Systematic investigation of alkali metal ion transfer across the micro- and nano-water/1,2-dichloroethane interfaces facilitated by dibenzo-18-crown-6

Facilitated alkali metal ion (M+= Li+, Na+, K+, Rb+, and Cs+) transfers across the micro- and nano-water/1,2-dichloroethane (W/DCE) interfaces supported at the tips of micro- and nanopipets by dibenzo-18-crown-6 (DB18C6) have been investigated systematically using cyclic voltammetry. The theory developed by Matsuda et al. was applied to estimate the association constants of DB18C6 and M+ in the DCE phase based on the experimental voltammetric results. The kinetic measurements for alkali metal ion transfer across the W/DCE interface facilitated by DB18C6 were conducted using nanopipets or-submicropipets, and the standard rate constants (k(0)) were evaluated by analysis of the experimental voltammetric data. They increase in the following order: k(Cs+)(0) < k(Li+)(0) < k(Rb+)(0) < k(Na+)(0) < k(K+)(0), which is in accordance with their association constants except Cs+ and Li+.

Study of facilitated potassium ion transfer across a water vertical bar 1,2-dichloroethane interface using different phase volume ratio systems

A study of potassium ion transfer across a water \ 1,2-dichloroethane (W \ DCE) interface facilitated by dibenzo-18-crown-6 (DB18C6) with various phase volume ratio systems is presented. The key point was that a droplet of aqueous solution containing a redox couple, Fe(CN)(6)(3-)/Fe(CN)(6)(4-), with equal molar ratio, was first attached to a platinum electrode surface, and the resulting droplet electrode was then immersed into the organic solution containing a hydrophobic electrolyte to construct a platinum electrode/aqueous phase/organic phase system. The interfacial potential of the W \ DCE within the series could be externally controlled because the specific compositions in the aqueous droplet make the Pt electrode function like a reference electrode as long as the concentration ratio of Fe(CN)(6)(3-)/Fe(CN)(6)(4-) remains constant. In this way, a conventional three-electrode potentiostat can be used to study the ion transfer process at a liquid \ liquid (L \ L) interface facilitated by an ionophore with variable phase volume ratio (r = V-o/V-w). The effect of r on ion transfer and facilitated ion transfer was studied in detail experimentally. We also demonstrated that as low as 5 x 10(-8) M DB18C6 could be determined using this method due to the effect of the high phase volume ratio.

Study of electron-transfer reactions across an externally polarized water/1,2-dichloroethane interface by scanning electrochemical microscopy

A novel method to study electron-transfer (ET) reactions between ferrocene in 1,2-dichloroethane (DCE) and a redox couple of K3Fe(CN)(6) and K4Fe(CN)(6) in water using scanning electrochemical microscopy (SECM) with a three-electrode setup is reported. In this work, a water droplet that adheres to the Surface of a platinum disk electrode is immersed in a DCE solution. The aqueous redox couple serves both as a reference electrode on the platinum disk and as an electron donor/acceptor at the polarized liquid/liquid inter-face. With the present experimental approach, the liquid/liquid interface can be polarized externally, while the electron-transfer reactions between the two phases can be monitored independently by SECM. The apparent heterogeneous rate constants for the ET reactions were obtained by fitting the experimental approach curves to the theoretical values. These rate constants obey the Butler-Volmer theory i.e., them, are found to be potential dependent.

Sol-gel derived carbon ceramic electrode containing methylene blue-intercalted alpha-zirconium phosphate micro particles

Methylene blue-intercalated a-zirconium phosphate (MBZrP) micro particles in deionized water were deposited onto the surface of graphite powder to prepare graphite powder-supported MBZrP, which was subsequently dispersed into methyltrimethoxysilane-derived gels to yield a conductive composite. The composite was used as electrode material to fabricate a surface-renewable, rigid, leak-free carbon ceramic composite electrode, bulk-modified with methylene blue (MB). In the configuration, alpha-zirconium phosphate was employed as a solid host for MB, which acted as a catalyst. Graphite powder ensured conductivity by percolation, the silicate provided a rigid porous backbone and the methyl groups endowed hydrophobicity and thus limited the wetting section of the modified electrode. Peak currents of the MBZrP-modified electrode were surface-confined at low scan rates but diffusion-controlled at high scan rates. Square-wave voltammetric study revealed that MBZrP immobilized in carbon ceramic matrix presented a two-electron, three-proton redox process in acidic aqueous solution with pH ranged from 0.44 to 2.94. In addition, the chemically modified electrode showed an electrocatalytic activity toward nitrite reduction at +0.15 V (vs. Ag/AgCl) in acidic aqueous solution (pH=0.44). The linear range and detection limit are 1 x 10(-6)-4 x 10(-3) mol L-1 and 1.5 x 10(-7) mol L-1, respectively.

