Clay-modified electrodes as studied by the quartz crystal microbalance: Redox processes of ruthenium and iron complexes

The electrochemical quartz crystal microbalance (EQCM) has been employed to investigate the mass transport processes on a clay-modified electrode. The systems investigated were the redox couples of [Ru(bpy)(3)](2+/3+) (bpy = 2,2'-bipyridine), [Ru(NH3)](6)(2+/3+) and [Fe(CN)(6)](4-/3-). A clay-modified electrode was prepared by depositing synthetic saponite onto a gold coated quartz crystal. An electrode was allowed to swell for more than 50 h in 0.01 M Na2SO4 or NaCl or NaClO4 prior to the adsorption and electrochemical measurements. The EQCM results revealed that the charge balancing during a redox reaction was accomplished by leaching or incorporating mobile ions in the clay film. For the [Ru(bpy)(3)](2+/3+) couple, one [Ru(bpy)(3)](3+) molecule was eliminated from the clay film when three [Ru(bpy)(3)](2+) ions were oxidized. For the [Ru(NH3)(6)](2+/3+) couple, one SO42- ion, which was co-adsorbed with the ruthenium complex, was removed from the clay film when two [Ru(NH3)(6)](3+) molecules were reduced. For the [Fe(CN)(6)](4-/3-) couple, a part of the excess charge generated by the initial oxidation of [Fe(CN)(4)](4-) was canceled out by the elimination of a sodium ion bound by a clay layer. No mass transfer was detected during the oxidation at the later stage. This was probably because sodium ions in the aqueous medium within a clay film carried excess charge. The results are discussed in relation to the reported electrochemical behavior of these metal complexes. (C) 1998 Elsevier Science S.A. All rights reserved.