Application of Voltammetric Techniques at Microelectrodes to the Study of the Chemical Stability of Highly Reactive Species

Abstract

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 on a short time scale. With the above aim, simple explicit solutions are presented in this article 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.

Normal Pulse Voltammetry and Steady State Voltammetry of the Square Mechanism at Spherical Microelectrodes

“…that can be applied to other electrode geometries by replacing d d by the correspondinge xpression of the Nernst diffusion layer [13].F rom Equation 31, the half-wave potential is derived easily:…”

“…Note that Equation 32 offers as imple diagnosis criterion to discriminate between two situations where as ingle wave is obtained with the same limiting current and shape:t he reversible Em echanism and the squarem echanism in the fully-labile limit. As can be deduced from Equation 32, when the concentration of species L( c L *) can be controlled, the variation of c L within the proper range (see Figure 2i n [ 13]) affects the position of the voltammogram of the square mechanism, unlike in the case of reversible electron transfers without coupled homogeneous reactions. Let us consider now the partially-labile situation, which is the most interesting case given that the electrochemical response is sensitive to the chemical kinetics.Asingle wave is obtained when the absolute value of the difference between the formal potentials is smaller than 100 mV in Figure 1.…”

“…

[a] Dedicated to Professor R. G. Compton on the Occasion of his 60 th Birthday

1IntroductionThes quare mechanism given in Scheme 1d escribes many experimental systems where electron transfer processes are coupled to structural changes (isomerisation [1][2][3][4], conformation modifications [5,6]), complexation and ligand exchange [7],e ncapsulation [8],( de)protonation and substitution [9][10][11][12],i on pairing [13][14][15],e tc. This is also the case of ion transfer processes across liquid j liquid interfaces where complexation of the ion may take place in the organic and/or aqueous phases [16].M oreover, more simple situations,s uch as the CE, EC and ECE mechanisms,c an be studied as particular cases.T herefore, the theoretical and practical interest of the square scheme is well justified.

…”

Effects of Unequal Diffusion Coefficients and Coupled Chemical Equilibria on Square Wave Voltammetry at Disc and Hemispherical Microelectrodes

“…Thus, one and even two orders of magnitude differences in the diffusion coefficients have been reported between free species and species associated with large (bio) molecules or nanoparticles [1][2][3][4][5]. Examples of coupled chemical processes commonlyencountered in electrochemical studies include structural rearrangements [6], protonations [7,8], ion pairing [9,10] and complexations [11,12].…”

“…As a consequence, the theoretical description of the system is much more difficult. Thus, whereas simple, analytical expressions can be deduced for SWV whatever the electrode size and shape when the effective diffusion coefficients are equal [9,23,24], the use of numerical treatments is mostly necessary when the values of the diffusion coefficients differ significantly.…”