Electrooxidation of dihydroxybenzenes at a mechanically renewed nickel electrode

The dependence of the potentials and peak currents of the electrooxidation of isomeric dihydroxybenzenes on the polarization mode of a mechanically renewed nickel electrode is studied by direct-current cyclic voltammetry. The results indicate that the oxidation peaks of hydroquinone, pyrocatechol, and resorcinol appear in alkaline (0.05–0.10 M KOH), neutral (0.02–0.10 M Na₂SO₄) and acidic (0.02–0.05 M H₂SO₄) supporting electrolytes. The peak shape and parameters depend on the composition of the supporting electrolyte, which creates the conditions for the formation of different nickel oxides on the electrode surface then involved in the electrooxidation of dihydroxybenzenes. The regeneration of the electrode surface also affects the peak parameters, especially for resorcinol, whose signals completely disappear without the electrode renewal. The analytical signals for three isomeric dihydroxybenzenes are peaks in an alkaline solution, and also hydroquinone and pyrocatechol peaks in neutral and acidic solutions.     

Voltammetric determination of sorbite at a mechanically renewed copper electrode

Potentials and currents of D-sorbitol oxidation peaks as a function of polarization conditions for a copper electrode in situ renewed by mechanically cutting a 0.5-μm surface layer are studied by direct-current cyclic voltammetry. Oxidation peaks of sorbite emerge in cyclic voltammograms recorded in alkaline supporting electrolytes (0.05–0.10 M KOH and NaOH solutions) upon scanning the potential to the anodic region (E _p = 0.50–0.58 V) and in the reverse direction (E _p = 0.60–0.62 V). The shape and parameters of these peaks depend on the concentration of KOH, because of the different copper oxides involved in the oxidation of sorbite formed at the electrode surface. The regeneration of the electrode surface is the necessary condition for good reproducibility of the peak parameters. The signals obtained on the surface of the unrenewed electrodes are almost halved and less reproducible. The calibration graph of the current of the sorbite oxidation peak as a function of its concentration is linear in the range from 5 × 10⁻⁴ to 1 × 10⁻² M.     

Voltammetric determination of pyrocatechol at a mechanically renewed electrode made of a graphite-epoxy composite

The reversibility of a two-electron pyrocatechol/o-benzoquinone system (ΔE _p = 28 ± 1 mV) is found by cyclic voltammetry at a mechanically renewed electrode from a graphite-epoxy composite. The closeness of the found and theoretical values of ΔE _p indicates the high electrochemical activity of the graphite-epoxy composite electrode renewed in situ by cutting a thin surface layer. The potentials of pyrocatechol anodic and cathodic peaks are proportional to E ₀ ± 0.059pH in the pH range of 1–9. The pyrocatechol anodic and cathodic peak currents remain linear functions of pyrocatechol concentration in the concentration range from 0.01 to 0.08 and from 0.08 to 0.9 g/L, respectively. A procedure for the voltammetric determination of pyrocatechol in its individual solutions and in the presence of hydroquinone is proposed. Good precision and the absence of a systematic error in the determination of pyrocatechol by measuring its cathodic peak are demonstrated.     

Voltammetric determination of thallium(I) at a mechanically renewed Bi-graphite electrode

A composite of high-purity grade carbon powder and an epoxy resin is used to in situ obtain a bismuth-film electrode. The surface of the indicator electrode is renewed by cutting a thin surface layer before each determination. The in situ obtained Bi-film electrode is virtually no different from an Hg-film electrode in sensitivity, reproducibility, and the ability to separate signals from Tl, Cd, and Pb. The calibration graph is linear in the range of thallium concentrations from 0.01 to 1 mg/L (RSD varies from 4 to 2%). A significant excess of Pb(II), Sn(II), and Cd(II) does not interfere with the determination of thallium. A procedure for determining down to 5 × 10^?6% thallium in lithium carbonate is developed.     

