Mercury Electrode In Aqueous Solutions Research Articles

Theoretical Treatment of a Cathodic Stripping Mechanism of an Insoluble Salt Coupled with a Chemical Reaction in Conditions of Square Wave Voltammetry. Application to 6‐Mercaptopurine‐9‐D‐Riboside in the Presence of Ni(II)

AbstractCathodic stripping mechanism of an insoluble salt coupled with a homogenous chemical reaction is considered both theoretically and experimentally under conditions of square‐wave voltammetry. For the mercury electrode in aqueous solution, the electrode reaction is described as L(aq)+Hg(l)=HgL(s)+2e−, where L(aq) is the reactive ligand that forms a sparingly soluble compound HgL(s). The electrode reaction is coupled with a homogenous, first‐order chemical reaction, A(aq)=L(aq). Theoretical predictions are confirmed by experiments with 6‐mercaptopurine‐9‐D‐riboside in the presence of nickel(II) ions.

Quantitative Voltammetry in Weakly Supported Media. Two Electron Transfer, Chronoamperometry of Electrodeposition and Stripping for Cadmium at Microhemispherical Mercury Electrodes

Double potential step chronoamperometry for the two-electron transfer amalgam-forming deposition and stripping of cadmium at a hemispherical mercury electrode in aqueous solution containing different concentrations of supporting electrolyte is compared with simulated results using the Nernst-Planck-Poisson system of equations. The latter were generated using an approximation of zero electric field at the interface [J. Phys. Chem. C 2008, 112, 13716−13728]; the approximation gives a good agreement between theory and experiment, so validating its use for the description of macroelectrode and microelectrode (but not nanoelectrode) problems involving low or negligible levels of supporting electrolyte.

Adsorption phenomena on a mercury electrode in aqueous cryptand[2.2.2] solutions by the background of tetraalkylammonium salts

The electrochemical behavior of cryptand[2.2.2] (Cry) is studied on a mercury electrode in aqueous solutions of tetraalkylammonium tetrafluoroborates (Me4N+, Et4N+, and Bu4N+). Cryptand [2.2.2] is shown to exhibit high surface activity in Me4 NBF4 nd Et4NBF4 solutions. Based on the model of two parallel capacitors supplemented by the Frumkin adsorption isotherm, the adsorption parameters of Cry by the background of Me4BNF4 were calculated using the regression analysis methods. The calculated dependences of the differential capacitance on the potential adequately agree with experimental curves. The adsorption characteristics of Cry in the studied solutions are compared with those in MgSO4 solutions. By the background of Bu4NBF4, Cry molecules and Bu4N+ cations exhibit very close surface activity and form a mixed adsorption layer.

Refining adsorption parameters of n-butyl alcohol at the interface of mercury electrode with aqueous NaF and Na2SO4 solutions by correcting the adsorbate concentration

As an adjunct to the regression analysis of differential capacitance curves, which allows refining the adsorption parameters, a program, which takes into account possible errors in volume concentrations of organic substances, is developed. Using this procedure, the earlier data on the differential capacitance of a mercury electrode in aqueous solutions of normal butanol (1-BuOH) containing either 0.1 M NaF or 0.05 and 0.5 M Na2SO4 as the supporting electrolyte are analyzed. This allows obtaining the most accurate values of adsorption parameters for the systems mentioned above within the framework of the model of two parallel capacitors and the Frumkin isotherm. It is shown that, when a linear dependence of the intermolecular interaction parameter on the electrode potential is taken into account, the standard deviation of experimental capacitance values from those calculated using the mentioned model is 6.8–8.8%, which points to very high accuracy of this phenomenological model.

New Analysis of Data on the Differential Capacitance of a Mercury Electrode during the Adsorption of tert-Butanol

A regression analysis of data on the differential capacitance of a mercury electrode in aqueous solutions of tert-butanol, which were published previously, is performed. It is shown that, in contradistinction to an analogous analysis of electrocapillary curves (ECC), in order to describe the adsorption of this compound in the framework of a model of two parallel capacitors, it is necessary to take into account the linear dependence of the parameter of an intermolecular interaction on the electrode potential. It is established on the basis of modeling calculations that, in order to obtain the same result in the case of a regression analysis of ECC, the relative accuracy of measurements of the interfacial tension must be two orders of magnitude the relative accuracy of measurements of the differential capacitance.

