Nickel hydroxide electrodes as amperometric detectors for carbohydrates in flow injection analysis and liquid chromatography

The present paper describes the utilization of nickel hydroxide modified electrodes toward the catalytic oxidation of carbohydrates (glucose, fructose, lactose and sucrose) and their utilization as electrochemical sensor. The modified electrodes were employed as a detector in flow injection analysis for individual carbohydrate detection, and to an ionic column chromatography system for multi-analyte samples aiming a prior separation step. Kinetic studies were performed on a rotating disk electrode (RDE) in order to determine both the heterogeneous rate constant and number of electrons transferred for each carbohydrate. Many advantages were found for the proposed system including fast and easy handling of the electrode modification, low cost procedure, a wide range of linearity (0.5-50 ppm), low detection limits (ppb level) and high sensitivities. (C) 2009 Elsevier B.V. All rights reserved.

Prigogine-Defay Ratio for an Ionic Glass-Former: Molecular Dynamics Simulations

The pressure dependence of the glass-transition temperature, T(g)(P), of the ionic glass-former 2Ca(NO(3))(2) center dot 3KNO(3), CKN, has been obtained by molecular dynamics (MD) simulations The liquid-glass difference of thermal expansivity, Delta alpha, heat capacity, Delta C(p), and isothermal compressibility, Delta kappa, have been calculated as a function of pressure. It has been found that the Ehrenfest relation dT(g)/dP = TV Delta alpha/Delta C(p) predicts the pressure dependence of T, but the other Ehrenfest relation, dT(g)/dP = Delta kappa/Delta alpha, does not. Consequently, the Prigogine-Defay ratio, Pi = Delta C(p)Delta kappa/TV Delta alpha(2), is Pi similar to 1.2 at low pressures, but increases 1 order of magnitude at high pressures. The pressure dependence of the Prigogine-Defay ratio is interpreted in light of recent explanations for the finding Pi > 1.

All solid-state electrochromic devices with gelatin-based electrolyte

6 x 8cm(2) electrochromic devices (ECDs) with the configuration K-glass/EC-layer/electrotype/ion-storage (IS) layer/K-glass, have been assembled using Nb2O5:Mo EC layers, a (CeO2)(0.81)-TiO2 IS-layer and a new gelatin electrolyte containing Li+ ions. The structure of the electrolyte is X-ray amorphous. Its ionic conductivity passed by a maximum of 1.5 x 10(-5) S/CM for a lithium concentration of 0.3g/15ml. The value increases with temperature and follows an Arrhenius law with an activation energy of 49.5 kJ/mol. All solid-state devices show a reversible gray coloration, a long-term stability of more than 25,000 switching cycles (+/- 2.0 V/90 s), a transmission change at 550 nm between 60% (bleached state) and 40% (colored state) corresponding to a change of the optical density (Delta OD = 0. 15) with a coloration efficiency increasing from 10cm(2)/C (initial cycle) to 23cm(2)/C (25,000th cycle). (c) 2007 Elsevier B.V. All rights reserved.

Amylopectin-rich starch plasticized with glycerol for polymer electrolyte application

This paper describes the preparation and characterization of a solid polymer electrolyte based on amylopectin-rich starch plasticized with glycerol. The samples were characterized through ionic conductivity (sigma) measurements, scanning electron microscopy, thermal analysis, and spectroscopy in the UV-Vis-NIR region. The results showed that the highest sigma (1.1 x 10(-4) Scm(-1) at 30 degrees C) was obtained for the sample with n = [O]/[Li] = 6.5 ratio. In addition, the samples plasticized with 30-35 wt.% of glycerol presented high ionic conductivity, transparency and conduction stability. The ionic conductivity measurements as a function of lithium salt contents showed a maximum for n=6.5. The ionic conductivity as a function of time for amylopectin-rich starch plasticized with 30 wt.% of glycerol and containing [O]/[Li] = 10 showed conduction stability over 6 months (sigma similar to 3.01 x 10(-5) S cm(-1)). (C) 2010 Elsevier B.V. All rights reserved.

