Optimisation of mercury film deposition on glassy carbon electrodes: evaluation of the combined effects of pH, thiocyanate ion and deposition potential

The combined effects of pH, thiocyanate ion and deposition potential in the characteristics of thin mercury film electrodes plated on glassy carbon surfaces are evaluated. Charges of deposited mercury are used as an experimental parameter for the estimation of the effectiveness of the mercury deposition procedure. The sensitivity of the anodic stripping voltammetry (ASV) method for the determination of lead at in situ and at ex situ formed thin mercury films are also examined. It was concluded that, in acidic solutions (pH 2.5-5.7) and fairly negative deposition potentials, e.g. −1.3 to −1.5 V, thiocyanate ion promotes the formation of the mercury film, in respect both to the amount of deposited mercury and to the mercury deposition rate. Also, the mercury coatings produced in thiocyanate solutions are more homogeneous, as depicted by microscopic examinations. In the presence of thiocyanate there is no obvious advantage of using high concentrations of mercury and/or high deposition times for the in situ and ex situ preparation of the mercury film electrodes. The optimised thin mercury film electrode ex situ prepared in a 5.0 mM thiocyanate solution of pH 3.4 was successfully applied to the ASV determination of lead and copper in acidified seawater (pH 2). The limit of detection (3σ) was 6×10⁻¹¹ M for lead and 2×10⁻¹⁰ M for copper for a deposition time of 5 min. Relative standard deviations (R.S.D.s) of <1.2% were obtained for determinations at the nanomolar of concentration level.     

Silver based mercury film electrode: Part II. The influence of aging of electrode on the electrode processes of various metal ions

The cyclic voltammetric behaviour of 8 metal ions at solid silver amalgam electrodes prepared by aging of a thin silver based mercury film electrode (SBMFE) and by deposition of silver and mercury on platinum were investigated. It was established that such electrodes behave in relation to some metals (Pb, Bi, Sn) similarly as silver electrodes i.e. the cyclic curves obtained with these electrodes at concentration 10^?3M range show a prepeak-postpeak system corresponding to deposition and dissolution of the monolayer of deposit. On the other hand under the same conditions no prepeaks were observed for cadmium, zinc and thallium. In all cases investigated the heights of anodic stripping peaks were lower on curves obtained with aged SBMFE than on those obtained with fresh SBMFE having a mercury layer 1 μm thick.     

General characteristics of the copper-based mercury film electrode

The optimum conditions for the preparation, storage, conditioning and renewal of copper-based mercury film electrodes (CBMFEs) are given. The voltammetric results obtained at these electrodes are compared with the predictions of the theory of cyclic and stripping voltammetry at the mercury film electrode, as well as with the results obtained at the silver-based and the platinum-based mercury film electrodes. The advantage of a CBMFE is prolonged life-time, whereas the disadvantages ar the decreased range of usable positive potentials and the possibility of interfering reactions of the electrodeposited metals with the copper substrate or copper dissolved in the mercury phase. The presence of copper has no essential influence on the behaviour of lead and thallium; it affects the behaviour of zinc markedly and that of cadmium and indium slightly. The conditions allowing the minimization of the harmful action of copper on the behaviour of cadmium and indium have been found.     

Theory of cyclic voltammetry at the mercury film electrode: Reversible case with substrate initially present in the mercury phase

The theory of cyclic voltammetry has been developed for a case describing the behaviour of a reversible system at the mercury film electrode with a metal present initially in the mercury phase. Theoretical results are presented in the form of a simple equation coupled with a set of the calculated current function values. In addition, several simple relationships are given for the parameters characterizing the cyclic curves. The rapid exhaustion of the metal from the mercury film and the generation of the metal ions into the solution free of metal ions offer better conditions for the resolution of adjacent peaks than those existing in cyclic voltammetry at the HMDE and at the MFE with substrate present initially in solution. Theoretical predictions were compared with experimental results obtained for lead and thallium at the silver-based mercury film electrode.     

