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Reversible Process (reversible + process)

Distribution by Scientific Domains

Chemistry	70%

Selected Abstracts

Interference from Trace Copper in Electrochemical Investigations Employing Carboxylic Acid Terminated Thiol Modified Gold Electrodes

ELECTROANALYSIS, Issue 6 2009
Xiaohu Qu
Abstract Unexpectedly, electrochemistry at variable chain length carboxylic acid terminated alkylthiol self-assembled monolayers (SAMs) on gold electrodes gives rise to a Faradaic process in buffered aqueous electrolyte solution. In particular, the three-carbon chain length, 3-mercaptopropionic acid (MPA), exhibits a chemically reversible process with a mid-point potential of 175,mV vs. Ag/AgCl under conditions of cyclic voltammetry. This process is associated with the presence of trace (parts per billion) amounts of copper(II) ions present in the chemical reagents used to prepare the aqueous electrolyte and also from the gold electrode itself. The carboxylic acid moiety on the SAM concentrates Cu2+ ions by coordination and this surface confined layer is then reduced. Methods to minimize the interference of Cu2+ ions at carboxylic acid terminated SAM are discussed and caution with respect to the interpretation of protein electrochemistry is recommended when using carboxylic acid functionalized SAMs to provide biocompatible electrochemical transduction surfaces, unless a metal free environment can be obtained. [source]

Fourier Transformed Large Amplitude Square-Wave Voltammetry as an Alternative to Impedance Spectroscopy: Evaluation of Resistance, Capacitance and Electrode Kinetic Effects via an Heuristic Approach

ELECTROANALYSIS, Issue 15-16 2005

Abstract A detailed simulation of Fourier transformed large amplitude square-wave voltammetry is presented in the frequency domain for the process Red,Ox+e,. The simulation takes into account the influence of the electrode kinetics (Butler,Volmer model), uncompensated resistance (Ru) and double layer capacitance (Cdl). Of particular significance is the prediction that the even harmonic responses are only detected in the presence of quasi-reversibility or uncompensated resistance, and also are essentially devoid of charging current. In contrast, the DC and odd harmonic AC components exhibit much larger faradaic currents and also contain charging current. Conveniently, detailed analysis of the simulated DC and AC harmonic components reveals the presence of readily recognised patterns of behaviour with unique levels of sensitivity to electrode kinetics, Ru and Cdl, that facilitate quantitative analysis of these terms. These electrochemical parameters are generally calculated by small amplitude impedance spectroscopy and utilisation of linear analysis of equivalent circuits. Experimental studies on the one electron oxidation of ferrocene in dichloromethane (0.1,M Bu4NPF6) and the one electron reduction of [Fe(CN)6]3, in aqueous 0.5,M KCl electrolyte analysed via heuristic forms of data analysis based on recognition of patterns of behaviour, are presented as examples of a reversible process with significant uncompensated resistance and a quasi-reversible process with minimal ohmic drop, respectively. Results demonstrate the advantages of a more intuitively implemented form of data analysis than presently available with conventional forms of impedance spectroscopy. [source]

Kinetics of N -bromination of 2-oxazolidinone

INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 12 2004
F. A. Amoedo
The kinetics of N -bromination of 2-oxazolidinone by transfer of Br from sodium hypobromite, N -bromosuccinimide (NBS), or N -bromoacetamide (NBA) were determined spectrophotometrically, at pH between 4.6 and 12.45 (depending on the brominating agent). The reaction with hypobromite was of first order with respect to both the hypobromite and the substrate. The bromination of oxazolidinone with NBS (or NBA) has been found to be a reversible process of order one with respect to both NBA (or NBA) and oxazolidinone in the forward direction, and order one with respect to SI (or ACAM) and the resulting N -bromo-oxazolidinone in the other. The pH dependence of the reaction rate was in keeping with a mechanism in which all the brominating agents (HOBr, BrO,, NBS and NBA) react predominantly with the anion of the substrate. Bimolecular bromination rate constants increased in the order BrO, < NBA < NBS < HOBr. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 642,649, 2004 [source]

