Velia M. Solis

Amperometric determination of dopamine on an enzymatically modified carbon paste electrode

The amperometric determination of dopamine under controlled convective conditions on an enzymatically modified carbon paste electrode was studied using potato tissue as a source of polyphenol oxidase. Enzymatic kinetic constants and substrate diffusional-resistance parameters were determined and an analysis of the most common interferents was made. It is concluded that the electrode is suitable for the quantification of dopamine with a detection limit as low as 2.5 × 10−6 M when the hydrodynamic conditions are carefully controlled. The electrode is active towards dopac, ascorbic acid and pyrocatechol but not l-tyrosine, l-dopa, homovanillic acid, adrenaline, normetanephrine and vanilmandelic acid.

Amperometric determination of dopamine on vegetal-tissue enzymatic electrodes. Analysis of interferents and enzymatic selectivity

Abstract The amperometric determination of phenol and catechol derivatives in general, and dopamine in particular, is studied, using enzymatically modified carbon paste electrodes. Both the electrochemical behaviour of the substrates and the enzymatic selectivity of polyphenol oxidase (PPO) according to the source are taken into account. In this way, the quantification of dopamine in the presence of metabolically related compounds is carried out with reasonable accuracy. Fresh tissues of pear, peach, mushroom and potato are used as the enzymatic source. For fresh mushroom PPO, apparent Michaelis-Menten constants ( K ′ M ) for each substrate are determined electrochemically. Purified mushroom PPO is used for comparative purposes. The effect of the substituent group of catechol derivatives on the enzymatic affinity was also analysed.

A further investigation of the catalysis by lead ad-atoms of formic acid oxidation at a platinum anode

Abstract Potential step experiments permit a more detailed investigation of the oxidation of formic acid in aqueous perchloric acid and at a platinum electrode covered by submonolayer amounts of lead. It is confirmed that lead atoms are very effective catalysts allowing high rates of oxidation over long periods of time. Indeed, at low concentrations of formic acid ( M ) the rate of oxidation can be determined by diffusion of formic acid in solution. At higher concentrations (10–500 m M ); the current is kinetically controlled at short times, but later becomes diffusion controlled; it is suggested that the rate-determining step at short times is dissociative adsorption of formic acid at two platinum atoms adjacent to a lead ad-atom. The mechanism of catalysis is discussed.

Competitive transfer of H+ and Li+ ions through the water-1,2-dichloroethane interface mediated by 1,10-phenanthroline

Abstract The transfer of H + and Li + ions through the water-1,2-dichloroethane interface in the presence of 1,10-phenanthroline is studied by cyclic voltammetry. These ions compete for phenanthroline in the organic phase; the pH and the concentration of the ligand relative to that of the cation determine which predominates in the transfer. It is concluded that the transfer of H + mediated by phenanthroline occurs via a direct mechanism, whereas “facilitated” transfer is operative in the case of Li + .

Comparative investigation of formic acid and formaldehyde electro-oxidation on palladium in acidic medium: Effect of surface oxides

Abstract A comparative study of the electro-oxidation of HCOOH and HCHO on oxidized Pd electrodes was carried out by CV combined with oxidation potential steps, chronopotentiometry and potential decay at open circuit. The HCOOH oxidation currents increased steadily when oxidation potential steps of increasing value were applied, provided that not very stable Pd oxides were formed. The opposite effect was observed for HCHO oxidation. Palladium oxides were chemically reduced by HCOOH but not by HCHO. The activating effect observed for HCOOH oxidation was associated with the chemical and electrochemical reduction of the less stable Pd oxides.

Effect of cyclodextrins on the electrochemical behaviour of ascorbic acid on gold electrodes

The study of the inclusion complexes formed by ascorbic acid with β-cyclodextrin at different pH is reported. Special attention is paid to the electrochemical behaviour of ascorbic acid in the presence of the macrocycle on gold electrodes. The modifications observed can be explained by the decrease in the diffusion coefficient of the complex and the presence of an additional chemical step, the dissociation of the complex prior to the electron transfer steps. The inclusion of ascorbic acid in β-cyclodextrin increases the stability of the guest molecule to oxidising agents. Adsorption of β-cyclodextrin on gold is also detected, depending on the pH. No effect of ascorbic acid behaviour owing to the addition of α-cyclodextrin was observed.

Quantitative determination of erythromycin and its hydrolysis products by cyclic voltammetry at the interface between water and 1,2-dichloroethane

Abstract An electrochemical method for quantifying erythromycin (Er) salts and their hydrolysis products is described. Cyclic voltammetry at the water/1,2-dichloroethane interface in a four-electrode system was used. A linear relationship between Ip and the erythromycin concentration was obtained up to the solubility limit of the antibiotic. The various hydrolysis products which can be obtained in acid or basic solutions were characterized by their respective transfer potentials. Similar hydrolysis products were observed for aged solutions of Er lactobionate. It is concluded that the proposed method is suitable for the quantification of Er and for distinguishing it from its hydrolysis products.

Square wave stripping voltammetry of Cd–oxine complexes; surface redox reactions

The effect of the reactant adsorption on square wave voltammetry of a 2-electron quasi-reversible redox reaction is analysed experimentally and theoretically. Current potential profiles and the characteristics of the peak curve and the peak potential were analysed at different pH and adsorption potentials for Cd–oxine complexes. The rate constants ks=6, 1.3 and 0.4 s−1 were obtained at pH 6.4, 7.8 and 11, respectively. Analytical applications were also considered.

Oxidation of formic acid on palladium anodes in acidic medium. Effect of Pd(II) ions

Abstract Electro-oxidation of HCOOH on Pd in acidic medium is studied by cyclic voltammetry. Homogeneous oxidation of HCOOH by Pd(II) ions is followed by potential decay at open circuit and by spectrophotometry. An enhancement of HCOOH electro-oxidation current is found by Pd(II) addition to the working solution or when electrogeneration of Pd(II) occurs. The oxidative properties of these ions, both in a homogeneous reaction with HCOOH and in a heterogeneous one with electroformed HCOOH intermediates are taken into account to explain the observed effects.

Cathodic stripping square wave voltammetry of Cu(II)–oxine complexes. A mechanistic study

The reaction mechanism for the reduction of Cu(II)–oxine complexes on mercury electrodes is studied by cyclic voltammetry (CV) and square wave voltammetry (SWV). The quasi-reversible two-electron transfer is analysed from a theoretical point of view according to the EED and EDE mechanistic schemes based on an analysis of the Butler–Volmer formalism. The effects of SW parameters and solution composition are analysed. Fitting experimental data with theoretical curves allowed the evaluation of charge transfer rate constants, as well as discrimination between adsorbed or soluble species. In buffered solution at pH 7.7, a ks value of 0.8 s−1 was determined, for a one-step process involving adsorbed reactants and soluble products. In unbuffered solutions, a split in the redox wave is observed with ks values of 60 and 4 s−1 for each process, assigned to the reduction of CuOx+ and Cu(Ox)2, respectively.