Peculiarities of anodic behavior of gold electrode in thiosulfate electrolytes

Using the methods of quartz microgravimetry and voltammetry, the anodic behavior of gold electrode in thiosulfate electrolytes is studied in the pH range of 7 to 11. It is found that, in the potential range from 0.15 to 1.0 V (NHE), the anodic current is associated predominantly with the oxidation of thiosulfate ions, and the gold dissolution rate in this electrolyte is negligibly low (< 0.02 mA/cm²). It is shown that the study of anodic processes in the neutral thiosulfate electrolytes requires stabilization of solution acidity, because the near-anode layer can be acidified to the pH values, which are sufficient for the formation of elemental sulfur. It is found that the use of Britten-Robinson buffer solution with pH 7 as the supporting electrolyte changes significantly the polarization curve of thiosulfate ion oxidation, but does not raise the gold dissolution rate. An increase in the solution pH to 11 and an exposure of electrode at various potentials (−0.5 and 0.15 V) prior to the onset of potential scanning also do not accelerate considerably the gold dissolution in the thiosulfate electrolyte. A comparison between the regularities of gold anodic behavior in the thiosulfate solutions and earlier studied gold dissolution in the cyanide and thiocarbamide electrolytes showed that they are similar. It is supposed that the specific features of anodic processes in these cases are of a similar nature: the metal dissolution proceeds with the formation of two-ligand complexes with linear structure, which is typical for all aforementioned ligands.     

Electrochemistry of nanopore electrodes in low ionic strength solutions

The steady-state voltammetric behavior of truncated conical nanopore electrodes (20-200 nm orifice radii) has been investigated in low ionic strength solutions. Voltammetric currents at the nanopore electrode reflect both diffusive and migrational fluxes of the redox molecule and, thus, are strongly dependent on the charge of the redox molecule and the relative concentrations of the supporting electrolyte and redox molecule. In acetonitrile solutions, the limiting current for the oxidation of the positively charged ferrocenylmethyltrimethylammonium ion is suppressed at low supporting electrolyte concentrations, while the limiting current for the oxidation of the neutral species ferrocene is unaffected by the ionic strength. The dependence of the limiting current on the relative concentrations of the supporting electrolyte and redox molecule is accurately predicted by theory previously developed for microdisk electrodes. Anomalous values of the voltammetric half-wave potential observed at very small nanopore electrodes (<50 nm radius orifice radii) are ascribed to a boundary potential between the pore interior and bulk solution (i.e., a Donnan-type potential).     

Streaming maxima in the iodine electroreduction at Hg electrodes in DMSO solutions

A current maximum was observed by d.c. polarography during the electroreduction of iodine at Hg electrodes in DMSO solutions. i?t curves during the drop life were performed to characterize the observed maxima. Changes in the cation of the supporting electrolyte do not. appreciably affect the results. The dependence of the streaming with depolarizer and supporting electrolyte concentration, as well as its independence of flux rate confirmed by linear sweep potential experiments at a hanging mercury electrode, permit us to classify the streaming under study as a positive first kind streaming. Regular current fluctuations at a certain depolarizer concentration and low supporting electrolyte concentration are also reported.     

Molecular Description of the Persulfate Ion Reduction on a Mercury Electrode

The polarization curves for the S₂O₈ ^2- electroreduction on a mercury electrode at high overvoltages and various concentrations of a surface-inactive supporting electrolyte are modeled within modern theory of charge transfer in polar media and quantum-chemical approaches. Based on an analysis of the reactant adsorption in terms of a cluster model, the conclusion is drawn that the persulfate ion is localized in the diffuse part of EDL. When calculated the current, it was assumed that the transfer of the first electron, accompanied by the bond cleavage, is the limiting stage of the total two-electron process. The integration if performed over the entire electron spectrum of the metallic electrode and an attempt is made to account for electrostatic and solvation effects on a molecular level. It is shown that the experimentally-studied overvoltage interval corresponds to the occurrence of the process near the activationless region. The increase in the current at high negative charges of the surface is due to an increase in the reaction layer thickness. This effect arises from a change in the ratio between contributions made by the reactants at the distance of closest approach and the species farther away.     

Elucidation of Mercury Ion Binding Property of a New Aryl Amide Type Podand by Electrochemical and Fluorescence Measurements

Abstract

A new aryl amide type podand (AAP), 1,8-bis(o-amidophenoxy)-3,6-dioxaoctane, was synthesized and its mercury ion binding property was elucidated by electrochemical and fluorescence measurements. The effects of supporting electrolyte, pH and scan rate on the electroreduction of AAP and on cyclic voltommogram of mercury ion were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). It was observed that the complex formation between AAP and mercury ion exhibited pH-dependent behavior in different electrolyte solutions. Elucidation of such properties makes the AAP intriguing candidates as a modifying agent in sensor application for determination of mercury ion in mixture of aqueous solution.     