Convective Available Potential Energy (CAPE) in mixed phase cloud conditions

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 624 2007
B. Früh
Abstract An approximate but pragmatic approach is presented to define Convective Available Potential Energy (CAPE) in mixed phase cloud conditions. The underlying process calls for mixed (i.e. liquid and ice) phase parcels and assumes the liquid fraction to be a unique function of temperature. The approach is meant to represent average conditions. Differences between this and more traditional approaches are quantified and discussed for mean tropical conditions. Generally freezing increases parcel temperature and, hence, buoyancy. If freezing occurs isobarically (as was often assumed in the past), all water changes phase at a single level resulting in a discontinuity in buoyancy at that level. By contrast, the mixed phase parcel process implies a continuous phase transition in a finite range of temperatures Tfs , T , Tfe, leading to a gradual change of buoyancy with altitude and preventing any temperature inversion. The details of this gradual change depend on the choice of the specified temperature range [Tfs, Tfe]. High in the troposphere, where all water is frozen irrespective of the details, the differences between the buoyancy profiles are small (but finite). CAPE is very sensitive to the treatment of the freezing process. Isobaric freezing at a relatively high temperature (e.g. , 5 °C) in a reversible process may increase CAPE by a factor of 2 to 3, and this increase is similar in magnitude to the difference between the pseudo-adiabatic and the reversible processes for pure water parcels. Both of these processes are considered less realistic than the reversible mixed phase process with continuous freezing over a broad temperature range [Tfs, Tfe] = [,5 °C, , 40 °C]; the corresponding CAPE lies about half way between the reversible and irreversible pure water processes. For clouds with finite precipitation efficiency the effect of freezing is less pronounced than for reversible conditions. Copyright © 2007 Royal Meteorological Society [source]

Kinetics and mechanism of Cr(VI) adsorption onto tea-leaves waste

ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 4 2008
Md. Salatul Islam Mozumder
Abstract Adsorption equilibrium and kinetic experiments have been conducted in batch mode to evaluate Cr(VI)-tea-leaves waste system. The equilibrium data followed the Langmuir adsorption isotherm and the adsorption was viewed as a physicochemical reversible process. A unified approach model was used to describe the adsorption system from both equilibrium and kinetic viewpoints. The model satisfactorily described both kinetic and equilibrium data. The adsorption and desorption rate constants were evaluated from the model fittings and were not dependent on initial concentration and adsorbent doses. pHzpc of the adsorbent was evaluated as 4.2 ± 0.1, and below that pH the adsorbent surface is positively charged. Adsorption of Cr(VI) was found highly pH-dependent, and the removal efficiency dropped sharply from 95 to 10% when pH of the system changed from 2 to 5. The surface functional groups of tea-leaves waste (before and after adsorption) were analyzed by Fourier transform infrared (FTIR) and the amine groups were found to take part in the adsorption of Cr(VI). The experimental result inferred that electrostatic attraction between the surface and the species is one of the major adsorption mechanisms for binding metal ions to the tea-leaves waste. Copyright © 2008 Curtin University of Technology and John Wiley & Sons, Ltd. [source]

Proton- and Redox-Controlled Switching of Photo- and Electrochemiluminescence in Thiophenyl-Substituted Boron,Dipyrromethene Dyes

CHEMISTRY - A EUROPEAN JOURNAL, Issue 3 2006
Holger Röhr Dipl.-Phys.
Abstract A luminescent molecular switch in which the active thiol/disulfide switching element is attached to a meso -phenyl-substituted boron,dipyrromethene (BDP) chromophore as the signalling unit is presented. The combination of these two functional units offers great versatility for multimodal switching of luminescence: 1) deprotonation/protonation of the thiol/thiolate moiety allows the highly fluorescent meso - p -thiophenol-BDP and its nonfluorescent thiolate analogue to be chemically and reversibly interconverted, 2) electrochemical oxidation of the monomeric dyes yields the fluorescent disulfide-bridged bichromophoric dimer, also in a fully reversible process, and 3) besides conventional photoexcitation, the well separated redox potentials of the BDP also allow the excited BDP state to be generated electrochemically (i.e., processes 1) and 2) can be employed to control both photo- and electrochemiluminescence (ECL) of the BDP). The paper introduces and characterizes the various states of the switch and discusses the underlying mechanisms. Investigation of the ortho analogue of the dimer provided insight into potential chromophore,chromophore interactions in such bichromophoric architectures in both the ground and the excited state. Comparison of the optical and redox properties of the two disulfide dimers further revealed structural requirements both for redox switches and for ECL-active molecular ensembles. By employing thiol/disulfide switching chemistry and BDP luminescence features, it was possible to create a prototype molecular ensemble that shows both fully reversible proton- and redox-gated electrochemiluminescence. In dieser Arbeit wird ein lumineszierender molekularer Schalter vorgestellt, bei dem das aktive Thiol/Disulfid-Schaltelement an einen meso-phenylsubstituierten Bordipyrromethen (BDP) Farbstoff als Signal gebende Komponente gekoppelt ist. Die Kombination dieser beiden funktionellen Einheiten offeriert eine hohe Vielseitigkeit für das multimodale Schalten der Lumineszenz: 1) die Deprotonierung/Protonierung der Thiol/Thiolat-Gruppe erlaubt es, das stark fluoreszierende meso-p-Thiophenol-BDP und das analoge, nicht fluoreszierende Thiolat chemisch reversibel ineinander zu überführen, 2) die elektrochemische Oxidation der monomeren Farbstoffe ergibt das fluoreszierende, disulfidverbrückte, bichromophore Dimer,dieser Prozess ist ebenfalls vollständig reversibel, und 3) neben der konventionellen Anregung mit Licht erlauben die deutlich getrennten Redoxpotenziale des BDPs ebenfalls eine elektrochemische Generierung des angeregten BDP-Zustandes, d.h., Prozesse 1) und 2) können dazu eingesetzt werden, sowohl die Photo- als auch die Elektrochemilumineszenz (ECL) des BDP zu steuern. Diese Arbeit stellt die Charakteristika der verschiedenen Zustände des Schalters vor und diskutiert die zu Grunde liegenden Mechanismen. Die Untersuchung des ortho-Analogen des Dimers vermittelte zudem Einsicht in mögliche Chromophor,Chromophor-Wechselwirkungen im Grund- wie im angeregten Zustand von solchen bichromophoren Architekturen. Der Vergleich der optischen und Redoxeigenschaften der beiden Disulfid-Dimere gab des Weiteren Aufschluss über strukturelle Voraussetzungen von Redoxschaltern sowie ECL-aktiven molekularen Ensembles. Unter Einsatz schaltbarer Thiol/Disulfid-Chemie und mit den Lumineszenzeigenschaften der BDPs war es möglich, ein erstes molekulares Ensemble zu entwickeln, das vollständig reversible protonen- als auch redoxgesteuerte Elektrochemilumineszenz zeigt. [source]

