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

Abstract

When iodide ions (I−) are electrochemically oxidized in an aqueous solution containing a high concentration of I−, an iodine (I2) 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 I2 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 I2 molecules. In the theoretical calculations, AN showed a behavior that it promoted hydration of an I2 molecule by coordinating with the I2 molecule while EtOH formed hydrogen bonds with H2O molecules and did not show the behavior. AN desorbed from the I2 molecule when the coordinated I2 molecule was hydrated. The aprotonic organic compounds were considered to act as a catalyst to promote the hydration of the I2 molecules contained in the I2 film and thereby promote dissolution of the I2 film.