(9) Rice, E. W., Lukasiewicz, D. B., Clin. Chem. 3, 160-2 (1957). (10) Rosenthal, H. L., Pfluke, M. L., Buscaglia, S., J. Lab. Clin. Med. 50, 318-22 (1957). (11) Sperry, W. M., Webb, Merrill, J. Biol. Chem. 187, 97-110 (1950). (12) Swell, Leon, Treadwell, C. R., Ibid., 212, 141-50 (1955). (13) Wycoff, H. D., Parsons, Janis, Science 125, 347-8 (1957). (14) Youden, W. J., “Statistical Methods for Chemists,” Wiley, New York, 1951. (15) Zak, Bernie, Am. J. Clin. Pathol. 27, 583-8 (1957).
5421-2 (1953); Ora. Syntheses 35, 43-9 (1955). (3) Green, Joseph, Marcinkiewicz, S., Watt, P. R., J. Sci. Food Agr. 6, 274—82 (1955). (4) Hansen, P. W., private communica-
tion.
(5) Hansen, P. W., Dam, H., Acta. Chem. Scand. 11, 1658-62 (1957). (6) Henlev, A. A., Analyst 82, 286-7
(1957). (7) Langan, T. A., Durrum, E. L., Jencks, W. P., J. Clin. Invest. 34, 1427-36 (1955). (8) MacIntyre, I., Ralston, M., Biochem. J. 56, xliii (1954).
on
the Electro-oxidation of Iodide Ion
Sir: Recent voltammetric studies of the oxidation of iodide ion in acetonitrile at a rotating platinum electrode, by Kolthoff and Coetzee (4) and Popov and Geske (6), have shown that electro-oxidation to iodine proceeds in two steps. The first involves oxidation of iodide ion to triiodide ion and is a two-electron step; the second involves oxidation of triiodide ion to iodine and is a one-electron step
·
612I3—
2I3- +
=
=
3I2
4e
2c
Because solvent effects on the oxida-
tion-reduction potential of inorganic redox couples are, at present, receiving a great deal of attention, we wish to report additional data of interest on this oxidation, to call attention to solvent effects on the mechanism of electro-oxidation and electroreduction of inorganic substances. C'hronopotentiograms for the electrooxidation of iodide ion at a platinum foil electrode in aqueous 0.1M sodium perchlorate indicated a single one-electron step for the formation of iodine, as expected (7, 5). The Em and 0.059/n values were 0.51 volt. vs. S.C.E. and 0.057, respectively. One-step oxidation of iodide ion to iodine was also observed in: methanol, 1-butanol, 2-propanol, and acetic acid. Although ethylene glycol favored the two-step oxidation of iodide ion to iodine, the Em values for the two steps differed by only 0.1 volt. The following solvents along with acetonitrile were found to favor twostep oxidation of iodide ion to iodine: acetone, 1,2-dimethoxyethane, dimethyl sulfoxide, and acetic anhydride. Electro-oxidation of iodide ion in pyridine was also found to proceed via a two-step mechanism but of a different type. The Em values are 0.37 and 0.89 volt vs. S.C.E., chronopotentiometric transition times indicate that the electrode reactions are
Received for review March 24, 1958. Accepted November 3, 1958. Presented in part before the Division of Biological Chemistry, Methods and Procedures, 133rd Meeting, ACS, San Francisco, Calif., April 1958. Work supported in part by grants from the John A. Hartford Memorial Fund, the Albert and Mary Lasker Foundation, New York, the Nutrition Foundation, Inc., New York, and the Fund for Research and Teaching, Department of Nutrition.
COMMUNICATIONS
SCIENTIFIC Solvent Effects
(16) Zlatkis, Albert, Zak, Bernie, Boyle, A. J., J. Lab. Clin. Med. 41, 486-92 (1953).
21" I2
=
I2
=
+
2e
It is somewhat surprising that the oxidation should proceed by this mechanism, in view of the fact that the triiodide ion is known to be more stable than P\T2 (3). Interestingly, the electro-oxidation of iodine in pyridine also proceeds via a two-step mechanism; the Em values are 0.5 and 0.9 volt vs. S.C.E. When a small amount of sodium iodide was added to the solution of iodine in pyridine, a three-step chronopotentiogram with E1/4 values of about 0.35, 0.5. and 0.9 volt vs. S.C.E. was obtained. It appears that the first step in the electrooxidation of iodine in pyridine involves oxidation of triiodide ion to iodine, followed by further oxidation of iodine to iodine(I) in the second step. This supports evidence previously presented for the following dissociation of iodine in pyridine (2, 7). 2I2
+ Pv
Table
21+ + 2e
=
Pvl
+
+ L-
The electro-oxidation of iodide ion in mixed solvent systems of acetone-water and 1 to 1 ethylene glycol-1,2-dimethoxyethane was found to proceed in two steps. In the case of acetone-water system, two steps were evident up to 60 to 70% water. The mechanism favored by a particular solvent is evidently more subtle than could be explained on the basis of an aqueous vs. nonaqueous effect, as shown by the fact that the alcohols, acetic acid, and pyridine favor the same process as water; that pyridine, a base, and acetic acid, an acid, favor the same mechanism; that acetic acid and acetic anhydride favor different processes; and that, in general, solvents containing the OH group favor one-step electro-oxidation of iodide ion to iodine. Table I lists the Eiu vs. S.C.E. values for the oxidation of iodide ion in the solvents studied.
I.
E¡u vs. S.C.E. Values
for
Oxidation of Iodide Ion Solvent (0.1.1/ NaC104)
Water Methanol 1-Butanol" 2-Propanol" Acetic acid Ethylene glycol Pyridine Acetonitrile Acetone Acetic anhydride 1,2-Dimethoxyethane Dimethyl sulfoxide 1 to 1 acetone/water 1
mixture to 1 ethylene glycol1, 2-dimethoxvethane
mixture 0.2.1/LiClOi. I -» I+ step.
Step
Step 2
1
0.51 0.40 0.42 0.42 0.44 0.42 0.37 0.25 0.22 0.20 0.39 0.48
0.52 0.89*
123456
0.55 0.62 0.60 0.65 0.70
0.42
0.54
0.37
0.56
“
6
2
A more detailed report on this and related studies will be presented in the near future. Reynold T. Iwamoto Department of Chemistry University of Kansas Lawrence, Kan. LITERATURE CITED
(1) Adams, R. N., McClure, J. H., Morris, J. B., Anal. Chem. 30, 471 (1958). (2) Audrieth, L. F., Birr, E. J., J. Am. Chem. Soc. 55, 668 (1933). (3) Kleinberg, J., Colton, E., Sattizahn, J., VanderWerf, C. A., Ibid., 75, 442 (1953). (4) Kolthoff, I. M., Coetzee, J. F., Ibid., 79, 1852 (1957). (5) Kolthoff, I. M., Jordan, J., Ibid., 75, 1571 (1953). (6) Popov, A. I., Geske, D. H., Ibid., 80, 1341(1958). (7) Zíngaro, R. A., VanderWerf, C. A., Kleinberg, J., Ibid., 73, 88 (1951).
Received for review January 27, 1959. Accepted March 2, 1959. VOL. 31, NO. 5, MAY 1959
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