A model for the coupled transport of ions and electrons in redox conductive microcrystals

Coupled diffusion of ions and electrons in microcrystals of insertion compounds immobilized at an electrode surface is theoretically analysed by a lattice-gas model without interactions. The transport in the direction perpendicular to the electrode surface depends on Wagner's factor for electrons, while the transport parallel to the electrode depends on this factor for ions. The iso-concentration profiles may depend on the orientation of the particle on the electrode surface. Chronoamperometric responses of volume and surface redox reactions are calculated. Received: 5 June 1998 / Accepted: 22 August 1998     

Electrochemical Determination of Epinephrine with a Pyrolytic Graphite Electrode in the Presence of Ascorbic Acid and Dopamine

Epinephrine (EP) is one of the most important neurotransmitters in mammalian central nervous systems, existing in the nervous tissue and body fluid in the form of large organic cations, and controlling the nervous system in the series of biological perfor…     

Evaluation of GaN and In0.2Ga0.8N Semiconductors as Potentiometric Anion Selective Electrodes

The response of potentiometric anion selective electrodes consisting of undoped GaN or In_0.2Ga_0.8N films grown on Al₂O₃ (sapphire) was measured in electrolyte solutions of F^?, NO₃^?, Cl^?, SCN^?, ClO₄^? or Br^? anions at concentrations ranging from 10^?6 to 10^?1 M. The slope of the linear regions varied between ?32.8 and ?51.9 mV/decade for the GaN electrode and between ?31.0 and ?72.0 mV/decade for the In_0.2Ga_0.8N electrode. The drift of the GaN electrode reached 1.57 mV/day in KNO₃ solutions, whereas the drift of the In_0.2Ga_0.8N electrode could not be evaluated due to large drops in the slope of its linear range over time. Both electrodes were sensitive to pH variations over the pH range from 12.8 to 1.3. The GaN electrode surface could be electrochemically etched under anodic polarization; however, both GaN and In_0.2Ga_0.8N electrodes remained chemically stable and mechanically intact under open circuit conditions even after prolonged use.     

Electrochemical Studies on New Chelating Compounds of the Mono‐ and Bis(imidazolyl)pyridine Type

Recently designed and synthetized mono‐imidazolinone (I and III) and bisimidazolinone (II and IV) chelating ligands were electrochemically characterized at mercury and carbon paste electrodes. Based on polarographic, voltammetric and coulometric investigations in buffered aqueous media, the general reduction pathway has been suggested. Reduction of the mono‐imidazolinone derivatives proceeds in acidic and neutral media in two two‐electron steps. In the first step, the 2,3‐C?N double bond of the imidazolone ring is reduced yielding a mixture of two diastereoisomers (V and VI). In the second step, the 2,3‐C? N single bond (in protonated form) is further reductively split and as the only final product the compound VII was formed. Both intermediates (V and VI) as well as the final product (VII) were prepared using controlled potential electrolysis on the first and second wave, respectively, isolated and identified by means of NMR. The reduction of bis‐derivatives proceeds most probably in an analogical way: in the first step, both imidazoles are reduced simultaneously at the same potential, whereas the following reduction (ring‐opening) proceeds stepwise. In the case of compound III, the covalent hydration of the parent compound takes place in acidic media, partially preventing its reduction. Finally, voltammetric behaviour of mono‐ and bisimidazolinones at carbon paste electrode is also discussed and, in prospect, possible electroanalytical applications outlined.     

在双电极对激光器中激光感生介质扩散的影响

从理论上研究了在ＣＯ２激光器中双电极对对激光感生介质扩散的影响，讨论了激光感生介质的扩散系数，传播速率以及感生介质密度并采用复化Ｓｉｍｐｓｏｎ公式进行了数值计算。所得结果在双电级对激光器的设计中有重要意义。     

Role of acidity on the electrochemistry of Prussian Blue at plain and Nation filmcoated electrodes

Prussian Blue (PB) coated on plain platinum (Pt) shows a redox wave at 0.44V vs SCE in addition to the two usual redox waves at 0.82 and 0.12 V vs SCE when the electrodes were dipped in acidic KC1 solution. PB incorporated into Nation film-coated on Pt electrode exhibited the same behaviour even in the presence of neutral KG solution. In acidic KC1, the additional redox wave observed for PB incorporated into Nation film shifted positively to 0.39V vs SCE and the peak separation was reduced to 30mV. The observed additional redox wave for PB coated on plain Pt electrode and incorporated into Nafion film-coated Pt electrodes was assigned to the partial reduction of PB occurring due to the insertion of protons into the film. The effect of various cations on the electrochemistry of PB incorporated into Nafion film-coated electrode was also studied.     