Fast separation of selective serotonin reuptake inhibitors antidepressants in plasma sample by nonaqueous capillary electrophoresis

A simple, fast, and sensitive liquid-liquid extraction method followed by nonaqueous capillary electrophoresis (LLE/NACE) was developed and validated for Simultaneous determination of four antidepressants (fluoxetine, sertraline, citalopram and paroxetine) in human plasma. Several experimental separation conditions using aqueous and nonaqueous media separation were tested by varying the electrolyte pH value (for aqueous medium) and the ionic strength concentration considering the similar mobility of the compounds. High-resolution separation was achieved with a mixture of 1.25 mol L(-1) of phosphoric acid in acetonitrile. The quantification limits of the LLE/CE method varied between 15 and 30 ng mL(-1), with a relative standard deviation (RSD) lower than 10.3%. The method was successfully applied in therapeutic drug monitoring and should be employed in the evaluation of plasma levels in urgent toxicological analysis. (C) 2009 Elsevier B.V. All rights reserved.

Electrochimie analytique en absence d'electrolyte indiferent : transports de matiere lors de la reduction des acides a une ultramicroelectrode

O transporte de massa por migração relativo à redução do proton em um ultramicroeletrodo de platina foi investigado. O efeito da migração sobre as correntes limite foi primeiramente estudado para uma só espécie eletroativa em solução através da comparação dos voltampérogramas obtidos na ausência de eletrólito suporte assim como na presença de um grande excesso do mesmo (efeito do eletrólito). O comportamento do proton na água e dos ácidos do tipo BH+ e HA- em acetonitrila foi estudado e uma expressão para quantificar o efeito do eletrólito é proposta. Ela considera as condutâncias equivalentes e as cargas das espécies iônicas em solução, o número de elétrons envolvido na reação eletroquímica e o tipo de eletrodo utilisado. Os fenômenos de exaltação da corrente de migração que podem se manifestar quando a redução de uma espécie eletroativa é precedida pela redução de uma segunda espécie presente simultâneamente na solução foram igualmente estudados. Observa-se que a exaltação da corrente de migração de uma espécie iônica ocorre mesmo quando sua transformação eletroquímica precede aquela da espécie molecular. Nêste caso, se as mobilidades iônicas são próximas, a altura da onda da espécie molecular é duas vezes maior que na ausência da espécie iônica.

The effect of ionic strength on the structural organization of dioctadecyldimethylammonium bromide in aqueous solution

High-curvature and stabilized vesicles of dioctadecyldimethylammonium bromide (DODABr) can be formed spontaneously in aqueous electrolytic solution. It is shown by cryo-transmission electron microscopy that 5.0 mM DODABr molecules associate in water at a temperature above its gel-to-liquid-crystalline phase transition temperature (T(m)approximate to45 degreesC) in a variety of complex bilayer structures. However, in the presence of NaCl the preferred structures formed are unilamellar and bilamellar vesicles with high curvature and the dispersion is polydisperse in size and geometry, but the main vesicle population contains spherical, flattened and smoothed structures. It is, however, less polydisperse than the corresponding salt-free dispersion, and the size polydispersity and the vesicle curvature radius tend to decrease with NaCl concentration. Long cylindrical bilamellar vesicles, with a very thin water layer separating the bilayers are also formed in the presence of 10 mM NaCl. The effect of the ionic strength on T-m, obtained by differential scanning calorimetry, is shown to depend on the nature of the counterion: Br- decreases, whereas Cl- increases Tm of DODABr, indicating different affinity of these counterions for the vesicle surfaces.

Solution of the Poisson-Boltzmann equation for a system with four ionic species

The analytical solution of the Poisson-Boltzmann equation in an electrolyte with four ionic species (2:2:1:1), in the presence of a charged planar membrane or surface is presented. The function describing the mean electrical potential provides a convenient description that helps the understanding of electrical processes of biological interest.