Polarography in hexamethylphosphoramide: Effect of supporting electrolyte cations

Polarographic reductions of various metal ions such as the silver, cupric, zinc, cobaltous, nickel, ferric, ferrous ions and hydrogen ion in hexamethylphosphoramide (HMPA), have been investigated in the supporting electrolytes with various perchlorates. The reduction of most of these ions is strongly influenced by the cation of the supporting electrolyte. In the presence of the tetraethylammonium ion, when the size of the cation of the supporting electrolyte is small and easily adsorbed on the negatively charged electrode surface, the reductions of metal ions are controlled by some preceding processes and are naturally irreversible. The rate of reduction becomes more rapid with the increase of the size of the cation. Thus, in Hex₄NClO₄ or LiClO solutions, the reduction of these various metal ions takes place almost totally under diffusion control, although the waves of most of metal ions show a maximum. These effects of the cation of the supporting electrolytes on reduction can be explained as a phenomenon occurring on the electrode surface. This phenomenon has been reported in previous papers [1] on the reductions of the alkali and alkaline earth metal ions. The difference in the electrocapillary curves in these solutions is rarely shown at the potential around the electrocapillary maximum, but it is very obviously shown at more negative potential. The difference in the effect of the size of the cation of the supporting electrolyte on reduction of metal ion coincides well with the difference in the electrocapillary curves in these solutions: the effect of the size of the supporting electrolyte cation on the polarographic reduction is rarely shown at the potential around the electrocapillary maximum, but it is very obviously shown at more negative potential; therefore this effect is due to the electrode double-layer difference.     

Electrochemical evaluation of 4-(dimethylamino)pyridine adsorption on polycrystalline gold

Using differential capacity and chronocoulometry, we have studied the electrosorption of 4-(dimethylamino)pyridine (DMAP) on polycrystalline gold electrode surfaces. Our results indicate that the orientation of DMAP is highly dependent on the electrode potential and electrolyte pH. At pH values at or above the primary pKa, the adsorbed species is DMAP and orients vertically on the electrode surface via the lone pair of electrons on the pyridine ring's nitrogen atom. At very low pH values (<3) the adsorbed species is the protonated ion, DMAPH+, which can be desorbed from the electrode surface when the metal's surface charge density is made appreciably positive of the potential of zero charge. At intermediate electrolyte pH, either DMAP or DMAPH+ is adsorbed on the surface depending on the electrode's potential. At negative charge densities, DMAPH+ lies nearly flat on the gold electrode and the surface coverage is correspondingly low. When the electrode is positively charged, the adsorbate undergoes a phase transition to a vertical orientation and is simultaneously deprotonated to DMAP. Our results rationalize the stability of DMAP-ligated gold nanoparticles as a function of pH and demonstrate that the ligand's surface coverage is the principal factor in determining the stability of the colloidal system.     

磁场效应对理解水分解反应物的可能意义

电催化水分解由两个基元反应构成,即析氢反应(HER)和析氧反应(OER).开发强大的HER和OER技术需要在分子层面理解反应机理,然而,目前水分解反应的反应物还没有完全确定.本文利用磁场来研究HER中的质子传输和OER中的氢离子传输,对确定HER和OER中真实的反应物具有重要意义.磁场是改变离子等带电物质运动的一种有效...     

Voltammetric Determination of Aflatoxin B1

For the first time the possibility of voltammetry used for the determination of aflatoxin B1 on a glassy carbon electrode was shown. The effect of pH of a supporting electrolyte on the analytical signal of aflatoxin B1 has been investigated and it was shown that there is a more pronounced peak with a maximum current at pH of 5.33. The most favorable supporting electrolyte for a linear range of detectable concentrations of aflatoxin B1 – 0.1 M (NH₄)₂SO₄ was determined. The results of research on the development of conditions of voltammetric measurement of aflatoxin B1 are presented.     

Anodic stripping voltammetric determination of mercury by use of a sodium montmorillonite-modified carbon-paste electrode

A sodium montmorillonite (SWy-2)-modified carbon-paste electrode has been examined for determination of trace levels of mercury. Because of its strong cation-exchange and adsorptive characteristics, SWy-2 greatly improves the sensitivity of determination of Hg(2+). Hg(2+) is preconcentrated and reduced on the modified electrode surface at -0.40 V and then stripped from the electrode surface during the positive potential sweep. The conditions used for determination, e.g. supporting electrolyte, pH, amount of SWy-2, accumulation potential, and accumulation time, were optimized. The peak current was linearly dependent on the concentration of mercury from 1 x 10(-9) to 5 x 10(-7) mol L(-1). The detection limit (signal-to-noise ratio=3) was 1 x 10(-10) mol L(-1) after accumulation for 6 min. When the SWy-2-modified carbon-paste electrode was used to detect mercury in water samples the average recovery was 101.11%.     

