Joaquín González

General analytical solution for a catalytic mechanism in potential step techniques at hemispherical microelectrodes : Applications to chronoamperometry, cyclic staircase voltammetry and cyclic linear sweep voltammetry

An easy, general analytical solution, applicable to any multipotential step technique for a catalytic process in planar and spherical electrodes (including ultramicrohemispheres), is presented. The solution has been deduced rigorously using the mathematical method described by Molina (A. Molina, J. Electroanal. Chem. 443 (1998) 163) based on the application of the superposition principle. In this paper it has been applied to multistep chronoamperometry, cyclic staircase voltammetry (CSCV) and cyclic linear sweep voltammetry (CLSV) by analysing the influence of the electrode radius and the equilibrium and rate chemical constants over the different I/t and I/E responses. Different voltammetric steady states (kinetic, microgeometrical and geometrical-kinetic) can be distinguished and methods for calculating the kinetic parameters of the chemical reaction are also proposed. In the case of an irreversible chemical reaction in linear sweep voltammetry our results are in agreement with those recently deduced by Diao and Zhang (G. Diao, Z. Zhang, J. Electroanal. Chem. 429 (1997) 67).

Differential pulse voltammetry for ion transfer at liquid membranes with two polarized interfaces.

A simple analytical expression for the response of the double-pulse technique differential pulse voltammetry (DPV) corresponding to ion transfer processes in systems with two liquid/liquid polarizable interfaces has been deduced. This expression predicts lower and wider curves than those obtained with a membrane system with a single polarizable interface. Moreover, the peak potential of these systems is shifted 13 mV from the half-wave membrane potential. We have applied this expression to study the ion transfer of drugs with different pharmacological activities (verapamil, clomipramine, tacrine, and imipramine), at a solvent polymeric membrane ion sensor.

Pulse Voltammetry in Physical Electrochemistry and Electroanalysis

Introduction.- Single Potential Step Voltammetry.- Double and Triple Potential Step Techniques.- Multipotential Step Voltammetry.- Linear and Cyclic Sweep Voltammetry.

Theory for cyclic reciprocal derivative chronopotentiometry with power and exponential programmed currents applied to electrodes coated with reversible electroactive molecular films

Theoretical expressions corresponding to the responses of adsorbed redox molecules exhibiting reversible charge transfer in cyclic reciprocal derivative chronopotentiometry with programmed currents of the form I(t) ( 1) i 1 I0t i and I(t)(1) i 1I0 e vti are presented. The effects of the interfacial potential distribution are considered in both cases by using the Smith and White model (Anal. Chem. 64 (1992) 2398). These equations become those previously deduced by Honeychurch (J. Electroanal. Chem. 445 (1998) 63) in reciprocal derivative chronopotentiometry with constant current on making u 0a ndv 0. The total analogy between the reciprocal derivative chronopotentiograms (dt i 1 :dE versus E or d e vti :dE versus E curves) and the cyclic voltammograms (I:E curves) is proved.

Application of cyclic reciprocal derivative chronopotentiometry with programmed currents to the study of the reversibility of electrode processes

The general equations corresponding to the dt:dEE curves obtained in cyclic reciprocal derivative chronopotentiometry with programmed currents of the form I(ti) ( 1) i 1 Iit i with u] 0 are deduced. These expressions are valid for reversible, quasirreversible and irreversible electrode processes. Easily applicable criteria are proposed for the characterisation of reversible and irreversible processes based on the influence of the power of time of the programmed current in the apparent reversibility of the charge transfer. This technique is able to evaluate electrochemical rate constants of the order of those that can be calculate in voltammetric techniques by operating in a time scale ten times greater. These theoretical equations have been tested experimentally for the system Fe(CN)6 : Fe(CN)6 in Na2SO4 which behaves as totally reversible when a current ramp is applied (u 1) whereas it can be considered as quasirreversible when two successive programmed currents with u5 1:2 are applied. We have also studied the system Ce 4 :Ce 3 in H2SO4, which behaves as totally irreversible for any value of u.