The Reduction of 2,6‐Dimethoxy‐4‐chloro‐1,3,5‐triazine on Mercury Electrodes in Aqueous Solutions in Relation with the Reduction of s‐Triazine Herbicides

AbstractThis paper presents polarographic and voltammetric studies on the electroreduction of 2,6‐dimethoxy‐4‐chloro‐1,3,5‐triazine on mercury electrodes in aqueous solutions and in the acidity range 2 M H2SO4 to pH 7. Above pH 6.5 no signals were obtained. In both DC and DP polarography and voltammetry, one or two partially overlapped reduction waves were observed, depending on the pH of the medium. The overall process corresponded to a four‐electron irreversible reduction. In strongly acidic media (pH<4.5) the protonated reactant was reduced through a two‐electron process to give a dechlorinated molecule that is subsequentely reduced at the potentials of the second wave. It was concluded that the reduction of the chlorinated s‐triazine rings seems to occur through the reductive cleavage of the chlorine irrespective of the nature of the groups bonded to the ring in positions 2, 6.

Impedance voltammetry of electro-dimerization mechanisms: Application to the reduction of the methyl viologen di-cation at mercury electrodes and aqueous solutions

The faradaic impedance for an electrode mechanism with a reversible homogeneous dimerization reaction following the electron transfer step is derived. The chemical reaction shows up in the frequency dependence of the faradaic impedance and admittance in a similar way as deduced by Sluyters-Rehbach and Sluyters (J. Electroanal. Chem. 23 (1989) 457; J. Electroanal. Chem. 26 (1990) 237) for a homogeneous first-order chemical reaction. Two limiting cases can be distinguished in which the general expression reduces to the simpler Randles or pseudo-Randles expression. Under those conditions, the presence of the dimerization reaction can be inferred from the potential dependence of the impedance parameters. The theory is applied to the reduction of the methyl viologen di-cation at mercury electrodes in aqueous solution. The rate and the equilibrium constants for the dimerization reaction and the standard potential for the electron transfer step are obtained from the Warburg coefficient, while the potential dependence of the irreversibility coefficient allows the calculation of the standard rate constant and the transfer coefficient for the electron transfer step.

Electrochemistry of Covalently Mercurated Uridine Nucleotides. Diffusion and Surface-Confined Pathways for the Two-Electron Reduction of the Carbon−Mercury Bond

The electrochemical reduction of 5-mercuriuridine monophosphate, diphosphate, and triphosphate nucleotides at mercury electrodes in aqueous solution is reported. Both diffusion-controlled and surface-confined electrode reactions were observed. In aqueous KCl electrolytes, the diffusion-controlled pathway involved the two-electron reduction of the carbon−mercury bond to give the parent nucleotide. In D2O solution, the corresponding 5-deutero nucleotide was formed. All three mercurated nucleotides formed compact monolayers on the mercury electrode surface in accord with the Langmuir isotherm. Anomalies in the voltammetric behavior and the adsorption isotherm of the 5-mercuriuridine monophosphate in KCl solution are ascribed to the formation of polymeric anions by coordination of the mercury substituent with the nitrogen of the pyrimidine ring. The reduction mechanism of the surface-confined nucleotides is more complex than the diffusion-controlled reaction. For the surface reaction, two one-electron steps (...

Reorientation of adsorbed 2-mercaptopyridine N-oxide on mercury electrodes in the presence of triton x-100

The effect of the addition of Triton x-100 on the electrochemical oxidation of 2-mercaptopyridine N-oxide (NOS) on mercury electrodes in aqueous solutions is investigated. The shape of dc polarograms changed, (the prewave observed in the absence of Triton is now absent) and the dc polarographic data conform with the oxidation mechanism previously proposed. The differential pulse polarographic data and the cyclic voltammetric data were explained on the basis of a reorientation of the radical NOS adsorbed on the mercury surface, directed by the electrode potential. The Gibbs free enthalpy of the reorientation process was found to be around 3.6 kJ mol −1.

