Potential window, ITIES

The range of polarized potential (the potential window) is limited by the transfer of ions constituting supporting electrolytes. Let us consider an ITIES formed by the contact of an organic phase containing a supporting electrolyte KA and of an aqueous phase containing an electrolyte K A, ... 

In ac impedance measurement at ITIES, admittance due to the transfer of supporting electrolyte ions is significant even in the middle of the potential window, as was first suggested and treated quantitatively by Samec et al. [35]. This imposes a difficulty in accessing double layer capacitance from the admittance, particularly when the transfer of supporting electrolyte ions is not reversible. There is no straightforward way to deconvolute the admittance ascribable to double layer capacitance and that ascribable to ion transfer admittance [30]. A nonlinear least-squares... 

Slow ET between redox species confined to two immiscible solvents was first observed by Guainazzi et al. (18). Several different theoretical and experimental studies of ET between redox species at the ITIES have been reported in the last several years (6-8,15,19-21). Severe experimental problems complicate extraction of the kinetic parameters from conventional electrochemical measurements at the ITIES (e.g., by cyclic voltammetry). Besides the difficulty of discrimination between ET and IT, there are also distortions from the double-layer charging current and /R-drop in the highly resistive nonaqueous solvents, and the limited potential window for studying ET in the absence of currents controlled by IT (1,16). 

A potential window, where negligible ion re-partitioning occurs, then exists, with limits defined by the lowest absolute values of the standard Galvani potentials. Experimentally, this window is on the order of 0.5-1.0 V, for an ITIES composed of aqueous electrolyte phases and solvents such as 1,2-dichloroethane (DCE) or nitrobenzene (NB). In this type of system, the interfacial potential difference can be imposed externally using potentiostatic control. 

The SECM is capable of quantitative determination of ET rates at the ITIES in a straightforward manner as long as appropriate account of possible coupled ion transfer processes is taken [82]. The study of ET kinetics at the liquid-liquid interface is an area of topical interest [134-138]. Conventional electrochemical measurements employ a four-electrode cell to drive the ET reaction, however, in the SECM studies, the electrodes are all in one phase and the problems associated with double layer capacitance, iR drop, and restricted potential window are therefore avoided. In particular, the study of ET rates at high driving force is possible. 

In 1991, Kihara et al. pioneered the concept of voltammetry for organic liquid membranes with two ITIES in series, that is, for a system aqueous electrolyte (AEl)-organic electrolyte-aqueous electrolyte (AE2) [292,293], Using a 6-elec-trode cell, they have clearly shown that, as we polarize a liquid membrane, AE2-AEl, the two polarized ITIES in series, are coupled by the equation of continuity of current, and the overall potential window is the sum of the two individual potential windows in series. In 1998, Beriet and Girault used this approach to selectively transfer metal ions for metal recovery [294],... 

The dual-pipette technique allows quantitative separation of different IT and ET processes simultaneously occurring at the liquid/liquid interface (e.g., simple transfer of potassium, facilitated transfer of the same ion with a crown ether, and IT of supporting electrolyte). It can also be used to overcome potential window limitations and study numerous important reactions occurring at high positive or negative voltages applied across an ITIES (e.g., transfers of alkali metals from water to organic media). 

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