ANALYTICAL EDITION
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the course of this investigation on the apparatus which he designed.
OUR suggestions during
Vol. 5, No. 5
(10) Jander and Beste, 2.anorg. allgem. Chem., 133,46 (1924).
(11) Jannasch, “Praktischei Leitsaden ber Gewichtsanalyse,” 1897, n. 2x3 F
LITERATURE CITED Beck, Z. anal. Chem., 47, 465 (1908). Buokwalter and Wagner, E. C., J . Am. Chem. Soc., 52, 5241 (1930). Bunsen, Ann., 86, 265 (1853). Chapin, J . Am. Chem. Soc., 41,351 (1919). Ebell, Rep. anal. Chem., 141 (1886). Farsoe, 2.anal. Chem., 46,308 (1907). Finkener, Zbid., 43,656 (1904). Fleck, Pha~m.Zentralhalle, 22, 152 (abstr. Z . anal. Chem., 21, 444 (1882). Fresenius. “Quantitative Analysis,” 6th ed., Vol. 1, p. 476.
(12) Le Blanc and Ebsrius, Z . anal. Chem., 89, 81 (1932). (13) Lunge, G., and Berl, E., “Chemeurisch-technische Untersuchungs methoden,” 7th ed., Vol. 1, p. 972. (14) Marc, Chem.-Ztg., 26,556 (1902). (15) Roark and McDonnell, J. IND.ENQ.CHEM.,8,327 (1916). (16) Rupp, 2.anal. Chem., 57, 226 (1918). (17) Sherer and Rumpf, Chem. News, 20,302 (1869). (18) Topf, 2. anal. Chem., 26,295 (1887). (19) Treadwell and Christie, 2.angew. Chem., 18, 1930 (1905). (20) Ullman, Chem.-Ztg., 18, 487 (1884). (21) Wagner, E. C., IND.ENQ.CHEM.,16,616 (1924). R ~ C E I YMay ~ D 16, 1933.
T T
Automatic Pressure Regulators for vacuum Distillation 11. Sulfuric Acid as a Manostat Fluid E. B. HERSHBERG AND E. H. HUNTRESS Research Laboratory of Organic Chemistry, Massachusetts Institute of Technology, Cambridge, Mass.
I
N A PREVIOUS communication (2) a simple portable
apparatus was described for the automatic control of reduced pressure during the vacuum distillation of organic compounds. I n connection with further more exacting work, however, it was desired to increase the precision of regulation. In order to do this the authors have designed a second simple control (Figures 1 and 2) in which the final pressure adjustment is effected by means of a thermionically controlled flutter valve, and in which concentrated sulfuric acid is substituted for mercury as the manostat liquid. This combination is approximately ten times as sensitive as the previous unit and permits a regulation of *0.015 mm. mercury at any pressure up to atmospheric.
THETHERMIONIC RELAY The manostat contacts actuate the g r i d c i r c u i t of a 71-A t y p e vacuum tube, which controls the operation of a 2000-ohm magnetic relay. The circuit is similar to that described in the previous paper, but operates in the opposite sequencethat is, closing of the grid circuit causes the plate current to flow and this in turn closes the relay. The latter lifts a rubber pad from a capillary inlet allowing air to leak into the system and establish the correct pressure. Needle valves of the c o n v e n t i o n a l design (Hoke No. 304) are employed for rough adjustment of the air inlets, leaving the final flutter control to be effected by the relay system.
THE MANOSTAT
The manostat shown in Figures 1 and 4 combines several desirable features. It operates throughout the entire range of pressure, uses a minimum of fluid, and presents but litble frictional resistance. The bulbs B and C (Figure 4) provide for a sudden change of Dressure. The diameter of B is such that most of the motion is confined to the right-hand arm A and by tilting alone a range of *2.5 mm. m e r c u r y may be secured. The manostat is constructed of Pyrex c h e m i c a1-r e si s t a n t glass with sealed-in platinum contacts F and G. External connection is made through mercury pools H and the center e l e c t r o d e is joined to a standard taper ground-glass joint I to facilitate cleaning and filling the manostat. The stopcock J is l u b r i c a t e d with v a s e l i n e only, rubber lubricants being more or less attacked by the acid. The manostat is sealed into the ground-steel joint with Picein or de Khotinsky cement. Flexible rubber connections are avoided and manostat s e t t i n g is f a c i l i t a t e d by p i v o t i n g on the ground steel joint K , which serves as an axis for rotation. The joint is of s t a i n l e s s steel ( A l l e g h e n y metal) both parts being cut a t the same lathe setting with about a 14’ taper and lightly ground together with 600-mesh s i l i c o n c a r b i d e . V a s e l i n e is used as l u b r i c a n t and the pressure of the a t m o s p h e r e o r c h e c k n u t s suffices to VIEW OF MANOSTAT FIGURE 1. FRONT
September 15,1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
prevent turning when once the manostat is set a t a particular pressure.