Electrochemical behaviour of isomers of a tetraazamacrocyclic Ni(II) complex in solutions saturated with argon, CO and CO 2

Isomers of [Ni(CRH)] 2+ have been isolated and studied by d.c. and a.c. voltammetry on mercury electrodes in aqueous solution. The configurational isomers of [Ni(CRH)] 2+ were found to differ in their stability in the solution and the isomerization process was monitored voltammetrically. The adsorptivity of the isomer A on mercury electrodes was found to be stronger than that of the isomer B, while no differences were seen for the adsorbed Ni(I) complexes. The solution resident [Ni(CRH)] + complexes are more labile than the corresponding Ni(II) compounds, hence upon reduction the isomer B undergoes fast transformation into the more stable isomer A. The electroreduction of [Ni(CRH)] 2+ in the presence of CO and CO 2 is described and the influence of the isomeric form used in the processes is discussed.

Electroreduction of thionicotinamide on mercury electrodes in aqueous solutions

Electroreduction of thionicotinamide on mercury electrodes in aqueous solutions

Studies of electroreduction of nitropentane and nitrohexane on mercury electrodes in the presence of adsorbed aliphatic alcohols

AbstractElectroreduction of 1‐nitropentane and 1‐nitrohexane was studied on mercury electrodes in aqueous solutions, together with the inhibitors: n‐pentanol, n‐hexanol and n‐heptanol. The electrode behavior of these species was not found to be influenced by inhibitors at concentrations lower than 10 mM and 2 mM for n‐pentanol and n‐hexanol, respectively, because adsorption of nitrocompounds is stronger than that of alcohols. For high alcohol concentrations, when a total surface coverage by inhibitor molecules is almost achieved, the reaction of nitroalkanes is inhibited. Steric factors evaluated using the dependence of rate constant on inhibitor concentration were used in the determination of the orientation of reactant molecule on the electrode. For alcohol concentrations near saturation, the rate constant becomes independent of inhibitor concentration. In this case a reaction of nitroalkanes dissolved in the surface organic phase was postulated.

ED mechanisms: electrodimerization of N′-methyl nicotinamide on mercury electrodes in aqueous solutions

The electrodimerization of the NAD + model compound N′-methyl nicotinamide has been studied in aqueous media in the pH range 4–11 by direct current and differential pulse polarography and linear-sweep cyclic voltammetry. Tafel slopes and reaction orders were obtained from I- E curves traced at potentials corresponding to the foot of the polarographic wave. Controlled-potential electrolysis indicated that at pH 5 and 8 one electron is transferred per reactant molecule and a dimer must be obtained. The reduction pathways consisted of the reversible transfer of one electron, preceded by the transfer of either two (first wave) or one (second wave) H + ions, followed by an irreversible electrodimerization which was the rate-determining step, in agreement with that which occurs in the reduction of other model compounds and the NAD + coenzyme itself at these pH values. The dimerization reaction is a radical-radical coupling taking place in the reaction layer.

Adsorption of nicotinamide and nipecotamide from aqueous solutions at the mercury electrode

Differential capacity measurements are reported for the adsorption of nicotinamide (NcA) and nipecotamide (NpA) at the mercury electrode in aqueous solutions containing KF as a supporting electrolyte. The experimental data were analysed to obtain the surface excess and adsorption parameters of NcA and NpA as a function of the electrode charge density and the bulk concentration of the adsorbate. The results obtained indicate that NcA molecules are adsorbed flat on the mercury surface. The adsorbed layers for both substances contain mixtures of adsorbate and water molecules. The role of the π electrons of the pyridine ring in the adsorption process and the charge dependence of the standard Gibbs energy are discussed.

