE effusion method for the determination of the specific
gravity of gases is widely used where grcat accuracy is not required, and is particularly adaptable to routine and control work. The method is based on the observation that the square of the time required for equal volumes of two gases under the same pressure gradient to pass through the same orifice is proportional (approximately) to their specific gravities. Thus, the specific gravity of any gas (compared with air as 1) is conveniently obtained by means of the following equation: Specific gravity of gas =
time for gas to escape time for air to escape
The most widely used effusiometer is that designed by Schilling, in which a constant volume of gas or air confined over water in a glass tube is forced through a small, thin, standardized platinum orifice by the water seeking its level. The time for the water level to pass two marks on the tube is measured with a stop clock. Mitchell (3) among othen has discussed the limitations and uses of this method.
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holder. The capillary tube or metal holder is joined to one arm of the capillary stopcock with rubber tubing. The orifice may be protected from the adverse effects of drafts by the modfication suggested by Mitchell (3). The electric stop clock is a 4-watt, magnetic starter ty manufactured by the Standard Electric Time Co., Springfieg Mass. The electric circuit 1s the series-parallel one used by Feskov, but became the salt solution between A and B acts as a resistor, only one 1000-ohm external resistor is needed. This 1000-ohm resistor may be the wire-wound, vitreous porcelain enamel ty e commonly used in modern radios and obtainable from r a i o repairmen. Some clocks may require an additional resistor, which should be placed between A and X. The glass U-tube is filled with a salt solution to a level sliglitly below the contact, B, and the whole a paratus is lowered into a water bath, so that the external water revel is slightly belowthe stopcock. The liquid in the U-tube may be either saturated sodium chloride solution or the acidified sodium sulfate solution suggested by Kobe and Kenton 2). This solution works articularly we! and is prepared by ad ing 40 ml. of concentrate! J36 N ) sulfuric acid to 200 grams of anhydrous sodium sulfate dissolved in 800 grams of distilled water. Air or gas is introduced until all three platinum contacts are covered with brine. Because of the drain of current through A-X, there is not enough current available to allow the stop clock to run. The stop clock is then set to 0 and the stopcock turned to allow the air or gas to escape through the orifice. When the brine level falls below A the stop clock starts and when it passes B it stops because the circuit is broken. For best results the salt solution must drain freely from the walls of the tube. When the apparatus is first uFd, .gas .bubble! will appear on the platinum wires, but these will diminish until they practically disappear. Several determinations are required to dilute and flush out the gas in the capillary tubing. When this is done, the time for the salt solution to pass from A to B is reproducible within 0.1 second, which allows specific gravities to be calculated to the third decimal place (1). Corrections for water vapor in the gas may be made (3).
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Other Uses
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Feskov (I) has described an automatic effusiometer in which he used mercury instead of water as the displacing medium and eliminated the visual observation of the moving mercury surface by employing a series-parallel electric circuit to start and stop an electric stop clock. His method eliminated the possibility of moisture condensing on the edge of the orifice, but it cannot be used with gases containing hydrogen suliide. This limitation is very serious with most petroleum gases and led to t h e development of the apparatus described herein.
The apparatus as described may be modified in a number of ways. The A contact may be made movable by supporting it with a slotted stopper. This allows a wide variation in the volume of gas forced through the orifice and a corresponding wide time range. By removing the orifice and attaching a rubber tube to the capillary tube it is possible to use the apparatus as a timing mechanism for other equipment; such a connection with a capillary viscometer tube will allow the operator to carry on other duties while the time is automatically recorded. The time of filling or emptying tanks can also be conveniently determined.
Literature Cited (1) Feskov, G . V., Im.ENQ.CHEM.,ANAL.E D . , 11, 653 (1939). (2) Kobe and Kenton, Ibid., 10, 76 (1938). (3) Mitchell, G. S., Refiner, 5, No. 8, 38, 40, 42; No. 9, 42, 44,47 (1926).
Improved Apparatus This improved apparatus employs salt solution as the confining liquid and is thus particularly adapted to gases that are normally stored over brine. High-priority mercury is not required. The apparatus consists of four parts: (1) a water-jacketed bulbed U-tube with a three-way stopcock closing one end, (2) standard orifice, (3) electric stop clock, and (4) electricr esistorsand connectors (see dia ram). The glass U-tube has three platinum wire contacts fusef into the open end arm of the tube. These are made of 2.5-cm. (1-inch) lengths of 20 B. & S. gage wire sealed directly into the glass. If the U-tube is made of Pyrex these seals are not gas-tight but are made water- or brine-tight by applying a thin coat of waterproof cement. The standard orifice (obtained from The American Meter Co., Philadelphia, Penna.) may be either cemented to 3.75-om. (1.5-inch) length of 2-mm. capillary tubing ground flat on fine emery cloth, or it may be obtained already mounted in a metal
P R E E E N Tbefore ~ D the Division of Petroleum Chemistry at the 105th Meeting of the AMERICAN CHEMICAL SOCIETY, Detroit, Mioh.
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