Studies on membrane concentration potentials across heavy metal soap discs

The electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes involving adsorbed reactants with a time of adsorption greater than the characteristic time of surface-diffusion jumps is considered. The validity of the most general form of the Frumkin correction term for double-layer effects, with ψ₁ equal to the local potential ₃ at the position occupied by the reactant in the transition state, is first examined on the basis of current molecular theories of electrode kinetics. Subsequently, approximate expressions of ₃ are derived for the case in which adsorbed reactant and adsorbed supporting ion are charged both of equal and of opposite sign. In both cases the logarithm Φ of the rate constant for the electrode reaction at constant applied potential, as corrected for diffuse-layer effects only, is expected to vary linearly with the charge density q_i due to the adsorbed supporting ion. |ΔΦ/Δq_i| is, however, much greater in the case of electrostatic attraction between reactant and adsorbed supporting ion than in the case of electrostatic repulsion. The influence of reactant adsorption with partial charge transfer upon the magnitude of ΔΦ/Δq_i is considered.     

Kinetics and Mechanism of Electroreduction of Palladium(II) Bisethylenediamine Complexes on a Dropping Mercury Electrode

Kinetics of electroreduction of complexes Pd(en)²⁺ ₂(2 × 10^–5M) on a dropping mercury electrode is studied in solutions with various concentrations of ethylenediamine and supporting electrolytes (NaF, NaClO₄) at pH 4.5–11 and different instantaneous current recording times. Diffusion coefficients for reducing complexes and kinetic parameters of the slow electrochemical stage are determined. The concentration of supporting electrolytes is found to affect the half-wave potential, which points to an inner-sphere mechanism of electrochemical stage at supporting electrolyte concentrations of 0.005 to 0.03 M and to a predominantly outer-sphere mechanism at higher concentrations.     

Oligonucleotide Functionalized Microporous Gold Electrode for the Selective and Sensitive Determination of Mercury by Differential Pulse Adsorptive Stripping Voltammetry (DPAdSV)

Abstract

A novel electrode modified with oligonucleotide and microporous gold was fabricated for the determination of mercury by differential pulse adsorptive stripping voltammetry (DPAdSV). Microporous gold was synthesized by electrochemical reduction using dynamic hydrogen bubble template. The oligonucleotide was immobilized on microporous gold by self-assembly. The prepared electrode exhibited an improved electrochemical response for mercury(II) ion because of the large surface area and excellent electron transfer capacity provided by microporous gold and the specific coordination between mercury ion and thymine bases in oligonucleotides. Under the optimal experiment conditions, the oligonucleotide functionalized microporous gold electrode had a linear relationship between the stripping current and mercury ion concentration in the range from 0.5 to 30?µg/L with a detection limit of 0.021?µg/L. Moreover, the prepared electrode exhibited good selectivity, reproducibility, repeatability and stability. Furthermore, the prepared electrode was applied to detect mercury in tap water with satisfactory results.     

Multistage Electrochemical Reactions with the Transfer of Intermediates between the Near-Electrode Layer and the Bulk Solution: The Accumulation of the Intermediates and the Current Redistribution between the Stages during Electrolysis

Dependences of reactant concentrations and current densities of individual stages on the electrolysis time for a multistage electrode process complicated by the transfer of intermediates between the near-electrode layer and the bulk electrolyte are obtained by numerical computer-aided modeling. Effect of various factors on the dynamics of variations in the concentration of the initial reactant and intermediate in the bulk electrolyte is analyzed. The possibility of a substantial increase in the current efficiency of the second stage of discharge during electrolysis is shown.     

The Effect of Supporting Electrolyte Concentration on Zinc Electrodeposition Kinetics from Methimazole Solutions

The kinetic parameters of Zn²⁺ ion electroreduction in sodium perchlorate used as the supporting electrolyte on the mercury electrode in the presence of methimazole were determined using electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and DC polarography. The two‐stage nature of this process was demonstrated. Both steps are catalysed by a methimazole. The size of the acceleration effect depends on the concentrations of methimazole and the supporting electrolyte. The acceleration of the electrode process involves the formation of active complexes between the depolarizer ions and methimazole on the electrode surface. These complexes facilitate the exchange of charge between the electrode and zinc ions during electroreduction process. The change of the hydrating sphere of the zinc ion is also important here. This in turn depends on its oxidation state and the concentration of the supporting electrolyte.     

