Kohji Maeda

A voltammetric study of Keggin-type heteropolymolybdate anions

Abstract The voltammetric behavior of several Keggin-type heteropolymolybdate anions, [XMo12O40]n− (XS, P, As, V, Si, Ge; n = 2–4) at a glassy carbon (GC) electrode was investigated in acetone or acetonitrile. The one-electron reduction waves for the Keggin anions were converted into two-electron waves as a function of acid concentration. The effect of protons on the successive reduction steps was verified experimentally, and the reduction mechanism of the conversion process was elucidated.

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.

Charge dependence of one-electron redox potentials of Keggin-type heteropolyoxometalate anions

Abstract The two-step one-electron reduction processes of Keggin-type heteropolymolybdate and heteropolytungstate anions were investigated in several solvents using cyclic voltammetry. The slope of a plot of standard redox potentials E° against the ionic charge ΔE° was evaluated. It was found by theoretical considerations that ΔE° consists of a constant term independent of the solvent and another term comprising the charge-dependent component of the solvation energy. The difference between ΔE° for water and for 1,2-dichloroethane (1,2-DCE) was in good agreement with the value estimated from the transfer energies across the 1,2-DCE|water interface which were measured by ion transfer voltammetry. The values of ΔE° in various solvents cannot be explained by the Born-type electrostatic solvation energy, suggesting that the short-range interaction is important for the interaction of a Keggin anion with solvent molecules.

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.

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.

Voltammetric study of the partition of amines between water and an organic solvent

Ion-transfer voltammetry of organic amines at an organic solvent/water (O/W) interface is studied. A theoretical equation of the current vs. potential curve of amines at a dropping electrolyte electrode is derived. The partition processes of both protonated and neutral forms of amine at O/W interface are taken into account. Kinetics of the protonation reaction between the neutral and protonated forms in W phase is also taken into account using the reaction layer theory. The theoretical prediction is in good agreement with the experimental results obtained with procaine at nitrobenzene/water interface using a dropping electrolyte electrode. Ion-transfer voltammetry and polarography is useful for studying the partition of amines between O and W phases in both neutral and protonated forms and the protonation reaction of amines in W phase as well.

Membrane oscillations and ion transport

Oscillations observed at such membranes in the absence of any channel like substances as liquid membranes, lipid-doped Millipore filters, artificial bilayer lipid membranes (BLM), etc. are reviewed with the intention of extracting the fundamental information significant for the understanding of oscillations at biomembranes in living organisms. Since the ion transfer at the aqueous/membrane interface and the interfacial adsorption are deeply concerned with the oscillations (especially those at liquid membranes), an outline is given on the voltammetry for the ion transfer at an interface of two immiscible electrolyte solutions, (VITES), which is very powerful in the elucidation of the ion transfer processes and the adsorption phenomena. The ion transfer through a liquid membrane or a BLM is also discussed based on the results obtained by the VITIES. The usefulness of the VITIES and the voltammetric concept in the elucidation of mechanisms of oscillations is demonstrated taking, for an example, the analysis of novel oscillations of membrane potential and membrane current found recently by the authors with liquid membranes under applied currents or potentials. One of the novel oscillations which was observed with the transfer of Na+ showed characteristics similar to those of the oscillation at a biomembrane with “sodium channel”.

Solvent extraction of lithium and sodium with 4-benzoyl or 4-perfluoroacyl-5-pyrazolone and TOPO

The synergic solvent extraction of lithium and sodium into benzene or cyclohexane with 4-benzoyl or 4-perfluoroacyl-5-pyrazolone and trioctylphosphine oxide (TOPO) has been investigated. Quantitative extraction (>99%) of lithium, which is one of the most poorly extractable metal ions, can be achieved with 1-tolyl-3-methyl-4-perfluoroacyl-5-pyrazolone and TOPO. The extraction of sodium is somewhat poorer than that of lithium under the same conditions. The perfluoroacyl group at the 4-position of the pyrazolone ring enhances the extraction and increases the maximum percentage extracted. Cyclohexane is found to be suitable for a quantitative extraction as an organic phase when the reagents are soluble in it. Improved separation of lithium and sodium can be attained when they are extracted into benzene.

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.