INDUSTRIAL AND EPiGIKEERIIiG CHEMISTRY
752
galvanometer was then increased by changing the setting of the Ayrton shunt from f = 0.08 to f = 0.40, a small readjustment of the compensating e. m. f. was made to bring the galvanometer deflection exactly to zero, and the wave of the zinc was recorded starting a t an applied e. m. f. of 0.8 volt. Curve 2 was obtained G
R,
1
Damping Shun1 10,000n
Ayrton Shunt
Polarograph
I
FIGURE 4.
IMPROVED COMPENSATION CIRCUITEMPLOYIXG A CONDEKSER TO DECREASEGALVANOMETER OSCILLA-
TIONS
C. 2000- to 5000-microfarad electrolytic condenser
after compensating the cadmium wave without the condenser in the circuit, whereas curve 3 was obtained with the condenser (2000 microfarads). Without the condenser (curve 2) the galvanometer oscillations are so large that the measurement of the wave height is difficult, but with the condenser (curve 3) the oscillations are greatly decreased and the wave height is easily measurable. Without the use of a condenser the compensation method is impractical when the concentration of the interfering
VOL. 12, NO. 12
major constituent is greater than about ten times that of the minor constituent, because of the exceedingly large oscillations of the galvanometer. On the other hand, when a condenser of the proper capacitance is used it is possible under optimum conditions to employ the compensation method up to a concentrabion ratio of the major and minor constituents of about fifty. Any tendency of the diffusion current to change with changing applied e. m. f . is greatly magnified in the compensation method. The diffusion current usually tends to decrease Tvith increasing negative potential beyond about -0.G volts us. the S.C. E. owing to the decrease of the drop time with increasing negative potential. K i t h the ordinary method of measurement this change is usually not very noticeable (3)but it becomes very pronounced when the compensation method is used, as shown by curves 2 and 3 in Figure 3. This fact limits the application of the compensation method to very well-defined waves.
Summary A method for decreasing the magnitude of the galvanometer oscillations in polarographic measurements uses an electrolytic condenser connected across the galvanometer shunts. By the proper choice of capacitance the galvanometer oscillations can be practically completely eliminated without affecting the diffusion current. The application of this principle greatly extends the practical applicability of the compensation method of measuring small diffusion currents. An improved compensation circuit is described.
Literature Cited (1) Heyrovsky, J., and Shikata, M., Rec. trav. chim., 44, 496 (1925). (2) Hohn, H., "Chemische Analysen mit dem Polarographen", Berlin, Julius Springer. 1937; 2. Elektrochem., 43, 127 (1936). (3) Kolthoff, I. M..and Lingane, J. J., Chem. Rev., 21, 1-94 (1939). Review of fundamental principles of polarographic analysis. (4) Lingane, J. J., and Kolthoff, I. M., J. Am. Chem. Soc., 6 1 , 825 (1939). (5) Thanheiser, G., and Willems, J., Mitt. Kaiser-Wilhelm Inst., Eisenforschung, Diisseldorj, 21, 65 (1939).
A High-Output Electric Flask Heater HAROLD A. KRANTZ
AXD
RALPH W. HUFFERD, Kendall Refining Co., Bradford, Penna.
