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a p H value (using a glass electrode) of 6.0. When air free from ammonia and carbon dioxide was blown through the still, the conductance changed to between 0.5 x 10-8 and 0.3 X mho. with a pH value of 6.8. Variation of the acid feed showed that the best results were obtained when the pH value of the overflow water from the still was around 3.5. After running with this type of treatment for several months, the still was found to be in very good condition. A black coating appeared below the water level but the tin coating was still intact beneath it. A typical balance of acid feed and distilled water generated is as follows: The acid chromate solution was made up with 9 cc. of concentrated sulfuric acid and 10 grams of sodium chromate to 5 liters of water. This was added by means of a continuous feeding device at the rate of 30 cc. per minute. About 15 liters per hour of condensed steam were mixed with the acid feed and fed t o the still, the excess above the amount distilled overflowing
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to waste. Nine liters per hour of distilled water‘were generated. The water overflowing from the still had a pH value of about 3.5.
The steam from the heating system contained a small amount of oil, giving a faint opalescence to the condensed steam, but the distillate from the still was perfectly clear, The oil appeared to carry over and deposit in the tin piping and from time to time globules of oil about 0.16 cm. in diameter came through with the water. A separator shown in Figure 2 was installed. The oil collected on the top of the solution and was drawn off through a valve, so that the resulting water was free from oil. RECEIVEDJuly 3, 1935. Presented before the Division of Industrial and Engineering Chemistry a t the QOth Meeting ot the dmerican Chemical Society, San Francisco, Calif., August 19 to 23, 1935. Part of the research conducted in cooperation with the Utilities Research Commission, Chicago, Ill. Published by permission of &I.L. Enger, Director, Engineering Experiment Station, University of Illinois.
A Precision Oil Gage S. PALKIN, Bureau of Chemistry and Soils, U. S. Department of Agriculture, Washington, D. C.
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NCREASING application of vacuum practice to fractional distillation has stimulated the development of a variety of pressure control devices (2, 3, 5, 6 ) . Marked progress in this direction has been made in recent years, and control devices capable of a high order of precision have been developed (3, 6 ) . Progress has lagged, however, in the development of gages capable of measuring the small pressure changes often involved-changer; of a magnitude comparable with the precision control now possible by the improved devices. The ordinary mercury manometer of the U-tube type, the most widely used in laboratory practice, permits a wide range of absolute pressure measurements but is comparatively coarse and insensitive. Pressure changes less than 0.25 mm. cannot be read with any reasonable degree of accuracy. As may be observed from vapor pressure curves generally, there is a marked decrease in pressure per unit of temperature as one approaches the low pressure region. Temperature changes in the higher pressure regions generally conform to relatively marked changes in pressure and are not apt to be confusing. But a t 20 mm. or under, a temperature change of 0.2’ and sometimes more in a distillation may be due to pressure changes (in the system) which cannot be detected on the manometer rather than to any change in composition of the vapors-the inference usually drawn. An urgent need therefore exists for pressure gages of high precision, particularly in the range of 40 mm. or less. The McLeod gage, useful in high-vacuum work, has a limited usefulness in this field since it provides a very narrow range, necessitates separate manipulation for each observation, requires tedious calibration, and cannot be used with condensable gases. A mercury gage developed by Dubrovin (1) intended to fill this need, has a pressure range up to 20 mm. and permits a magnified reading of about 9 to 1, but the magnification is not uniform over the 20-mm. range, so that for precision work calibration throughout the entire range is here also necessary. In addition, reconditioning of the gage, occasionally required, makes recalibration necessary after each reconditioning. A sensitive U-manometer constructed of large diameter tubing and utilizing sulfuric acid as the manometric liquid was described in a previous publication (6). One millimeter on this manometer is equivalent to less than 0,125 niin. of
mercury. The high absorptive capacity of strong sulfuric acid for olefin, terpene, and other unsaturated vapors, however, renders this manometer, in the course of time, unreliable when used in distillations involving these compounds. Even with efficient trapping devices, occasionally “reboiling” out of this instrument becomes necessary.
