Chemical Education Today
Letters Improved Hittorf Apparatus The article by Jinqing et al. (1) describing an improved Hittorf apparatus raised some concerns for me on a number of issues. 1. The apparatus is shown as a simple U-tube with a stopcock in the middle. This type of arrangement was rejected years ago as unworkable since on electrolysis there are large changes in concentration and density that can cause both mixing and siphoning, which defeat the entire experiment. This is the reason that Hittorf included a central buffer compartment (2). Table 1 shows a 65% change of concentration of the HCl and NaOH solutions, which supports this conclusion.
Literature Cited 1. Jinqing, K; Qin, X; Ke, C. J. Chem. Educ, 2001, 78, 937–938. 2. Glasstone, S. Textbook of Physical Chemistry, 2nd ed.; D. Van Nostrand Co.: New York, 1946; pp 910–921. Morris Bader Department of Chemistry Moravian College Bethlehem, PA 18018
2. The basic premise of the Hittorf method is that the water does not migrate (whether this is true or not, it still is the premise). Consequently all concentrations should be either weight percent or molal since the weight of water in the compartment must be obtained. Molarity (a volume measure) should never be used for serious work (2).
The authors reply We thank Morris Bader for his interest in our manuscript. While some statements in the article may have been ambiguous, we hope readers will consider these facts.
3. The electrode system is critical for determining the exact changes in concentration. In Table 1 the authors neglect to tell us what the electrode system is and which compartment is being analyzed, and suggest that one equation serves for all systems.
The following example will disprove that assumption. Consider a solution of CuCl2 in the anode compartment electrolyzed with (A) a Cu metal electrode and (B) a Ag/AgCl electrode. At the anode the electrode reactions are: (A) Cu → Cu2+ + 2e᎑ and (B) Ag + Cl᎑ → AgCl + e᎑ The balance sheet for the changes in concentration in (A) is as follows: + Ne eq. of Cu2+ (electrolysis) − t+ Ne (migration) = Nf − Ni There is a gain of Cu2+ due to electrolysis and a loss due to migration, where Ne is the total charge transferred in chemical equivalents (i × t /F ), Ni and Nf are the equivalents of Cu2+ before and after as determined by some analytic method. Then: Nf − Ni = Ne − t+ Ne = (1 − t+) Ne = t ᎑ Ne or:
t ᎑ = (Nf − Ni ) / Ne
(1)
In (B) the changes are the same as (A) but no Cu2+ is formed by electrolysis and there is only a loss of Cu2+ by migration. Then: Nf − Ni = ᎑ t+ Ne or: t+ = (Ni − Nf ) / Ne
742
Note that equations 1 and 2 are different and the transport numbers switch from the negative to the positive ion. Also, in (A) the solution becomes more concentrated, and in (B) more dilute. To those teachers who want to confound students, all that is needed is to stabilize the solution with some inert KCl. The transport numbers may turn out negative or greater than one since CuCl42᎑ complex is formed and the Cu ion will migrate in the wrong direction.
(2)
1. Please remember that the concentration value for HCl and NaOH solutions found in Table 1 is much less than 0.1 mol dm᎑3. 2. We did not reject “the basic premise of the Hittorf method” in our article. On the contrary, we stated that “It yields an apparent transference number, which cannot be distinguished from the results obtained by the moving-boundary method” (1). Strictly, the concentration unit in electrochemistry should be mol kg᎑1 (i.e., molal), but in diluted solutions there is no difference between molarity (i.e., mol dm᎑3) and molal, so molal can be displaced by molarity in diluted solution. 3. The electrode used in our experiment is inactive (a graphite or Pt electrode); we obtained the RSD values after running the experiment eight times.
We also refer interested readers to the relevant sections of the texts listed below (2, 3). Literature Cited 1. Jinqing, K.; Qin, X.; Ke, C. J. Chem. Educ. 2001, 78, 937– 938. 2. Moelwyn-Hughes, E. A. Physical Chemistry; Pergamon Press: London, 1957; pp 838–839. 3. Erkang, Sun, et al. Experimental of Physical Chemistry; Nanjin University Press: Nanjin, 1998; pp 72–75. Kan Jinqing Department of Chemistry Yangzhou University School of Sciences Yangzhou 225002 P. R. China
Journal of Chemical Education • Vol. 80 No. 7 July 2003 • JChemEd.chem.wisc.edu
