Polarography, ion-transfer

This chapter is focused on our recent research topics regarding the analysis of complexation reactions at L/L interfaces. We first describe the hydrogen-bond-mediated anion recognition as studied by ion transfer polarography and interfacial tensiometry [22,23], and then alkali metal ion recognition as studied by SHG spectroscopy [24,25]. 

A hanging electrolyte drop has also been applied to determine ionic species in solution using differential-pulse-stripping voltammetry procedures [69]. Particular emphasis was given to assessing the selectivity and sensitivity of the method. The technique of current-scan polarography has also been applied in the study of electron-transfer [70] and coupled electron-transfer-ion-transfer [71,72] reactions at the ITIES in this configuration. 

The facilitated transfers of Na+ and K+ into the NB phase were observed by the current-scan polarography at an electrolyte-dropping electrode [12]. In the case of ion transfers into the DCE phase, cyclic voltammetry was measured at an aqueous gel electrode [9]. Both measurements were carried out under two distinctive experimental conditions. One is a N15C5 diffusion-control system where the concentration of N15C5 in the organic phase is much smaller than that of a metal ion in the aqueous phase. The other is a metal ion diffusion-control system where, conversely, the concentration of metal ion is much smaller than that of N15C5. Typical polarograms measured in the both experimental systems are shown in Fig. 2. 

The electrodes used in conventional polarography and voltammetry are electronic conductors such as metals, carbons or semiconductors. In an electrode reaction, an electron transfer occurs at the electrode/solution interface. Recently, however, it has become possible to measure both ion transfer and electron transfer at the interface between two immiscible electrolyte solutions (ITIES) by means of polarography and voltammetry [16]. Typical examples of the immiscible liquid-liquid interface are water/nitrobenzene (NB) and water/l,2-dichloroethane (DCE). 

In 1982, Samec et al. studied the kinetics of assisted alkali and alkali-earth metal cation-transfer reactions by neutral carrier and conclnded that the kinetics of transfer of the monovalent ions were too fast to be measured [186]. In 1986, Kakutani et al. published a study of the kinetics of sodium transfer facilitated by di-benzo-18-crown-6 using ac-polarography [187]. They concluded that the transfer mechanism was a TIC process and that the rate constant was also high. Since then, kinetic studies of assisted-ion-transfer reactions have been mainly carried out at micro-lTlES. In 1995, Beattie et al. showed by impedance measurements that facilitated ion-transfer (FIT) reactions are somehow faster than the nonassisted ones [188,189]. In 1997, Shao and Mirkin used nanopipette voltammetry to measure the rate constant of the transfer of K+ assisted by the presence of di-benzo-18-crown-6, and standard rate constant values of the order of 1 cm-S were obtained [190]. A more systematic study was then published that showed the following sequence,, which is not in accordance with... 

The concept of ac measurements for the kinetic study of ion transfer had been applied previously by Senda et al, who studied the transfer of tetramethylammonium, picrate and the tetraalkylammonium series by ac polarography. The study of picrate transfer was interesting because it showed that the measured rate constants were independent of the supporting electrolyte concentration and because it casted some serious reservations the validity of the use of the Frumkin correction as practiced by Samec et al vide supra). Their results indicated that the observed potential dependence of the rate constants, obtained in moderately concentrated electrolyte solutions, may reflect the real potential dependence of the rate constants. The study of the series of tetraalkylammonium ions comprising tetramethylammonium to tetrapropylammonium, for which the true rate constants should ndt differ a lot from the measured apparent values, did not seem to corroborate the Br0nsted relationship. 

The addition of hydroxyde ion to nitrosobenzene produces azoxybenzene186. Three techniques (electronic absorption spectroscopy, linear sweep voltammetry and d.c. polarography) have been used to study the equilibrium between nitrosobenzene and hydroxyde ions. The probable reaction pathway to obtain azoxybenzene is indicated by Scheme 4. The importance of the nitroso group in the reduction of nitro derivatives by alkoxide ions, when the electron-transfer mechanism is operating, has been explained187. 

The potential applications of NIR OFCD determination of metal ions are numerous. The detection of metal contaminants can be accomplished in real-time by using a portable fiber optical metal sensor (OFMD). Metal probe applications developed in the laboratory can be directly transferred to portable environmental applications with minimal effort. The response time of the NIR probe is comparable to its visible counterparts and is much faster than the traditional methods of metal analysis such as atomic absorption spectroscopy, polarography, and ion chromatography. With the use of OFMD results can be monitored on-site resulting in a significant reduction in labor cost and analysis time. 

The effects of solution acidity on the polarography of organic compounds have been reviewed, principally in aqueous solution. A thorough discussion of kinetic and catalytic currents that involve hydronium ion has been presented,52 and the irreversible polarographic and voltammetric curves that involve proton transfer in unbuffered and poorly buffered solutions have been discussed.59... 

When initiation is more complex, the elementary reactions can sometimes be studied separately. This is the case for initiation of the polymerization of 1,3-diox-olane with trityl salts. In the first reaction, hydride transfer takes place and then the newly formed cation reacts again with monomer. This second process is considerably slower than the former one. The first hydride abstraction was studied by (disappearance of the trityl cation absorption at X = 430 nm, Cmax =" 3-6 10 ) and by polarography (observation of (C6H5)3C giving a reversible one-electron wave for the trityl ion reduction with Eyz 0.51 V). 

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