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completely (a few minutes). With pure reagents both layers will be colorless or nearly so. Carefully tap off the lower phosphoric acid layer into a 50-cc. Nessler tube. Wash the chloroform layer with 2.5 cc. of phosphoric acid and add it t o the acid in the tube. Make up to volume with acetic acid. The characteristic purple-red color develops upon the addition of the acetic acid ( 5 ) , the intensity being in direct proportion t o the amount of indole present. By varying the amounts of indole, the color intensities produced may be used as standards for comparison, the strength of the color decreasing only slightly after 1 or 2 days. The comparison may also be made photometrically in a neutral-wedge (extinction) photometer such as that devised by Clifford. The spectral curve shows a maximum absorption in the neighborhood of 570 mu. By means of the photometer as little as 0.5 micro ram of indole may be detected. CAUTION.The reaction m a y t e obtained by first adding the dimethylaminobenzaldehyde to the chloroform solution of indole and then shaking with phosphoric acid, but if added in reverse order, tbe indole is apparently polymerized by the acid and will not react with the benzaldehyde.
be washed with a small amount of dilute hydrochloric acid, and an aliquot may then be treated with Ehrlich’s reagent as above described. If emulsions form during the alkaline extraction, they may be broken usually by stirring in or shaking with sufficient powdered ammonium sulfate to cause separation of a clear liquid, after which the mixture may be centrifuged or decanted and filtered. (The treatment with ammonium sulfate in alkaline solution, as indicated, also removes formaldehyde, which, if present, may prevent the color formation.) Sometimes the separation of the chloroform and phosphoric acid layers may be difficult. The addition of 5 or 10 cc. more of acetic acid and somewhat longer standing (about 30 minutes) may obviate this difficulty. It is important that this separation be clean, as otherwise a turbidity may develop in the final solution when made to volume with acetic acid.
Discussion Indole may be extracted by chloroform from either slightly alkaline or acid aqueous solution. Since interest in the determination of indole is almost entirely with reference to biological material which may contain many interfering substances of acidic or alkaline nature, it is suggested that such material be first made slightly alkaline with dilute sodium hydroxide and shaken vigorously with a measured amount of chloroform. After separation, the chloroform solution may
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Literature Cited (1) Clarke et al., J. Assoc. Oficiul A g r . Chem., 20, 481-5 (1937). (2) Clifford, P. A., U. S. Food Drug Admin., Food Control 8tatement, 41, p. 1 (JuneDec., 1935). (3) Conant, J. B., et al., “Organic Synthesis”, Vol. 11, p. 17, New York, John Wiley & Sons, 1922. (4) Ehrlich,Deut. med. Wochschr., 1901, 1-15. ( 5 ) Macchia, Boll. soc. ital. biol. sper. 10, 723-5 (1935). (6) Urk, H. W. van, Pharm. Weekblad, 66, No. 6, 101-8 (Feb. 5, 1929).
Pressure Regulator for Vacuum Distillation MELVIN S. NEWMAN Ohio State University, Columbus, Ohio
T
HE regulation of pressure during vacuum distillation is
usually accomplished with the aid of a manostat which keeps the pressure constant by causing a pump to operate intermittently or by c o n t r o l l i n g a n air leak (1). Such devices require a good deal of time, labor, and materials before they can be placed in operation. A pressure regulator which can be quickly ass e m b l e d f r o m apparatus and materials available in a n y laboratory is shown in Figure 1. The principle upon which this regulator is based is simple. The gas in the system whose pressure is to be controlled must overcome the pressure of a column of liquid before i t can be pumped out. I n principle all that is required is a gas washing bottle conFIGURE1
taining a quantity of a liquid having an insignificant vapor pressure a t room temperature. In practice the two refine ments shown in Figure 1 make operation easier. When the pump, connected at A , is first started the bulk of gas in the system, connected at B , is removed through the open stopcock, C. When the pressure has almost reached the desired pressure C is closed, forcing the remaining gas t o be pumped through the head of liquid, h. The end of the gas inlet tube is constricted, so that when the system has come to equilibrium the constant leaks therein (including the distillation capillary) cause a slow steady stream of bubbles instead of the more intermittent larger bubbles that result if no constriction is made. The inside diameter of the constriction should be about 1 t o 1.5 mm. Before admitting air when the distillation has been completed, C is o ened in order to avoid violent splashing of the liquid in the bottre. Although the author has used this device for regulating pressure for only a few months, it has proved to be extremely efficient. After allowing about 15 minutes for the entire system to come to equilibrium, the pressure, as read on the usual type of mercury-filled manometer, has remained constant during distillations requiring as long as 10 hours. A pressure range of 1.5 to 16 mm. is conveniently covered by the ordinary gas washing bottle. Theoretically taller wash bottles or a series of short ones would allow for a wider pressure range. The author has used ethyl phthalate as a suitable liquid, but undoubtedly other liquids could be substituted.
Literature Cited (1) Morton, A. A , , “Laboratory Technique in Organic Chemistry”, pp. 105-9, New York, McGraw-Hill Book Co., 1938.