Voltammetric determination of dihydroxybenzenes at a mechanically renewed electrode made from a graphite-epoxy composite

Differences of potentials of anodic and cathodic peaks (ΔE _p) are determined in cyclic voltammograms of dihydroxybenzene/p-benzoquinone redox systems at an electrode made of a graphite-epoxy composite in a wide pH range. The data obtained (ΔE _p = 29 ± 1 mV) are close to the thermodynamic values for two-electron reversible reactions. This indicates that the electrode mechanically renewed by cutting a 0.5-μm surface layer directly in a test solution exhibits a high activity in such electrochemical reactions. The potentials of anodic and cathodic peaks are proportional to the pH of the supporting electrolyte solution in the range from 1.0 to 9.0. A change of 58 ± 1 mV in E _p per unit pH for all isomers shows that the first stage of the oxidation of each dihydroxybenzene isomer involves one electron and is accompanied by the detachment of one hydrogen ion, that is, an intermediate oxidation product, semiquinone, is formed. Despite the closeness of the potentials of hydroquinone and pyrocatechol peaks (ΔE = 100 mV), a scheme is proposed for the selective voltammetric determination of dihydroxybenzene isomers in a 0.1 M HCl solution in hydroquinone-pyrocatechol, pyrocatechol-resorcinol, and hydroquinone-resorcinol binary mixtures. The concentrations of hydroquinone and pyrocatechol are found from cathodic peaks and that of resorcinol, from the anodic peak. The results are well reproducible and contain no systematic error.     

Voltammetric determination of hydroquinone on a mechanically renewed graphite-epoxy electrode in developing photographic solutions

Effect of various components of developing photographic solutions on parameters of electro-oxidation-reduction peaks of hydroquinone in neutral and acid supporting solutions on a graphite-epoxy composite electrode with surface mechanically renewed directly in solution prior to each polarization cycle was studied. A procedure is suggested for voltammetric determination of the content of hydroquinone from the cathodic peak in a freshly prepared developing photographic solution and from the anodic peak in a solution allowed to stay upon preparation for 1 day and more.     

Reduction of metol on mechanically renewed metal electrodes

The electroreduction of metol on mechanically renewed metallic electrodes is studied by direct voltammetry with linear potential sweep. Reduction peaks of metol are found in a neutral supporting electrolyte (0.02–0.5 M Na₂SO₄) on nickel, silver, and mercury electrodes before the potential of hydrogen liberation from the supporting electrolyte. The shape and parameters of the cathodic peak depend on an electrode material, and also on the composition and pH of the supporting solution. A probable mechanism of the electroreduction of metol is proposed. The regeneration of nickel and silver electrodes by in situ mechanical cutting of a 0.5-μm surface layer provides good reproducibility of the value of peak current; it is proportional to the concentration of metol in the range 2 × 10⁻³–1.8 × 10⁻² M.     

Electrochemical behavior of hydrazine at mechanically renewed solid electrodes

The electrooxidation of hydrazine was studied at metallic wire electrodes made of Co, Ni, Ag, Cu, and a graphite-epoxy composite by anodic and cyclic voltammetry with a linear potential sweep in alkaline-supporting electrolytes. Electrode working surfaces were regenerated by mechanically cutting a thin 0.5-μm layer in situ before each polarization cycle. The effects of the electrode material and the renewal of its surface on the parameters of anodic voltammograms of hydrazine were demonstrated. Hydrazine anodic peaks obtained at clean surfaces of electrodes fabricated from Ni, Ag, and a graphite-epoxy composite and also peaks at the potentials of the oxide formation on Ni and Cu electrodes can be used as analytical signals for the voltammetric determination of hydrazine.     

Behaviour of the platinum electrode as a pH electrode

The platinum wire electrode behaves as a good pH electrode in the titration of 1.0 and 0.1N sulphuric or hydrochloric acid with the equivalent concentration of sodium hydroxide. The efficiency is greater when sulphuric acid is used. The end-points obtained are very close to those obtained with a glass-calomel system.     

