Electrooxidation of the hypophosphite ion on a palladium electrode

Anodic oxidation of sodium hypophosphite on smooth Pd and Pd/Pt electrodes and a Pd membrane is studied. Thei vs.E curves for the Pd electrode exhibit two anodic current peaks. One is caused by oxidation of H₂PO ₂ ^- , and the other, by simultaneous ionization of Pd and oxidation of H₂PO ₂ ^- . The hypophosphite ion adsorbed on the Pd surface hinders the formation of the passive film. This brings about a rapid dissolution of Pd in the oxygen region and its subsequent deposition with the formation of palladium black. The oxidation probably includes a slow heterogeneous chemical reaction, specifically, a cleavage of the P-H bond of the hypophosphite ion. The change in the reaction stoichiometry following an increase in solution pH and in anodic polarization is probably due to changing conditions of the H₂PO ₂ ^- adsorption and the number of adsorption sites occupied by H₂PO ₂ ^- on the surface. Following an increase in polarization, the phosphite ion may undergo oxidation to phosphate. Deceased.     

Voltammetric determination of linuron at a carbon-paste electrode modified with sepiolite

The determination of linuron by differential pulse voltammetry with a carbon-paste electrode modified with 20% w/w sepiolite has been studied. The linuron is preconcentrated under open-circuit conditions at pH 2.0. With 0.01M potassium nitrate at pH 1.7 in the measurement cell, a sweep rate of 30 mV/sec and a pulse amplitude of 100 mV, an oxidation wave with a peak potential of 1.2 V is obtained. Under these conditions, determination limits of 75 ng/ml have been obtained, with a relative error of +2.8% and a relative standard deviation of 8.0%. The method has been applied to the direct determination of linuron in river water with no previous separation of the pesticide. Determination in sea-water is not possible, as chloride interferes at high concentration.     

Electrooxidation of insulin at silicon carbide nanoparticles modified glassy carbon electrode

For the first time silicon carbide nanoparticles (SiC) was used for electrode modification and electrocatalytic oxidation of insulin. In comparison to bare glassy carbon (GC) electrode, the oxidation of insulin at GC electrode modified with SiC nanoparticles occurred at reduced overpotentials. The modified electrode was applied for insulin detection using cyclic voltammetry, differential pulse voltammetry (DPV) and flow injection analysis (FIA). Flow injection amperometric determination of insulin at this modified electrode yielded a calibration curve with the following characteristics; linear dynamic range up to 600 pM, sensitivity of 710 pA pM^?1 cm^?2 and detection limit of 3.3 pM. In addition interference effect of the electroactive existing species (uric acid, glucose, lactic acid, l-cysteine and cholesterol) was diminished and for ascorbic acid eliminated by covering the surface of modified electrode with nafion film. This electrode shows many advantages as an insulin sensor such as simple preparation method without using any specific electron transfer mediator or specific reagent, high sensitivity, excellent catalytic activity, short response time, long term stability and remarkable antifouling property toward insulin and its oxidation product. Sensitivity, detection limit and antifouling properties of this insulin sensor are better than all of the reports in the literature for insulin detection at physiological pH solutions.     

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.     

Cyclic voltammetry of Tifluadom at a C18-modified carbon-paste electrode

Tifluadom, N-[5-(2-fluorophenyl)-2,3-dihydro-methyl-1H-1,4-benzodiazepine]-2-4-methyl-3-thiophene carboxamide, was determined by using a carbon-paste electrode modified with C₁₈ μBondapak. Adsorption on the electrode served as a preconcentration step which improved the limit of detection. Preconcentration for 5 min (open circuit) gave a linear range of 2.2×10^?7 M?4.5×10^?6 M with a detection limit of 1.3×10^?7 M (%C₁₈=25, w/w) for Tifluadom in Britton-Robinson buffer pH 6. The determination of Tifluadom added to urine required no preliminary treatment; the detection limit was 1.3×10^?6 M.     

