Mitsuko Suzuki

The electron transfer at a liquid/liquid interface studied by current-scan polarography at the electrolyte dropping electrode

Abstract Some novel systems are introduced as suitable for observing the electron transfer voltammograms and/or polarograms at the aqueous (W)/nitrobenzene (NB) interface. Among them, the polarographic processes with systems composed of hexacyanoferrate(II), hydroquinone or hydroxyl ion in W/7,7,8,8-tetracyanoquinodimethane (TCNQ) in NB, hexacyanoferrate(III) or Ce4+ in W/ferrocene (FC) or tetrathiafulvalene (TTF) in NB and permanganate in W/tetraphenylborate ion in NB were found to be reversible under appropriate conditions. The relation between the limiting currents in polarograms and the concentration of reactants either in W or NB is presented. On the basis of analysis of reversible polarograms at the W/NB interface and a comparison with voltammograms at a platinum electrode in W or in NB, the half-wave potentials are connected to the oxidation-reduction potentials of the reactants in each individual solution, W or NB.

The transfer of anions at the aqueus/organic solutions interface studied by current-scan polarography with the electrolyte dropping electrode

The transfer of such anions as halides, their oxo-acid anions and other polyatomic anions was investigated at the aqueous/1,2-dichloroethane, nitrobenzene, or chloroform interface. The polarograms were reversible for most of the anions and therefore it was concluded that the transfer processes of these anions are controlled by the diffusion of anions in the aqueous and/or organic solutions. The half-wave potentials, ΔV12, of the polarograms, which can be related directly to the transfer energies, are discussed on the basis of the ionic radii, crystallographic forms, and charges of the anions. As for monovalent anions, linear relationships were obtained between ΔV12 and the inverse of the thermochemical radii, irrespective of the kind of organic solvent. The analytical aspects of ion-transfer polarography in the determination of anions are also discussed.

The transfer of carboxylate and sulphonate anions at the aqueous/organic solution interface studied by polarography with the electrolyte solution dropping electrode

The transfer of carboxylate, RCOO−, and sulphonate, RSO3−, anions was investigated at the aqueous/1,2-dichloroethane or nitrobenzene interface, based on the half-wave potentials, ΔV12s, of the polarograms which can be related to the transfer energies, δgtrs. It has been demonstrated that ΔV12 is determined by the electrostatic solvation energy, ΔGsolv(el), of the charged moiety in the anion, COO− or SO−3, as well as the non-electrostatic one, ΔGsolv (non-el), of the uncharged moiety, R, in both solutions when the objective anion is aliphatic carboxylate or sulphonate anion. The ϵGsolv(el) depends on the charge density of COO− or SO−3. The non-electrostatic contribution is attributable mainly to cavity formation for the anion in the aqueous solution and hence depends on the volume of the anion. For aromatic carboxylate or sulphonate anions, whose charges are partly distributed to R due to resonance, Δ12s were less negative, i.e. ΔGtrs were smaller, than those expected from Δv12s of aliphatic anions. In this connection, the substituent effects on ΔV12s of these aromatic anions were investigated. Results obtained using chloroform as the organic solution, in which ion-pair formation is serious, are also presented as reference data. Finally, the analytical aspects of the ion transfer polarography in the determination of carboxylate and sulphonate anions are discussed.

Voltammetric study on the oscillation of the potential difference at a liquid/liquid or liquid/membrane interface accompanied by ion transfer

Abstract The oscillations of potential difference at the interface between an aqueous solution containing MgSO4 and an organic solution containing dilute Cs+ or tetramethylammonium ion (TMA+) and fairly concentrated tetrapentylammonium tetraphenylborate (TPenA+· TPhB−) were investigated by forcing the transfer of Cs+ or TMA+ at the interface through an applied current. The organic solvents employed were nitrobenzene, 1,2-dichloroethane and o-nitroanisole. The necessary conditions for the oscillation were confirmed to be identical to those for the appearance of the maximum in the voltammogram for the ion transfer at the interface. By referring to the voltammogram and the drop tune-potential curve and taking into account the ion-pair formation equilibrium in the organic, such characteristics of the oscillation as the amplitude, the pulse width, the period and the range of applied current were elucidated, and a mechanism for the oscillation is proposed. The result at the aqueous/organic interface is connected to the oscillation of the potential at a liquid membrane.

Reduction of flavin mononucleotide at the aqueous/organic solution interface in the absence and presence of oxygen☆

Abstract The electron transfer at the interface between an aqueous solution (W) containing flavin mononucleotide (FMN) and an organic solution (Org) containing decamethylferrocene in the absence and presence of oxygen was studied by polarography. The reduction of FMN in the absence of oxygen at the W/Org interface differed from that observed by voltammetry at metal or carbon electrodes. The reduction of FMN proceeds as a one-electron process at the W/Org interface, whereas a two-electron process is observed at the metal or carbon electrodes. In the presence of oxygen, the reduction of oxygen by chemical reaction with the reduced FMN was observed at the W/Org interface. Mechanisms of reduction processes of FMN and oxygen at the W/Org interface are discussed.

