The Potentiodynamic Electrooxidation of Glycerol on Platinized Platinum Electrodes

Abstract

The electrooxidation of adsorbed glycerol species on platinized platinum electrodes in 1 N H₂SO₄ was studied by cyclic voltammetry and the potential step method. The main conclusions are: the electrooxidation of glycerol at potentials lower than 1.2 V (vs, RHE) is a complex reaction in which more than one adsorbed intermediates are probably involved. The adsorbed intermediates exhibit ageing effects, at higher potentials a chemical interaction between glycerol and adsorbed oxygen occurs and the overall process is controlled by mass transfer.     

Iodide-Selective PVC Membrane Electrodes Based on Five Transitional Metal Chelates of bis-furfural-semi-o-tolidine

Abstract

Some new PVC membrane electrodes based on Co(II), Mn(II), Ni(II), Cu(II), and Zn(II)chelates of bis-furfural-semi-o-tolidine as carriers are described. The electrodes exhibited different selectivity behaviour compared with the electrode using a classical anion exchanger such as tetraalkyl ammonium. The results showed that the Cobalt(II) chelate-based electrodes had a Nernstian response to iodide ion ranging from 1.0 × 10^?1 to 1.0 × 10^?6 mol.L^?1 in a phosphate buffer solution of pH 2.0 with a detection limit of 6.4 × 10^?7 mol.L^?1 and a slope of 57.8 mV/dec at 25deg;C. The response mechanism was also investigated by use of both a.c. impedance and SPQC techniques. The Co(II) chelate-based electrodes were used to determine the iodide content of a drug with satisfactory results.     

Enzyme Electrodes Using Bioelectrocatalytic Reduction of Hydrogen Peroxide

Abstract

Amperometric electrodes have been constructed using the direct electron transfer between electrode and peroxidase for mediatorless hydrogen peroxide detection at a potential of ?0.010 V. For this purpose peroxidase was either adsorbed on pyrographite or immobilized in electrochemically synthesized polypyrrole layers on pyrographite or platinum electrodes.     

Tetracycline Sensitive Membrane Electrodes Based on Poly (Vinyl Chloride) Matrices and their use in Drug Analysis

ABSTRACT

Tetracycline hydrochloride-selective electrodes of both the coated wire and the conventional polymer membrane types have been prepared. They are based on incorporating the tetracycline-phosphotungstate ion-associate in plasticized poly(vinyl chloride) film. A Nernstian response is shown by these electrodes within certain concentration ranges depending on the type of electrode. The effect of pH of the test solution and time of soaking on the electrodes' performance are studied. The electrodes are highly selective for tetracycline with respect to several inorganic cations, sugars and some amino-acids of significant importance in biological fluids and pharmaceutical preparations. The standard cell potentials, E°, were determined at different temperatures and used to calculate the isothermal temperature coefficient of the cell. Tetracycline is determined successfully in pure solutions and in pharmaceutical preparation using the standard additions method and potentiometric titration.     

Adsorption and determination of bismuth with 4-(2-pyridylazo)resorcinol on a fibrous ion exchanger

The adsorption of bismuth was studied on a fibrous material filled with a Chelex 100 ion exchanger with iminodiacetic groups and a KU-2 cation exchanger, and as thiosulfate and iodide complexes on a fibrous AV-17 (PANV-AV-17) anion exchanger. Bismuth adsorbed on the solid phase was determined with 4-(2-pyridylazo)resorcinol (PAR). The optimal conditions for adsorption and determination were selected and selectivity was studied. The system bismuth-iodide-PANV-AV-17-PAR was proposed for the determination of bismuth in the solid phase. A procedure was developed for the adsorption-spectroscopic determination of 0.04–0.4 mg/L of bismuth in fresh water and urine with a detection limit of 0.03 mg/L and RSD < 20%.     