Voltammetric study of ammonium amalgam

Ammonium amalgam formation has been studied at mercury ultramicroelectrodes in various organic solvents and at standard size mercury electrodes in aqueous solution. Numerical simulations were carried out in order to establish the kinetics of the process. Ammonium amalgam is formed via a fast charge-transfer process, but it undergoes a decomposition according to the first-order rate law with a rate constant of about 0.6 s −1. In addition, some NH 4 + ions are irreversibly discharged at the electrode surface, presumably to form H 2 and NH 3. This competing process is affected strongly by the nature of the solvent. Matching of experimental and simulated curves suggests that the diffusion coefficient of NH 4 in mercury is about 1.8 × 10 −5 cm 2 s −1. The standard potential of NH 4 +(aq)/NH 4(Hg) was determined to be −1.723 V/SHE. The absolute chemical potential of ammonium ions in mercury was calculated and compared with that of alkali metal ions.

ELECTROCHEMICAL PROPERTIES OF ADSORBED ALKANETHIOL MONOLAYERS ON MERCURY ELECTRODE SURFACES

n-Alkanethiols (CH3(CH2)nSH, n=3, 5, 11, 13, 15, 17) have been adsorbed at mercury electrodes prom ethanolic solutions. A. c. voltammetry has been carried out of the adsorbed alkanethiol on mercury electrodes in aqueous solutions containing 1M KCl at 25°C. Cyclic d.c. voltammetry of several metal ions also has been performed at these electrodes. The results show that long-chain alka-nethiols n_??_11 can form densely packed monolayers with fully extended alkyl chains in the potential range between 0 and -0.7 volt vs. Ag/AgCl. Metal ions such as Pb2+ are unable to penetrate into the monolayer. In contrast, short-chain alkanethiols with n=3 and 5 form less ordered and loosely packed adsorption layers, which are permeable to the metal ions. A pH dependent a.c. voltammetric peak is observed at a mercury electrode with adsorbed alkanethiol(n=5) in buffer solutions, which is attributable to the redox reaction of the adsorbed thiol: [CH3(CH2)5S]adHg+H++e-_??_[CH3(CH2)SH] adHg.

Heterogeneous electron-transfer rate measurements of cytochrome c3 at mercury electrodes

The electron-transfer rate constant of cytochrome c 3 at mercury electrodes in aqueous solution was measured by the galvanostatic double pulse method at 25°C. The resulting rate constant and transfer coefficient were 1.2 cm s −1 and 0.48, respectively. The measured rate constant agreed well with that estimated using Marcus theory and the homogeneous self-exchange rate measured by NMR.

Anodic behaviour of the mercury electrode in aqueous solution of 1,10-phenanthroline: The role of adsorption and nucleation processes

The anodic dissolution of the mercury electrode in 1,10-phenanthroline (phen) aqueous solution has been investigated by polarographic and voltammetric methods. The adsorption processes affecting the entire mechanism of oxidation and the complex formation have been observed and analysed introducing the random adsorption model (Frumkin isotherm) and the nucleation-growth collision model. The results suggest the occurrence of different surface structures of phenanthroline itself (the ligand) as well as of the product of the oxidative process, [Hg(phen) 2] 2+, upon varying the ligand concentration in the bulk.

A study on the reduction mechanism of VO 2+- diethylenetriaminepentaacetate at a mercury electrode in aqueous solution

Le mecanisme de la reduction du diethylenetriamine pentaacetate de vanadyle est examinee par voltammetrie cyclique a differentes vitesse de balayage et de pH

Electroanalytical Study of the Reduction of Sb(III) in Tartrate Medium

Abstract A systematic study on the reduction of Sb(III) on a mercury electrode in aqueous solution of tartrate ion has been carried out. The nature of the electrode reactions has shown to be a function of the acidity of the solution. Results from polarographic, coulometric and voltammetric experiments allow us to outline a model of electrode reaction according to which different Sb-tartrate complex ions can be reduced. Sensitivity and detection limits for the polarographic determination of Sb(III) in tartrate medium are given using several polarographic waves obtained at different pH. In this way, the selection of the basic medium is vindicated.