Investigation of ion transfer across the micro-water/nitrobenzene interface facilitated by a fullerene derivative

A functionalized fullerene derivative containing a monoaza-18-crown-6 moiety was investigated by facilitated ion (such as Li+, Na+, K+, NH4+, Mg2+, and Ca2+) transfer across the micro-water/nitrobenzene interface supported at the tip of a micropipet. The current responses were detected by cyclic voltammetry and Osteryoung square wave voltammetry, which demonstrated that the facilitated ion transfer does occur by an interfacial complexation-dissociation process. The diffusion coefficient of this compound in nitrobenzene was approximately (5.90 +/- 0.04) x 10(-7) cm(2) s(-1), which is 1 order of magnitude less than other common ionophores due to the large size of the molecule. The selectivity of this molecule toward the metal ions followed the sequence Na+ > Li+ > K+ > NH4+ > Ca2+ similar to Mg2+. In addition, this compound was also easy to form film at the water/nitrobenzene interface to inhibit the simple ion transfer of tetramethylammonium ion. However, the adsorption of this ionophore has less influence on the facilitated metal ion transfer.

Organic-phase enzyme electrode for phenolic determination based on a functionalized sol-gel composite

An amperometric biosensor for monitoring phenols in the organic phase was constructed by the silica sol-gel immobilization of tyrosinase on a glassy carbon electrode. The organic-inorganic hybrid materials with different sol-gel precursors and polymers were optimized, and the experimental conditions, such as the effect of the solvent, operational potential and enzyme loading were explored for the optimum analytical performance of the enzyme electrode. The biosensor can reach 95% of steady-state current in about 18 s, and the trend in the sensitivity of different phenols is as follows: catechol > phenol >p-cresol. In addition, the apparent Michaelis-Menten constants (K-m(app)) and the stability of the enzyme electrode were discussed. (C) 2000 Elsevier Science S.A. All rights reserved.

Electrochemiluminescence of the Ru(bpy)(3)(2+)/S(2)O(8)(2-)system in purely aqueous solution at carbon paste electrode

The electrochemiluminescence (ECL) of the Ru(bgy)(3)(2-)/S2O82- system in purely aqueous solution at a carbon paste electrode can be clearly seen with the naked eye for Ru(bpy)(3)(2+) concentrations higher than 1 mmol L-1. The log-log plot of the emmitted light intensity vs. Ru(bpy)(3)(2+) concentration is linear over the region 10(-3)-10(-7) mol L-1 with a correlation coefficient of 0.997. The ECL intensity increases linearly with the S2O82- concentration from 10(-6) mol L-1 up to 0.3 mmol L-1 and drops off sharply at concentrations higher than 1 mmol L-1. In addition, a weak ECL signal was obtained when the potential was biased more negative than -0.6 V even in the absence of S2O82-.

The effect of substrate and solvent properties on the response of an organic phase tyrosinase electrode

The responses of a cryohydrogel tyrosinase enzyme electrode to four substrates in three pure water immiscible organic solvents were investigated. Kinetic parameters, the maximum kinetic current, I-max, the apparent Michaelis-Menten constant, K-m(app), and I-max/K-m(app), were calculated. The I-max/K-m(app) value was taken as an indicator of the catalytic efficiency of the sensor. The effect of the substrate hydrophobicity on I-max/K-m(app) and response time of the sensor were discussed. The effects of both hydrophobicity (log P) and dielectric constant (epsilon) of the organic solvent on the catalytic efficiency of the enzyme in the organic phase were studied. (C) 1997 Elsevier Science S.A.

Liquid chromatography amperometric detection of nitrite using a polypyrrole modified glassy carbon electrode doped with tungstodiphosphate anion

Chromatography-amperometric detection of nitrite with a polypyrrole modified glassy carbon electrode doped with tungstodiphosphate anion (Dawson-type P2W18O626-/PPy/GC electrode) based on its electrocatalytic reduction of nitrite is described. The cyclic and hydrodynamic voltammetry of nitrite at the P2W18O626-/PPy/GC electrode was studied. The factors affecting the detection of nitrite and the analytical performance of the modified electrode in flowing stream were investigated. The results show that the modified electrode has a good sensitivity (the limit of detection is 1 mu mol dm(-3)) and a satisfactory reproducibility (RSD = 3.78%, N = 21). The modified electrode was used in the chromatographic detection of nitrite spiked in the liquid from a tin of mushrooms and the mineralized spring water. It was found that the modified electrode exhibited good selectivity for nitrite.

