Electrode mechanism and voltammetric determination of selected guanidino compounds

The present review describes the recent results on the electrochemical activity of bio-guanidino compounds, such as famotidine, metformin, acyclovir, ganciclovir, zanamivir, moroxydine as well as guanidino compounds, such as S-[(2-guanidino-thiazol-4-yl)methyl]isothiourea hydrochloride, 2-guanidino-1,3-thiazole, 2-guanidinobenzimidazole. The focus is on analyzing the electrode mechanism of the guanidino compounds at the hanging mercury drop electrode and at the silver amalgam film electrode, as well as on the character of the square wave (SW) voltammetric signals. It has been stated, that the compounds can act as electrocatalysts — they are protonated and adsorbed at the surface of the electrode, after which the protonated forms of the compounds are irreversibly reduced, yielding their initial form and hydrogen. The experimental adsorption data obtained by measuring the differential capacity of the double layer, the zero charge potential, and the surface tension at the zero charge potential have established the adsorption processes underlying their electrochemical activity. The analytical application of the obtained voltammetric signals in the determination of these compounds in biological samples is also presented. This review concentrates on our own results in the context of general developments in the field.

Atypical electric behavior of the double layer. Experimental case studies: Rh(111) in aqueous HCl solutions, and Au(111) in an ionic liquid, BMIPF6

Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and zero-charge-potential measurements have been performed for the title systems. The results of the Rh(111)/aqueous HCl solutions demonstrate that in solutions of binary electrolytes with adsorbing anions the interfacial impedance is a single, indivisible element, even if its frequency dependence implies a four-element circuit. The measurements on Au(111) in 1-butyl-3-methylimidazolium hexafluorophosphate (BMIPF6) reveal that double-layer formation and rearrangement in ionic liquids are very slow processes manifesting themselves as a parallel combination of a capacitance and a constant phase element (CPE).

Adsorption of Butyl Acetate on Mercury Electrode and Its Effect on Electroreduction of Zn Cations

Adsorption of butyl acetate on the mercury electrode in 1 M, 0.5 M or 0.1 M NaClO4 is described. Differential capacity curves obtained in these solutions for various butyl acetate concentrations point to the strongest decrease in capacity in 0.1 M NaClO4 for the smallest concentration of the ester tested. At the same time the heights of desorption peaks in the solutions tested decrease in the following order: 1 M > 0.5 M > 0.1 M NaClO4. The obtained results show dynamic competitive adsorption in the ClO4--H2O-ester system. In all the systems studied the zero charge potential values determined with a streaming electrode are shifted towards positive potential with increasing ester concentration. These results suggest that the polar molecule of the ester adsorbs on the mercury electrode with its hydrophobic end while hydrophilic ester group is directed towards the solution. The values of the relative surface excess obtained in the range of potentials where the strong adsorption occurs, virtually do not depend on the base electrolyte concentration. The values of the Gibbs energy of adsorption ∆G0 determined from the Frumkin isotherm have also similar values at base concentrations of the electrolyte tested. The values of interaction constant A are radically different: the adsorbed molecules undergo repulsive force in 1 M NaClO4, whereas in 0.5 M and 0.1 M NaClO4 they are under weak attraction. In the range of more negative potentials, the adsorption layer was investigated following the kinetics of the reduction of Zn++ as a pilot ion. It was stated that the concentration of the base electrolyte fundamentally affects the process: the inhibition of butyl acetate decreases in the order 1 M > 0.5 M > 0.1 M NaClO4.