Direct electrochemistry of recombinant tobacco peroxidase on gold

Direct electron transfer (DET) reactions of recombinant tobacco peroxidase (rTOP), namely direct electroreduction of Compound I/Compound II and heme Fe^3+/2+ conversion, were studied on gold electrodes. rTOP of wild type, non-glycosylated, was produced using an Escherichia coli expression system. At pH 5.0, the redox potential for direct electrochemical transformation of the Fe^3+/2+ of the peroxidase heme was −143 mV vs. AgAgCl, and 0.26 ± 0.07 pmol of the adsorbed rTOP were in DET contact with the gold electrode. The total amount of the adsorbed rTOP estimated from QCM data was 53 ± 5 pmol/cm² or 1.67 pmol when referred to the surface area of the electrodes used for electrochemical measurements. Of 1.67 pmol of adsorbed rTOP, only 0.76 pmol were catalytically active. DET between Au and the enzyme was also studied in the reaction of the bioelectrocatalytic reduction of H₂O₂ by cyclic voltammetry and amperometric detection of H₂O₂ at +50 mV with rTOP-modified Au electrodes placed in a wall-jet flow-through electrochemical cell. Maximal bioelectrocatalytic current response of the rTOP-modified gold electrodes to H₂O₂ was observed at pH 5.0 and stemmed from its bioelectrocatalytic reduction based on DET between Au and the active site of rTOP. Kinetic analysis of the DET reactions gave 52% of the adsorbed rTOP molecules active in DET reactions (0.4 pmol of adsorbed catalytically active rTOP, correspondingly), which correlated well with the non-catalytic-voltammetry data. DET was characterised by a heterogeneous ET rate constant of 13.2 s⁻¹, if one takes into account the QCM data, and 19.6 s⁻¹, if the amount of rTOP estimated from the data on DET transformation of Fe^3+/2+ couple of rTOP is considered. The sensitivity for H₂O₂ obtained for the rTOP-modified Au electrodes was 0.7 ± 0.1 A M⁻¹ cm⁻². These are the first ever-reported data on DET reactions of anionic plant peroxidases on bare gold electrodes.     

A New Potential Gradient Detection System for Isotachophoresis

Abstract

Concerning the potential gradient detector for isotachophoresis, the authors solved the problem of bubble generation on the sensing electrodes which frequently resuls in suspension of isotachophoresis, which had been the biggest impediment to the practical development of this type of detector.

In order to obtain stable measurements of zone boundaries without descernible generation of bubbles even at 250 μA of migration current, several modifications were made. The projecting part of the sensing electrodes were scraped off resulting in reduction of the electric current between two sensing electrodes, the cross-sectional area of the cell was expanded resulting in reduction of the current density in the sensing cell, and finally, the sensing cell, which is situated at a high voltage to ground, was isolated from ground potential by converting the electrical signal provided by the sensing cell into an optical signal, resulting in a reduction of the current to ground.

The detection limit was 2×10^?9 gram equivalent for adipic acid. A boundary between zones was detectable when the mobility difference between the zones was about 1% in the case of a sample having a diffusion coefficient of about 10⁵ cm²/sec.     

Adsorption phenomena in the NAD+/NADH system at glassy carbon electrodes

A variety of electrochemical approaches has been used to investigate the adsorption of NAD⁺, NADH and the NAD-NAD dimer from aqueous solution at glassy carbon electrodes (GCE) with supplementary studies of adsorption at pyrolytic graphite and platinum electrodes from aqueous media and at GCE from DMSO solution. The following hypotheses are advanced concerning the adsorption orientation: at carbon electrodes, on which NADH is not adsorbed, NAD⁺ produced by anodic oxidation of the NADH is first rapidly adsorbed in a planar configuration relative to the electrode surface, which is probably bound to the surface through the adenine moiety; there is then a relatively slow reorientation of the adsorbed NADH molecules to a perpendicular orientation relative to the electrode surface, which adsorbate is more tightly bound to the surface than the planar oriented adsorbate and which likely involves interaction between parallel adenine and pyridinium rings. Reduction (one-electron process) of NAD⁺ at the GCE produces the NAD-NAD dimer, which, at a clean electrode surface, involves a diffusion-controlled process and an adsorption-controlled process; the latter is due to formation of adsorbed dimer, which is more strongly adsorbed than NAD⁺. The dimer is oxidized at the GCE only if it is adsorbed. The factors controlling and involved in the adsorption processes have been examined with particular reference to the use of anodic voltammetry for the analytical determination of NADH.     