Oil Gages The magnified reading and consequent increased precision that is possible with light liquids such as butylphthalate has long been recognized. But, as is well known, these liquids can only be used in the U-type manometers of the open-arm form, such as the Hickman gage (4), because of the impossibility of dislodging the liquid from the closed end. To provide the necessary reference vacuum, therefore, the Hickman gage utilizes a butylphthalate condensation pump as an integral part of the gage assembly. Its operation is necessarily a continuous function of the gage while the latter is in use. I n this paper, an improved gage is described which utilizes oil or other light nonvolatile medium as the manometric liquid and does not require a high-vacuum condensation pump for either its preparation or its use. For purposes of preparing or conditioning the gage a good laboratory pump, usually the same one used in connection with the vacuum distillation, is sufficient. After that, it functions much like any other type of gage, independent of the pump. Production of a reference vacuum, comparable to the Torricellian vacuum of the ordinary U-tube type, i5 made possible primarily by the construction of the gage. With reasonable safeguards, the reference vacuum can be maintained for a long time and the gage can be used either continuously or intermittently without the need for reconditioning, although conditioning is, in itself, a very simple process and can be repeated for checking purposes as often a , desired. With-paraffin oil as a manometric liquid, a pressure difference of 1 mm. oil level is equivalent to less than 0.067 mm. of mercury. Assuming an accurate reading of 0.5 (mm.) division, pressure changes of 0.033 mm. of mercury may therefore be read with precision. The oil level is, furthermore, very responsive to small pressure changes, no tapping, etc., as in mercury manometers, being required.
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TO PUMP TO W l N T X . stopcock can be used, the mercury-sealed, or The gage is in effect a double U-tube type CONTROL UNIT A-C grease-sealed stopcock (shown in detail) is premanometer in which both oil and mercury are ferred. The grease should be “high vac” type, employed, the latter serving, however, only as free from volatile matter, a mobile “backing” medium for the oil column. Reservoirs 30, 31, and 32 are joined hermetiThe bores, respectively, of the tube below the stopcock, the plug, and the tube above the c a 11y , as indicated, by thick-walled capillary stopcock are progressively larger, as shown, in tubes of approximately 1.5-mm. bore for the order to prevent the trapping of air bubbles. tube between reservoirs 30 and 31, and 2.5 mm. between reservoirs 31 and 32. Reservoir 30 serves as the mercury reservoir, 32 as the oil, Sonie indication may be bad of the uniand 31 as a temporary common reservoir for 35 formity of behavior of the oil gage throughboth, also as a degassing chamber and as a 6 o u t i t s p r e s s u r e range from the data in space for the Torricellian vacuum, comparable to the closed end of the simple U-type mercury Table I. Comparative readings were made manometer. with the oil gage and a well-made simple Stopcocks 33 a.nd 34 directly below reserU-type mercury manometer, using the presvoirs 30 and 32, respectively, serve t o lock sure control unit (described in another paper, the two liquids, mercury and oil, as necessary. The three-way stopcocks, 35 and 36, 6 ) to m a i n t a i n c o n s t a n t pressure a t the serve p r i m a r i 1y t o establish communication various points while making the observations. with the pumping system, the atmosphere, or Readings were checked by T. H. Harris of this the system whose pressure is t o be measured, laboratory. a n d f o r manipulating the movement of the liquid during preparation for use as explained The mercury gage could probably be read below. a t best to a fourth of a division (0.25 mm.). The three-way stopcock, 37,. provides a means The values given in column 3 of Table I for selective communication elther directly with were obtained by dividing the oil readings by the pumping system or with the gas system whose pressure is to be measured, as the case 15.53, the ratio of the specific gravities of may be. The relative position of the reservoirs, mercury and oil. their dimensions, etc., are in general as indiIt was found that pressure readings could cated in the illustration. The position of 30 must, however, be such as to insure against a be made more rapidly and accurately by using differential mercury height in