Browsing by Subject "Microelectrode"

Now showing 1 - 3 of 3
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    Application of voltammetric techniques at microelectrodes to the study of the chemical stability of highly reactive species
    (American Chemical Society (ACS), 2014-12-31) Laborda, Eduardo; Olmos, José Manuel; Torralba, Encarnación; Molina, Ángela; Química Analítica
    The application of voltammetric techniques to the study of chemical speciation and stability is addressed both theoretically and experimentally in this work. In such systems, electrode reactions are coupled to homogeneous chemical equilibria (complexations, protonations, ion associations,…) that can be studied in a simple, economical and accurate way by means of electrochemical methods. These are of particular interest when some of the participating species are unstable given that the generation and characterization of the species are performed in situ and in a short time scale. With the above aim, simple explicit solutions are presented in this paper for quantitative characterization with any voltammetric technique and with the most common electrode geometries. From the theoretical results obtained, it is pointed out that the use of square wave voltammetry in combination with microelectrodes is very suitable. Finally, the theory is applied to the investigation of the ion association between the anthraquinone radical monoanion and the tetrabutylammonium cation in acetonitrile medium.
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    Effects of unequal diffusion coefficients and coupled chemical equilibria on square wave voltammetry at disc and hemispherical microelectrodes
    (Elsevier, 2015) Olmos Martínez, José Manuel; Molina Gómez, María de los Ángeles; Laborda Ochando, Eduardo; Martínez-Ortíz, Francisco Asís; Química Analítica
    Square wave voltammetry (SWV) at disc and hemispherical microelectrodes is studied when the electroactive species show different diffusivities and/or take part in chemical equilibria in solution, under both transient and steady state conditions. Despite the use of microelectrodes, experiments in pulse techniques such as SWV frequently correspond to transient conditions since the pulse duration are typically very short (<100ms, f>5Hz). As will be shown, under such conditions the case of unequal diffusion coefficients gives rise to a complex behaviour of the SWV peak, the features of which deviate from the theory and criteria already established under the assumption of equal diffusion coefficients. Also, the theoretical treatment is notably more difficult and no analytical solution has been deduced for multipulse techniques.The effects of coupled chemical equilibria and, particularly, of unequal diffusion coefficients on SWV will be described in depth as a function of the electrode size and shape and they will be compared with those observed in differential double pulse voltammetry (DDPV). The main discrepancies in the peak height, width and position with respect to the case of equal diffusion coefficients will be studied. Also, appropriate methodologies and experimental conditions will be discussed for the determination of, formal potentials and equilibrium constants.
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    Guidelines for the voltammetric study of electrode reactions with coupled chemical kinetics at an arbitrary electrode geometry
    (ACS Publications, 2019-04-02) López Tenés, Manuela; Laborda Ochando, Eduardo; Molina Gómez, Ángela; Compton, Richard G.; Química Física
    A powerful, unified, and simplifying mathematical approach for the theoretical treatment of first-order chemical kinetics coupled to interfacial charge transfers at electrodes of arbitrary geometry and size, both uniformly accessible and nonuniformly accessible to the electroactive species, is presented. The general CEC mechanism at spherical and disc electrodes is considered to test the validity and benefits of such an approach, based on the application of the so-called kinetic steady state, that enables the reduction of the multivariable problem of kinetic-diffusive differential equations to a single variable problem of a diffusion-only differential equation. This is solved both analytically and numerically, showing how this approach leads to general, simple, and efficient solutions for the study of the influence of coupled chemical kinetics on the voltammetric response. The voltammetry of the CEC mechanism is analyzed as a function of the kinetics and thermodynamics of the preceding and subsequent chemical reactions and of the electrode size (from macroelectrodes to ultramicroelectrodes) and shape (spherical and disc). Comparison with the responses of the CE, EC, and E mechanisms is included, proposing diagnosis criteria and procedures for quantitative analysis of experimental data.