阳离子选择性膜修饰Ir/IrOx-pH电极增敏作用研究

采用阳离子选择性透过膜对Ir/IrOx-pH电极表面进行修饰,提高了电极抗阴离子氧化/还原性物质的干扰能力.利用SEM观察离子选择性透过膜的表面孔径分布;通过循环伏安法研究了氧化物电极在Fe(CN)63- 溶液中的电化学行为.实验表明,选择性透过膜对电极的E-pH线性、准确度、响应时间等无影响,显著改善了电极在含氧化/还原性物质中的使用性能.从而提高了电极的抗干扰能力和灵敏度.     

Photophysics of the fluorescent pH indicator BCECF

The photophysical behavior of BCECF [2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein]--currently the most widely used fluorescent pH indicator for near-neutral intracellular pH measurements--has been explored by using absorption and steady-state and time-resolved fluorescence measurements. The influence of ionic strength as well as total buffer concentration on the absorbance and steady-state fluorescence has been investigated. The apparent acidity constant of the pH indicator determined by absorbance and fluorescence titration is dependent on the added buffer and salt concentrations. A semiempirical model is proposed to rationalize the variations in the apparent pKa values. The excited-state proton exchange of BCECF at physiological pH becomes reversible upon addition of phosphate buffer, inducing a pH-dependent change of the fluorescence decay times. Fluorescence decay traces collected as a function of total buffer concentration and pH were analyzed by global compartmental analysis yielding the following values of the rate constants describing excited-state dynamics of BCECF: k01 = 3.4 x 10(8) s(-1), k02 = 2.6 x 10(8) s(-1), k21 approximately 1 x 10(6) M(-1) s(-1), k12(B) = 1.4 x 10(8) M(-1) s(-1), and k21(B) = 4.3 x 10(7) M(-1) s(-1).     

Salient features of the electrical double layer structure on mechanically renewed gold-silver electrodes in aqueous solutions of a surface-inactive electrolyte

The behavior of electrodes, which are made of binary Au-Ag alloys (Ag content 1–15 at %) and renewed by mechanical cutting in aqueous solutions of sodium fluoride, is studied with the aid of cyclic voltammetry and impedance methods. It is established that, in the region of potentials corresponding to ideal polarizability, the differential capacitance of the electrical double layer rapidly changes with time elapsed after the renewal of the surface of the electrodes. The change in the capacitance is brought about by the exit of silver atoms into a surface layer. The implication is that silver is the surface-active component in these alloys. The rate of the surface segregation of silver atoms depends on the electrode potential. The segregation rate substantially increases upon going into the region that corresponds to positive charges of the silver electrode surface and to the beginning of adsorption of atomic oxygen on the electrode. Based on phenomenological models, a method for processing capacitance curves is realized, which links experimentally observed time effects to variations that occur in the surface composition, and assumptions concerning the mechanism of relaxation processes that are responsible for the observed time effects are put forth. Explicit data on the effect, which is exerted by mechanical renewal on the composition of the surface layer of Au-Ag alloys at different distances from the interface with a vacuum, are obtained with the aid of an x-ray photoelectron spectroscopy method. It is established that the surface layer (～0.5 nm) is enriched by silver atoms as compared with the alloy’s bulk.     

pH measurements based on a palladium electrode

The feasibility of a metal palladium electrode as the pH sensor in air-saturated aqueous solutions was evaluated. The measurement was based on an “on-site” precleaning step and a potential decay process. The experimental results showed a linear pH response with a sensitivity of around 60 mV/pH and good reproducibiliry. The temperature effect on the measurement was also evaluated.     

