A APPARATUS FORDEMONSTRATING CONDUCTIVITY OF

ELECTROLYTES

HAROLD J. ABRAHAMS, WILLIAM BLITZSTEINAND FREDERICK LUBORSKY

Central High School, Philadelphia^ Pennsylvania

The electrical conductivity of polar molecules is a phenom-enon which is at once fundamental and interesting. It thereforerightfully finds a place in so elementary a treatment of chemistryas is given in the usual high school course. Yet the demonstra-tion of this property of electrolytes must suffer from the lack ofsuitable apparatus.

FIG. 1. An old form of apparatus

Various arrangements have been proposed for showing theelectrical conductivity of ionizable compounds. One very oldform consists of a drinking tumbler upon which rests a procelaincollar, serving as a mount for a pair of glass-encased mercuryelectrodes the ends of which are short lengths of platinum wire,

730

CONDUCTIVITY APPARATUS 731

sealed into the glass. This apparatus suffers from the fact thatit is clumsy, easily upset and uses a large volume of electrolyte.In addition it necessitates pouring from a wide-mouth vessel toa narrow-mouth bottle, when the particular liquid used is to bereturned to the original container at the end of the demonstra-tion. Furthermore, if one wishes to show the effect of dilutionupon conductivity, use of this apparatus results in wastage, bydilution, of a sizeable quantity of electrolyte, not to mentionthe danger involved if sulphuric acid is chosen for this demon-stration. Most teachers will want to demonstrate the behaviorof anywhere from three to six compounds. This results in theadditional annoyance of washing the tumbler and the electrodes,and, for some demonstrations, even drying them.

Most, though not all of these disadvantages are overcome bythe type of apparatus which consists of a set of bottles contain-ing solutions of electrolytes or non-electrolytes. These are fittedwith rubber stoppers which serve as mounts for mercury-filled,platinum-tipped glass electrodes. The circuit, including thelamp, is closed by immersing the wires from the lamp into themercury electrodes. The disadvantages of this apparatus arethat the bottles may be tipped over, spilling the mercury, orthe wires from the lamp may be accidentally brought too closeto each other, causing a short circuit. Mounting these wires on asmall piece of fibre-board at such a distance from each otheras to fit automatically into the mercury electrodes obviates thepossibility of a short-circuit. ^One very desirable feature, not possessed by either type of

apparatus discussed above, is the possibility of comparing thevarying degree of luminosity of the lamp produced by differentelectrolytes. As alpha,* the degree of dissociation, differs fordifferent electrolytes, it is a great advantage to be able to showthat this difference is observable in the conductivity demon-stration. Where, however, only one lamp is used in the appara-tus, the teacher must rely, as he goes from one electrolyte toanother, upon the students^ recall of how brightly the lampglowed for each of the tested liquids. The apparatus about to bedescribed has the advantage that each of the six liquids in thedemonstration has its own lamp and all liquids are tested si-multaneously so that it is possible to observe relative conduc-tivity. In addition the apparatus is inexpensive, compact, easily

* The most recent refinements in the ionization theory are beyond the scope of the elementary courseand are therefore not considered here.

732 SCHOOL SCIENCE AND MATHEMATICS

FIG. 2. Courtesy of Mr. Mook Hagmann, Philadelphia

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CONDUCTIVITY APPARATUS 733

portable, does not have to be washed, dried nor constantly re-charged, nor does mercury produce any problem, as none is usedin this design.A wooden test tube rack, of standard design, with a capacity

of a half dozen six-inch test tubes is nailed to a base board ofhalf-inch stock. The hemispherical depressions at the base of therack which normally accommodate the bases of the test tubesare enlarged by means of a suitable wood-bit as so to permit theglass cells to fit and rest therein.A box, of the design and dimensions shown in Figures 1 and

2, is so constructed that the back is easily removable for repairs,should these ever be necessary. The front and sides of the boxare made of half-inch stock, while the top and back may be ofone-quarter-inch material. No bottom is needed. This box isfastened to the same base-board, in back of the test tube rack,as shown in the illustrations.The cells are six ordinary 15 cc. medicine vials, fitted with

two-perforation No. 2 rubber stoppers, bearing in each perfora-tion a piece of 3 millimeter internal-bore glass tubing. Two ofthese six stoppers are cut away vertically to about one-third oftheir width, so that when fitted into the vials, a wide opening isleft. This is to permit dilution of electrolyte without removal ofstopper. The electrodes consist of pieces of soft telephone cord,around the uninsulated ends of which one-inch pieces of plati-num wire have been very carefully ^ind tightly wrapped, leavingas much as possible of the platinum wire for sealing into theglass tubing and protrusion beyond the end of the tube to theextent of 5/16 of an inch.The telephone cord is of such length as to create a goodly

amount of slack between the top of the vial and the front ofthe box. The purpose of this slack cord is to permit removal ofthe vial for recharging or cleaning.A rubber band may be passed around the test tube rack half

way between top and bottom so as to keep the vials more se-curely in their places, if this is found necessary. The cells maybe easily removed from the rack by lowering the rubber band,lifting out the stoppers and then lifting the vials out backwardinto the space between rack and box.The six lamp-sockets, each bearing a 7 watt lamp, are fas-

tened to the front of the box in two rows of three each, thussaving space, and making it easier for the eye to associate a

given cell with its lamp. The lamps and cells are connected in a

734 SCHOOL SCIENCE AND MATHEMATICS

series-parallel arrangement and placed across a 110 volt lineby means of a common plug (see illustration 2).The following electrolytes, in five cc. quantities, are used in

the glass-vial cells:

1. Concentrated hydrochloric aicd2. Ten per cent sodium hydroxide solution3. Ten per cent sodium chloride solution4. Ten per cent sugar solution5. Concentrated ammonium hydroxide6. Glacial acetic acid

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To use the apparatus for demonstration, the teacher merelyplugs the apparatus into a 110 volt line.The lamps attached to the cells of the first three electrolytes

burn very brilliantly. No illumination is, of course, visible withsugar solution. The lamp connected with ammonium hydroxideproduces a very dull glow. Glacial acetic acid, like sugar, allowsno visible illumination and students immediately conclude thatthese two compounds are non-electrolytes. At this point in thedemonstration the ammonium hydroxide and glacial acetic acidare diluted with water by inserting the capillary end of a longdropping pipette into the opening in the stopper of each of thecells containing these electrolytes. The effect of dilution uponeach of these is very striking. Due to increased ionization bothlamps glow brightly.A ten inch pipette may be ma3e by drawing out a piece of

glass-tubing to form a capillary at one end. The other end isflattened somewhat so as to produce a shoulder over which arubber aspirator bulb of about one inch diameter is passed.Such a pipette produces very vigorous stirring action in thecell.The apparatus described above may be used with gratifying

results for demonstrating the conductivity of electrolytes. Byusing weakly dissociated compounds in the demonstration, a

carry-over may be made to the subject of hydrolysis. Still fur-ther applications of this apparatus may suggest themselves toother teachers.

A research problem is not solved by apparatus, it is solved in a man^shead. The laboratory is the means by which it is possible to do the solvingafter the man has the idea clarified in mind.�CHARLES F. KETTERING.