Interfacial behavior and charge transfer reactions at chemically modified electrodes

Abstract

Platinum electrodes coated with a mixture of poly(4vinylpyridine- co-styrene) [PVPCS] and hexadecyl methanesulfonate [HDMS] show novel voltammetric features in aqueous solution. Platinum oxide and platinum-pyridyl complexes formed at anodic potentials are reduced at different potentials. The platinum-pyridyl complex is also formed in aqueous pyridine solutions. The formation of this complex competes with the adsorption of small polar organic molecules from solution. The diffusion of metal ions such as cu^2+/cu^+, Fe^3+/Fe^2+, and Co^2+/Co^0 through PVPCS+HDMS polymer coatings is complicated and cannot be explained by simple diffusion phenomena. The rate of diffusion at the polymer coated electrode is much slower than that at a bare electrode. The estimated apparent diffusion coefficients show a reciprocal relationship with the bulk concentration which is due to the formation of metal-pyridyl complexes. Metal ions in anionic form such as Fe(CN)6^3-/Fe(CN)6^4- do not form complexes and show only restricted diffusion. Adsorption of acetate onto platinum surfaces to form a platinum-acetate complex is apparent voltammetrically. The equilibrium between platinum-acetate and platinum-hydroxide complexes formed at anodic potentials has an equilibrium constant of 1.3X10^10. The apparent number of acetate ions adsorbed per adsorption site is 1.6. Adsorption of acetate is mimicked in studies of zinc acetate-coatings on platinum in aqueous solution. Metal acetate coatings on platinum electrodes are unusually stable due to the adsorption of acetate. Moreover, platinum electrodes with thick zinc acetate coatings behave as zinc electrodes which show the reduction of Zn2+ in aqueous solution. The voltammetry of zinc acetate and zinc acetate+HDMS coatings show a dramatic change in the presence of methanol. The sensitivity of organic molecules to PVPCS and metal acetate electrode coatings points toward the development of a new class of analytical sensors.