Access in clay-modified electrodes: effects of pH and film thickness on the electrochemical activity of gallery exchange ions and surface ion pairs

Abstract

Abstract The effects of pH and clay film loading (thickness) on the electrochemistry of Fe(bpy)2+3, An− ion pairs (An− = CH3CO−2, CCl3CO−2, SO2−4) and Ru(NH3)3+6 exchange cations in montmorillonite- and laponite-modified graphite electrodes are investigated. Included in the study is the effect of pH on the activity of Fe(CN)3−6 anions in a clay film. We find that the number of surface-bound Fe(bpy)2+3, An− ion pairs capable of accessing the electrode surface decreases with increasing acidity, with the more basic CH3CO−2 counter-anions exhibiting the greater pH dependence. In contrast, the electroactivity of Ru(NH3)3+6 exchange ions intercalated in the gallery regions of the clay film is pH independent. Also, the fraction of electroactive Fe(bpy)2+3, An− ion pairs is essentially independent of clay film loading, indicating that the electroactive sites are confined to a narrow region of the film close to the electrode surface. However, the electroactivity of the more mobile Ru(NH3)3+6 exchange ions increases with film loading. Fe(CN)3−6 anions are easily displaced from a clay film by washing, but at pH 3.0–4.0 the complex anion decomposes to give a cyclic voltamogram indicative of Prussian blue (Fe3[Fe(CN)6]2) deposited on the clay surface. These differences in electrochemical behavior for Fe(bpy)2+3, An− ion pairs, Ru(NH3)3+6 exchange cations and Fe(CN)3−6 anions are inconsistent with the clay-layer protonation mechanism proposed previously to explain the decrease in electrochemical activity with decreasing pH for Ru(bpy)2+3, SO2−4 and K+,Fe(CN)3−6 salts at clay-modified electrodes. Instead, the pH dependence for M(bpy)2+3 species is most likely due to changes in the degree of ion-pair formation caused by the protonation of the An− counter-ions. These results, together with those of earlier studies, provide insights regarding the electrochemical accessibility of redox active species in clay-modified electrodes.