Existence and stability of new nanoreactors: Highly swollen hexagonal liquid crystals

We report the preparation of direct hexagonal liquid crystals, constituted of oil-swollen cylinders arranged on a triangular lattice in water. The volume ratio of oil over water, rho, can be as large as 3.8. From the lattice parameter measured by small-angle X-ray scattering, we show that all the oil is indeed incorporated into the cylinders, thus allowing the diameter of the cylinders to be controlled over one decade range, provided that the ionic strength of the aqueous medium and rho are varied concomitantly. These hexagonal swollen liquid crystals (SLCs) have been first reported with sodium dodecyl sulfate as anionic surfactant, cyclohexane as solvent, 1-pentanol as co-surfactant, and sodium chloride as salt (Ramos, L.; Fabre, P. Langmuir 1997, 13, 13). The stability of these liquid crystals is investigated when the pH of the aqueous medium or the chemical nature of the components (salt and surfactant) is changed. We demonstrate that the range of stability is quite extended, rendering swollen hexagonal phases potentially useful for the fabrication of nanomaterials. As illustrations, we finally show that gelation of inorganic particles in the continuous aqueous medium of a SLC and polymerization within the oil-swollen cylinders of a SLC can be conducted without disrupting the hexagonal order of the system.

Mixed ionic-electronic YSZ/Ni composite for SOFC anodes with high electrical conductivity

The preparation of the ZrO(2):8 mol % Y(2)O(3)/NiO (YSZ/NiO) composites by a modified liquid mixture technique is reported. Nanometric NiO particles dispersed over the yttria-stabilized zirconia (YSZ) were prepared, resulting in dense sintered specimens with no solid solution formation between the oxides. Such a feature allowed for the electrical characterization of the composites in a wide range of relative volume fraction, temperature, and oxygen partial pressure. The main results indicate that the composites have high electrical conductivity, and the transport properties in these mixed ionic-electronic (MIEC) composites are strongly dependent on the relative volume fraction of the phases, microstructure, and temperature. These parameters should hence be taken into consideration for the optimized design of MIEC composites for electrochemical applications. In this context, the composite was reduced under H(2) for the preparation of high-conductivity YSZ/Ni cermets for use as solid oxide fuel cell anode material with relatively low metal content. (c) 2005 the Electrochemical Society. [DOI:10.1149/1.2149312] All rights reserved.

Environmental coordination chemistry: Binary systems comprising some bivalent cations and monocarboxylates in aqueous solution. Ionic medium effects on equilibrium constants

The molar single activity coefficients associated with propionate ion (Pr) have been determined at 25 degrees C and ionic strengths comprised between 0.300 and 3.00 M, adjusted with NaClO4, as background electrolyte. The investigation was carried out potentiometrically by using a second class Hg/Hg2Pr2 electrode. It was found that the dependence of propionate activity coefficients as a function of ionic strength (I) can be assessed through the following empirical equation: log y(Pr) = -0.185 I-3/2 + 0.104 I-2. Next, simple equations relating stoichiometric protonation constants of several monocarboxylates and formation constants associated with 1:1 complexes involving some bivalent cations and selected monocarboxylates, in aqueous solution, at 25 degrees C, as a function of ionic strength were derived, allowing the interconversion of parameters from one ionic strength to another, up to I = 3.00 M. In addition, thermodynamic formation constants as well as parameters associated with activity coefficients of the complex species in the equilibria are estimated. The body of results shows that the proposed calculation procedure is very consistent with critically selected experimental data.

