M. Fátima Bento

Steady state voltammetry at low electrolyte/reactant concentration ratios: what it means and what it does not mean

Abstract Voltammetric measurements performed at low [electrolyte]/[reactant] ratios are affected by migrational transport, as well as by ohmic drop contributions. The latter depend on the current as well as on the charge of the initial electroactive species because the local electrolysis changes the ionic composition in the vicinity of the electrode. Extraction of thermodynamic or kinetic data from wave shapes and positions is thus impossible without correction of these ohmic drop components. This work extends a previous experimental approach for eliminating ohmic drop contributions from experimental voltammograms obtained at low [electrolyte]/[reactant] ratios, by combining impedance measurements and voltammetric data. The results presented here confirm our previous independent conclusions that when the reactant is neutral, the variation of ohmic drop along the voltammetric curve (which reflects the progressive ionic enrichment of the diffusion layer) cannot be predicted by considering diffusional/migrational transport alone, but also requires consideration of the influence of natural convection.

About potential measurements in steady state voltammetry at low electrolyte/analyte concentration ratios

Abstract At low [electrolyte]/[analyte] ratios, besides being affected by tandem diffusional–migrational transport, voltammograms obtained at ultramicroelectrodes are altered by ohmic drop and junction potentials. This makes impossible the direct extraction of thermodynamic or kinetic data from wave shapes and positions. In this work, we propose two experimental strategies for eliminating distortions of voltammograms due to junction potential, and test them based on four one-electron reversible redox systems (oxidation and reduction of DPA; reduction of DCN; oxidation of ferrocene). Taking advantage of the availability of junction potential-free reconstructed voltammograms, an analysis of ohmic drop distortions is performed. It is concluded that ohmic drop has two origins. One, which has been considered in the presently available theory, relates to the convection-free diffusion layer and is current dependent because of the current dependent ionic enrichment or depletion of the diffusion layer. The other, not considered by previous theories, is due to the resistance of the unaffected bulk solution. When the current flow corresponds to the overall creation of ionic charges, the later component becomes critical at low [electrolyte]/[analyte] ratios and high currents, because the former is considerably reduced by the ionic enrichment of the convection-free diffusion layer. This duality is particularly important in describing the shapes of voltammetric waves recorded with low supporting electrolyte concentration and sets limits on the usefulness of presently available theories which consider only one component of ohmic drop.

Potential measurements in steady state voltammetry at low electrolyte/analyte concentration ratios. Role of convection on ohmic drop : a simplified model

Voltammetric measurements performed at low [electrolyte]/[analyte] ratios are affected by migrational transport, as well as by ohmic drop contributions. The latter depend on the current as well as on the charge of the initial electroactive species because the local electrolysis changes the ionic composition in the vicinity of the electrode. Extraction of thermodynamic or kinetic data from wave shapes and positions is thus impossible without correction of these ohmic drop components. In this work we extend and generalize a previous experimental approach for eliminating such ohmic drop contributions from an experimental voltammogram. The method is tested based on four one-electron reversible redox systems: oxidation and reduction of DPA; reduction of DCN; and oxidation of ferrocene. It is thus confirmed that under conditions where there is no excess of supporting electrolyte, the cell resistance has two origins. One, already recognized by previous theories, depends on the potential because the current flow affects the ionic content within the diffusion layer. The second, which was not considered in previous theories, is independent of the potential since it reflects the resistance of the solution in a range in which the ionic content is not affected by the electrode current. This occurs because convection precludes development of infinite diffusion layers, an important issue not considered in previous theories. At low [electrolyte]/[analyte] ratios, the second term becomes significant and may even become the major one for the portion of experimental interest in voltammetric waves. A simplified model is proposed to describe this phenomenon. Despite its simplicity and the crudeness of some approximations performed in order to achieve an analytical solution, this simple model predicts all the trends as well as the order of magnitude of the experimental deviations vis a vis previous theoretical predictions.

Antimicrobial and antioxidant properties of propolis ethanol extracts from Terceira Island (Azores, Portugal)

Bees produce a resinous mixture named propolis known to have several functions in the beehive, namely structural and sanitary [1,2]. Propolis presents a rich and complex chemical composition, imparted from the several plant materials collected, and several compounds and extracts have been associated with biological activities of high interest [3]. This study focuses on the antimicrobial activity and antioxidant capacity of ethanolic extracts (EE) obtained from propolis samples collected in 2010, in five different areas of the Terceira Island (Azores Archipelago). Antimicrobial activity was evaluated by dilution assay incorporating different EE concentrations in solid media. Bacteria and yeasts of clinical and agrofood relevance were used as indicators of susceptibility. The results showed that Gram-positive bacteria displayed more resistance (best MIC value 200 μg/ml) than the Gram-negative (100 μg/ml). Extracts with higher antibacterial activity (e.g. EE4 and EE5) were generally distinct from those with greater activity against yeast (EE1), but species-specific susceptibilities and extract-specific efficacies against particular microorganisms were also observed, only EE1 was effective against S. epidermidis. A relationship between propolis macroscopic characteristics (colour, hardness and odour) and its antifungal activity was noticed. If confirmed for a broader range of samples, this relationship may constitute an important diagnostic tool. The antioxidant capacity was assessed using cyclic voltammetry [4]. EE4 and EE5 showed the highest antioxidant capacities (39.6 and 23.5 mM eq. gallic acid, respectively), in agreement with results from the antimicrobial assays. Chemical characterization of all EE are underway. Despite being collected in a somewhat confined geographic area propolis samples exhibited diverse antimicrobial activities with different efficiencies and antioxidant capacities.