Multi-layer protruded electrodes for reducing the operating voltage and gamma shift of fringe-field switching LCDs

ABSTRACT

To reduce the operating voltage and gamma shift of the nematic liquid crystal display (LCD), a single-domain-protruded fringe-field switching (PFFS) electrode structure is designed. In this work, a kind of nematic liquid crystal (NLC) is introduced firstly. Then, the operating voltage and gamma shift of the proposed PFFS LCD are investigated under various electrodes’ parameters. Besides, its light leakage and contrast ratio are also discussed. The results show that the operating voltage of the PFFS LCD is only 1.40 V, here the electrodes’ width is 2 μm and electrodes’ gap is 4 μm. The gamma shift of the PFFS LCD can be reduced to the indistinguishable level under various electrodes’ sizes, if the height of the insulation layer is proper. For contrast ratio, it is larger than 200:1 at full viewing-angle, and the zone of 500:1 covers 60° polar angle.     

Removal of Cr(VI) from aqueous solutions using EDCC-MCM-41: Isotherm,kinetics and thermodynamic evaluation

Abstract

EDCC-MCM-41, a novel ethylenediamine derivative of MCM-41 was synthesized from a sugar industry waste, bagasse fly ash. Studies were carried out on its ability to remove Cr(VI) from aqueous solution. Its performance was also compared with that of unmodified MCM-41 and an already known derivative NH₂-MCM-41 (both also synthesized from bagasse fly ash) at the same experimental conditions. Results show that the novel adsorbent produced a better sorption performance (adsorption capacity of 49.04?mg g^?1) than MCM-41 and NH₂-MCM-41 which produced adsorption capacities of 13.78?mg g^?1 and 25.29?mg g^?1 respectively. Thermodynamics, kinetics and isotherm studies confirm that the sorption process involving the novel adsorbent is favorable for Cr(VI) and can best be represented by pseudo-second-order kinetics model. The sorption process was also found to be multistep, involving both film diffusion and intraparticle diffusion into the pores on the adsorbent’s surface.

Abbreviations: BFA: Bagasse Fly Ash; MCM-41: Mobil Composition of matter no. 41; NH2-MCM-41: mine functionalized MCM-41; EDCC-MCM-41: Ethylenediamine grafted MCM-41; qe: Mass of material adsorbed at equilibrium per mass of adsorbent; qt: Mass of material adsorbed at any time t per mass of adsorbent; C0: The initial concentration; Ce: The residual concentration at equilibrium; Ct: Residual concentration at any time t; W: The weight of adsorbents; rRMSD: Root mean square deviation; R: Gas constant     

Pulse Voltammetric Determination of Thiazolidine-4-Carboxylic Acids

Abstract

The electrochemical behaviour of thiazolidine-4-carboxylic acid (thioproline, Thz) and its derivatives 2-propyl-thiazolidine-4-carboxylic acid (PrThz) and 2-tetrahydroxybutyl-thiazolidine-4-carboxylic acid (AThz) has been studied by cyclic, normal and reverse pulse, differential pulse and Osteryoung square wave voltam-metrics. Anodic waves, resulting from the formation of adsorbed mercury thiazolidates, were obtained for all the compounds at mercury electrodes in acetate and phosphate buffers at pH from 3.8 to 8. the initially formed mercury compounds rapidly transform into the final products accumulating on the electrode surface. the oxidation products inhibit the anodic processes and distorted voltammetric curves were obtained at the covered electrodes. Various effects of these processes in various pulse voltammetric techniques were evident. Normal pulse voltammograms were least affected by the product transformations. Osteryoung square wave voltammetry permitted very fast determinations of Thz derivatives.     