Voltammetric interpretation of ion transfer coupled with electron transfer at a liquid/liquid interface

Abstract Ion transfer coupled with an electron transfer at an aqueous (W)/nitrobenzene (NB) interface was investigated. The ion transfer reactions studied were those of K + and Na + facilitated by valinomycin or dibenzo-18-crown-6 from W to NB and those of such anions as ClO 4 − , SCN − and I ∓ from W to NB. The electron transfer reaction was Fe(CN) 3− 6 /Fe(CN) 4− 6 in W and 7,7,8,8-tetracyanoquinodimethane (TCNQ)/TCNQ − in NB. Referring to the polarograms for the ion and electron transfer recorded at the W/NB interface, the interaction between these processes was elucidated quantitatively. The interaction depends not only on standard transfer energies for both transfers, but also on the ratio of redox agents in W or NB. The separation of ions based on the interaction is also discussed.

Oxidation of ascorbate and ascorbic acid at the aqueous|organic solution interface

An electron transfer reaction between ascorbate in an aqueous solution and oxidizing agents in an organic solution immiscible with water has been studied for the first time by polarography for charge transfer at the interface between two immiscible electrolyte solutions. A reversible electron transfer polarogram at the aqueous|organic solution interface could be observed when teterachlorobenzoquinone, dibromobenzoquinone and Meldolas Blue were used as oxidizing agents in the organic solution. The oxidation reaction of ascorbate at the aqueous|organic interface was discussed comparing with the reactions at the ordinary electrodes and in homogeneous solutions. The half-wave potentials of electron transfer polarograms at the aqueous|nitrobenzene interface were applied to evaluate the formal redox potential of ascorbate/ascorbate free radical.

Ion transfer from water to mixtures of formamide and water studied by polarography at the electrolyte | solution dropping electrode

Abstract Voltammetry for ion transfer at the interface of two immiscible electrolyte solutions was demonstrated to be feasible at the interface between various mixtures of formamide (FA) and water (W) and 2,6-dimethylnitrobenze. Standard Gibbs transfer energies ΔG0tr of ions from W to FA + W were evaluated from the differences in the half-wave potentials of the voltammograms. Values of ΔG0tr for ions of moderately small volumes, such as (CH3)4N+, BF−4 and ClO−4, were almost independent of the FA content, which was explained by the electrostatic and non-electrostatic interactions between the ion and solvents. Values of ΔG0tr for alkali metal ions decreased and those for halide ions increased with the increasing FA content, and the changes were remarkable in the range below 30% FA, which was explained in terms of short-range interactions between the ions and FA or W. The decrease in ΔG0tr for an uncharged moiety such as CH2 on addition of FA to W was interpreted in terms of the decrease in the cavity formation energy.

Effect of temperature on ion transfer at the aqueous/organic solution interface studied by current-scan polarography with the electrolyte solution dropping electrode

Abstract The transfer of I−, BF4−4, ClO−4, sulphonate anions (RSO−3) and alkylammonium cations (R4N+ or RNH+3) was investigated at the aqueous/nitrobenzene or 1,2-dichloroethane interface in the temperature range from 4 to 80° C. The ion transfers were polarographically reversible for all ions at any temperature investigated. The standard Gibbs transfer energies from the aqueous to the organic solution, ΔGo,w ⇌ orgtr, were calculated from the half-wave potentials of ion transfer polarograms. The effect of temperature on ΔGotr for ions of small volume was explained by the electrostatic interaction between the ion and solvents taking into account the change in the dielectric constants of the solvents and the structure of water. The effect for such voluminous ions as RSO−3 and RNH+3 was elucidated by considering both the electrostatic and non-electrostatic solvation energies of ions. The non-electrostatic contribution to the change of ΔGotr with temperature is discussed from the viewpoint of the structural change of water.

Ion transfer from water to formamide or 1,2-ethanediol studied by polarography at the electrolyte solution dropping electrode

Abstract Voltammetry for ion transfer at the interface of two immiscible electrolyte solutions was demonstrated to be feasible not only at the aqueous (W) ⋎ organic solution (Org) interface but also at the formamide (FA) ⋎ Org and 1,2-ethanediol (ED) ⋎ Org interfaces. The transfer of alkali metal ions, I − , BF 4 − , ClO 4 − , IO 4 − , PF 6 − , picrate ions and alkylammonium ions has been investigated at the FA ⋎2,6-dimethylnitrobenzene (DNB) and ED ⋎ DNB interfaces. Standard Gibbs energies of ion transfer ⋎ gD G tr 0 ⋎ from W to FA or ED were evaluated from the differences in the half-wave potentials at the FA ⋎ DNB, ED ⋎ DNB and W ⋎ DNB interfaces. The values of Δ G tr 0 were used as the basis of a discussion of features of the salvation of ions in FA, ED and W, taking into account the electrostatic, non-electrostatic and short-range ion-solvent interactions.