THEMANOSTAT FLUID Because of its peculiar properties mercury has hitherto been universally employed as a manostat fluid. Its negligible vapor pressure, low viscosity, and high electrical conductivity make it particularly valuable in this connection, but these are to a large degree offset by its high density, enormous surface tension, and failure to wet the glass walls of the manostat.
345
centimeters has been created. This describes an exaggerated case, but the factors are operative to a lesser extent even in very wide tubes. The conditions are at their worst after a slow motion in a tube which is free from vibration ( I ) .
Considered as a manometer fluid it is true that some of these objections may be eliminated by providing a lubricant, but with manostats where the mercury must also play the part of an electrical switch, such a lubricant acts as an insulator which must be broken down or squeezed out from between the contacts before electrical connection can be established. On the whole, it is simpler to abandon mercury as a manostat fluid and to seek some other liauid which will combine high electrical conductivity, low de;sity, low v a p o r p r e s s u r e , and reasonable v i s c o s i t y , with the property of wetting the manostat walls. Sulfuric acid possesses approximately the desired properties. It wets the glass manostat walls, its density is about one-seventh that of mercury, and its conductivity is sufficient to permit use of a sensitive thermionic relay. It is, however, nearly fifteen times as viscous as mercury, and in order to obtain a comparable sensitivity under a given pressure head the m a n o s t a t must be. about four F I G U R4. E SULFURIC ACIDMANOSTAT times the d i a m e t e r of that for mercury, considering the latter A. 2 om. inside diameter B . 6 em. inside diameter as a perfect fluid and neglecting C. 2.5 om. diameter D. 17.5 om. film friction. E . 5 om. on centers Sulfuric acid (sp. gr. 1.84) when F. 0.001-inch (0.026-mm.) dkmeter platinum contact kept in vacuum e v o l v e s gases wire FIGURE 2. SIDE VIEW OF MANOSTAT G. 0.005-inch (0.125-mm.) which tend to impair the setting diFmeter platinum contact It is readily attacked by common agents and the products of of the manostat. It was found, wire H . Tubes filled with merthis chemical attack are usually insoluble in the bulk of the however, that b y u s i n g acid of cury to establish contact between the fine platinum wires fluid, precipitating a t the interface and forming a continuous sp. gr. 1.71 gas e v o l u t i o n was and the external circuit a negligible minimum, reduced to I . No. 11 I. S. T. groundfilm which must be broken before motion can occur. joint provided the manostat had been glass J . 3-mm. bore stopcock K. Ground-steel joint Mercury adsorbs most substancespowerfully but it is especially given the proper preliminary treatactive towards its own compounds and it may refuse to recede ment. This consists in evacuating from n plug of mercuric oxide or sulfide until a tension of some the filled manostat to a pressure of about 0.1 mm., boiling gently for 5 minutes, and allowing it to cool under vacuum. - + The manostat should subsequently be kept under vacuum when not in use by closing the 4-mm. oblique bore stopcock a t the rear of the ground-steel joint. Under these conditions the rate of increase of pressure in the system per hour is less than the sensitivity of the manostat.
FIGURE3. UNIVEbaSAL 115-VOLT A. C.-D. C. THERMIONIC RELAY A . 2000-ohm magnetic relay C. 4-mfd. condenser, either foil or dry electrolytic type. If the latter is used the proper polarity must be observed. M . Manostat contacts Ri. 25-watt, 115-volt tungsten filament Mazda lamp Rz. 40- to 50-watt, 115-volt tungsten filament Mazda lamp Ra. 3- t o 5-megohm resistor, 2-watt capacity T. 71-A type vacuum tube
THERESERVOIR AND EXTERNAL CONNECTIONS A reservoir of about 2 liters capacity is ample to absorb surges and serves as a convenient point to fasten the tubes leading to the pump and apparatus. A low-temperature vapor trap similar to that previously described (8) may be used to protect the mechanical pump from condensable vapors. However, since there are no valves in the gas path, and since the regulator itself is constantly being swept out by the incoming air or inert gas, the trap may be dispensed with when using a water suction pump, corrosive gases discharging with the water stream. OPERATION In operation the right-hand needle valve is manipulated so that the pressure is reduced to within 1 or 2 mm. mercury of the desired point as indicated by a sensitive mercury or butyl phthalate gage, and the manostat stopcock is closed. As the pressure continues to fall the acid rises in the right-
ANALYTICAL EDITION
346
hand manostat arm. As soon as it touches the platinum contact the relay valve opens, allowing air to leak in until the electrical connection is broken. The manostat is then tilted until the desired pressure is reached, and a final close adjustment of the needle valves is made to eliminate surging. With the pump operating continuously the flutter valve oscillates at a frequency of from approximately 60 to 400 vibrations per minute. During operation excessive temperature changes of the room are to be avoided, since these will ultimately alter the setting.
