Sorin Kihara

Ion transfer through a liquid membrane or a bilayer lipid membrane in the presence of sufficient electrolytes

Abstract Voltammograms for the ion transfer from one aqueous solution (W1) to another (W2) through a liquid membrane (LM) (VITTM) were recorded under various conditions by scanning the potential applied between two aqueous phases and measuring the current between two aqueous phases. A method of analysing the VITTM was proposed based on the comparison of the VITTM with voltammograms at the W1|LM and LM|W2 interfaces observed simultaneously with the VITTM. The membrane transport was demonstrated to be controlled mainly by the complementary ion transfers at the W1|LM and LM|W2 interfaces, when W1, LM and W2 contain sufficient electrolytes. The influence of different ions in LM or W2 on the membrane transport of an objective ion and the change in ion transfer reactions at the W1|LM and LM|W2 interfaces during the electrolysis under an applied membrane potential were elucidated taking into account the relation among the VITTM and voltammograms at two interfaces. The ion transport through a bilayer lipid membrane (BLM) was found to be analogous to that through an LM when the BLM contains sufficient ions, although the BLM is very much thinner than the LM.

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 role of non-ionic polyoxyethylene ether surfactants on ion transfer across aqueous/organic solution interfaces studied by polarography with the electrolyte dropping electrode

Abstract The transfer of Li+, Na+, K+, NH+4, Mg2+, Sr2+, and Ba2+ from aqueous solution to 1,2-dichloroethane or nitrobenzene facilitated by polyoxyethylene ethers, Triton X, was studied by current-scan polarography using the aqueous electrolyte dropping electrode. The facilitated ion transfer is attributable to the formation of a hydrophobic complex of metal ion with Triton X adsorbed at the aqueous/organic solution interface, followed by the transfer of the complex from the interface into the organic solution. The stability and the composition of the metal-Triton X complexes were investigated by changing the length of the oxyethylene chain of the Triton X, taking into account the ionue5f8dipolar bond formation between metal ions and the oxyethylene chain. The adsorption-desorption behavior of the Triton X and metal-Triton X complex was elucidated by referring to drop time curves.

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 interpretation of the potential at an ion-selective electrode, based on current-scan polarograms observed at the aqueous/organic solution interface

The potential-generating process at ion-selective electrodes (ISE) of liquid-membrane types has been interpreted by comparing the ISE potential with the current-scan polarogram which indicates the transfer of a particular ion i(z+) at the aqueous/organic solution (w/o) interface. The potential at zero current, DeltaV(l = 0), in the composite polarogram observed with i(z+) in both w and o, corresponds to the ISE potential. A stable potential giving Nernstian response to the concentration of i(z+) in w is obtained at the ISE only when the w/o interface is depolarized byi(z+). The detection limits are controlled by the final rise and final descent of the residual current in the polarogram. The interference of a second ion,j(z+), in the ISE measurement of i(z+), and the role of an ionophore in the membrane of the ISE can be explained by considering the shift of DeltaV(l = 0) in the composite polarogram ofi(z+) in the presence of j(z+) in w or the ionophore in o. Equations which express the ISE potential, the interference at the ISE, and the effect of an ionophore on the ISE potential have been derived, connected with the polarographic equations for ion-transfer at the w/o interface.

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.

Simultaneous determination of tungsten and molybdenum in sea water by catalytic current polarography after preconcentration on a resin column

Abstract The simultaneous determination of tungsten and molybdenum in sea water is based on preconcentration by column extraction with 7-(1-vinyl-3,3,5,5-tetramethylhexyl)-8-quinolinol (Kelex- 100) resin, and measurement of the polarographic catalytic currents obtained in a solution of chlorate, benzilic acid and 2-methyl-8-quinolinol. When the concentration factor is 50, the detection limits are 2.4 pM for tungsten and 17 pM for molybdenum (for a signal-to-noise ratio of 3). The precision of the determination is ca. 10% for 67 pM tungsten and ca. 5% for 106 nM molybdenum in sea water ( n =4). Results for sea water and other natural waters are presented.

Adduct formation properties of mono- and bidentate phosphine oxide compounds in the liquid—liquid extraction of some divalent metals with 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone

Abstract The bidentate phosphine oxide compounds bis(diphenylphosphinyl)methane (BDPPM) and bis(diphenylphosphinyl)ethane (BDPPE) were synthesized and the liquid—liquid extraction equilibria of some divalent metal ions such as Co(II), Ni(II), Zn(II) and Cd(II) with 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (HPMBP) and the neutral phosphine oxide compounds were investigated. BDPPM was found to be much more powerful than BDPPE and the monodentate neutral ligand trioctylphosphine oxide (TOPO). The composition of the extracted species were determined by a graphical method to be M(PMBP)2(TOPO)s (s = 2 for Co, Ni, Cd and s = 1 for Zn), M(PMBP)2(BDPPM) and M(PMBP)2(BDPPE). The adduct formation constants between the acylpyrazolone chelates and the neutral ligands were determined and are discussed in terms of the molecular structure.

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.