Sensing of picric acid with a glassy carbon electrode modified with CuS nanoparticles deposited on nitrogen-doped reduced graphene oxide

We describe the preparation of a nanohybrid consisting of nitrogen doped reduced graphene oxide and CuS nanoparticles (N-rGO/CuS) by in-situ microwave irradiation at weight ratios of 25/75, 50/50, and 75/25. The resulting nanohybrids were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, FTIR, spectroscopy, scanning electron and transmission electron microscopy, electrochemically by cyclic voltammetry and electrochemical impedance analysis. It is shown that the CuS nanoparticles are evenly decorated onto the N-rGO surface. The nanohybrids was placed on glassy carbon electrode (GCE) where they showed electro-reductive activity towards picric acid, typically at working voltages between ?0.2 and ?0.8 V (vs. SCE). Effects of pH value and scan rate were evaluated, and it is shown that two electrons are involved in electro-reduction. The detection limits of the GCE modified with various N-rGO/CuS hybrids (with 25/75, 50/50, and 75/25 wt%) are 6.2, 3.2, and 0.069 μM respectively. The method demonstrates its applicability in sensing of picric acid with good reproducibility.

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Graphical abstract Nitrogen doped reduced graphene oxide nanohybrids was synthesized for the detection of picric acid. A straightforward and preconcentration free analysis of picric acid was successfully demonstrated at nanomolar levels using the nanohybrids.

     

Electrocatalytic oxidation of captopril using a carbon-paste electrode modified with copper-cobalt hexacyanoferrate

A carbon-paste electrode was modified with copper-cobalt hexacyanoferrate by consecutive potential cycling. The kinetic parameters were calculated for the electroactive species. The resulting electrode exhibited electrocatalytic activity towards the oxidation of captopril. The kinetics of the electrocatalytic reaction was studied. A linear relationship was observed between anodic current and the concentration of captopril in the range of 5.0 × 10^?6–3.1 × 10^?5 μM with a detection limit of 4.2 μM (S/N = 3). The modified electrode was used in the analysis of captopril tablets successfully.     

Preconcentration and voltammetric measurement of mercury with a crown-ether modified carbon-paste electrode

Carbon-paste electrodes modified with crown-ethers were constructed by mixing the crown-ethers into a graphite powder/Nujol oil matrix. The electrodes so formed were able to bind mercuric ions chemically, and gave greater voltammetric response to mercury than that of ordinary carbon-paste electrodes. The response was characterized with respect to paste composition, crown-ether, preconcentration period, mercury concentration, reproducibility, possible interferences, and other variables. Best results were obtained with 18-crown-6 and an acetate buffer (pH 4.0). The electrode gave good linearity for 1 x 10(-5)-6 x 10(-6)M mercury, a detection limit of 2 x 10(-6)M, and a relative standard deviation of 11%. The investigation may lead to a new class of modified (complexing) electrodes, with different patterns of reactivity.     

Reduction of palladium(II) monoglycinate complexes at rotating disc palladium electrode in acid media

Reduction of palladium(II) glycinate complexes in strongly acid 0.5 M NaClO₄ solutions (pH 0.6 and 1.0) with variable palladium(II) complex and free glycine concentration was studied by the taking of cyclic voltammograms at palladium rotating disc electrode. It is shown that it was a chelate monoglycinate palladium(II) complex that was present in all studied solutions and underwent the reduction. The diffusion coefficient of the chelate monoglycinate palladium(II) complex D = (6.5 ± 0.5) × 10⁻⁶ cm²/s was determined from the limiting diffusion current of the complex reduction. The monoglycinate palladium(II) complex reduction occurred in the double-layer segment of the palladium charging curve; it was not complicated by hydrogen adsorption at electrodes. The palladium(II) complex reduction half-wave potential was determined (E _1/2 = ∼0.300 to 0.330 V (SCE)). It is shown that the decreasing of the number of ligands coordinated by palladium via nitrogen atom facilitates the complex reduction process. In particular, the reduction potentials of palladium(II) complexes with different ligand number at palladium electrode shifted markedly toward negative potentials in the series: Pdgly⁺ < Pd(gly)2 < Pd(gly)₄²⁻.     

