Carmen Serna

Conditions of applicability of the superposition principle in potential multipulse techniques: implications in the study of microelectrodes

Abstract In this article we establish the necessary mathematical conditions for the superposition principle to be applicable to obtaining the faradaic response to a multipulse sequence on plane and spherical electrodes (whether increasing with time or not) and on cylindrical electrodes. These conditions have been partially established by some authors, but to date have not been derived in a rigorous and general form. The general expressions derived in this paper for the current I j corresponding to any pulse j are much simpler than those obtained previously in the literature for some specific cases and can easily be used in practice. To illustrate this, we apply these solutions to staircase and square-wave voltammetry. The effect of the size of the electrode on the final solution is also analysed.

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).

Derivative and Differential Voltammetry and Reciprocal Derivative Chronopotentiometry Identical Behavior Verification for Electrode Reversible Processes

In the present paper, we carry out the experimental verification of the analogy between the responses obtained in derivative and differential voltammetric techniques and those corresponding to a new modality of the reciprocal derivative chronopotentiometric techniques using a programmed current of the form I(t) = I 0 t u with u ≥ - 1/2. Identical information can be obtained from derivative voltammetry and reciprocal derivative chronopotentiometry although the experimental application of the latter is simpler and signals obtained are less noise influenced due to the use of programmed current. In order to show the above mentioned analogy in the case of reversible electrode processes, we have carried out an experimental study of the systems Fe(C 2 O 4 ) 3 3 in 0.2 M K 2 C 2 O 4 (pH 4.49) at a static mercury drop electrode and ferrocene in CH 3 CN/0.25 M PF 6 NBu 4 at a Pt electrode.

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.

General solutions for the I/t response for reversible processes in the presence of product in a multipotential step experiment at planar and spherical electrodes whose areas increase with any power of time

Abstract General explicit expressions are presented for the current I p corresponding to the p th step of any sequence of potential steps E 1 , E 2 ,…, E p , when both oxidised and reduced species are present initially. These equations are applicable to reversible processes at planar and spherical electrodes whose areas increase with any power of time ( A ( t )= A 0 t z , z =0 for planar and spherical static electrodes and microelectrodes, and z =2/3 for the DME). For an SMDE we have also considered those situations in which the diffusion coefficient values of both species are not equal and the reaction product is amalgamated initially into the electrode. As an example of the application of these equations, we have analysed the influence of the presence of the reaction product in multistep chronoamperometry for an SMDE and also when microhemispheres are used by considering that the diffusion coefficients are equal and the product is soluble in the electrolytic solution. All equations are applicable to any single, double or multipotential step techniques and also to linear sweep and cyclic voltammetry and polarography.

Study of multistep electrode processes in double potential step techniques at spherical electrodes

Analytical solutions for reversible multistep electrode processes in double potential step techniques at spherical electrodes of any size, including plane electrodes and ultra-microelectrodes, are derived. These solutions are valid when the diffusion coefficients of all species are equal, and they are applicable for any number of the species initially present in the solution, for any values of the formal potentials of the different steps and for any duration of the two potential steps. In differential pulse voltammetry (DPV) and in additive differential pulse voltammetry (ADPV) the effects of the applied potential and of the electrode sphericity on the current are found to be separable and, therefore, the position of the peaks and the cross potentials are not dependent on the size of the electrode. The convenience of using ADPV in determining the formal potentials when some electrochemical steps are not completely separated is shown. It is also demonstrated that the reproportionation/disproportionation homogeneous reactions have no effect either on the surface concentrations nor on the currents in any multipotential step technique under the conditions considered here.

Pulse polarography: Part IX. A method of discrimination between the catalytic, CE, ECE and EC mechanisms. Calculation of the rate constants of the chemical reaction for the catalytic, CE and ECE mechanisms

A method to discriminate between the catalytic, CE, ECE and EC mechanisms is proposed. This method is based on the kinetic effects that are observed on different time scales in normal pulse polarography (NPP) and in dc polarography (DCP). Thus, if we consider the experimental observable, consisting of the ratio of the instantaneous limiting current in NPP over the instantaneous limiting current in DCP, and we plot this observable vs. log χDCP, the curves obtained are different for each of the mechanisms given above. In these plots χDCP=(k1+k2) τ, where k1 and k2 are the rate constant values of the chemical reaction and τ is the drop time. These plots also allow the values of the rate constants to be found for the catalytic, CE and ECE mechanisms when the diffusion coefficient or the bulk concentration of the species involved in the electrode process are not known.

Pulse polarography: Part VI. Kinetic currents with a CE mechanism

Abstract An equation for the instantaneous limiting current in pulse polarography with a CE mechanism has been derived. This equation is valid without any limitation in the magnitude of the rate constants of the chemical reaction, i.e. the steady-state approximation was not used. In addition, both the expanding plane and the expanding sphere electrodes have been taken into account. Finally, it is shown that in pulse polarography the interval of the rate constant values that may be determined is larger—about 10 3 —than in dc polarography.

Studies of ion transfer across liquid membranes by electrochemical techniques

The fundamentals and recent advances in ion transfer across the interface between two immiscible electrolyte solutions (ITIES) are reviewed. The different strategies developed to overcome the limitations of the traditional experimental studies with ITIES and to broaden its scope of applications are discussed. Special attention is given to studies of ion transfer through liquid membranes which contain two ITIES, one or both of which can be polarized. Theoretical and experimental studies on the application of different galvanostatic and potentiostatic electrochemical techniques to the study of such systems are described, emphasizing their unique characteristics. The article also includes sections devoted to facilitated ion transfer, liquid/liquid micro-interfaces and the use of weakly supported media.

Physical insights of salt transfer through solvent polymeric membranes by means of electrochemical methods.

A combined voltammetric study of the joint transfer of the two constituting ions of a water-soluble salt has been carried out using normal-pulse voltammetry, linear-sweep voltammetry and square-wave voltammetry in a system with two liquid-liquid polarized interfaces. As a result, we have explained the voltammetric features that allow us to distinguish this uptake from that corresponding to two equally charged ions, in spite of the appearance in both situations of two current peaks with the same sign in both square-wave and linear-sweep voltammograms, and we have found that linear-sweep voltammetry and square-wave voltammetry complement each other excellently.A theoretical comparison with a system of a single polarized interface has also been made, showing that these systems are much less appropriate for characterizing these salt-ion transfers.