Peculiarities of the adsorption behavior of adamantanol-1 on a mechanically renewable silver electrode

Using the method of electrochemical measurements on electrodes with mechanically renewable surface, the adsorption behavior of adamantanol-1 (AdOH) on the Ag electrode interface with solutions of a surface-inactive electrolyte (NaF) is studied. Based on the results of impedance and voltammetric measurements, it is shown that the kinetics of AdOH adsorption in the ideal polarizability potential range (near the zero charge potential) is described within the framework of the mechanism of the quasichemical reaction Ag(H₂O)_ads + AdOH = Ag(AdOH)_ads + H₂O on metal surfaces with energy uniformity. A phenomenological model is proposed that makes it possible to consistently describe the temporal effects on renewable Ag electrodes in the potential range of its initial oxidation in solutions containing AdOH.     

Potential transients on a renewed electrode as a source of information on the surface concentration of catalytically active adatoms

Potential transients on renewable gold and silver electrodes in cyanide solutions containing catalytically active ions of lead and thallium are measured. The transients are recorded both on freshly renewed electrodes and on renewed electrodes kept in solution for specified time periods t. The behavior of transients following a change in the concentration of lead and thallium ions, current density, and t qualitatively conforms to regularities that could be expected if the catalytically active adatoms were inserted into the deposit similarly to the admixtures. Methods for determining coefficients of capture of such adatoms by the growing metal deposit are suggested. Factors that hamper determination of these coefficients and make it worthwhile to select a method for their determination that would use potential transients are considered. The capture coefficient for the incorporation of lead adatoms in the growing silver deposit is nearly 20 times that for gold.__________Translated from Elektrokhimiya, Vol. 41, No. 4, 2005, pp. 460–467.Original Russian Text Copyright © 2005 by Bek, Shuraeva.     

Effect of pH on the response of a cyanide ion-selective electrode

The influence of pH on the cyanide response of a silver iodide membrane electrode has been found to depend on the buffer system employed. A theoretical explanation of the results is attempted by assuming a pH-gradient in the vicinity of the electrode surface.     

The mechanism of the platinum indicator electrode in argentometry

A study has been made of the behaviour of platinum and some other inert electrodes in silver nitrate titrations. Where the metal surface has been subjected to a reducing treatment, such as cathodic polarization, before use, the electrode will often function as a silver indicator electrode throughout the titration. It has been shown experimentally that this is due to the formation of a layer of metallic silver upon the electrode by interaction of the surface with silver ions in solution. If the metal surface undergoes such pretreatment that it is oxidized, then its potential normally remains at the oxide value during a silver nitrate titration until the silver ion concentration is sufficiently great for this value to be exceeded by the silver silver ion potential; formation of metallic silver then again takes place and from this point onwards the electrode behaves as a silver electrode. A detailed interpretation of the behaviour of platinum after various pretreatments has been made along these lines.     

Potentiometric detection in ion chromatography using a metallic copper indicator electrode

Summary A metallic copper electrode housed in a suitable flowcell is shown to be a sensitive and versatile potentiometric detector for ion chromatography. This electrode may be used for direct or indirect detection of many inorganic anions and cations and also for organic acids. In the direct detection mode, electrode response is based on either complexation of copper ions at the electrode surface by eluted species, or on oxidation and reduction reactions for eluted species which are strong oxidants or reductants. Direct detection is therefore applicable to such species as amino acids, organic acids, chloride, bromide, iodide, chlorate, bromate and iodate. Indirect detection is possible for anions which do not complex copper ions, provided a copper complexing ligand (such as phthalate) is used in the eluent; cations which complex this ligand are also detectable. Indirect detection may be used for species such as nitrite, nitrate, acetate, formate, succinate, benzoate, alkaline earth ions and transition metal ions. Electrode calibration relationships are discussed and sample separations are presented, together with some typical detection limits attainable in the direct and indirect detection modes.Presented in part at a National Symposium on Ion Chromatography, held at the University of New South Wales, Kensington, Australia, Nov. 21, 1984.