A Dendrimer Chiroptical Switch Based on the Reversible Intramolecular Photoreaction of Anthracene and Benzene Rings

CHEMISTRY - AN ASIAN JOURNAL, Issue 8 2010
Wenjie Liu Dr.
Abstract A series of Fréchet-type dendrimers with 9-benzyloxymethylanthracene cores were synthesized and characterized. The chiral source for the dendrimers was an (S)-2-methyl-1-butoxy group in the 3-position of the benzene ring. Irradiation at 366,nm of a dilute benzene solution led to the formation of two diastereomers (1:1) through a quantitative intramolecular [4,+4,] cycloaddition between the central anthracene ring and the neighboring benzene ring. The process can be reversed with 254,nm UV light or heat. The benzene rings in the dendrons work as a light-harvesting system. The optical rotation values measured for the reversible process showed fatigue resistance. Thus, a promising new type of chiroptical switch has been created that has optical rotation values as output signals. [source]

Effects of cold-girdling on flows in the transport phloem in Ricinus communis: is mass flow inhibited?

PLANT CELL & ENVIRONMENT, Issue 1 2006
ANDREAS D. PEUKE
ABSTRACT The effects of cold girdling of the transport phloem at the hypocotyl of Ricinus communis on solute and water transport were investigated. Effects on the chemical composition of saps of phloem and xylem as well as of stem tissue were studied by conventional techniques and the water flow in the phloem was investigated by NMR imaging. Cold girdling reduced the concentration of sucrose but not that of inorganic solutes or amino acids in phloem saps. The possibility that cold treatment inhibited the retrieval of sucrose into the phloem, following leaching from the sieve tubes along a chemical gradient is discussed. Leaching of other solutes did not occur, as a result of missing promoting gradients in stem tissue. Following 3 d of cold girdling, sugar concentration increased and starch was synthesized and accumulated in stem tissue above the cold girdling region and along the cold-treated phloem pathway due to leaching of sugars from the phloem. Only in the very first period of cold girdling (< 15,30 min) was mass flow inhibited, but recovered in the rest of cold treatment period to values similar to the control period before and the recovery period after the cold treatment. It is concluded that cold treatment affected phloem transport through two independent and reversible processes: (1) a permanent leaching of sucrose from the phloem stem without normal retrieval during cold treatment, and (2) a short-term inhibition of mass flow at the beginning of cold treatment, possibly involving P proteins. Possible further mechanisms for reversible inhibition of water flow are discussed. [source]

The structure of short-lived excited states of molecular complexes by time-resolved X-ray diffraction

ACTA CRYSTALLOGRAPHICA SECTION A, Issue 2 2005
Philip Coppens
Experimental and computational methods for time-resolved (TR) diffraction now allow the determination of geometry changes on molecular excitation. The first results indicate significant changes in the interatomic distances and molecular shape on photo-excitation, but also a dependence of the induced changes on the molecular environment. Though the use of high-brightness synchrotron sources is essential, it limits the time resolution to the width of the synchrotron pulse which is currently 70,100,ps. The experiments discussed fall into two categories: (i) picosecond powder diffraction experiments on the molecular excitation to a singlet state, and (ii) microsecond experiments on the excited states of inorganic complexes. Both involve reversible processes for which a stroboscopic technique can be applied. [source]