ANALYTICAL EDITION
386
the upper left-hand object in Figure 3. The hole must be cut exactly in the center of the stopper. Failure to do this results in an inverted stopper having a flange of unequal width and a n oval shape. A little 3-in-1 Oil is used to lubricate the cork borer. With large cork borers, cutting straight through the stopper is easier than with small borers. The cork borer can be withdrawn from a partially cut stopper for application of additional lubricant. The lower outside surface of the inverted stopper can be sandpapered to smooth out slight imperfections as a result of an imperfect cut. If the inverted stopper% too tall it is slipped over the cork borer and a razor blade is passedmound and through the stopper to shorten it
Vol. 6, No. 5
to the desired extent. The sanding of the lower outside edge is carried out with the stopper held as for shortening. The apparatus and materials described and illustrated in this work are all standard laboratory equipment, with the exception of the large cork borers which should be made available as standard laboratory apparatus.
LITERATURE CITED (1) Bailey, J. IND.ENQ.CHEM.,5,756 (1913). (2) Spencer, Ibid.,4,614 (1912). (3) Thomas Co.,A. H.,Catalog, 1931 ed., Items 4196, 4198,4200. 4204,and 4206. RECEIVED Julyi2, 1934.
A Differential Pressure Control Mechanism for Vacuum Distillation S. PATXINAND 0. A. NELSON,Bureau of
D. C.
H E advantages of pressure-regulating devices which pensation becomes a t once an audible warning of leaks and require only intermittent operation of the pump over inadequacies in the condensing system. It is thus particularly those requiring continuous pumping have only re- valuable as a safeguard against the loss of important volacently been appreciated. In a review of the literature on the tile constituents. The pressure-control scheme described in this paper takes subject by Hershberg and Huntress ( I ) only five of some thirty publications cited relate to the intermittent pumping advantage of the sensitive Hershberg-Huntress (2) sulfuric type, and these, with but one exception, have all appeared acid manostat, so adapted as to make possible accurate preswithin the past two years. To these may be added one more sure control with the pump operating intermittently. Sulfuric acid is used as a manometric liquid, and a convenient recent publication (4). A decided advance in pressure-control technic was con- simple U-type manometer of high sensitivity is included as a tributed by Hershberg and Huntress (2) in their publication part of the system. A manometer of this kind, in view of its describing a novel manostat in which sulfuric acid constitutes simplicity, and high sensitivity, should prove a generally usethe manostatic liquid. The latter, with an appropriate ful device for laboratory purposes. As a rule, lighter liquids, thermionic relay and flutter valve, is claimed to permit such as dibutylphthalate and oils, cannot be used in U-type manometers, except in openpressure control within arm forms such as the Hick+0.015 mm. of m e r c u r y . man mercury vapor pump To use this device, however, gage, as it is virtually imthey found it necessary to possible to dislodge the liquid make the pump o p e r a t i o n from the closed end even a t continuous and to use three very low p r e s s u r e s , owing valves to bleed in air a t atprobably to surface tension. mospheric pressure for the For similar reasons inclined adjustment. mercury manometers (4),alIn a distillation system inthough they provide magnit e n d e d f o r precision work, fied movement of the mercury which should be reasonably thread, are not sufficiently . tight to begin with, any unrapid in response to gradual avoidable minor leaks should pressure change to be useful require but very infrequent for accurate readings. Mcpumping for a d j u s t m e n t . Leod gages require tedious The use of continuous pumpc a l i b r a t i o n , provide very ing is t h e r e f o r e n o t only l i m i t e d range, and do not annoying, particularly where permit of continuous observadistillations over long periods tion. are involved, but makes it T h e s u l f u r i c acid mavery difficult to detect losses nometer, while i t d o e s n o t of the more highly volatile possess quite the degree of substances, a source of error a c c u r a c y of the Hickman generally not fully apprecigage, is relatively simple in ated. With i n t e r m i t t en t construction. It provides a pumping such losses would I convenient and ready means, ordinarily be detected early, particularly with pressureas any increase in frequency of pumping over that occaMOTOR PUMP controldevices,for setting and maintaining fixed pressures. sionally necessary for comFIGURE 1. DIAGRAM OF APPARATUS
T
c
C h e m i s t r y and Soils, Washington,
September15,1934
INDUSTRIAL AND ENGINEERING CHEMISTRY
APPARATUS Two distinct pressure systems are maintained with a pressure difference of from 4 t o 30 mm. These are in intermittent communication with one another by way of a valve operated by a magnet, controlled in turn by a HershbergHuntress (2) sulfuric acid manostat a n d t h e r m i o n i c relay. I n the lowpressure system, pressure is maintained s-6 only approximately constant by means of a mercury manostat relay, and may be from 4 to 30 mm. lower than the pressure in the distillation system. The flask containing the valve which is in FIGURE 2 direct communication with the distillation apparatus represents the other system, in which the pressure is accurately controlled. The valve-operating mechanism enclosed in flask F-1 consists of a small simple magnet (old type of telephone bell-ringing