excess of 76 cm. the top of the wide meniscus (reservoir 32) (barometric height), so as to make possible free for the 0 reference point and reading the dismovement of the mercury through t,he whole tance between it and the b o t t o m of t h e a paratus when so desired. Thus, the greater meniscus in the capillary rather than reading tfe oil reading range desired, the higher the DETAIL OF STOPCOCK 34 mercury reservoir 30 must be set up. the distance between the bottom of the two In order to maintain a Torricellian reference menisci. The d i f f e r e n c e between top and FIGURE 1. DIACROIOF vacuum, the oil or other light liquid must be GAGE bottom should, however, be determined as suit’able for manometric work, have a very low vapor pressure and reasonably low viscosity, and also the e r r o r du’e t o c a p i l l a r i t y . The be free from volatile impurities, to insure against the filling of latter is best determined on a separate piece of capillary reservoir 31 with vapors. tubing (2.5 nim.). These error? are rompensating and their difference, which is generally within 1 mm. of oil (0.06 mm. Conditioning Gage for Use of mercury), is applied as a correction for absolute pressure The requisite quantity of purified mercury is introduced into reading. reservoir 30 by way of opening 38 (ordinarily kept closed with rubber stopper) and stopcock 35 by applying slight suction TABLE1. COMPARATIVE READINGS through 37 and is allowed to fill the capillary tube up to bottom of reservoir 31. The mercury is then locked in position by stopOil Reading Reading on Calculatedc Reading on cock 33. The rest of the system (reservoirs 31,32, capillary, etc.) Oil Gagea Hg U-Manometerb to Mm. Hg is then evacuated by communication with the pump by way of Mm. Mm. jtopcocks 34, 36, and 37. Stopcock 36 is then locked and the requisite quantity of oil or other nonvolatile manometric liquid 543 35 34.97 is introduced through opening 39 and drawn into reservoir 32 through stopcock 36 and thence into reservoir 31. By proper manipulation of appropriate stopcocks the mercury layer and its overlying oil layer are then allowed to rise in reservoir 31 to a point about one-half or two-thirds full. Stopcock 33 is then locked. Suction is applied to both sides of the system by way of the pump and pumping continued until no further gas evolution is evident. Heat may be applied to hasten the degassing process. Reservoir 30 is then opened to the atmosphere by way of stopcock 35 and by careful manipulation of stopcock 33, mercury and oil are pushed upward, the oil layer being sent through the capilOil used was dpieson-B (J. Biddle and Co., Philadelphia), sp. gr. 0.8712 lary and the major part into reservoir 32 until the top of the at. ~. 2.5/250. . mercury column is at the top of reservoir 31. The oil layer is then 6 M&ury manometer readings accurate to about 0.3 mm. Calculated ,values obtained by dividing oil readings by 15.53-the locked by stopcock 34 and, with the pump operating continuously, ratio of the specific gravity of mercury to that of the oil. the three-way stopcock is turned to evacuate reservoir 30. When the pressure has been sufficiently lowered, the mercury thread will break a t the mercury-oil juncture and the mercury column is then allowed to drop and empty reservoir 31. Stopcock 33 is Literature Cited then locked. There is thus produced a Torricellian vacuum between the mercury and oil. Moreover, since reservoir 31 is (1) Dubrovin, U.S. Patent 1,928,096(Sept. 26,1933). relatively large, the effect of any trace of residual gas in the mer(2) Hershberg and Huntress, IND.ENQ.CHEM.,Anal. Ed., 5, 144 cury or oil that had not been removed in the degassing process is (1933). (Contains comprehensive bibliography to 1933.) (3)Ibid., 5,344 (1933). so minimized as to constitute a ne ligible factor. With the pump operating continuously, stopcocf 34 is now opened, the oil 52,4714(1930). (4) Hickman and Weyerts, J.Am. Chem. SOC., column drawn over to a convenient level, locked again by 34, Anal. Ed., 7,70(1935). (5) Jacobs, IND.ENG.CHEM., (6) Palkin and Nelson, Ibid., 6,386(1934). and is now ready for use. Communication with the system to be measured is made through stopcock 37. Stopcock 34 is opened and differential pressure read directly as in the ordinary mercury RECEIVEDJuly 15, 1935. Presented before the Division of Agrioultural U-type manometer. and Food Chemistry a t the 90th Meeting of the American Chemical Society, SPECIALSTOPCOCK 34. While any well-ground, long-barreled San Francisco, Calif., August 19 to 23, 1935.
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