Preconcentration at a carbon-paste electrode and determination by adsorptive-stripping voltammetry of rutin and other flavonoids

The non-Faradaic preconcentration behaviour of nine flavonoids (six flavones: fisetin, galangin, morin, quercetin, rhamnetin, rutin, and three flavanones: hesperidin, hesperitin, naringin) at a carbon-paste (nujol/graphite) electrode and the factors affecting it (pH, accumulation potential, presence of various surfactants) for their subsequent differential pulse voltammetric determination are examined. All flavones tested are readily accumulated on the carbon paste electrode resulting in a considerable signal enhancement making determinations feasible down to 10⁻⁸ − 10⁻⁷ M after preconcentration for 1–4 min. Flavanones are not preconcentrated so their lower determination limits are of the order of 10^-6 M. A simple voltammetric procedure for the determination of rutin in a multivitamin preparation is presented.     

Nanomolar amperometric sensing of hydrogen peroxide using a graphite pencil electrode modified with palladium nanoparticles

We report on a simple and rapid method for the preparation of a disposable palladium nanoparticle-modified graphite pencil electrode (PdNP-GPE) for sensing hydrogen peroxide (H₂O₂). The bare and PdNP-modified GPEs were characterized by cyclic voltammetry and SEM. The two electrodes displayed distinct electrocatalytic activities in response to the electrochemical reduction of H₂O₂. The amperometric detection limits were 45 nM and 0.58 mM, respectively, for the PdNP-GPE and bare-GPE, at an S/N of 3. The electrodes can be prepared simply and at low cost, and represent a promising tool for sensing H₂O₂.

Determination of theophylline in plasma by oxidation at the stationary carbon-paste electrode

A method for the determination of the concentration of theophylline in plasma at therapeutic levels is described. The method is based on the oxidation of theophylline at the stationary carbon-paste electrode and involves a simple extraction procedure and differential pulse voltammetry. Possible interferences are discussed and typical calibration curves are presented.     

Voltammetric determination of N-alkylated anilines by adsorption/extraction at a carbon-paste electrode

The voltammetric behaviour of aniline and some of its N-alkylated derivatives at a carbon-paste (Nujol/graphite) electrode is examined. The N-alkylated anilines, in contrast to aniline, are accumulated at the electrode by a combined adsorption/extraction process. The organic bases are determined in the rane 10^?7?2×10^?6 M after a 2-min preconcentration period by differential pulse voltammetry; N,N-dimethylaniline can be determined in the presence of excess of aniline (10^?5 M) by using medium-exchange. The voltammetric method is used for evaluation of the distribution ratios of the organic bases in the Nujol/aqueous buffer system. The enhancement of the voltammetric signal of each base is correlated with the respective distribution ratio.     

Electrooxidation of phenol at exfoliated graphite electrode in alkaline solution

The present paper reports on exfoliated graphite (EG) used for the cyclic electrochemical process of phenol oxidation in alkaline solution. It is shown that the electrochemical activity of anode-produced EG decreases considerably in the second cycle due to the deposition of an oligomer film, composed of the products of phenol oxidation, on the EG surface. Thermal treatment of the inactive graphite anode in air at 500 °C provided a regenerated material of activity three times higher for the first cycle and 2.6 times higher for three cycles as compared to the original anode. The reason for such a behavior is assigned to a carbon film formed on the EG surface during the carbonization/oxidation processes involving the products of phenol oxidation. Comparative studies showed that electroactivity of the original EG can also be enhanced if before the process of phenol oxidation the original EG is activated by heat treatment. Unfortunately, the electrochemical activity of the product of such a treatment is higher only for the first cycle of phenol oxidation and drops dramatically in the following cycles.Contribution to the 3rd Baltic Conference on Electrochemistry, Gdask-Sobieszewo, 23–26 April 2003. Dedicated to the memory of Harry B. Mark, Jr. (28 February 1934–3 March 2003)     

