Electroinactive Ions Research Articles

On the analysis of ionic mass transfer in the electrolytic bath of an aluminum reduction cell

AbstractAn electrolyte typically used in an aluminum electrolysis cell is composed of different ions moving in the electromagnetic field generated by the high intensity current needed for the industrial application. The flux of these ions has an important impact on the functional parameters of the cell, like current efficiency. In this study, the transient behaviour of these ions in the NaF‐AlF3‐Al2O3 mixture is modelled using a numerical finite element method. The electric potential field equation governed by electrochemical reaction kinetics at electrodes is solved to obtain the electric potential field, current density, and consequently heat generation in the cell. Subsequently, the concentration field is solved for ionic species in the bath. The results indicate formation of a high concentration gradient of electroactive ions like Al2OF62− and AlF4− at the corresponding reacting electrodes with time and diffusion as the main mechanism for these ions transfer. It is found that from the early stages of the 3 minute simulation of the electrochemical process, the difference between bulk concentration and surface concentration of electroactive ions remains constant. Moreover, the results indicate that although the flux of electroactive species is dominated by diffusion, especially for larger times, migration is the controlling mechanism of transport for the electroinactive ions.

Sensitivity and Selectivity of Polypyrrole Based AC‐Amperometric Sensors for Electroinactive Ions – Frequency and Applied Potential Influence

AbstractOxidation/reduction of polypyrrole films coupled with ion exchange on the polymer/solution interface can be utilized for amperometric sensing of electroinactive ions. Anion or cation exchanging films (polypyrrole doped by chloride or poly(4‐styrenesulfonate) ions, respectively) can be used to determine common anions (as Cl−, NO$\rm{ {_{3}^{-}}}$, SO$\rm{ {_{4}^{2-}}}$ etc) or cations (K+, Na+, Li+, Ca2+, Mg2+) under conditions of alternating current (AC) amperometry in the range 10−4–1 M. A sensitivity can be tuned by choosing appropriate electrode potential, corresponding to polypyrrole oxidation (anion‐exchanging films) or reduction (cation‐exchangers). Electrochemical impedance spectroscopy and AC‐voltammetry studies have shown that applied frequency and potential could also affect the observed dependence of the signal (admittance or AC‐current) on ion concentration. For high frequency the sensitivity is higher but selectivity lower, due to influence of solution conductivity on the response. For low frequencies the sensitivity is lower; however, a selectivity increase was observed due to diverse mobility of ions in the polymer film. Selectivity of AC‐amperometric responses was studied both in separate and mixed solutions.

Polymer (bio)materials design for amperometric detection in LC and FIA

This review covers the penetration of polymer-based electrode coatings for amperometric detection in continuous flow systems of analysis. Redox hydrogels, loaded ionomers, and conductive electroactive polymers form the basis of such materials. When used as such, they can improve the detectability of electroactive substances with slow kinetics on classical electrode materials. Conductive electroactive polymers can detect electroinactive ions amperometrically, via an indirect mechanism. When combined with immobilized enzymes, the above materials can detect otherwise electroinactive substances in LC and FIA methods.

Voltammetric investigation of the complexation equilibria in the presence of a low level of supporting electrolyte. Experiments with an inert complex

The experimental voltammetric studies of an inert complex has been carried out in the absence of intentionally added supporting electrolyte. The chosen system consisted of europium(III) nitrate and 1,4,8,11-tetraazacyclotetradecane (cyclam) in a 4:1 mixture (v/v) of methanol and dimethylsulfoxide. The complex formed is reduced at glassy carbon electrodes at −1.4 V vs. the reference electrode used. The complex stoichiometry is 1:1. The formation constant in the presence of supporting electrolyte (0.1 M tetrabutylammonium perchlorate), log β=3.3, has been determined in the usual manner using a glassy carbon electrode. Under the conditions of the absence of excess supporting electrolyte, reduction wave height of the Eu 3+ ion depends on concentration of electroinactive ions in the way predicted by the Myland–Oldham theory. The complex stoichiometry (1:1) and the complex formation constant (log β=3.2), in the absence of supporting electrolyte, have been determined by nonlinear curve-fitting of the steady-state currents measured at a gold microelectrode for various concentrations of the ligand. A good agreement of these two values supports the theoretical model presented in the previous paper of this series.