Activity coefficients from the emf of liquid membrane cells V. alkaline earth hexacyanoferrates (III) in aqueous solutions at 25°C

The relative mean activity coefficients of the M₃[Fe(CN)₆]₂ salts, M=Mg, Ca, Sr, Ba, are measured down to about 5×10^–6 mol-kg^–1 using the liquid membrane cell method. In the dilute region these salts display negative instead of positive deviations from the limiting law, contrary to Debye-Hückel's theory predictions. An indirect method based on auxiliary emf measurements in MCl₂, K₃Fe(CN)₆ and KCl, rather than a theory-assisted direct extrapolation to zero of the relative activity coefficients, is used to identify the actual values of the activity coefficients. The data are compared with Mayer's theory, ion-pair theory and numerical integration of the Poisson-Boltzmann equation. Best-fit coefficients of Pitzer's equation which meet the activity coefficients of the M₃[Fe(CN)₆]₂ salts to be reproduced, are reported.     

Dynamics of electron hopping in redox polymer electrodes using kinetic Ising model

The analysis of diffusion-migration equations pertaining to electron hopping and physical displacement in redox polymer electrodes is carried out using kinetic Ising model formalism. It is shown that, by the appropriate choice of transition probabilities obeying detailed balancing conditions, a hierarchy of transport equations can be derived. Earlier transport equations due to Nernst-Planck and Savéant are derived as special cases. The dependence of apparent diffusion coefficient on number density of redox centres, polymer morphology etc. are pointed out. Several new insights concerning the microscopic basis underlying the hitherto known phenomenological equations are demonstrated. Received: 27 May 1997 / Accepted: 24 July 1997     

Electrocatalytic oxidation of NADH at carbon paste electrodes modified with meldola blue adsorbed on zirconium phosphate: effect of Ca2+ and polyethyleneimine

The basic electrochemistry of carbon paste electrodes modified with Meldola Blue adsorbed on zirconium phosphate (MB-ZP-CPEs) and their ability to oxidize NADH have been investigated. Three types of carbon powder (graphite and glassy carbon-type Sigradur K and G) were used to obtain MB-ZP-CPEs. On comparing cyclic voltammograms recorded at MB-ZP-CPEs, similarly prepared from the three different carbon powders, those made with Sigradur K exhibited the lowest background current, and the best MB electrochemistry, seen as the highest peak intensities and smallest peak separation. Using MB-ZP-CPEs based on Sigradur K a study on NADH oxidation was done focusing on the effect of the Ca²⁺ concentration in the contacting solution and on the addition of polyethyleneimine (PEI) into the paste. It can be stated that MB-ZP-CPEs based on Sigradur K and containing 1.23% (w/w) PEI exhibited the best behavior for NADH oxidation, measured by the highest electrocatalytic rate constant (8.2×10³ M^–1 s^–1).     

Low-potential stable detection of β-NADH at sol–gel derived carbon composite electrodes

Sol–gel derived carbon composite electrodes, prepared from different non-silicate metal alkoxide precursors, offer a substantial decrease in the overvoltage of the NADH oxidation reaction (compared to ordinary carbon electrodes). Such promotion is attributed to acceleration of the proton-transfer step by the metal-oxide component of the composite. Passivation problems, accrued by accumulation of reaction products, are also greatly minimized. Both titania–, zirconia–sol–gel carbon composite electrodes thus offer a highly sensitive and stable anodic detection of NADH at +0.2 V. Greatly improved retention of the redox mediator Meldola Blue within the sol–gel network permits convenient measurements at NADH at −0.1 V. These improvements indicate great promise for the design of dehydrogenase-based amperometic biosensors. An intrinsic activation action by the metal-oxide component is also reported towards the oxidation of hydrazine, hydrogen peroxide, ascorbic acid and catechol. Low-potential detection of NADH is also illustrated at microfabricated titania/carbon screen-printed electrodes.