A DEVICE FOR MEASURING THE COMPARATIVE CONDUCTIVITY OF ELECTROLYTES BEN H.
PETERSON, COECOLLEGE, CEDAR RAPIDS, IOWA
Various methods have been devised to show the conductivity of electrolytes as an experiment in general chemistry. The most common type consists of an electric light bulb connected in series with two terminals which are immersed in the solution to be tested. The intensity of the light is taken as a measure of the dissociation of the electrolyte. Such a demonstration will show observable differences in the conductivity of strong and weak electrolytes but it is difficult to ohserve any difference in the behavior of, say, hydrochloric and sulfuric acids of equivalent concentrations. Furthermore, the effect of dilution on the conductivity cannot be so measured. The device shown in Figure 1 has been used in our laboratories and found very satisfactory. It permits a quantitative measurement, which is a desirable feature in any experiment in general chemistry, and the students get the idea of strong and weak electrolytes more readily. The figure is self-explanatory. The terminals are made of 7-mm. capillary tubing into which small pieces of platinum wire are sealed. Contact is made to the copper leads with mercury. The milliammeter, reading to 250 milliamperes, is made by the Lundqnist Tool and Manufacturing Company of Worcester, Massachusetts, for the radio industry, and may be purchased a t radio repair and supply shops. Using a six-volt battery, the terminals of platinum being one inch apart and a half-inch long, the following data were obtained with half-normal acids. Acids Milliamps.
HCI 165
HnSOh
HsPOd
HIGOl
135
65
63
923
HC~H~OP 5
JOURNAL OF CHEMICAL EDUCATION
924
MAY,1932
Normal sulfuric acid served as an excellent starting point to show the effect of dilution on the conductivity. The reading is obtained for the normal acid and then for successive dilutions such as, half, eighth, sixteenth, etc. The reading for each dilution is multiplied by the dilution number. The results show an approach of the conductivity to a limiting value as illustrated by the following data taken from a typical experiment.
Dilution Conductivity D X C
1 210 210
2 170 340
4 122 488
8 80 640
16 50 800
32 28 896
64 15 960
128 8 1024
These data are shown as (dilution X conductivity) plotted against dilution in Figure 2. The approach to a maximum dissociation is plainly demonstrated.