Silver based mercury film electrode: Part IV. Comparison of theoretical and experimental voltammetric results obtained for cadmium

Using the silver based mercury film electrode (SBMFE) and cyclic and stripping voltammetric techniques the behaviour of cadmium has been compared with the behaviour of lead as well as with the predictions of the theory of de Vries and van Dalen. At the SBMFE with film thickness between 0.1 and 2 μm lead behaves with good agreement with theoretical predictions and only at thicknesses higher than 2 μm some deviations occur due to collecting of the excess mercury at the bottom of the wire electrode. On the other hand even at thin film electrodes the behaviour of cadmium deviates significantly from the predictions of the theory. The height of the anodic peak decreases and its width increases; also the displacement of the potentials of both cathodic and anodic peaks is smaller than the values predicted theoretically. The deviations are caused neither by the formation of intermetallic compounds in the bulk of mercury phase nor by the formation of heterogenous cadmium amalgam; they reflect the interaction between cadmium dissolved in mercury and the solid silver amalgam which is the substrate of the mercury film. Owing to the effect discussed the stripping determination of cadmium at SBMFE is characterized by a lower sensitivity and reproducibility.     

Reduction of a simple ion at permeable film-coated stationary electrodes

Recently proposed chemically prepared electrodes are coated with a thin, permeable, insulating, inert film which does not react with the depolarizer, does not allow depolarization on its surface and does not change the standard constant of the depolarization rate. It only changes the diffusion coefficient of a certain ion near the surface of the electrode. In this article, the theory of a reversible reduction of a simple ion at a film-coated stationary planar electrode is developed. If the film thickness is comparable with a diffusion layer thickness, considerable changes on the i-t curves can occur, but the position of the half-wave potential will remain constant.     

Electrochemistry of and Redox‐Induced Metal Release from Metallothioneins at a Nafion‐Coated Bismuth Film Electrode

Nafion‐coated bismuth film electrodes (NCBiFEs) were used to investigate the redox behavior of and metal release from rabbit liver metallothioneins (MTs) in an acetate buffer. Owing to the permselective exchange between positively charged MT molecules and cations in Nafion and the absence of detectable MT adsorption at bismuth surface, a diffusion‐controlled Nernstian redox wave of MTs (E_pc=?0.869 V) was observed for the first time. The Nernstian behavior of the diffusing MTs is in contrast to the voltammetric responses of MTs at thin mercury films or Nafion‐coated mercury film electrodes, which either result in the replacement of the metals originally present in MTs by mercury or lead to a noticeable MT adsorption. By avoiding these undesirable features, the NCBiFE provides an excellent milieu for voltammetric studies of different types of MTs and related isoforms, paving the way for studying the redox‐modulated metal transfer of MTs and understanding the biological role of MTs in heavy metal detoxification and essential metal regulations.     

Anodic behaviour of the (222) macrobicyclic ligand on mercury electrode in propylenecarbonate

The anodic behaviour of the free macrobicyclic ligand (222) was examined by various electrochemical techniques on mercury and on platinum electrodes. From results on platinum electrode using cyclic voltammetry it appears that the anodic oxidation of (222) proceeds irreversibly at potentials more positive than +0.7 V (aq. SCE), however, its mechanism could not be determined because the anodic signal was not well developed. Under polarographic conditions a reversible anodic diffusion controlled wave at E_1/2=+0.08 V (aq. SCE) was observed, corresponding to a complex formation of the ligand (222) with the ions of mercury formed by anodic polarization of mercury electrode. In excess of mercuric cations a cathodic wave at slightly more positive potentals was found.     

Effects of the mercury ion concentration on the preparation of mercury deposits on platinum micro-disk electrodes

In this paper, the effect of mercury ion concentration on the preparation of mercury microelectrodes fabricated on a platinum microdisk of 10 microns radius was studied. The preparation of the mercury microelectrodes was followed by chronoamperometry, and the measurements were performed in Hg2(NO3)2 solutions with concentrations over the range 0.1-12.5 mM. The mercury microelectrode size was referred to as h/a, where h is the height of the mercury deposit and a is the radius of the inlaid microdisk. The mercury microelectrodes investigated had h/a values over the range 0-2. The results obtained indicated that from concentrated mercury ion solutions (> 0.5 mM) the mercury growth was somehow erratic due to the coalescence of small mercury droplets. On the other hand, from dilute solutions (< 0.5 mM), the mercury deposits grew smoothly. Under the latter conditions the geometric coefficient k, which characterises the steady state diffusion limiting current expression at a mercury microelectrode, was determined with good accuracy (within 5% error) from the chronoamperometric curves recorded during the mercury microelectrode preparation. In general, the coefficient k was calculated from theoretical expressions derived for a sphere cap and spheroidal geometry, which may model the shape of the mercury deposits. The comparison between theoretical and experimental k values suggested that both geometries equally modelled the experimental mercury deposit obtained. Finally, for the sphere cap geometry an algebraic expression relating k and h/a was derived for an easier k evaluation from experimental h/a values.     

