JOURNAL OF CHEMICAL EDUCATION
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A CONDUCTIVITY EXPERIMENT FOR GENERAL CHEMISTRY S. PORTER MILLER Monmouth College, hionmouth, Illinois
ONEof the most common conductivity experiments is acetate (0.1 M) in glacial acetic acid produces a much the neutralization of a saturated aqueous solution of barium hydroxide by sulfuric acid. The current carried by the reaction mixture is small because harium hydroxide is not very soluble, and the harium sulfate formed in the reaction precipitates out. As soon as the stoichiometrical point is reached, the excess sulfuric acid makes the solution a good conductor which is shown by an increase in the brightness of the light bulb, or recorded on the ammeter. This experiment can also be used to determine the strength of an unknown sulfuric acid solution. It is well adapted to beginning students and deserves a wider usage than is indicated by its appearance in only 35 per cent of the laboratory manuals we have examined. However, we wished to go a.step farther in connection with the discussion of the Brgnsted Theory. L. C. Flowers (1) used glacial acetic acid as the solvent and described the titration of sodium acetate by perchloric acid using basic fuchsin as the indicator. We are accustomed to using a solution of sulfuric acid in glacial acetic acid in place of the more dangerous perchloric acid. The indicator (Tropeolin 00) changes from light yellow in sodium acetate to purplish in sulfuric acid, but the end point is not very sharp. The sodium sulfate which is formed in the reaction is insoluble in the glacial acetic acid solvent and masks the color. Since this reaction is similar to the neutralization of barium hydroxide by sulfuric acid, described earlier in this article, which lends itself to conductometric titration, it was decided to try the same method of measurement on the sodium acetatesulfuric acid reaction. Sulfuric acid (0.1 M) in glacial acetic acid is a very weak acid and produces a faint glow on a neon bulb (3) used to measure the passage of the current. Sodium
brighter illumination with the same apparatus. Therefore, the sulfuric acid solution was placed in a beaker, and the copper electrodes immersed in the acid. It does no harm to add a drop of the indicator. Then the sodium acetate solution was added slowly with vigorous stirring. No change in the glow around the edges of the plate in the neon bulb was observed, except that there was a flickering during the stirring because more of the reaction mixture would strike the electrodes, and then more current could he carried temporarily. When the end point was reached, the purplish color disappeared, and the neon bulb showed a marked increase in brightness. These two reactions performed with the same equipment serve as the basis for the consideration of the Brgjnsted Theory and reactions in nonaqueous solvents. They can also he performed using the indicator method, hut we prefer the conductometric method herein described, because it emphasizes the role of ions in these reactions, and eliminates the mystery and complexity of indicators. In the 6rst experiment, using water as the solvent, the hydroxide ion is the base, the hydronium ion is the acid, and the equivalent point is reached when these two react to form water, the solvent. OH-
+ H.O+ = 2H90
In the second experiment, the acetate ion is the base, and some such ion as HCJ&Oo.H+ is the acid. Neutralization is reached when acetic acid, the solvent, is produced. CnH,02-
+ HGH80rH+ = 2HCzHsOz
LITERATURE CITED
FLOWERS, L. C., J. CEEM.Enuc., 13, 219 (1936). (2) TRTESSEN, G. W., ibid., 23, 267 (1946). (1)