Parameters influencing transport across conducting electroactive polymer membranes

The parameters which influence electrochemically facilitated transport of electroinactive ions across conducting electroactive polymer membranes have been investigated. The design of membranes and the materials used as well as transport cells and systems have been addressed to improve selectivity and flux. Polypyrrole- para-toluenesulfonate (PPy-pTS) was compared with the copolymer of pyrrole with 3-carboxy-4-methylpyrrole (PPy/PCMP-pTS) and their different chemistries resulted in different membrane selectivities for ions. Platinum mesh was found to be the most suitable auxiliary electrode material and its placement in the cell chamber(s) facilitated ion incorporation/expulsion at the membrane working electrode. This enhanced the flux of ion transport. The flux can also further enhanced by narrowing the distance between the membrane working electrode and the platinum mesh auxiliary electrode(s), and/or by stirring to improve the hydrodynamics. An alternative cell design, namely a dual membrane flow through cell, also proved to be more efficient for ion transport. Good connection geometry to the membrane as well as the application of a square wave pulsed potential waveform to the membrane was found to be essential for achieving high and sustainable flux in ion transport.

Development of a conducting polymer‐based microelectrode array detection system

AbstractAn individually addressable microelectrode array system has been developed in the course of this work. The performance of this system was investigated using conventional electrochemistry (oxidation/reduction or ferrocene carboxylic acid) to demonstrate that independent potential control and current measurement at each of the array components was possible. The practical utility of this system has been demonstrated using a conducting polymer based microarray detection system subsequent to Ion Chromatography. The detection of simple electroinactive ions (e.g., Cl−, SO42−, K+, NH4+) using the array system was achieved and the response at each of the array components was studied as a function of polymer composition.

Chronoamperometry and pulse voltammetry of uncharged species at microelectrodes in the presence of a very low amount of supporting electrolyte

Normal pulse voltammetric waves obtained under conditions of substantial deficiency of supporting electrolyte may have heights identical to those recorded in the presence of an excess of supporting electrolyte. This is due to fast accumulation of ions in the microelectrode neighbourhood. Even such a small amount of electroinactive ions as that which may be adsorbed at the microelectrode surface can significantly speed up the formation of an “excess electrolyte zone”, and correspondingly can lower the total resistance between the electrodes. Solutions of oxygen and ferrocene, and undiluted methanol were taken for the normal pulse and chronoamperometric experiments. The discussion of the experimental results was supported by digital simulation.

Detection of electroinactive ions using conducting polymer microelectrodes

AbstractThe detection of electroinactive ions using conducting polymers has been described previously. In this work, we show that the analytical performance of this method is greatly improved using microelectrodes. The use of microelectrodes overcomes the problems associated with the implementation of this detection scheme in low conductivity eluents.

Electroconducting conjugated polymers: New sensitive matrices to build up chemical or electrochemical sensors. A review

Electroconducting conjugated polymers, ECPs, appear very attractive for use in sensors either as sensitive components or as a matrix for easy immobilization of specific substrates. This is due to their intrinsic properties: a one-step electrosynthesis in the form of an adherent film deposited at the surface of the electrode with anionic species being included by doping. This doping reaction makes it possible to modulate the conductivity reversibly over several orders of magnitude via redox interactions. ECP-based gas sensors are sensitive to gases (NO 2, NH 3, etc.) affecting the doping level, which results in a straightforward conductance monitoring. ECP-based ionic sensors amperometrically detect electroinactive ions, since the doping of the ECP results in a current flow. The ion-sieving effect allows some selectivity by cut-off size. SGFET microelectrochemical 'transistor' devices are sensitive to pH. Immobilization with a good 'electrical wiring' of enzymes in ECP films explains the increasing interest in research on ECP-based biosensors. An interesting approach is the simultaneous inclusion of the enzyme (glucose oxidase) with an anionic electron relay. A lifetime of 60 days has been reported. The versatile properties of ECP promise improvements in specificity, enzyme wiring and lifetime.

Electrocatalysis and its application to analysis. Plenary lecture

Many redox processes are rate limited by the kinetics of the electron-transfer rates and not the rates of diffusion. These heterogeneous electron-transfer rates can be dramatically controlled by the composition of the electrode double layer with respect to specific adsorption of electroinactive inorganic ions and surface active organic species. The control of these species can be employed for a variety of analytical applications.

Liquid chromatography with electrochemical detection : New applications in ion chromatography and reversed-phase chromatography

Electrochemical detection is shown to have the potential to be applied to an even wider range of techniques and conditions. It is useful in ion chromatography for detecting both electroaactive and electroinactive ions. Carbon paste electrodes may be suitable for work at high concentrations of organic solvent and sample clean-up can sometimes be simplified without a substantial decrease in performance.

Effect of specific adsorption of ions on the kinetics of electrode processes

The problem of statistical averaging of the probability of the elementary act of the electron transfer electrochemical reaction at the metal/solution interface in the presence of specifically adsorbed electroinactive ions is considered. The analytical expressions for the polarization characteristic of the electrode are obtained for the cases of localization of the centers of the ionic reagents in the reaction state, both inside the compact and the diffuse parts of the double layer. Within the framework of the developed theory the available experimental data on the hydrogen-ion discharge on mercury in the presence of halogenide ions are analyzed.

