The author replies to Vogelezang.

I agree with Michiel Vogelezang’s letter regarding Lederer’s demonstration using a light-bulb conductivity apparatus (1), where the dilution of a 1.0 M acetic acid solution to a 0.5 M solution produced an (apparent) unanticipated increase in the solution’s conductance. The focus of our exchange (1, 2) was my earlier paper (3) and not on rationalizing Lederer’s demonstration.

In a subsequent analysis, I calculated the concentration of the ions in the system containing 0.5 M versus 1.0 M acetic acid using an “ICE” table (3, 4). The results show that the dilution of a 1.0 M acetic acid solution increases the percent ionization of acetic acid and decreases the concentration of the sum of all ions in solution. As the solution’s conductance depends on the concentration of ions in solution (5), a decrease in the concentration of ions in solution would decrease the solution’s conductance, which is inconsistent with Lederer’s observations (1).

Thomas Newton (6) measured a lower conductance in a 0.5 M acetic acid solution than in a 1.0 M solution, which I was able to confirm. The box below shows the solution conductance as a function of the depth of electrode immersion and the concentration of acetic acid. The conductivity measurement apparatus consists of a gel box power supply (which displays the voltage and current in the system), wires with alligator clips, and a pair of graphite electrodes (0.25 in. diameter rods, 2.7 cm apart).

These experimental measurements are inconsistent with Lederer’s observations (1).

Without specific information on Lederer’s experimental protocol and equipment, I am unable to resolve the contradiction between Lederer’s observations and the conductivity measurements or my calculations. A possible explanation could be an increase in the surface area of the electrodes in contact with the solution upon dilution (5, 6; see boldface items in the box above). Other factors may be electrode geometry (6, 7).

Literature Cited

1. Lederer, R. J. Chem. Educ. 2005, 82, 1149.
2. Matsumoto, P. S. J. Chem. Educ. 2005, 82, 1150.
3. Matsumoto, P. S. J. Chem. Educ. 2005, 82, 406–407.
4. Brown, T. L.; LeMay, H. E.; Bursten, B. E. Chemistry. The Central Science, 7th ed.; Prentice Hall: Upper Saddle River, NJ, 1997.
5. Hille, B. Elementary Properties of Ions in Solution. In Ionic Channels in Excitable Membranes, 2nd ed.; Sinauer Associates, Inc.: Sunderland, MA, 1992; Chapter 10.
6. Newton, T. personal communication.
7. Jovov, B.; Wills, N. K.; Lewis S. A. Am. J. Physiol. 1991, 261: C1196–1203.

See also Lederer's reply and the Editor's Note.