magnet), the movable part (hinged lid) of which is covered with a strip of smooth rubber. The magnet and a long 0.125-inch (0.32-cm.) tube, t, the end of which has a capillary opening, c, are securely fastened to a base as indicated. The movable part of the magnet effects intermittent closure of the ca illary. The sulfuric acid manostat and thermionic regy, M and TR, are essentially as described by Hershberg and Huntress (B), a mercury-sealed ground-glass joint (Figure 2) being substituted for the ground-steel joint used in their apparatus. The mercury manostat (Hg-M) is similar to the sulfuric acid manostat in construction and provides for a wide range of pressures. It is rendered less sensitive than the ordinary U-type mercury manostat by making bulb b relatively large. The manometer, M-2, is a simple U-type made from Pyrexglass tubing at least 2 cm. in inside diameter, to compensate for the high viscosity of the acid (d). Sulfuric acid, of the concentration used in the sulfuric acid manostat (sp. gr. 1.71), may be used as a manometer liquid, the apparatus being properly trapped to prevent diffusion of gases or vapors to or from the acid. Melville (3) mentions the use of sulfuric acid for such a purpose. After filling, it was carefully boiled out by gentle heating, the pressure having been reduced to 1 to 2 mm. of mercury; 7.9 mm. difference in acid level corresponded to 1 mm. pressure of mercury. Flasks F-1 and F-2 are used to eliminate possible pressure fluctuations when the valve opens. It was noticed that with a pressure difference of 5 to 6 mm. in the two systems and with flask F-1 of 2-liter capacity, there was a movement of about 1 mm. in the sulfuric acid levels on opening add closing the valve. Substituting a 5-liter flask eliminated this difficulty entirely, a pressure difference of 32 mm. of mercury in the two sysFlask %-2 may be of any convenient size; 2-, 5-, and 12flasks and a 20-gallon (75.7-liter) tank gave equally good results, the only difference being somewhat more fre uent starting and stopping of the vacuum pump with the smJler vessels. T I is a trap filled with quicklime. Other traps may be used at TzrFa,etc. The entire apparatus with the exception of the pump was made portable as in the case of the Hershberg-Huntress scheme (I), by mounting i t on an instrument board with a fixed base, connections with distillation apparatus being made with rubber tubing at XI,Xz,and XI. The general principle of dual pressure control described above can be applied to some advantage even with the pump (mechanical or water-aspirator type) continuously operated. When so used the mercury manostat, Hg-M, and pump relay are eliminated. Leakage or an inadequate condensing system would manifest itself by too frequent operation of the thermionic relay, T R , and valve. OPERATION For use with a fractionating column the following sequence has been found desirable and permits the taking of fractions during the distillation without disturbing the pressure: To evacuate the system the manostat stopcock, 5-6, is opened and stopcocks 8-1, 5-3,and S-4 are turned so as to connect F-1 with F-2, and the by-pass between receivers R, and Rz is opened
387
through S-2, so that the entire system is now connected directly with F-2 and the vacuum pump. When the required pressure is reached, 8-2 is closed and 5-3 is turned so as to connect RI and Rz. At the same time the sulfuric acid manostat is adjusted so that capillary tip c of tube t inside F-1 is closed by the rubber pad as discussed above and the manostat stopcock is closed. 5-1 is turned so as to connect F-1 with R1. Evacuation of F-2 is continued until the desired pressure difference between the two systems is reached, as observed on manometers M-1 and M-3, when the mercury manostat, Hg-M, is adjusted. To remove a fraction of the distillate from the receiver RS, the following procedure is recommended:
5-3is turned to shut off R1 from Rz, and 5-4 is opened to the atmosphere. Through 5-3, Ra is brought to atmospheric pressure, and the fraction is run out through 8-5. During this operation, the pressure in the distillation apparatus as a whole is being maintained constant. After the fraction has been taken, the pressure in Rz is reduced to that which prevails in the still, in two stages as follows: Rz is connected with F-2 through 8-4, and evacuation continued until the pressure recorded on M-3 has practically reached that of M , and 8-4is promptly turned off. Final adjustment of the pressure in Rz is accomplished by establishing communication between it and F-1 through 8-1, after which all stopcocks are turned to their original positions. SUMMARY
I n the pressure-control assembly here described, a dual pressure system is maintained with the two systems about 4 to 30 mm. apart. One vessel, in continuous communication with the distillation system, constitutes the higher pressure system, the pressure being maintained constant by means of a valve operated by a magnet controlled by a HershbergHuntress sulfuric acid manostat and thermionic relay. Another vessel, in continuous communication with the pump but in intermittent communication with the first vessel, constitutes the lower pressure system, the pressure being maintained only approximately constant. The pump operates only occasionally and is controlled in turn by a mercury manostat and relay. The sulfuric acid U-type manometer described is about eight times as sensitive as the ordinary mercury manometer.
LITERATURE CITED (1) Hershberg and Huntress, IND. ENG. CHEM.,Anal. Ed., 5, 144 (1933). (2) Ibid., 5,344(1933). (3) Melville, H.J., J . Chem. SOC.,1931,2509. (4) Palkin, S.,IND.ENG.CHEM.,25,95 (1933). R ~ C ~ I May T ~ D 18, 1934.
A Simple Absorption Pipet ELDONA. MEANESAND EDWARD L. NEWMAN Wichita Testing Laboratories, Wichita, Kans.
T
HE accompanying dia-
gram illustrates a carbon dioxide absorption tube for organic combustion analysis. Besides being simple in construction, it has the added advantages of being in one c o m p a c t u n i t , easy to fill, easily hung on the balance, and much more rugged than the Liebig or Geissler bulbs. V e r y s a t i s f a c t o r y results have been obtained from the use of this pipet. RECJDIYED July 7, 1934.
SOOP
LIME TUBE
*
I I
