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SIMPLE CELL for the STUDY of CONDUCTANCE H. B. GORDON Alabama Polytechnic Institute, Auburn, Alabama
W
HILE a student learns much from lectures and textbooks he acquires a more vivid realization of natural phenomena when he himself collects data in the laboratory to illustrate them. Thus a student learns that certain types of substances, when dissolved in water, conduct an electric current, while other types do not. To impress this distinction upon him the electrical conductances of various solutions are determined, either in laboratory or as a lecture experiment. It is also well for him to observe the changes in conductance which take place when two solutions are mixed, either with or without chemical change. This has sometimes been done by placing one of the solutions under investigation in a beaker provided with electrodes, placing a sugar solution on top of this, and then adding the second solution of the study.* The con-
A modification of this procedure is to introduce the denser liquid beneath the other by means of a funnel-tube or similar device. The writer first saw this procedure used about twentyfive years ago. It is described in "An Introduction to General McGraw-Hill, 1920 edition, Chemistry" by McCoy AND TERRY, page 243. The writer is informed that the experiment has also been performed using a diaphragm of paper floating on the lower liquid to delay mixing. These procedures are also open to the objection that they permit more unintentional mixing of the solutions than is desirable.
ductance of the three solutions is determined, they are then mixed, and the conductance again determined. This experiment is fairly satisfactory hut unless great care is exercised there is too much unintentional mixing of the solutions to yield results as striking as if no mixing occurred before i t is desired. The presence of the sugar is also objectionable since it affects the degree of ionization besides being a component of the system in addition to those under consideration. To make the experiment more satisfactory for the use of the student a t the best stage of his development to profit by it a cell has been constructed which is illustrated by the accompanying isometric sketch. The cell is made of any size desired. A convenient size is 10 cm. long, 2.5 cm. wide, and 6 cm. deep. It may he made of wood, fastened together by small nails, either with or without the assistance of glue. Glue alone should not be depended upon. Platinum electrodes, A , are placed a t the ends of the cell. These should cover the ends of the cell as high as the solutions will extend. They are attached by platinum wires to the binding posts, B. About the mid-height of the cell, on both sides, narrow wooden cleats, C, are attached. These run the entire length of the cell be-
tween the electrodes and help to hold them in place. A cover or diaphragm (not shown) is made from a thin board cut to such a size that it will just slip easily into the cell, and when resting on the cleats, C, will separate the cell into two horizontal compartments. The diaphragm is provided with a handle. This may consist of a slender bolt held firmly in place by a washer and nut on each side of the diaphragm. If corrosive liquids are to be studied both the diaphragm and inside of the cell should be treated with paraffin before the electrodes are put in place. In use the cell is connected in series with a variable resistance and a low-reading ammeter and connections are made with a low-potential source of direct
current. The resistance is so adjusted that if the cell is short-circuited the current will still bewell within the range of the ammeter. Since the cell polarizes quickly a switch should be placed in the circuit and closed only momentarily when the current is to be read. The student can infer readily that the conductance of the solution is essentially proportional to the current flowing. He will understand the significance of negligible currents with water and solutions of such substances as alcohol or sugar, the low currents with ammonium hydroxide and acetic acid, and the high currents with salts and strong acids and bases. When he compares cupric chloride with cadmium or mercuric chloride he may be a little surprised. In this connection he should explain the separation of copper and cadmium by the use of salt solution and hydrogen sulfide.' To study the effect of mixing two solutions the student places a quantity of one solution in the cell just sufficient 1
F. E. BROWN,"Qualitative Analysis," The Century Co.,
N e w York City, 1932, p. 172.
to fill it up to the tops of the cleats. He then puts the diaphragm in place and makes i t tight by means of a little paraffinwhich may be spread as desired by means of a hot wire. The second solution is next placed on top of the diaphragm. When the solutions are such that they will react chemically when mixed they should be taken in chemically equivalent quantities. To mix the solutions the diaphragm is lifted out and the solutions stirred. The current is read before and after mixing, the switch being closed only momentarily for each reading. By the time the student has observed and explained a slight increase in current due to mixing sodium chloride and potassium nitrate, a sharp decrease with hydrochloric acid and sodium hydroxide, a sharp increase with acetic acid and ammonium hydroxide, a change from high current to almost zero on mixing sulfuric acid and barium hydroxide, and a change from almost zero to a high current on mixing water and concentrated sulfuric acid, he will usually have learned enough to make this extremely simple experiment well worth performing. An obvious improvement in construction of the apparatus is to substitute glass or other corrosion-resisting material in place of wood, but to make the cell of such material is beyond the skill of the average laboratory technician. If the cell is to be made of glass it would probably be well to let the cleats extend across the ends as well as along the sides of the cell, and to have the contact surfaces of cleats and diaphragm ground. By this arrangement the procedure could probably be simplified by dispensing with the paraffin or substituting stopcock grease for it.