Mass transport and effective diffusion coefficient in the reduction of hydrogen ions from aqueous sulfuric acid solutions: Numerical modeling

This work presents results of the numerical solution to a system of equations of material balance and the movement of particles in solution under the influence of the forces of diffusion, migration, and convection, which describe the process of mass transport during the reduction of hydrogen ions at a rotating disk electrode from aqueous sulfuric acid solutions with and without excess supporting electrolyte. Results of the calculations show that the diffusion kinetics of hydrogen ion reduction can be observed only with measurements in dilute (≤10^?3 M) sulfuric acid solutions with an excess of indifferent supporting electrolyte. For more acidic solutions it is necessary to take into account the simultaneous diffusion of hydrogen and bisulfate ions. In the study of the regularities of hydrogen ion reduction in sulfuric acid solutions with a sulfate supporting electrolyte, it is necessary to take into account that with excess supporting electrolyte, the limiting current of hydrogen reduction is caused solely by the diffusion of bisulfate ions, but for small concentration ratios of the supporting electrolyte to acid, the influence of migration effects is significant.     

Parameters affecting the determination of mercury by anodic stripping voltammetry using a gold electrode

The electrochemical determination of aqueous Hg(II) by anodic stripping voltammetry (ASV) at a solid gold electrode is described. The aim of this work is to optimise all factors that can influence this determination. Potential wave forms (linear sweep, differential pulse, square wave), potential scan parameters, deposition time, deposition potential and surface cleaning procedures were examined for their effect on the mercury peak shape and intensity. Five supporting electrolytes were tested. The best responses were obtained with square wave potential wave form and diluted HCl as supporting electrolyte. Electrochemical and mechanical surface cleaning, aimed at removing the amount of mercury deposited onto the gold surface, were necessary for obtaining a good performance of the electrode. Response linearity, repeatability, accuracy and detection limit were also evaluated.     

Electrogenerated chemiluminescence of luminol on a gold-nanorod-modified gold electrode

Electrogenerated chemiluminescence (ECL) of luminol on a gold-nanorod-modified gold electrode was studied, and five ECL peaks were obtained under conventional cyclic voltammetry in both neutral and alkaline solutions. Among them, four ECL peaks (ECL-1-4) were also observed on a gold-nanosphere-modified gold electrode, but the intensities of these ECL peaks were enhanced about 2-10-fold on a gold-nanorod-modified gold electrode in neutral solution. One new strong ECL peak (ECL-5) was obtained at -0.28 V (vs SCE) on a gold-nanorod-modified gold electrode in both neutral and alkaline solutions and enhanced with an increase in pH. In strong alkaline solutions, ECL-1 and ECL-2 on a gold-nanosphere-modified electrode were much stronger than those on a gold-nanorod-modified gold electrode, while ECL-3-5 appeared to only happen on a gold-nanorod-modified gold electrode. The emitter of all the ECL peaks was identified as 3-aminophthalate. The ECL peaks were found to depend on the scan direction, the electrolytes, the pH, and the presence of O(2) and N(2). The reaction pathways for ECL-4 have been further elucidated, and the mechanism of the new ECL peak (ECL-5) has been proposed. The results indicate that a gold-nanorod-modified gold electrode has a catalytic effect on luminol ECL different from that of a gold-nanosphere-modified gold electrode, revealing that the shape of the metal nanoparticles has an important effect on the luminol ECL behavior. The strong ECL of luminol in neutral solution obtained on a gold-nanorod-modified electrode may be used for the sensitive detection of biologically important compounds in physiological conditions.     

Double layer effects in the electrode kinetics of amalgam formation reactions: Part II. A comparison of the CE and IE mechanisms

The potential dependence of two possible mechanisms for the reduction of a divalent cation at a mercury electrode are compared. In one mechanism, the reactant undergoes a chemical step in the double layer, followed by electron transfer (CE mechanism). The other mechanism involves transfer of the ion to a site closer the electrode which is followed by electron transfer (IE mechanism). It is shown that these mechanisms are easily distinguished on the basis of double layer effects. The present analysis suggests that the IE mechanism is more probable when the cation is reduced at a negatively charged electrode. If a chemical step occurs in the diffuse layer, then a corrected Tafel plot with a negative slope is expected, a result which has not been observed to date.     

Precapacitive processes in single potential-step chronoamperometry

Extremely short-lived anodic currents were observed in the early parts of the transient response following the application of a cathodic potential step to a mercury working electrode. It is proposed that this phenomenon is due to the existence of a brief precapacitive period, which precedes full development of the double-layer charging current, and which allows momentary reaction (reduction) of species present at the electrode surface. The observed anodic currents are explained in terms of a re-oxidation of such "resident" species that were reduced during this precapacitive period. The subsequent capacitive surge produced by the charging of the electrical double layer leads to an anodic shift of the electrode potential that can be sufficient for the re-oxidation of the precapacitive amalgam. The anodic peak is linearly related to depolarizer concentration and varies with supporting electrolyte concentration, ion mobility and potential step size. Cathodic preconcentration of the depolarizer enhances the effect.