Determination of hydroxylamine by capillary electrophoresis electrochemical detection with a palladium-particle modified carbon fiber microdisk array electrode

A highly dispersed ultramicro palladium-particle modified carbon fiber microdisk array electrode (Pd-CFE) was employed for capillary electrophoresis-electrochemical (CEEC) detection of hydroxylamine (HA). The Pd particles obtained were in the nanometer scale, had a high electrocatalytic activity towards HA and exhibited good reproducibility and stability. A linear relationship between the current and the analyte concentration was found between 5 x 10(-6) and 1 x 10(-3) mol/l of HA with a correlation coefficient of 0.9992. The detection limit was 5 x 10(-8) mol/l. The applicability of the method for the determination of HA in river water and waste water was investigated.

Detection of menadione sodium bisulfite (vitamin K-3) by reversed-phase high performance liquid chromatography with series dual-electrode amperometric detector

A reversed-phase high-performance liquid chromatography with series dual glassy carbon electrodes for the amperometric detection of water-soluble menadione is described. The complex post-column derivatization reaction and the high background currents were avoided. The menadione sodium bisulfite was reduced at -0.3 V vs. SCE at the upstream (generator) electrode and oxidized at +0.2V vs. SCE at the downstream (collector) electrode. The mobile phase was 0.2moll(-1) HAc-NaAc aqueous buffer (pH 5.50) and 40% (v/v) methanol. The linear response was in the range of 35 ng to 15 mu g, with a detection Limit of 15 ng (S/N=3). The correlation coefficient was 0.9997 (n=6). The electrochemical detection with series dual electrodes has a higher selectivity for menadione (vitamin K-3) compound than with UV detection.

Electrocatalytic reduction of nitrite using Dawson-type tungstodiphosphate anions in aqueous solutions, adsorbed on a glassy carbon electrode and doped in polypyrrole film

The electrochemical behavior of Dawson-type P2W18O626- adsorbed on a glassy carbon electrode and doped in a polypyrrole film electrode was described. These modified electrodes all display catalytic activity for nitrite reduction, either in acid solutions or in pH > 4.0 solutions.

Impedance spectrum measurement in liquid liquid interface electrochemistry .1. Analysis of various kinds of interfacial impedance in the four-electrode system

Applying impedance spectrum technique to liquid/liquid interfacial electrochemistry, we present the theoretical expression of the liquid/liquid interfacial impedance in the four - electrode electrolytic cell measured by alternating current impedance method. The influence of the electrolytic cell parameters and input passage parameters of the impedance apparatus on impedance spectrum curves are theoretically studied.

RAPID-DETERMINATION OF KINETIC-PARAMETERS FOR THE ELECTROCATALYTIC REDUCTION OF DIOXYGEN BY VITAMIN-B12 ADSORBED ON GLASSY-CARBON ELECTRODE

In this paper, the electrochemical behavior of vitamin B-12, ie cyanocobalamin (abbr. VB12) in a weak acidic aqueous solution and adsorbed on glassy carbon (GC) surface (abbr. VB12(ad)/GC) in different pH buffer solutions have been described by using cyclic voltammetry (cv). It is found that VB12 and VB12(ad)/GC exhibit catalytic activity for the electroreduction of O2 according to two reduction peaks at -0.50 and -1.00 V vs. sce; but their electrocatalytic activity is very unstable. Based on the method of hydrodynamic amperometry [B. Miller and S. Bruckenstein, J. electrochem. Soc. 117, 1033 (1970)], some kinetic parameters for the electrocatalytic reduction of O2 by VB12(ad)/GC have been determined rapidly by using a linear rotation-scan method [Rongzhong Jiang and Shaojun Dong, Electrochim. Acta 35, 1451 (1990)]. These kinetic parameters indicate that the reduction of O2 on VB12(ad)/GC gives water predominantly in both potential ranges which correspond to those two reduction peaks. Possible reaction mechanisms have been suggested.

EFFECT OF PH ON THE ELECTRON-TRANSFER OF CYTOCHROME-C ON A GOLD ELECTRODE MODIFIED WITH BIS(4-PYRIDYL)DISULPHIDE

The adsorption of bis(4-pyridyl)disulphide (PySSPy) and 4.4'-bipyridyl (PyPy) on a gold electrode was studied using cyclic voltammetry. The adsorption isotherms and equilibrium constants (1 X 10(6) mol-1 l for PyPy and 6 x 10(6) Mol-1 l for PySSPy) were determined. The effect of pH on the electrochemical behaviour of cytochrome c was studied on the PySSPy-modified gold electrode. The results show that cytochrome c can only transfer electrons on a deprotonated electrode surface. When the pH is decreased, the standard heterogeneous rate constant of cytochrome c on the modified gold electrode decreases and the electrochemical behaviour changes from a quasi-reversible to an irreversible process.