Arvydas Survila

Cathodic RDE voltammograms obtained for the reduction of Ag(I) complexes with glycine, sulfite and cyanide were analysed using a modified Koutecky–Levich approach. To extend this method to processes involving ligands, the composition of the electrically active complex was taken into account. For this purpose, the initial experimental data were modified using special correction functions. Experimental data were analysed with the involvement of information on the complexation characteristics and the mechanism of charge transfer. The scope of the basic model and factors requiring further evaluation were discussed.

The applicability of the Koutecky–Levich (KL) approach for the processes occurring in the systems of labile metal complexes is considered. According to the analysis of simulated RDE voltammograms, the use of classical KL relationships is possible if the predominant complexes are involved in the charge transfer process. In the opposite case, corrections to the initial voltametric data, taking into account the surface distribution of complexes and ligands, are required. The rationale and methodology for correction functions are presented. Inaccuracies in the estimation of kinetic and diffusion characteristics arising from ignoring the peculiarities of mass transfer of chemically interacting particles are analysed.

Anodic LS voltammograms of the alkaline Cu|Cu(II), glycine system were converted into mass-transport corrected Tafel plots considering that the glycinate anion L⁻ takes part in the first stage of copper ionization. The anodic current density was normalized relative to the surface concentration of L⁻ species, which was determined using the mass transfer model of chemically interacting species. The lability of the system was assessed based on available kinetic characteristics and specific experimental data. To linearize the Tafel plots, corrections were made to account for the insufficient mobility of the proton attached to glycine amino group. Obtained Tafel constants indicate that the second step of copper ionization, involving the oxidation of the intermediate copper(I)-glycine complex, is the rate-determining step.

Anodic voltammograms using rotating disc electrode (RDE) and linear sweep (LS) techniques were obtained for alkaline (9 < pH < 11.2) Cu (II)-glycine solutions. The nature and properties of the RDE anodic pseudo-limiting currents i _lim and LS peak currents i _p were considered within the framework of a theoretical model, taking into account the peculiarities of mass transfer of chemically interacting species. Both i _lim and i _p increase with solution pH and vary linearly with the concentration of glycinate anion l ⁻ . The degree of irreversibility of the process significantly affects the i _p values, but has only small influence on i _lim . Both characteristic current densities could serve as a measure of deprotonated ligand species, which is capable of forming stable Cu (II) complexes. The applications of modified equations, which were derived for i _p in the case of simple redox systems, are discussed in detail.

The distribution of species in Cu(II)-glycine solutions, containing sulfate or perchlorate as a supporting electrolyte, is considered. Substantial differences are observed at pH < 4.5, where the molar fraction of monoligand complex (associate) CuSO4 reaches up to 20 percent of the total Cu(II) concentration. Cu(I)-containing species are generated at the Cu|solution interface. These are Cu+ aqua-complexes (acid media) or mono- and bidentate Cu(I)-glycine complexes (alkaline media). The addition of sulfate reduces the content of Cu+ ions, but practically does not affect the distribution of Cu(I) complexes. The study of acidbase equilibria involving Cu+ ions shows that the thermodynamic probability of Cu2O formation remains low in both perchlorate and sulfate media. HSO4– ions formed in acid media should be treated as rather labile proton donors. This is quantitatively confirmed by experimental data, according to which the rate of hydrogen evolution increases correspondingly when perchlorate is replaced by sulfate in glycine solutions.

Linear sweep voltammetry data, obtained for hydrogen evolution on a copper electrode were used to estimate the lability of weak monoprotic acids as proton donors. Analysis of differential equations involving diffusion and kinetic terms shows that the total mass transport of proton donors and acceptors does not depend on the kinetics of chemical steps. Their surface concentrations as functions of the electrode potential can be easily obtained by convolution of experimental voltammograms and used in further analysis. The results show that acetic and glycolic acids are sufficiently labile, whereas the dissociation of the carboxylic group in gluconic acid is kinetically inhibited. The zwitterions formed in glycine solutions cannot be regarded as labile proton donors due to the inertness of the protonated amino group. Transforms necessary for constructing linear Tafel plots are discussed.

The kinetics of hydrogen evolution on a copper electrode in acetic acid solutions is studied by the method of linear potential sweep. At рН < 2.5, double current peaks are observed with the height...

Electrochemical lability of different glycine species is considered on the basis of voltammetric data obtained for hydrogen evolution on the copper electrode in weakly acidic glycine solutions at different potential sweep rates (v). Voltammograms contain well-defined current peaks the height of which depends on both solution pH and glycine concentration. Current peaks and their potentials vary linearly with √−v and log v, respectively, this being typical of the electrochemical processes controlled by diffusion and charge transfer.To characterise system lability, peak coordinates and convolutive transforms of voltammograms are analysed. The results show that protons attached to different groups in a glycine molecule exhibit different mobility. In contrast to the protonated amino group, the release of a proton from the carboxylic group proceeds significantly faster. Consequently, protonated species +H3N–CH2–COOH can be treated as labile proton donors, whereas zwitterions +H3N–CH2–COO– do not fall into this category.

Electrochemical processes, involving mass transport of chemically interacting metal complexes and ligands, are considered using the quantitative model based on the second Fick’s law equations supplemented with kinetic terms. Concen­tration profiles obtained at different diffusion and kinetic parameters are used for the characterization of system lability. When chemical steps are fast enough and the interrelation between concentrations of species is specified by stability constants of metal complexes, all complexes in the system might be treated as electrically active species yielding the respective partial currents. In the case of the systems with limited lability, certain deviations from the equilibrium distribution of species is observed in the diffusion layer. The reaction layer concept is analysed and consideration is given to the feasibility of an idealized mass transport model in the electroanalysis.