Equipment for Studying Electrolysis and Conductivity G. B. HEISIG a n d H. P. KLUG
University of Minnesota, Minneapolis, Minnesota
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XPERIMENTS to show the differencesin the conductivlty . . of solutions are included in practically all laboratory manuals of inorganic chemistry. Usually an apparatus is improvised by using as electrodes two copper wires. The electrodes are connected in series with a lamp, the change in whose brightness serves to show changes in the conductivity of the electrolyte between the electrodes. Ordinarily the lighting circuit is the source of the electric current. The equipment is assembled into a unit which is given out a t the stock room or, in some cases, the apparatus is mounted permanently in some comer. As the experiment is usually camed out, small differences in the conductivity of electrolytes cannot be detected by a change in the brightness of the lamp. Furthermore, detectable differencesin the brightness of the lamp may be caused by a change in the resistance of the cell due to the change in the distance between the electrodes or in the depth to which they are immersed in the electrolyte. The use of improvised equipment, also, necessitates considerable supervision, since there is danger of a student receiving a severe shock if he is grounded while touching one of the electrodes.
mounting strap (I.L. 1348) was fitted into each hole and the outside was covered with a brown bakelite plate (91021). The lamp was connected to the "live" side of a 110-v. A.C. circuit or the positive side of a source of direct current and to one terminal of the outlet. The other terminal of the outlet was connected to a toggle switch F, which was connected to the other side of the 110-v. line. Each unit was connected in parallel with the others. (This unit with appropriate changes in the wiring is used as an outlet for electric irons.) If both AS. and D.C. are available a double-throw switch appropriately placed makes i t possible to use either current. It is desirable though not necessary to place a master switch and fuse on the main line. The metal box was given a crackle finish and grounded. A work shelf, two inches wider than the cabmet to prevent students from running into the corners, was mounted four feet from the floor and ten inches below the cabinet. The shelf may be hinged so that it may be dropped along the wall when it is not needed. The electrodes, G, made of 16-gage B & S (0.051") chrome1 or nichrome wire, were connected to the terminals of a soft-rubber, flat grip cap, H, such as is used frequently in making an extension cord. The wires were forced through a rubber stopper (No. 00) cut to an appropriate length: The stopper was then forced into 'the opening in the cap, holding the wires firmly in place. When the plug is inserted in the outlet in the bakelite plate, the apparatus is ready for use. The electrodes are about '/4" apart and extend 13/4* w5. below the handle of the cap. .The containers for the solutions whose conductivity is'to be determined are 10 X 75-mm. test tubes placed in a test-tube block (21/4" X 81/p") holding 16 test tubes. When not in use f the cap is attached to the block by slipping the holes in 2,' the prongs over a headless nail driven into the wood. 1 By filling the test tubes to the same level and bringing PLUG OUTLET FOR H each test tube to the end of the cap, the characteristics BACK OF LAMP of the cell remain constant and the conductivities of I------- e" different solutions can be compared. Since the fila(a) (a) ment and the volume of the pilot lamp are small, the F I G ~1E change in the intensity of the light with a change in the conductivity of the solution is very pronounced. The apparatus may be used to c a w out the ekeTO avoid these difficulties the apparatus shown in cross section in Figure 1 was constructed for the use of trol~sisof electrolytes by using the socket as a source the large number of students enrolled in the beginning of direct current. Since a 7-watt lamp is in series with courses in inorganic chemistry. ~h~ cabinet A and the the cell, the maximum current which can pass through removable cover B, in long sections, were constructed of the cell is about 70 milliamperes if the voltage is 110. 16.gage sheet iron. openings about 2" x 3" were If both alternating and direct current are available, made in the bottom of the cabinet a t intervals of 18". their dierence in behavior can be determined. The apparatus built as described is rugged, attractive A unit E, built up of a pilot lamp (Bryant, I.L. 1340) and an outlet (Bryant, I.L. 1320) held in place by a in appearance, safe, and easily operated. 314
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