José Manuel Olmos

Voltammetric speciation studies of systems where the species diffusivities differ significantly

A rigorous and simple analytical solution is reported for the square scheme in single pulse techniques at (hemi)spherical electrodes when the electron transfers are reversible and the coupled chemical reactions are at equilibrium. The solution presented imposes no restriction to the values of the diffusion coefficients of the different species, and then it fully describes the cases where the coupled chemical processes involve significant changes of diffusivity. Simple criteria are discussed to understand and predict the effects of electrode radius, experiment time-scale and chemical thermodynamics on the results obtained in normal pulse voltammetry and derivative voltammetry. Unlike at macroelectrodes and ultramicroelectrodes, the position of the voltammograms (in these and other voltammetric techniques) is time dependent at spherical electrodes of intermediate size when the diffusion coefficients of the species are unequal. An analytical expression for the half-wave potential is given to describe this behaviour and also to assist the quantitative determination of the diffusion coefficients, formal potentials and equilibrium constants with the electrochemical techniques abovementioned.

Characterization of inclusion complexes of organic ions with hydrophilic hosts by ion transfer voltammetry with solvent polymeric membranes

The quantitative characterization of inclusion complexes formed in aqueous phase between organic ions and hydrophilic hosts by ion-transfer voltammetry with solvent polymeric membrane ion sensors is studied, both in a theoretical and experimental way. Simple analytical solutions are presented for the determination of the binding constant of the complex from the variation with the host concentration of the electrochemical signal. These solutions are valid for any voltammetric technique and for solvent polymeric membrane ion sensors comprising one polarisable interface (1PI) and also, for the first time, two polarisable interfaces (2PIs). Suitable experimental conditions and data analysis procedures are discussed and applied to the study of the interactions of a common ionic liquid cation (1-octyl-3-metyl-imidazolium) and an ionisable drug (clomipramine) with two hydrophilic cyclodextrins: α-cyclodextrin and 2-hydroxypropyl-β-cyclodextrin. The experimental study is performed via square wave voltammetry with 2PIs and 1PI solvent polymeric membranes and in both cases the electrochemical experiments enable the detection of inclusion complexes and the determination of the corresponding binding constant.