Effect of specific adsorption of ions on the kinetics of electrode processes

The problem of statistical averaging of the probability of the elementary act of the electron transfer electrochemical reaction at the metal/solution interface in the presence of specifically adsorbed electroinactive ions is considered. The analytical expressions for the polarization characteristic of the electrode are obtained for the cases of localization of the centers of the ionic reagents in the reaction state, both inside the compact and the diffuse parts of the double layer. Within the framework of the developed theory the available experimental data on the hydrogen-ion discharge on mercury in the presence of halogenide ions are analyzed.

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes Part VII. Case of a charged adsorbed reactant. Dibromoacetate electroreduction in the presence of Br−, N−3, SCN− and T1+ ions

The logarithmof the rate constant for CHBr2COO− electroreduction at constant applied potential, corrected for diffuse-layer effects according to Frumkin, varies linearly with the charge density qi due to specifically adsorbed supporting ions. The rate of decrease ofwith increasing |qi| as observed in the presence of the adsorbed anions Br−, N3−, and SCN− is about 5 times less than the rate of increase ofwith increasing the charge qi due to adsorbed Tl+ cations. This behaviour, analogous to that observed in the reduction of the CCl3COO− ion [4], has been explained by considering that the time of adsorption of the electroactive anion CHBr2COO− is long enough to cause a perturbation in the distribution of the adsorbed supporting ions in the neighbourhood of the adsorbed anionic reactant.

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes Part VIII. Cu2+ reduction and Cu0(Hg) oxidation in perchloric acid

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes Part VIII. Cu2+ reduction and Cu0(Hg) oxidation in perchloric acid

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes: Part VIII. Cu 2+ reduction and Cu°(Hg) oxidation in perchloric acid

The effect of the specific adsorption of the ClO 4 − ion upon the kinetics of Cu 2+ reduction and Cu 0(Hg) oxidation in aqueous solutions of HClO 4 was investigated using chronocoulometry, d.c., normal pulse, and a.c. polarography. For this purpose three different sets of data for ClO 4 − adsorption were used. In the first set the absolute surface excess of water as reckoned at the outer Helmholtz plane (o.H.p.) was disregarded, in the second set it was accounted for by postulating a complete adsorbed monolayer of water molecules [32], whereas in the third set partial displacement of water molecules from this monolayer by adsorbed ClO 4 − ions was accounted for. Kinetic data were found to the compatible with the latter set, indicating that ClO 4 − adsorption affects the rate-determining step Cu 2++ e→Cu + only by altering the average electric potential σ d-effect). The absence of any additional electrostatic effect points out that, in the transition state for Cu 2+ reduction to Cu +, the centre of charge of the reacting particle falls in the proximity of the o.H.p.

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes: Part VII. Case of a charged adsorbed reactant. Dibromoacetate electroreduction in the presence of Br −, N 3−, SCN − and Tl + ions

The logarithmof the rate constant for CHBr 2COO − electroreduction at constant applied potential, corrected for diffuse-layer effects according to Frumkin, varies linearly with the charge density q i due to specifically adsorbed supporting ions. The rate of decrease ofwith increasing | q i| as observed in the presence of the adsorbed anions Br −, N 3 −, and SCN − is about 5 times less than the rate of increase ofwith increasing the charge q i due to adsorbed Tl + cations. This behaviour, analogous to that observed in the reduction of the CCl 3COO − ion [4], has been explained by considering that the time of adsorption of the electroactive anion CHBr 2COO − is long enough to cause a perturbation in the distribution of the adsorbed supporting ions in the neighbourhood of the adsorbed anionic reactant.

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes Part V. Case of a charged adsorbed reactant. Trichloroacetate electroreduction in the presence of Br−, I−, SCN−, N3−, Tl+, and C(NH2)3+

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes Part V. Case of a charged adsorbed reactant. Trichloroacetate electroreduction in the presence of Br−, I−, SCN−, N3−, Tl+, and C(NH2)3+

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes Part VI. Dimethylglyoxime electroreduction from weakly acidic aqueous solutions in the presence of I− and SCN−

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes Part VI. Dimethylglyoxime electroreduction from weakly acidic aqueous solutions in the presence of I− and SCN−

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes Part IV. Carbon tetrahcloride electroreduction in the presence of Cl−, Br−, SCN−, and N3−

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes Part IV. Carbon tetrahcloride electroreduction in the presence of Cl−, Br−, SCN−, and N3−