M. Březina

Electrolysis with electrolyte dropping electrode: II. Basic properties of the system*

Abstract In a theoretical discussion the conditions have been pointed out where an interface of two immiscible electrolyte solution behaves as an equilibrium system metal ion-metallic electrode, as an ideally polarized electrode and as an electrode under faradaic current flow. The basic equations for current-electrical potential difference across the interface have been deduced for the cases of ion as well as electron transfer. Experimentally, various base electrolyte systems were studied, the most advantageous among these are LiCl in water+tetrabutylammonium tetraphenylborate in nitrobenzene and MgCl 2 in water+tetrabutylammonium dicarbollyl cobaltate in nitrobenzene. S-shaped polarographic curves were observed with the tetramethylammonium ion. The limiting current is directly proportional to concentration. The limiting currents are somewhat higher than those predicted by the Ilkovic equation which has been ascribed to the tangential movement of the interface.

Adsorption and kinetics of oxidation of ascorbic acid at platinum electrodes

Summary As shown by cyclic voltammetry and by polarography with a rotating disc electrode, ascorbic acid is oxidized at platinum electrodes in a two-electron process. There are two reaction paths of the oxidation, one corresponding to an irreversible electrode reaction of the substance without appreciable adsorption with α n a ≈0.4, the other being the oxidation of the adsorbed substance in the potential range of Pt surface oxide formation. The adsorption of ascorbic acid is reversible and non-destructive, attaining its maximum value (68% of sites of hydrogen adsorption) even at low concentrations of ascorbic acid. The process is controlled by kinetics of adsorption proper together with diffusion. One molecule of ascorbic acid occupies two hydrogen adsorption sites.

Electrochemical reduction of monovalent cation complexes of macrocyclic ionosphores. I. crown polyether complexes

Abstract The complex formation of alkali metals Na, K, Rb, Cs and of Tl(I) with cyclic polyethers (“crowns”) and the stability of the complexes were studied in methanol and acetonitrile by the polarographic method. The influence of a number of members in the cycle and of substituents in the polyether ring on the stability of complexes was discussed. The character of the solvent was found to be an important factor in view of its ability to solvate the cation. In methanol where the alkali cations are solvated nearly as in water, the stability constants of sodium complexes are lower than those found in the present paper in acetonitrile. They are sometimes comparable or higher than the stability constants of cyclic polyethers with the potassium ion.

Electrochemical reduction of monovalent cation complexes of macrocyclic ionophores. II. Valinomycin and macrotetrolide complexes

Abstract The stability and selectivity of complex formation of natural macrocyclic ionophores with alkali metal ions and monovalent thallium ion was studied by polarography. With valinomycin both stability constants and homogeneous dissociation rate constants were determined from polarographic kinetic currents. The macrotetrolides gave diffusion controlled currents. The stability of their complexes increases with the degree of substitution from nonactin to trinactin. The properties of natural ionophores were compared wiht those of crown polyethers. The selectivity of complex formation of valinomycin almost coincides with its effect on the increase of the conductivity of bilayer lipid membranes.

Electroreduction of oxygen and hydrogen peroxide catalyzed by hemine and phthalocyanines

Abstract By means of polarographic average and instantaneous current and drop time measurements it was shown that the catalysis of hydrogen peroxide reduction by hemine and of oxygen reduction by phthalocyanine takes place via adsorbed catalyst. In the case of oxygen a two-electron process in the absence of the catalyst changes to a four-electron process in its presence.

The dissociation-rate dependent electrochemical reduction of valinomycin complexes of monovalent ions

Abstract The polarographic reduction of valinomycin complexes of alkali metal and thallium (I) ions takes place via dissociation of the complex while the free metal ions are reduced. The stability constants determined from the half-wave potentials and the dissociation-rate constants determined from the polarographic (limiting currents show a distinct ion-specificity. The values for the thallium (I) complex lie outside the sequence based on crystallographic ionic radii.

A new model of membrane transport: Electrolysis at the interface of two immiscible electrolyte solutions*

Summary A simple membrane model is the interface between water and an organic liquid immiscible with water, with a strongly hydrophilic electrolyte dissolved in the aqueous phase and a strongly hydrophobic electrolyte in the organic phase. This interface can be electrochemically polarized in the same way as the interface electrode electrolyte solution using various modes of voltammetry or the galvanostatic method. A fourelectrode potentiostatic system is required for such studies. An electrolyte dropping electrode, analogous to Heyrovskýs DME, was also constructed. The voltammograms fully resemble those obtained with metallic electrodes. The faradaic processes studied so far are mainly connected with the transfer of hydrophobic ions across the interface. These processes are quite rapid and the half-wave potential of a particular ion is related to its standard Gibbs transfer energy. Observed electron-transfer effects model redox processes at membranes. Macrocyclic ionophores facilitate transfer of alkali metal ions across this interface. Very fast ion transfer as well as complex formation was observed in the systems under investigation so that, generally, the diffusion of the ionophore toward the interface and of the complex into the organic phase is the rate-controlling step, no surface reaction retarding the overall process. Apart from the investigation of membrane processes, this approach can be used for elucidation of processes in ion-selective electrodes and in phase-transfer catalysis.

Decomposition of hydrogen peroxide at a rotating silver disc in alkaline medium

Summary The decomposition of hydrogen peroxide at the interface silver-alkali metal hydroxide solution, which, as shown previously, is of an electrochemical nature, was studied by a rotating silver disc after various pretreatments of the surface. In all cases the reaction order was unity. In some cases a partial controlling step in the decomposition process is the transport of H 2 O 2 to the interface.