The origin and significance of the anodic prepeak (peak A0), seen in cyclic voltammetric studies of hydrous Ir oxide films in acidic solution just prior to the large Ir(III)/(IV) redox peaks (A1/C1), has been examined in this work. By also studying only very thin films, a corresponding cathodic feature (peak C0) has also been identified. The pH dependence of the A0/C0 peaks, assumed to also depict the Ir(III)/(IV) process, has been examined in sulfuric acid solutions, indicating that a ca. −80 mV shift per pH unit applies to this reaction. By also taking into consideration the measured in situ mass changes as the potential is scanned through the A0/C0 peaks, it is shown that these peaks are associated with a greater involvement of anions and more extensive water exchange than is the reaction in the main A1/C1 peaks. Other results also indicate that the film sites that react in the A0/C0 peaks are located deep within the oxide film, likely near the Ir – oxide film interface. Keywords: Ir oxide, hydrous oxide films, cyclic voltammetry, quartz crystal microbalance (QCMB), prepeak, pH dependence, electrochemistry.