A 3-liter flask was used as a mold about which the heater was constructed. T o support the heating coils during tlie application of the refractory material, a framework of wires was constructed about the flask. A ring of 16-gage wire was formed around the neck of the flask, and ten 30-cm. (12-inch) lengths of 26-gage wire u-ere fastened at one end to the ring. The wires were evenly a Bunsen burner. Constructed with three separate heating spaced about the neck of the flask and brought longitudinally to units, i t furnishes a %,ide range of heat output, is simply the bottom, where the ends were twisted together. Three made, inexpensive, and durable. heating coils were formed by winding separate 10.5-meter (35foot) lengths of 20-gage oxidized Xichrome xire on an arbor 3 mm. (0.125 inch) in diameter. Each coil was slightly stretched, so that adjacent turns did not touch. The coils were placed parallel to each other on 1.56cm. (0.625-inch) centers and wound about the flask as shown in Figure 1 (left and right). Thread was used to fasten the I coils firmly to the longitudinal 2 3 wires. The heating coils, now firmly fastened to the flask, were en/OF RESISTRTANCE COILS tirely filled with softened paraffin wax. A thin layer of wax was then applied to the outside RElNFORCED 6.M M. 0 D. FIR€ CLAY PYREX TUBING of the coils and to the area of the
A
N E L E C T R I C flask heater TVas designed with sufficient heat outputto distill higher boiling petroleum fractions (up to 343' c.,650" F.) and utilize maximum flask area for heat transfer. Its design permits supplementary heat with
DECEMBER 15, 1940
ANALYTICAL EDITION
flask upon which the refractory material was to be plastered (Figure 1, center and right). A conduit for the terminal ends of the resistance wire (Figure 1, center) was formed by 3.75-em. (1.5-inch) lengths of 6-mm. Pyrex tubing. Refractory material-Hitempite, Quigley Co., 56 West 45th St., New York, N. Y., a plastic used for binding firebrick in furnaces-was plaqtered over the assembly in four separate layers. Time was allowed after each application for drying, and between the second and third applications 1.56-mm. (0.067-inch) braided asbestos tubing was used for reinforcing material. The total thickness of the four layers of fire clay was approximately 3.1 em. (1.25 inches). The heating unit was removed from the flask by cutting the longitudinal wires and filling the flask with hot water to melt the paraffin. The wires still in place were removed after cutting the threads attaching them to the heating coils. The assembly was then placed under a hood, and a small current passed through each heating coil to volatilize the remaining paraffin and further dry the fire clay. (Care was taken to remove any small amounts of refractorymaterial from the exposed side of the heating coils IThere
753
they had become entirely encompassed in application.) When the wax had melted away, about two thirds of the outside diameter of each turn of the heating coil was surrounded by refractory material. Thus the coils were held firmly in the heater body. A support was made for the heater from a length of 0.625cm. (0.25-inch) pipe, bent into a circle of proper diameter to fit approximately the outer circumference of the heater at about half its horizontal height (Figure 1, center). One end was bent to extend laterally from the side of the heating unit and the pipe was cemented into place with sufficient refractory material to make it an integral part of the heater. A clamp can be used to support the heater on a ring stand. Any desired heat output can be obtained within the maximum limits of the heater. Each resistance coil draws 4.5 amperes a t 110 volts, giving approximately 500 watts maximum output per individual unit. The heating coils were arranged so that one, two, or three could be connected into a single lead. The heater also operates conveniently with a 2-liter flask. -4 Bunsen burner can be used to furnish supplementary heat through the 10-cm. (4inch) opening in the base of the heater.
Accurate Measurement of X-Rav Diffraction Films J
HAROLD P. KLUG, University of Minnesota, Minneapolis, Minn.
T
HE accurate measurement of line positions on powder and rotation photographs is a problem in all precision diffraction studies. This problem is complicated by the fact that x-ray lines cannot be subjected to high magnification, since they then tend to fade into the background. The usual types of comparators with high magnification are therefore unsatisfactory. Xumerous devices for measuring x-ray films have been described ( 1 4 , and all of them are based on the use of a highly accurate engraved scale or an accurate micrometer screw. The chief problem is to set the movable pointer or cross hair accurately on the edge or center of the line. To accomplish this some have incorporated costly auxiliary equipment such as photometers ( I ) , cathode-ray oscillographs ( 6 ) , etc. The average device for these measurements is thus very costly.
The instrument described involves only minor changes
in design, but it has proved to be simple to manipulate, sufficiently accurate, satisfactory for both powder and rotation photographs, and economical to construct, since it requires only standard available parts and simple machine work. The design of the movable pointer, means for holding the films in place, and source of illumination are improved features.
Apparatus Figure 1 shows a line drawing of the complete measuring device. The base is a wooden case, A , with sloping front. A hinged back on the case gives ready access to the source of illumination mounted therein. The light source is a Day Brite fluorescent lamp unit (Westinghouse catalog, No. 8930) with a 2.5 X 45 cm. (1 X 18 inch), I5-watt, daylight Mazda lamp. A piece of milky glass placed between it and the film support results in a very
FIGURE1. MEASURING DEVICE