苯醌在银-汞(膜)微环电极上的循环扫描伏安波

首次报道采用化学镀法制作银－汞（膜）微环电极,并用于探讨苯醌的循环伏安曲线。还对伏安曲线的性质进行了详细的论述。结果表明：苯醌在Ｍｃｌｌｖａｉｎｅ缓冲液中的还原过程属地单电子可逆反应。根据不同扫描速度时还原波的峰电流值可以计算得到苯醌的扩散系数值。     

Anodic stripping voltammetry at a micro-disc mercury film electrode: Theory of the reversible case

For p=(ar²/D_O)^1/2<0.12 and H=al²/D_R<0.14, simple theoretical equations are proposed to characterize the reversible anodic stripping voltammetric peaks of amalgams obtained with a micro-disc mercury film electrode. The calculated, normalized peaks were higher, thinner and shifted towards more negative potentials compared with the peaks described for the regular size mercury film electrodes. The ADI method was used in calculations.     

Carbon nanotubes as a secondary support of a catalyst layer in a gas diffusion electrode for metal air batteries

In this paper, we report the use of binary carbon supports (carbon nanotubes (CNTs) and active carbon) as a catalyst layer for fabricating gas diffusion electrodes. The electrocatalytic properties for the oxygen reduction reaction (ORR) were evaluated by polarization curves and electrochemical impedance spectroscopy (EIS) in an alkaline electrolyte. The binary-support electrode exhibits better performance than the single-support electrode, and the best performance is obtained when the mass ratio of carbon nanotubes and active carbon is 50:50. The results from the electrode kinetic parameters indicate that the introduction of carbon nanotubes as a secondary support provides high accessible surface area, good electronic conductivity, and fast ORR kinetics. Furthermore, the effect of CNT support on the electrocatalytic properties of Pt nanoparticles for binary-support electrodes was also investigated by different loading-reduction methods. The electrocatalytic activity of the binary-support electrodes is improved dramatically by Pt loading on CNT carbon support, even at very low Pt loading. Additionally, the EIS analysis results indicate that the process of ORR may be controlled by diffusion of oxygen in the electrode thin film for binary-support electrodes with or without Pt catalyst.     

Adsorption and inhibition effects of tribenzylamine in ethanol+water mixtures on a mercury electrode

The adsorption properties of tribenzylamine on mercury were studied using electrocapillary and C-E curves measurements in ethanol+water mixtures in various supporting electrolytes. With the increase of ethanol content a sudden decrease of the differential capacity to a minimum value was observed followed at negative potentials by an abrupt increase of the capacity to the value of the supporting electrolyte. These transition potentials were dependent on the percentage of ethanol and on the concentration of tribenzylamine.The observed behaviour was attributed to a two-dimensional association of the adsorbate in the double layer. The quantitative evaluation of the results fulfilled the theoretical relations for association in the film. Another support for this idea was obtained from the inhibition effect of tribenzylamine on several fast charge transfer reactions at the DME. The potential of the sudden decrease of the limiting diffusion controlled current was very near to the positive transition potential at the C-E curves. The inhibition effect decreased with ethanol content.     

Constant-potential electrolysis: determination of diffusion coefficients of thallium by using a hanging mercury drop electrode

The technique of constant-potential electrolysis at a stationary spherical electrode (hanging mercury drop) was investigated for the determination of the diffusion coefficient of Tl(I) in sodium citrate-sodium hydroxide medium and of Tl in mercury. Current-time curves, at controlled potential, were obtained, covering periods from 1 to 25 sec after the start of the electrolysis. The influence of applied potential, time of electrolysis, convection and shielding of the electrode was studied.     

