MASS-TRANSPORT ON AN ANIONIC CLAY-MODIFIED ELECTRODE AS STUDIED BY A QUARTZ-CRYSTAL MICROBALANCE

A quartz crystal microbalance (QCM) has been used to investigate mass transfer coupled with electron transfer through a hydrotalcite clay fi lm. The electrode was prepared by casting an aqueous suspension of hyd rotalcite (denoted as Mg-Al-HT) or hydrotalcite ion-exchanged with [Fe (CN)(6)](3-) (denoted as Mg-Al-HT-[Fe(CN)(6)])(3-). Adsorption studies using QCM and W-visible spectroscopic measurements revealed that [Fe( CN)(6)](3-) was incorporated as an ion-paired species with a counter c ation. The adsorbed species on an electrode in contact with a 0.1 M Na Cl solution was identified as a divalent ion-pair (Na[Fe(CN)(6)](2-)) while the adsorbed species in contact with a 0.1 M Na2SO4 or Na2CO3 so lution was a monovalent ion-triplet (Na-2[Fe(CN)(6)](-)). Electrochemi cal QCM measurements on an electrode modified with Mg-Al-HT-[Fe(CN)(6) ](3-) revealed that one Na[Fe(CN)(6)](2-) or Na-2[Fe(CN)(6)](-) ion wa s leached from a film when one or two [Fe(CN)(6)](3-) ions were reduce d, respectively. The leaching of these anions took place in order to c ompensate the excess negative charge generated in a film. (C) 1998 Els evier Science S.A. All rights reserved.