Rapid and precise coulometric determination and separation of redox inert ions based on electrolysis for ion transfer at the aqueous|organic solution interface.

Flow systems for precise and accurate coulometric determinations of ions that were developed on the basis of electrolytic ion transfer at the aqueous|organic solution (W|O) interface are reviewed. The electrolysis cell in the system is composed of a porous poly(tetrafluoroethylene) tube (1.0 mm inner diameter), a metal wire (0.8 mm diameter) inserted into the tube, O into which the tube is immersed, a reference electrode in O and a platinum wire counter electrode in O. The electrolysis is carried out by forcing W containing a species of interest to flow through the narrow gap between the tube and the metal wire. The coulometric determination can be performed with an efficiency of more than 99% and a precision of better than 0.2% based on the ion transfer under an optimum condition, even if the ion is redox inert such as Na(+), K(+), Mg(2+), Ca(2+), ClO(4)(-), picrate or alkylsulfonates. The system can be applied to selective electrolytic solvent extraction of ions.

Development of a simple column electrode for sensitive and rapid coulometry.

A simple column electrode, S-CE, with a glassy carbon fibers working electrode stuffed into a Teflon tube was developed. The current-potential curves for the reductions of [Fe(CN)(6)](3-) and Fe(3+) observed at the S-CE were analyzed based on a theory for those at an ordinary column electrode. A quantitative electrolysis was performed at the S-CE rapidly within 20 s. An accurate and precise coulometric determination could be attained at the S-CE even with a fairly dilute solution. For example, coulometric reductions of 5 x 10(-5) and 5 x 10(-6) M Fe(3+) were attained with efficiencies (n = 5) of 99.7 +/- 0.2 and 101.9 +/- 1.1%, respectively.

Voltammetric study on ion transport across a bilayer lipid membrane in the presence of a hydrophobic ion or an ionophore.

This review describes voltammetric studies on ion transport from one aqueous phase (W1) to another (W2) across a bilayer lipid membrane (BLM) containing a hydrophobic ion, valinomycin (Val) or gramicidin A (GA). In particular, the ion transport mechanisms are discussed in terms of the distribution of a pair of ions between aqueous and BLM phases. By addition of a small amount of hydrophobic ion into W1 and/or W2 containing a hydrophilic salt as a supporting electrolyte, the hydrophobic ion was distributed into the BLM with the counter ion to maintain electroneutrality within the BLM phase. It was found that the counter ion was transferred between W1 and W2 across the BLM by applying a membrane potential. Facilitated transport of alkali ions across a BLM containing Val as an ion carrier compound, could be interpreted by considering not only the formation of the alkali metal ion-Val complex but also the distribution of both the objective cation and the counter ion. In the case of addition of GA as a channel-forming compound into the BLM, the facilitated transport of alkali ions across the BLM depended on the ionic species of the counter ions. It was discovered that the influence of the counter ion on the facilitated transport of alkali ions across the BLM could be explained in terms of the hydrophobicity and the ionic radius of the counter ion.

Development of rotating liquid membrane disk electrode and rotating liquid membrane ring-liquid membrane disk electrode and evaluation of characteristics of ion transfer reactions at the rotating aqueous/organic solution interface.

A liquid membrane disk electrode, LMDE, and a liquid membrane ring-liquid membrane disk electrode, LMRE-LMDE, were developed by placing a gelled polyvinyl chloride thin membrane impregnated with 2-nitrophenyl octyl ether, NPOE-LM, on the surface of a glassy carbon, GC, disk or ring electrode. The voltammogram for the ion transfer at the interface between an aqueous solution, W, and NPOE-LM was recorded by setting the developed electrode in W and rotating at a rate, omega, between 0 and 4000 rpm. The sensitivity of the ion-transfer current at the WINPOE-LM interface, I, was enhanced to be more than 100 times better than that at the WINPOE (solution) interface when LMDE was rotated at omega higher than 200 rpm. The reversibility of the ion transfer reaction could be evaluated based on the dependence of I on omega of LMDE, and the reaction product at LMDE could be identified at LMRE when the rotating LMRE-LMDE system was adopted.