Extension of the voltammetric theory to inert,permeable thin-film coated rotating disc electrodes: Part II. Behaviour of ferrocene-ferricinium ion and quinone-hydroquinone systems on polymer-coated electrodes

The voltammetric behaviour of ferrocene-ferricinium ion and quinone-hydroquinone systems is investigated on electrochemically thin film polymer coated electrodes. Ferrocene oxidation is studied in 0.1 M NBu₄ ClO₄-nitromethane on rotating disc platinum electrodes coated with poly(2-hydroxymethyl-1,4-phenylene) oxide (I), poly-(2,6-dimethyl-1,4-phenylene) oxide (II), and poly[4-(2-aminoethyl)-1,2-phenylene] oxide (III) films. The quinone-hydroquinone system is investigated in aqueous medium (1 M HClO₄) with (I), (II) and poly-(2-cyano-1,4-phenylene) oxide films. Experimental results are in good agreement with the previously calculated voltammetric curves in steady-state mass transfer conditions. The charge transfer and diffusion parameters on these polymer-coated electrodes are calculated The quinone-hydroquinone system is rendered reversible by coating a platinum electrode with I.     

Effect of the modification of mercuric oxide on the properties of mercury films at HgO-modified carbon paste electrodes

The influence of modifications of the mercuric oxide on the voltammetric properties of mercury film carbon paste electrodes was studied. The mercury film was formed electrochemically from the bulk red or yellow mercuric oxide-modified carbon paste electrodes. Differential pulse anodic stripping voltammetry and optical microscopy in polarised light were used to characterise the properties of the mercury films. The results were compared with those obtained using the conventional preparation of mercury-plated carbon paste electrodes when the mercury film is deposited on the surface of the electrode by reduction of Hg(II) ions in solution utilising a sufficient negative potential. It was shown that the mercury film formed from the yellow modification of the mercuric oxide provides better voltammetric characteristics than the red one owing to the high distribution of its small particles, i.e. the mercury droplets after electrochemical treatment. Such a mercury film has similar properties to those of a mercury film generated from solution. Received: 06 December 1999 / Accepted: 16 March 2000     

Anion induced adsorption of Cd(II) from acetonitrile solutions

Solutions of Cd(II) in acetonitrile show no adsorption on mercury electrodes with sodium perchlorate as supporting electrolyte but strong adsorption of Cd(II) is produced by the addition of thiocyanate anion. The stoichiometry of the adsorbed species was shown to be Cd(NCS)₂ by means of chronocoulometric measurement of the quantities of both Cd(II) and NCS^? on the surface. The surface appears to reach a saturation coverage corresponding to ca. two monolayers of a tightly packed film. Speculations on the forces driving the adsorption are offered and similarities with previous results obtained in aqueous solutions are pointed out.     

Electrochemical Parameters of Phenoxazine Derivatives in Solution and at Monolayer‐Modified Gold Electrodes

Electrochemical properties of β‐(10‐phenoxazinyl) propylamine (APPX) and β‐(10‐phenoxazinyl) propionic acid (PPX) have been studied in solution, and in immobilized state on gold electrodes modified with monolayers of cystamine and mercaptoundecanoic acid. A reversible diffusion‐controlled process of APPX and PPX was observed at a bare gold electrode. The electrochemical conversion of both compounds at modified gold electrodes was a quasi‐reversible diffusion‐controlled process. The redox potential of immobilized APPX (443 mV) was similar to the potential in solution, while the value of the immobilized PPX was 131 mV higher than in solution. The immobilized mediators were electrocatalytically active in the fungal peroxidase‐catalyzed hydrogen peroxide reduction.     

An iridium based mercury film electrode: Part II. Comparison of mercury-film electrode behaviors: theory versus reality

An experimental study of a 2 mm diameter iridium based mercury film rotating disk electrode (IrMFE) is presented. Using anodic stripping voltammetry and test ions of cadmium, lead and zinc, the effects of Hg film thickness on peak potential, half-width and peak height are compared with the predictions given by the De Vries and Van Dalen theory. The response of the Ir-MFE was found to be theoretical for Hg film thicknesses less than 1 μm. Application of this classical theory to real Hg film electrodes, on any type of substrate, is found to be inappropriate when the Hg film thickness begins to exceed 1 μm. It is also shown that the stripping peak characteristics at a semi-spherical film electrode can be estimated by assuming that it is composed of a large number of annular flat films. The individual responses of the flat films can be summed to give the overall semi-spherical response.     

The equation of chronoamperometric current on an oblate hemispheroid microelectrode and its verification

With the aspect of equivalent diffusion layer an analytical chronoamperometric current equation on an oblate hemispheroid microelectrode for reversible electrochemical condition is derived. To verify this equation the chronoammograms have been obtained with benzoquinone in McIlvaine buffer solution (pH 7.0) at a mercury oblate hemispheroid microelectrode. The experimental results agree with the theoretical conclusions.