JULY 1947
503
spontaneously and it is necessary to begin again with a fresh sample. PROCEDURE
To 1 ml. of a glycerinated (50yo glycerol) protein solution containing 0.1 to 0.3 mg. of nitrogen add 1 ml. of 50% sulfuric acid in a 100-ml. Kjeldahl digestion flask. Heat directly over a low flame with care until the solution turns light brown and then immediately add 2 t o 3 drops of pure bromine. Continue heating with caution and after the solution again assumes a brown color, add a second portion of bromine and reheat. At this stage extreme care must be taken to prevent spontaneous carbonizaiion. Add several drops of bromine and >upplement with 2 to 3 drops ~. of Superoxol. The solution is apt to carbonize with great speed, and if the reaction is allowed to continue without the addition of SuDer0x01, an un&eldy mass of carbon forms which renders the sample unfit for assay. After four or five oxidations with bromine and Superoxol, the material can be heated without special precautions against overheating. The latter stages of oxidation employ only Superoxol; usually two or three additions of several drops each complete the oxidation.
Xitrogen is determined in the digested solution by the microKjeldahl or any modified method. A blank is prepared using approximately equivalent amounts of bromine and Superoxol and digested in the same manner as the sample. It is assayed by the method used for the sample and the results are applied as a correction to those obtained from the sample. The method can be applied to other solutions of high carbonnitrogen ratio, such as tannic acid and carbohydrates, that readily carbonize. The oxidation can be completed within 20 minutes, after a little experience. There are no reagents present that interfere with nesslerization in a colorimetric determination. LITER4TURE CITED
(1) Hoffman, W. S., “Photelometric Clinical Chemistry,” p. 83, New York, William Morrow & Co., 1941. (2) “Scott’s Standard Methods of Chemical Analysis,” 5th ed., Vol.11, p. 2153, New York, D. Van Nostrand Co., 1939. (3) Snell, F. D., and Snell, C. T., “Colorimetric Methods of dnalysis,” p. 30, New York, D. Van Nostrand Co., 1937.
Safety Control Device for Use with Glass Cloth Heating Mantles HAROLD I. SCHIFF, C h e m i s t r y D e p a r t m e n t , University of Toronto, Toronto, Canada MANY instances more or less continuous distillation is
I necessary in the laboratory in order to provide an adequate
supply of organic solvents. The general use of the glass cloth heating mantle in such work suggests the desirability of providing a safety device which will automaLically turn off the heating current should the distillation pot go dry. Moreover, it is desirable to have a device which will turn off the current a t any predetermined time. Level-control devices such as that used by White and Secor ( 1 ) find their chkf application in permanent distillation units, while the single-junction thermocouple usually built into such mantles cannot be used without additional complicated amplifying stages. The circuit represented diagrammatically in Figure 1 consists essentially of a modification of the Wheatstone bridge, the voltage output of which is independent of line voltage fluctuations. The voltage applied to the bridge is reduced to 30 volts by means of R, a 650-ohm resistance. R1 and Ra are fixed 250-ohm resist38 gage nickel wire ances; RZis made of a 250-ohm length of KO. sewn into a circular pad of glass cloth. The pad is inserted be-
A
tween the bottom of t’he distillation flask and the heating mantle; its exact position in the mantle is not at all critical. The temperature coefficient of resistance for nickel is about 0.006 per degree; a change in temperature of 100” C. causes a difference of 150 ohms in the resistance of the nickel coil. With the circuit described, this corresponds to a change of about 4 milliamperes in the plate current’of the 50L6 tube. The circuit is adjusted by means of the fourth arm of the bridge, Rs-a 350-ohm variable rheostat-to give a reading on the . milliammeter, MA, 4 milliamperes below the activation value of the relay, LL,at ordinary operat,ing temperatures of the st’ill. An increase of 100’ thus causes the circuit to be broken. Once L1 is activated LP is deactivated, thus breaking the heating circuit, and current cannot pass again until the momentary switch, S , is pressed. The values of R and of the bridge resist‘ors can be varied considerably, to give different ranges of plate current and sensitivity of control. The resistances of the relays are not critical; in this circuit L; has a resistance of 4000 ohms and is activated at’ 15 milliamperes, while Lz has a resistance of 2000 ohms and is activated at 27 milliamperes. If direct current is not available, the circuit, can be modified for use with alternating current by using either alternating current relays or a 16-microfarad condenser, C, across relay LI. The 50L6 tube acts as a rectifier, the grid volt’age being 180” out of phase with the plate voltage, so that the plate current rises as the grid voltage decreases. The sensitivity of control is decreased somewhat in alternating current operation as compared with direct current; this can be overcome by suitably alt,ering the voltage applied to the bridge. Alternatively two nickel coils can be used in the pad, forming diagonally opposite arms of 6he bridge. An ordinary mechanical alarm clock inserted in the position indicated can be adapted to serve as a single-throw switch. Contact is made between a copper stgp attached to the alarm winder of the clock and a fixed contact’. When the alarm goes off the winder rotates and the circuit is broken, thus deactivating L,. The ot,her details are self-explanatory. The simplicity of this circuit is an obvious advantage and it has proved extremely useful in greatly reducing the amount of supervision required in long distillations. ACKNOW LEDGMENT
/
The author would like to thank F.’E. \I-.\Tetmore of this department for his assistance in designing the circuit.
I!
f Figure 1. Diagram of Circuit
LITERATURE CITED
(1) White, L. XI., and Secor, G. E., IXD. C S G . CHEM.,ASAL. ED., 18, 332 (1946).
