ACCESS IN CLAY-MODIFIED ELECTRODES - EFFECTS OF PH AND FILM THICKNESS ON THE ELECTROCHEMICAL ACTIVITY OF GALLERY EXCHANGE IONS AND SURFACE ION-PAIRS

The effects of pH and clay film loading (thickness) on the electrochemistry of Fe(bpy)(3)(2+), A(n-) ion pairs (A(n-) = CH3CO2-, CCl3CO2-, SO42-) and Ru(NH3)(6)(3+) 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)(6)(3-) anions in a clay film. We find that the number of surface-bound Fe(bpy)(3)(2+), A(n-) ion pairs capable of accessing the electrode surface decreases with increasing acidity, with the more basic CH3CO2- counter-anions exhibiting the greater pH dependence. In contrast, the electroactivity of Ru(NH3)(6)(3+) exchange ions intercalated in the gallery regions of the clay film is pH independent. Also, the fraction of electroactive Fe(bpy)(3)(2+), A(n-) 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)(6)(3+) exchange ions increases with film loading. Fe(CN)(6)(3-) 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 (Fe-3[Fe(CN)(6)](2)) deposited on the clay surface. These differences in electrochemical behavior for Fe(bpy)(3)(2+), A(n-) ion pairs, Ru(NH3)(6)(3+) exchange cations and Fe(CN)(6)(3-) anions are inconsistent with the clay-layer protonation mechanism proposed previously to explain the decrease in electrochemical activity with decreasing pH for Ru(bpy)(3)(2+), SO42- and K+,Fe(CN)(6)(3-) salts at clay-modified electrodes. Instead, the pH dependence for M(bpy)(3)(2+) species is most likely due to changes in the degree of ion-pair formation caused by the protonation of the A(n-) counter-ions. These results, together with those of earlier studies, provide insights regarding the electrochemical accessibility of redox active species in clay-modified electrodes.