The dependence of the kinetics of some simple outer-sphere electrode reactions on the nature of the electrode material

The kinetics of a number of simple inorganic electrode reactions that are known or expected to follow outer-sphere pathways have been examined at mercury, silver, platinum, and gold-aqueous interfaces in order to explore the effects of varying the electrode material on outer-sphere reactivity. The electroreduction kinetics of Co(III) ammine complexes exhibited only mild dependences on the nature of the electrode material which were compatible with the expected variations in double-layer effects. However, the electrooxidation of Cr²⁺ proceeded at strikingly higher overpotentials on the solid surfaces compared with mercury electrodes. Similar effects were also seen for the electrooxidation of V²⁺, Eu²⁺ and Ru²⁺ in the presence of Cr²⁺. Much larger rate constants were observed for these aquo reactions at solid surfaces in the absence of Cr²⁺, although Cr²⁺ had no influence on Co(NH₃)₆³⁺ electroreduction, or any reaction at mercury electrodes. It is speculated that the very large substrate effects upon the electrode kinetics of aquo couples arise from the influence of the inner-layer water structure on the reactant-solvent interactions experienced by these “structure-making” reactants at their plane of closest approach. The inhibiting influence of Cr²⁺ may be due to its ability to efficiently remove adsorbed catalytic contaminants by incorporation into a substitutionally inert Cr(III) electrooxidation product by means of a ligand-bridge mechanism.     

Usefulness of Hysteresis Curves for Studying Temperature Effects on Ises Responses

Abstract

A procedure for study of the effects of the temperature on the response of ion-selective electrodes in which isothermal hysteresis cum determined at several concentrations are used for obtaining the temperature variation parameters (isothermal slope and potential coefficients and isopotential points) is proposed. The procedure was tested against the classic procedure in which calibration curves are obtained at different temperatures. 'All -solid-state' Ag₂S and Ag₂S/Agl electrodes with sensors applied to silver- and copper-loaded epoxy supports (and commercial electrodes, for comparison) were used for this purpose. The values of the temperature parameters obtained by the two procedures (in the raw 10–60°C) are compared. This comparison show that the present procedure yields acceptable values of temperature parameters for cases like the studied where the hysteresis is not very produced.     

Catalysis of dioxygen reduction at graphite electrodes by an adsorbed cobalt(ii) porphyrin

The electroreduction of dioxygen to hydrogen peroxide is catalyzed when the Co(II) porphyrin shown in Fig. 1 is adsorbed on the surface of graphite electrodes. The electroreduction of the adsorbed catalyst occurs at potentials well separated from those where the dioxygen is reduced showing that more than a simple redox catalysis is involved. The porphyrin also catalyzes the electro-oxidatoin of hydrogen peroxide. In 1 M NaOH catalyst-coated electrodes yield reversible, almost nernstian responses to the $\text{O}{}_2\text{O}{}_2\text{H}{}^{\mathrm{?}}$ couple. Cyclic and rotating disk voltammetry were used to examine the kinetics of the catalyzed reactions and mechanistic schemes are proposed to account for the observed kinetics.     

Ion Exchange Application of Overpressured Thin-Layer Chromatography

Abstract

The application of overpressured thin-layer chromatography introduced into the field of ion exchange chromatography. The basic differences between overpressured thin-layer chromatography and classical thin-layer chromatography are discussed including the distinction between the separations performed on thin-layer plates containing silica gel and a mixture of ion exchanger material and silica gel. The basic increase of flow velocity of solvent front with the aid of a pressurized ultra-micro chamber and the effect of flow velocity on the height equivalent of the theoretical plates are also presented. For basic amino acids, the flow velocity vs plate height curves show optima at a moderately high rate of development.     

