Promotion Effect by Organic Compounds on Dissolution of an Iodine Film Formed by Electrochemical Oxidation of Iodide Ions in an Aqueous Solution

When iodide ions (I − ) are electrochemically oxidized in an aqueous solution containing a high concentration of I − , an iodine (I 2 ) film is formed on the electrode surface and inhibits further oxidation of I − . The effect of adding an organic compound as an additive on the I 2 film was investigated and discussed in the light of electrochemical investigations, spectroscopic analyses, and quantum chemical calculations. Aprotonic organic compounds such as acetonitrile (AN) increased the current of I − oxidation while protonic organic compounds such as ethanol (EtOH) did not increase it. The effect did not simply correspond to the increase in solubility of I 2 molecules. In the theoretical calculations, AN showed a behavior that it promoted hydration of an I 2 molecule by coordinating with the I 2 molecule while EtOH formed hydrogen bonds with H 2 O molecules and did not show the behavior. AN desorbed from the I 2 molecule when the coordinated I 2 molecule was hydrated. The aprotonic organic compounds were considered to act as a catalyst to promote the hydration of the I 2 molecules contained in the I 2 film and thereby promote dissolution of the I 2 film.