Ionic medium effects on equilibrium constants .1. Proton, copper(II), cadmium(II), lead(II) and acetate activity coefficients in aqueous solution

The molar single ion activity coefficients associated with hydrogen, copper(II), cadmium(II) and lead(II) ions were determined at 25 degrees C and ionic strengths between 0.100 and 3.00 M (NaClO4), whereas for acetate the ionic strengths were fixed between 0.300 and 2.00 M, held with the same inert electrolyte. The investigation was carried out potentiometrically by using proton-sensitive glass, copper, cadmium and lead ion-selective electrodes and a second-class Hg\Hg-2(CH3COO)(2) electrode. It was found that the activity coefficients of these ions (y(i)) can be assessed through the following empirical equations:log y(H) = -0.542I(0.5) + 0.451I; log y(Cu) = -1.249I(0.5) + 0.912I; log y(Cd) = -0.829I(0.5) + 0.448I(1.5);log y(Pb) = -0.404I(0.5) + 0.117I(2); and log y(Ac) = 0.0370I .

Temperature and concentration dependence of density of model liquid foods

Density of binary solutions and combinations of sucrose, glucose, fructose, citric acid, malic acid, pectin, and inorganic salts were measured with an oscillating tube density meter in the temperature range from 10degrees to 60degreesC, at varying concentrations. Density can be predicted with accuracy better than 5 x 10(-5) g cm(-3) using predictive equations obtained by fitting the experimental data. Available literature values agreed well with experimental data. Relations for the excess molar volume of these solutions were derived in terms of mole fraction and temperature. A thermodynamic model for the volumetric analysis of multicomponent aqueous solutions containing electrolyte and non-electrolyte compounds was also proposed. These models can be used for prediction of density of liquid food systems, specially fruit juices and beverages, based on composition and temperature, with high accuracy and without elaborate experimental work.

Electric and magnetic properties of polymer electrolyte/carbon black composites

Measurements of 1H Nuclear Magnetic Resonance (NMR) relaxation times, Electron Paramagnetic Resonance (EPR) and AC Impedance Spectroscopy (IS) are reported for composites based on PEO8:LiClO4 and carbon black (CB), prepared by two methods: solvent and fusion processing. Three nuclear relaxation processes were identified for 1H nuclei: (i) belonging to the polymer chains in the amorphous phase, loosely bound to the CB particles, whose dynamics is almost the same as for unfilled polymer, (ii) belonging to the polymer chains which are tightly attached to the CB particles, and (iii) belonging to the crystalline phase in the loose polymer chain. The paramagnetic electronic susceptibility of the composite samples, measured by EPR, was interpreted by assuming a contribution of localized spin states that follow a Curie law, and a Pauli-like contribution of delocalized spins. A significant change of the EPR linewidth was observed at 40 K, which is the temperature where the Curie and Pauli susceptibilities equally contribute to the paramagnetic electronic susceptibility. The electrical properties are very sensitive to the preparation methods of the composites, which conditions the interaction between carbon particle-carbon particle and carbon particle-polymer chain. Classical statistic models to describe the conductivity in these media were not satisfactory. © 1998 Published by Elsevier Science B.V. All rights reserved.

Chemical stability and electrical properties of Nb doped BaCe 0.9Y 0.1O 3-δ as a high temperature proton conducting electrolyte for IT-SOFC

BaCe 0.9-xNb xY 0.1O 3-δ (where x=0, 0.01, 0.03 and 0.05) powders were synthesized by solid-state reaction to investigate the influence of Nb concentration on chemical stability and electrical properties of the sintered samples. The dense electrolyte pellets were formed from the powders after being uniaxially pressed and sintered at 1550 °C. The electrical conductivities determined by impedance measurements in temperature range of 550-750 °C in different atmospheres (dry argon and wet hydrogen) showed a decreasing trend with an increase of Nb content. For all samples higher conductivities were observed in the wet hydrogen than in dry argon atmosphere. The chemical stability was enhanced with increasing of Nb concentration. It was found that BaCe 0.87Nb 0.03Y 0.1O 3-δ is the optimal composition that satisfies the opposite demands for electrical conductivity and chemical stability, reaching 0.8×10 -2 S cm -1 in wet hydrogen at 650 °C compared to 1.01×10 -2 S cm -1 for undoped electrolyte. © 2012 Elsevier Ltd and Techna Group S.r.l.