A Modified Mattuck Apparatusi x
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to Demonstrate Conductivity of Solutions LOUIS WEISS and ALEX OPOCHINSKY Metropolitan Vocational High School, New York City
T N USING Mattuck's apparatus to demonstrate conductivity of electrol~~es it became apparent that some modifications were desirable. The pressure bottle and its hoses were too distracting and these were entirely eliminated. To facilitate the addition of solutions and wash water from the rear by the instructor, the separatory funnel was raised so that the lip came above the top of the board. The aspirator was also eliminated as the wide bore of the funnel stem permits a rapid flow through the wide rubber tubing to the sink. In order to improve the visibility and understanding of the circuit, a few simple changes were effected. The base board was replaced by two parallel strips of wood which support the board. All wiring was then done on the vertical board for the pupils to see. A few inches below the end of the funnel stem a hole was bored in the wood and the hose slipped through to the rear of the apparatus, to the sink. In the modified apparatus then, the entire circuit is in full view with all obstruction of the wiring eliminated. Since copper wires are so thin, the circuit was not seen very well by pupils in the rear seats. To overcome this difficulty the wood was painted black and a narrow strip of white paint was used to represent the circuit (~i&re 1). he actual wiring may then be done on the hack of the board. The parallel circuit used. by Mattuck to show that solids will not conduct the current is superfluous. Powdered solids can be placed directly into the funnel. Water can then be added, whereupon the incandescent lamp will begin to glow almost instantly. When the stopcock is opened, the undissolved solid is easily forced out by further addition of water. Another distracting feature is thus removed. One of the most attractive features of the Mattuck conductivity apparatus is that i t should be possible to demonstrate the effect of dilution on the conductivity of weak acids and bases. A small amount of concentrated acetic acid or ammonium hydroxide, enough to cover the ends of the electrodes, is poured into .the funnel. The lamp wili not glow until the water is added, and the intensity will increase as additional water is added. However, if the switch is kept closed while the solution is permitted to drain, the glow will diminish steadily. This indicates that the active electrode surface is an added variable in this experiment during volume changes. This dictates that in the
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original. apparatus the volumes used in all the experiments must be kept constant and all dilutions must be made outside the apparatus in order to make the observations valid. It was decided that an attempt should be made to make the effective electrode surface constant regardless of the volume of the solution. This was achieved by arranging the entire active electrode surface in a horizontal plane near the bottom of the funnel. The electrodes were made of No. 18 enameled copper wire. Three-eighths (3/s) inch of the enamel insulation was removed from the end of a piece of wire and the exposed ,.Lecture table apparatus to demonstrate section was bent into a semicircle. The wire was maniconductivity of solutions," J. CHEM.EDUC., 21, 502 (1944). pulated so that the exposed semicirclelay in a horizontal 177
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plane in the bottom of the funnel, the remaining insulated part of the electrode rising along the contour of the glass to the top of the separatory funnel. Here i t was hooked over the lip of the funnel for support and then connected to the binding post (Figure 2). When finally assembled, the exposed sections of the electrodes should form a split circle of approximately J/s-inch diameter in the base of the funnel. There should also be a '/s-inch clearance between the exposed sections of the electrodes and the glass. This will prevent current
flow through electrolyte films on the glass and will make washing of the funnel more efficient. Now the area between the electrodes is independent of volume changes in the vessel. The enamel insulation on the electrodes bas withstood the action of various chemicals exceedingly well Demonstrations and trial experiments were carried out repeatedly using dilute solutions of weak and strong acids and bases, varying concentrations of salt solntions, glacial acetic acid, and concentrated ammonium hydroxide. Also nonelectrolytes such as metliyl and isopropyl alcohol, glycerin, benzine, benzol, carbon disulfide, ether, and sugar solutions have been used. The original electrodes are still being used. However, when the electrodes do become ineffective they can be replaced in a matter of a few minutes. The Mattuck apparatus, modified as described, is extremely effectivefor classroom demonstrations of the conductivity of solutions. The circuit is simple, and wisible even in a large classroom. There are no distractions such as parallel circuits, pressure bottles, or rubber hoses that obstruct the full view of the circuit. Solids in powdered form can be placed directly into the cell to show the lack of conductivity, and water can then be added to dissolve the solids. Dilution effect on conductivity can also be demonstrated in the cell, using glacial acetic acid or concentrated ammonium hydroxide. The authors wish to express their appreciation to Mr. Jacob Rosenkrantz for his suggestions on electrode surface.
