A Spring Pendulum Current Interrupter for a Magnetic Stirrer With Application t o an Experiment in Catalysis V. A. LAMB a n d M. M. HARING University of Maryland, College Park, Maryland
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T IS the policy inour laboratory to introduce laborsavlng apparatus for class use whenever feasible. In this couuection, it was desired to construct a shaking apparatus for. use in the experiment on the catalytic decomposition of hydrogen peroxide, described in the laboratory manual by Daniels, Mathews, and Williams,' since hand shaking of the reaction mixture proved to be tiresome and .not very efficient. Reciprocating type shakers which werevtried did not work well, so a magnetic stirrer was used which has proved very satisfactory. The use of magnetic stirrers is of course common knowledge, but the current interrupter we use is of ow own invention and design, and it is thought that it may be sufficiently useful to warrant description. The intermpter is a spring pendulum, itself actuated by a magnet to prevent damping out of its oscillations. A diagrammatic representation of the pendulum and wiring is shown in Figure 1. The two contacts P are for the pendulum magnet P M and S is the contact for the stirrer magnet SM. At each extreme of its oscillation the iron bob on the pendulum is pulled back to the center by the pendulum magnet, whiie a t one extreme contact to the stirrer magnet is made. Construction of the intermpter could be simplified if contact to the stirrer magnet were made at both ex-
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DANIELS, MATREWS, ~ rWILLIAMS, i ~ "Experimental physical chemistry." 2nd ed., McGraw-Hill Book Co., Inc., New York City, 1934, p. 151.
tremes, but if this were done the oscillation frequency would have to be so low as to require a clumsy length of oendulum.
The perspective drawing (Figure 2) shows the general features of the apparatus as finally constructed. The base is wood, 8/4U X 5" X 7". The support rod for the pendulum and contacts is made from '/r-inch copper rod, 10 inches long, threaded a t the lower end for bolting to the base. The horizontal part of the support for the pendulum and contacts consists of two pieces of 20-gage brass plate, %' X 3'/e8, with one end rounded, as in Figure 2, A, to fit around the support rod. Two 20-
gage brass strips, '/a" X l'/a', are bolted to the horizontal strips, one on either side, and bent outward to form the contact supports (Figure 2, B ) . The contact strips are cut from 30-gage brass plate and have the form and dimensions shown in Figures 2 and 3, C. A
strip of platinum foil is soldered to the contact faces of these strips. The tapered ends serve as lugs to which the lead wires are soldered. They are insulated from the support strips with fiber washers. These parts are all assembled with '/rinch stove bolts. The pendulum is cut from 30-gage brass plate and bas the form and dimensions shown in Figures 2 and 3, D. A certain amount of cut and t?y trimming of the pendulum strip is necessary in order to get the proper tension. A different mass on the end would require a slightly different width. The pendulum bob can be any iron weight, such as two nuts held one on each side with a screw running through both. The mass of the bob we are using is about 15 grams. The slot in the pendulum is for the purpose of making possible adjustment in the position of the platinum foil in case one wishes to adjust the oscillation frequency by changing the length of the pendulum. The platinum foil is held in place with a bolt through the slot. The core of the pendulum magnet is made from 8/sinch soft iron rod as shown in Figure 3, E. Wire is wound separately on two spool forms. The spools are slipped in place over the two pole pieces and these are then connected by screwing the soft-iron cross-yoke in place. Each spool has 2500 turns of No. 30 enameled copper wire. The spools are connected in series. No claim is made for scientific design of this magnet. However, it is neat in appearance, has ample power for
its purpose, and has sufficiently high impedance to operate satisfactorily directly from a 110-volt A.C. line. I t does not overheat and draws about 0.35 ampere on 110 volts. The magnet is fastened to the center of the base by countersinking the cross-yoke in a depression chiseled in the base. A strip of brass plate laid across the cross-yoke and screwed into the base at each end holds the magnet firmly in place (Figure 2, F). The leads from the magnet are connected to the binding posts G and H (Figure 2). Binding posts I and J (Figure 2) are for the leads to the stirrer magnet. The wiring, as far as possible, is concealed under the base, which is mounted on rnbber feet at each corner. The stirrer magnet core is of the dimensions shown in Figure 4. I t is the field magnet from a 1936 model "Polar Cub" electric fan, which was found to be ideal for our purpose. I t is wound with approximately 1000 turns of No. 30 enameled copper wire. I t draws about 0.9 ampere on 110 vole. The reaction vessel we use is a 1" X 8" test tube, which is supported by the magnet. A piece of rubber tubing 'I4inch long is slipped against the lip to raise the lip slightly from contact with the magnet and to bring the test tube stopper (not shown) high enough so the impact of the iron ring does .not knock it loose. (See Figure 4.) A '/4-inch copper rod, 8 inches long, is bolted to the magnet -and clamped in a stand to support the magnet and test tube over the constant-temperature bath. The iron ring which actuates the stirrer is '/a" long and %" in diameter. It is cut from 20-gage sheet iron and lacquered to prevent rusting. Copper wire, passed through smaU holes drilled .at-bpposite sides of one end
of the ring and twisted together in the center, is fastened to the glass stirrer with De Khotinsky cement. The stirrer-to-ring connection should be rigid. The stirrer is a 5'/*-inch length of 2-mm. cane glass terminated at the lower end by a flat spiral of about two turns. Several diierent kinds of wire were tried as stirrers but all were found to have considerable catalytic activity. In operation the test tube is fitted with a rubber stopper through which passes a tube to a gas buret.
Once the contacts are properly adjusted, the peudulum intermpter operates smoothly and continuously without attention other than starting at the beginning of a run. Before its construction other types of interrupters, such as a metronome or motor-driven commutator were considered, but it is felt that the peudulnm intermpter has advantages over both of these, particularly from the standpoint of economy. The cost of the complete apparatus, excluding labor, was about three dollars.
The experiment has been performed by classes using the above-described stirrer and also using hand shaking. Gas evolution is more uniform and results more reproducible using automatic stirring. The reproduci-
hility of results obtainable is illustrated in Figure 5. Curves I and I1 are plots of VJ V, against time and log (V, - V,) against time, respectively, for the decomposition of 5 ml. of three per cent hydrogen peroxide (containing acetanilide inhibitor) catalyzed by 5 ml. of
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approximately 0.05 N potassium iodide solution. Circles and solid circles indicate the results for duplicate runs. VJ is the volume of oxygen obtained on heating the reaction mixture to complete decomposition and Vt the volume evolved a t time t. The volumes were measured at the same temperature and pressure for each run. I t is seen that smooth curves are obtained and that both runs yield the same value for the velocity constant, within the limits of the experimental error. The velocity cimstant, calculated from the slope of the first half of the log curve, is 0.368, with time in minutes. An objection which might be raised against this type of stirring is that the heating effect of the stirrer magnet on the gas in the test tube wiq produce error in the measured volume of gas evolved: This error has been investigated and it is found to be negligible. Apparently the action of the iron ring stirs the air in the test tube sufficiently to keep it a t the thermostat temperature.