苯酚在Ti/PbO2电极上的电氧化反应

利用线性伏安法和恒电流电解法研究了苯酚在Ti／PbO2电极上的电催化氧化反应,通过电氧化过程中反应物和中间物的高效液相色谱测定,研究了苯酚在Ti／PbO2电极上电催化氧化的反应机理．实验发现,Ti／PbO2电极对水溶液中苯酚的电氧化具有显著的催化作用,25℃时,对初始浓度为2mmol／L的苯酚水溶液,恒定电流密度为50mA／cm2,电解3h,苯酚转化率为98．02％,有机碳去除率为54．36％．苯酚在Ti／PbO2电极上电催化氧化要经过对苯二酚或邻苯二酚、对苯醌、反丁烯二酸和草酸,最终变成二氧化碳和水的历程．第一步反应苯酚转化的主产物是对苯二酚．对苯二酚转化为苯醌和苯醌转化为反丁烯二酸的反应为慢反应,反丁烯二酸转化为草酸的反应为快反应．     

Stripping voltammetry of silver(I) with a carbon-paste electrode modified with thiacrown compounds

The accumulation behaviour and stripping voltammetry of silver(I) was investigated with a carbon-paste electrode modified with a thiacrown compound. Silver could be accumulated at the electrode in the absence of an applied potential by immersing the electrode in a solution of sodium perchlorate containing silver(I), then reduced at constant potential in 0.1M acetate buffer solution. Finally a well-defined stripping peak could be obtained by scanning the potential in a positive direction. The calibration curve for silver was linear over the range 0.5-2.5 muM with accumulation for 5 min. Studies of the effect of other metal ions showed that the silver was selectively accumulated at the electrode.     

Nanotopography and electrochemical impedance spectroscopy of palladium deposited on different electrode materials

The objective of this work was to describe the characteristics of chemically and electrochemically deposited Pd surface layers on HOPG and polycrystalline gold electrode, using in situ ECSTM and EIS measurements, and SEM-EDX element analysis. Pd surface layers were deposited, in successive voltammetric cycles, and anodically dissolved in 0.01 M HCl+0.01 M (NH₄)₂PdCl₄ aqueous electrolyte. Both of the electrode materials used in the study were treated as standard testing electrodes: (i) HOPG for STM/ECSTM measurements, and (ii) polycrystalline Au as the well known working electrode in various electro-analytical applications. The elements surface analysis and nano-surface pictures were used to interpret the EIS diagrams and electrical equivalent circuits. Pd chemical and electrochemical deposition on the HOPG surface was compared with the same process on the polycrystalline gold electrode, on which palladium can be electrodeposited only by means of electrochemical cathodic deposition. Surface topographies of the electrodeposited palladium layers on HOPG and Au were completely different. The equivalent electrical circuits were fitted and the surface roughness of the investigated electrodes calculated. Relations between the surface topography, EIS and SEM-EDX, and interface model of the electrolyte solution electrodeposited Pd layer matrix electrode were proposed.     

Electrocatalytic oxidation and flow-injection determination of ascorbic acid at a graphite electrode modified with a polyaniline film containing electrodeposited palladium

A method for forming a composite film on the surface of a graphite electrode is proposed. Conditions for detecting the maximum catalytic current under batch and flow conditions are determined. A procedure for the electrocatalytic determination of ascorbic acid at the graphite electrode modified with a polyaniline film containing palladium particles is proposed. The catalytic effect of this electrode manifests itself by a ～300-mV decrease in the peak potential of ascorbic acid oxidation and by a multiple increase in the peak current of ascorbic acid oxidation as compared to the unmodified electrode. The linear dependence of the electrocatalytic response of the composite electrode on the concentration of ascorbic acid is observed down to 1 × 10^?8 M and 2.5 nmol of ascorbic acid under batch and flow-injection analysis conditions, respectively.