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ACKNOWLEDGMENT Grateful acknowledgment of assistance is made to A. A. Morton, D. E. Bennett, and E. A. Averill.
LITERATURE CITED (1) Hiokman, J . Optical SOC.Am., 19, 193 (1929). (2) Huntress and Hershberg, IND. ENG.CHEM.,Anal. Ed., 5, 144-6 (1933). R ~ ~ C B IJune V ~ ~16, D 1933. Contribution No. 92 from the Research Laboratory of Organic Chemistry, Massachusetts Institute of Technology.
A Continuous Air-Lift Extractor Application to Quantitative Determination of Benzoic Acid RAY P. CHAPMANAND LOUISP. HAMMETT, Department of Chemistry, Columbia University, New York, N. Y.
H
AVING to determine the solubility of benzoic acid in sulfuric acid solutions over a wide range of concentrations, the authors have experimented with continu-
ous extractors with the hope of effecting a saving in time and labor over that involved in the ordinary separatory funnel method of extraction. The usual type of continuous extractor for liquids (S,4, 5 ) , which depends on refluxing of the solvent, was found to be unsatisfactory because of the co-distillation of benzoic acid, Therefore an extractor was de-
l
'I
ELFIGURE 1. DIAGRAM OF EXTRACTOR Inside diameter of A , 25 mm.; B , 16 mm.; C, D, and E , 5 mm.; F a , 10 mm.; cap N ,16 mm.
vised in which continuous circulation of the solvent; (carbon tetrachloride) through the solution to be extracted and through standard sodium hydroxide is caused by means of a current of air. The extractor with the dimensions adopted is shown in Figure 1. In operation approximately 40 cc. of carbon tetrachloride are placed in A and 30 cc. in B. In all the subsequent discussion these amounts will be assumed. A weighed sample of the benzoic acid solution to be analyzed is superirn osed on the carbon tetraohloride layer in A and water addefuntil the tetra-
chloride rises in the side tube B to the level of the overflow tube F. An excess of standard sodium hydroxide is pipetted into B and this tube filled with water almost to the inlet. B is tightly stoppered and A stoppered with the cap H . A current of air entering through D raises a steady stream of drops of carbontetrachloride through G. This tetrachloride trickles down through the acid solution, extracting benzoic acid, accumulates at the bottom of A , overflows through F and trickles down through the sodium hydroxide solution, which in turn extracts the benzoic acid, and from the bottom of B is returned by the air current t o A . Four hours' extraction was found by experiment to be satisfactory in the case of benzoic acid. A t the end of this time the air is turned off, the liquid in B withdrawn through the stopcock, the tube rinsed down with water, and the excess sodium hydroxide titrated with standard hydrochloric acid in the presence of carbon tetrachloride which does not interfere in the titration. The separation of the carbon tetrachloride and aqueous layers in the apparatus is astonishing1 complete. The air current is adjusted t o give an even flow o f solvent without undue splashing. Before entering the extractor, the air is passed through a tube of carbon tetrachloride to minimize evaporation of this substance in the extractor itself and through a 12-inch (30-cm.) U-tube filled with soda lime to remove carbon dioxide. The extractor was constructed of thick-walled Pyrex tubing throu hout. The large bore of tube P eliminates any possibility of sipfoning. Since G opens into the acid side, the pressure in each side is always the same. The cap H revents dangerous splashing if the air pressure in D accidentally {ecomes too stron , and the opening K in the tube attached to the cap forms an e?fective outlet for air and carbon tetrachloride vapor. The air inlet tube D i s fastened t o F by a short brace of glass rod (not shown). The apparatus was found to be of sufficient rigidity and strength so that a bank of five withstood continuous use over a period of several months. Although these five were built by a professional glass-blower, the method was first tried out with a model which was constructed by an amateur glassblower, Two spirals of glass rod (not shown) were inserted around the small tube C in order to break the tetrachloride into small drops and retard its rate of descent through the aqueous solution. The dimensions given are subject to considerable variation, but in changing them certain limitations should be kept in mind: The efficiency decreases with increase in cross section of A and B; xy must be of such length that the solvent will not be forced in the wrong direction by air pressure; and the relative heights of C and F must be decided with due regard to the relative densities of solution and solvent. Of the solvents possessing the high density which this a p paratus requires, chloroform and acetylene tetrachloride proved entirely unsatisfactory, probably because of hydrolysis of the solvent by the alkali (I, .e), There was a large and continuing loss of titer of the alkali as extraction proceeded. To a much less pronounced extent the same difficulty appeared when carbon tetrachloride was used (1); in fact, it was pos-
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