The electrochemistry of gold–platinum alloys

The electrochemical properties of gold, platinum and gold–platinum alloy electrodes under different heat treatment conditions have been studied in 0.5 M H₂SO₄ and 0.5 M NaOH. The electro-oxidation of 0.1 M ethylene glycol in 0.5 M NaOH at these electrodes has also been studied. It was found that all the gold–platinum electrodes are more active for ethylene glycol electro-oxidation than both pure gold and platinum, and that the gold–platinum electrodes in the solid solution condition are more active than the two-phase electrodes. Poisoning of all the electrodes occurs during electrolysis of ethylene glycol at a fixed potential. Potential pulsing is successful in removing the poisoning species formed at the pure gold and pure platinum electrodes. High apparent current densities are found during the first few cycles at the Au–Pt alloy electrodes. These high current densities are also associated with more severe poisoning – than at both pure gold and platinum – and longer cleaning cycles are needed to remove the poisons at these electrodes.     

Surface-enhanced Raman spectroscopy of 12CN− and 13CN− adsorbed at silver electrodes

Measurements are reported of the surface-enhanced Raman of ¹²CN and ¹³CN (and of isotopically labelled mixtures) adsorbed at silver electrodes. The spectra are shown to arise from a complex species whose coordination number does not change with electrode potential. This species is probably a [Ag(CN₂)]^? entity having C_2v symmetry; at very negative potentials a reduced form of this complex [Ag(CN)₂]^2? coexists with the formally Ag¹ species at the surface. The shifts in band position are interpreted in terms of changes in the bond character of the adsorbed CN^? species. The spectrum of water coadsorbed with CN^? is also markedly dependent on the charge density of the adsorbed CN^? groups.     

Electrochemical behaviour of Pt(100), Pt(111) and Pt polycrystalline surfaces in hydrogencarbonate solution

It was demonstrated that adsorbed CO is obtained from the reduction of NaHCO₃ solution when Pt(100), Pt(110), disordered Pt(111) and polycrystalline electrodes are employed. Reduction of CO₂ coming from the dissociation of the hydrogencarbonate anion is proposed as the reaction that produces CO. By using Fourier transform infrared spectroscopy, linear and multi-bonded CO were detected on polycrystalline platinum electrodes. The shape of the band associated with linearly adsorbed CO is monopolar as a consequence of the partial overlapping, at lower wavenumbers, of the absolute bands at both potentials (0.05 and 0.35 V).     

A First‐Principle Calculation of Sulfur Oxidation on Metallic Ni(111) and Pt(111), and Bimetallic Ni@Pt(111) and Pt@Ni(111) Surfaces

Sulfur, a pollutant known to poison fuel‐cell electrodes, generally comes from S‐containing species such as hydrogen sulfide (H₂S). The S‐containing species become adsorbed on a metal electrode and leave atomic S strongly bound to the metal surface. This surface sulfur is completely removed typically by oxidation with O₂ into gaseous SO₂. According to our DFT calculations, the oxidation of sulfur at 0.25 ML surface sulfur coverage on pure Pt(111) and Ni(111) metal surfaces is exothermic. The barriers to the formation of SO₂ are 0.41 and 1.07 eV, respectively. Various metals combined to form bimetallic surfaces are reported to tune the catalytic capabilities toward some reactions. Our results show that it is more difficult to remove surface sulfur from a Ni@Pt(111) surface with reaction barrier 1.86 eV for SO₂ formation than from a Pt@Ni(111) surface (0.13 eV). This result is in good agreement with the statement that bimetallic surfaces could demonstrate more or less activity than to pure metal surfaces by comparing electronic and structural effects. Furthermore, by calculating the reaction free energies we found that the sulfur oxidation reaction on the Pt@Ni(111) surface exhibits the best spontaneity of SO₂ desorption at either room temperature or high temperatures.     

Synthetic Inorganic Ion Exchangers. XIV Thin-Layer Chromatography of Metal Ions on Thorium Phosphate: Quantitative Separation of Au (III) from Several Metal Ions

Abstract

The analytical potential of thorium phosphate as an ion exchanger has been explored by thin layer chromatographic (TLC) technique. Binder-free thin layers of thorium phos -phate have been explored for several important binary and ternary separations. A TLC method has been developed for quantitative separation of microgram-quantities of Au(III) from several metal ions in pure butanol system.