Charge /potential and capacitance /potential curves corresponding to reversible redox monolayers

Abstract General expressions for the charge–potential (Q/E) and capacitance–potential (C/E) curves corresponding to reversible redox monolayers are deduced. These responses present time-independent and universal behaviour, which does not depend on the voltammetric or chronopotentiometric technique employed to obtain them. This holds even in the more complex case in which non-faradaic effects are taken into account. The simultaneous availability of both Q/E and C/E curves allows the total characterisation of the reversible electroactive monolayer. The initial presence of both species in the monolayer can be detected only from the Q/E curves which present a cross potential from which the ratio of initial excesses can be obtained. Experimental C/E data are easily available from chronopotentiometric or voltammetric measurements. Nevertheless, to obtain the experimental Q/E curve, chronopotentiometric data results are more suitable.

Application of the superposition principle to the study of CEC, CE, EC and catalytic mechanisms in cyclic chronopotentiometry. Part III

In this paper we have obtained the general analytical equations corresponding to the response of complicated charge transfer processes with coupled homogeneous chemical reactions in cyclic chronopotentiometry by applying the superposition principle. The analysis of different cycles of this response can be used to obtain accurate and contrasted values of kinetic parameters in each complex reaction scheme analysed. These parameters are of great interest for the chemist and the biochemist. These equations are also applicable to cyclic derivative chronopotentiometry which is a very useful method since the response is obtained with peaks in a similar way to cyclic voltammetry but its mathematical treatment is simpler.

Cyclic reciprocal derivative chronopotentiometry. Applications to the detection and characterisation of adsorption processes

Abstract Cyclic reciprocal derivative chronopotentiometry (CRDC) is applied to the characterisation of different electrode processes in which the adsorption of the reactant and/or the product takes place. The d t /d E – E curves have been obtained by making the analytical or numerical reciprocal of the time derivative of the analytical equations corresponding to the chronopotentiograms (potential time curves) deduced in a previous paper in cyclic chronopotentiometry. The expressions corresponding to the simpler case of strong adsorption of both species have been deduced as a limit case of these general equations. When two successive cathodic and anodic currents (one cycle) are applied, the d t /d E – E response presents two peaks whose ratio of the heights and difference between their potentials afford simple diagnosis criteria to characterise different adsorption processes. Moreover, these processes can be further confirmed through the use of more than one current cycle. Methods for determining adsorption parameters are also proposed.

Mass transport at electrodes of arbitrary geometry. Reversible charge transfer reactions in square wave voltammetry

The basis for mass transport of the electroactive species in different diffusion fields is examined, pointing out important insights in relation to the value of the surface concentrations for the case of fast electrode processes. Moreover, a general analytical solution for the transient current is given for several geometries, when an arbitrary sequence of potentials pulses is applied to macroelectrodes (planar), spheres, cylinders, discs and bands. The results are particularized for Square Wave Voltammetry. Explicit solutions for the net current and the forward and backward components are given. The effects of frequency, pulse amplitude and electrode size or shape on the peak current are studied. Moreover, the conditions for the attainment of the steady state response are analyzed in the different geometries as well as the characteristics of this response.

Reciprocal Derivative Chronopotentiometry with Programmed Current: Influence of the Reversibility

The behavior of the system Cd 2/Cd(Hg) in the absence and presence of an aliphatic alcohol in noncomplexing media is analyzed in reciprocal derivative and double derivative chronopotentiometry with programmed current (RDCP and RDDCP, respectively). These electrochemical methods have been compared with derivative and double derivative voltammetry (DV and DDV, respectively). When the above system behaves as non reversible, it is not possible to establish a total analogy between chronopotentiometric and voltammetric responses, contrary to the case of a reversible process. The peaks obtained in RDCP are better resolved than those obtained in DVand the different operational principles of RDCP compared to those of DV can reduce its susceptibility to ohmic drop effects. Moreover, RDCP and RDDCP are very versatile in the determination of kinetic parameters of electrode processes since, by varying the current amplitude and/or the power of time u in the programmed current, it is easy to influence the reversibility of the process.