Lecture Table Apparatus to Demonstrate Conductivity of Solutions' J . A. MAlTUCK Brooklyn Technical High School, Brooklyn, New York
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HE apparatus commonly used to show the con- of the funnel is connected with a piece of rubber tubing duc'uvlty . . of electrolytes in solutions involves a reaching into the sink. number of time-consuming operations such as the reA few drops of water are introduced in the funnel, moval of the electrodes from the cell and their washing; enough to have the ends of the electrodes immersed. the emptying and washing of the cell containing the The addition of a drop or two of the solution to be solution and its refilling, and the replacement of the tested will cause the bulb to glow. electrodes. These operations involve considerable To empty the funnel it is only necessary to open the moving about to and from the sink on the part of the stopcock, and as the solution is running out, plain water instructor. may be poured into the funnel, thus washing the The lecture table apparatus to show conductivity funnel and the electrodes a t the same time. When of solutions here described eliminates all of these opera- the bulb ceases to glow the stopcock may be closed, tions. The cell is emptied and refilled, and the elec- and the apparatus is then ready for the experiment with trodes and cell are washed all in one operation. another electrolyte. This operation may be made more rapid if a water aspirator is available. The funnel then can be ernptied quickly by connecting the rubber tube from the fnnnel to the aspirator. If compressed air is available, the water delivered to wash the funnel can be poured in quickly by forcing in the water under pressure, resulting in rapid rinsing and washmg of the funnel and the electrodes. This operation may be effected by the following arrangement. A 2l/Aiter acid bottle is fitted with a three-hole rubber stopper. Through one of these openings a glass tube reaches down to the bottom of the bottle; the upper end of the tube is connected with a piece of bent glass tubing which reaches over the top of-the funnel and serves as a nozzle. Another piece of glass tubing goes through a second opening in the rubber stopper and reaches just below the stopper. This tube is connected to the compressed air. When the bottle is filled with water and the compressed air is on, the closing of the third hole in the rubber stopper with the finger for a moment results in a rush of water from the bottle into the funnel. If no compressed air is available i t is possible ~ I connect the piece of glass tubing which serves as a nozzle directly with the faucet. A pinchcock placed The apparatus consists essentially of a separatory over the piece of rubber tubing will serve to keep the funnel of the cylindrical type. Two copper wires flow of water into the funnel under control. There is another feature that can be added to this reach down to the narrow portion of the funnel, and need not be more than one-eighth to one-quarter of an device, by means of which i t can be demonstrated that inch apart a t the bottom of the funnel. These wires solid NaOH or crystals of copper sulfate or rock salt may be kept apart a t the top of the funnel and in will not conduct the current. For this purpose two position by a cork. They serve as the electrodes and copper wires provided with insulating handles are conare connected to the source of current with an in- nected across the electrodes in the funnel and serve candescent bulb in the circuit. The end of the stem as a by-pass for the current. When the ends of these terminals are brought in contact the incandescent bulb
1 Presented before the Chemistry Teachers Club of New York at Brooklyn College, April 21, 1944.
(Cmrtinud rm Pagc 506)
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LECTURE TABLE APPARATUS T O DEMONSTRATE CONDUCTIVITY OF SOLUTIONS (Continued from F g e 502)
glows. If these are brought in contact witb a crystal of copper sulfate, or solid sodium hydroxide, no current flows to make the bulb glow. These terminals may be made to serve another
useful purpose. They may be used to compare the brilliance of the glow when the current flows directly through the wires witb the glow of the bulb when the current has to pass through the solution.
