Manuel Blázquez

Nitro radical anion formation from nimodipine

Abstract Voltammetric studies of nimodipine using a mixed aqueous dimethylformamide (DMF) solvent have allowed us to generate the one-electron reduction product, the nitro radical anion RNO 2 . . The cyclic voltammetry technique has been employed to study the tendency of RNO 2 . to undergo further chemical reactions. This subsequent chemical reaction corresponds to a second-order process, a dismutation reaction which is initiated electrochemically. Data for rate constants and half-lives at pH 8.2 were determined in aqueous DMF media. A method which is able to generate selectively the RNO 2 . species electrochemically and to study its in-situ reactions is proposed.

Derivation and experimental verification of approximate explicit equations in differential pulse polarography Part II. Second-order processes

Abstract An approximate method applicable to electrochemical processes either coupled or not to first order chemical reactions in differential pulse polarography is proposed. A program has been prepared in order to enable the authors to interpret DP polarograms. The method has been tested with the aid of well-known oxidation and reduction processes. The utility of the method is shown, particularly in those cases involving overlapping DP polarographic waves. Applications to determinations of kinetic parameters and quantitative analysis have also been made.

Diagnostic criteria for characterization of CE and CEC mechanisms in polarography

Abstract From a calculation based on the reaction layer theory, several expressions have been developed to differentiate EC and CEC mechanisms when the transfer is reversible. In order to check these equations, they have been applied to the pyridine-4-aldehyde reduction and close agreement between the experimental and theoretical results was obtained for a CEC mechanism, with the last stage being a dimerization reaction.

A voltammetric study of 6-mercaptopurine monolayers on polycrystalline gold electrodes

Abstract 6-Mercaptopurine (6MP) monolayers were formed spontaneously on different gold surfaces by chemisorption from 6MP solutions. The present cyclic voltammetric study reveals that the monolayer formed is stable in a wide potential range up to values where the oxidative desorption occurs. The reductive desorption of 6MP at a polyfaceted gold electrode shows a multi-wave response. This has been analysed by comparing the observed peak potentials with those obtained at low-index single crystal gold electrodes. The film has no apparent effect on the electron transfer rate constant of Ru(NH 3 ) 6 3+/2+ and Fe(CN) 6 3−/4− at the substrates studied, but modifies the electron transfer rate of the benzoquinone redox couple. This behaviour is typical of thin films where only the slower reactions should display a larger apparent decrease of the rate constant. In contrast, the 6MP-monolayer shows a strong ability to inhibit electroreduction of dioxygen at polyfaceted gold while it remains stable.

Cyclic voltammetric study of the nitro radical anion from nitrendipine generated electrochemically

Abstract Electrochemical studies on nitrendipine using mixed aqueous/dimethylformamide (DMF) solvent have allowed us to generate the one-electron reduction product, the nitro radical anion, RNO .− 2 . The voltammetric technique has been employed to study the tendency of RNO .− 2 to undergo further chemical reactions. The electrochemical process corresponds to a dimerization reaction that is initiated electrochemically. The cyclic voltammetric technique has allowed the rate constant and the half-life time to be determined for the decay of RNO .− 2 in aqueous DMF mixed media and by extrapolation in aqueous media.

Resolution of absorption spectra

Abstract Absorption spectra are resolved on the basis of lognormal distribution. A procedure is described for resolving absorption spectra in terms of skewed bands.

Derivation and experimental verification of approximate explicit equations in differential pulse polarography: Part III. Diagnostic criteria for characterization of first-order processes

Abstract From an approximate equation previously derived, several diagnostic criteria have been developed to differentiate electrochemical processes coupled or not to first-order reactions. To check these criteria, they have been applied to the reductions of Cd(II) ion, diacetyl and alloxan, and a close agreement between experimental and theoretical results was obtained in all cases. Moreover, the range of applicability of these equations is also discussed and tested.

A curve-fitting program set for handling of differential pulse polarograms

Abstract DP polarograms corresponding to both first- and second-order processes are treated on the basis of approximate equations. Overlapped peaks can also be analysed. BASIC curve-fitting programs running on a APPLE II+ are proposed and tested. Applications to electrochemical kinetics and routine analyses are also considered.

Systematic errors in the calculation of kinetic parameters by the polarographic method

In this paper we have determined the systematic errors introduced in the calculation of some kinetic parameters, such as Tafel slopes and reaction orders, on applying both the Butler—Volmer equation and the rate-determining step approximation at potentials corresponding to the foot of the polarographic waves. These determinations have been carried out using the general i — E relations for the waves, which are obtained from implicit functions found in the literature. Our results indicate that in the mechanism involving dimerization reactions, the errors must be taken into account (including the zone where i < 0.05 iL), since these are greater than those corresponding to the other mechanisms. In order to check these results, two processes have been studied: the reduction of monoethylfumarate and the reduction of benzophenone. Close agreement was found with the experimental results in all cases.

The direct electrochemistry of cytochrome c at a hanging mercury drop electrode modified with 6-mercaptopurine

Abstract The modification of a mercury electrode with 6-mercaptopurine (6MP) has been performed at pH 6 in acetate buffer and the electrode behaviour has been checked in the absence of the modifier in solution. Direct electrochemistry of cytochrome c has been observed on a 6MP-Hg modified electrode. At high coverage of 6MP, the monolayer acts as an effective promoter for the redox exchange of cyt c , which is not observed on a bare mercury electrode. The film is stable for more than 200 scans and must play a role on avoiding irreversible adsorption and denaturation of the protein. At pH 7, 25 mM phosphate buffer and 0.1 M NaClO 4 , cyt c exhibits a quasi-reversible electron exchange similar to that reported with gold modified electrodes, the electron rate constant being 4.1×10 −3 cm s −1 . As far as we know it is the first time that such a response has been observed on mercury.