Pulse polarograpmc study of the electrochemical reduction of lucigenin in aqueous medium

The electrochemical reduction of lucigenin (bis-N-methylacridinium nitrate) in aqueous solution was studied by normal pulse polarography, normal pulse polarography with differential detection of the current, and differential pulse polarography with cathodic and anodic pulses at several pulse amplitudes. The effects of pH and lucigenin concentration were studied. In confirmation of an earlier d.c. polarographic study, lucigenin is shown to be reduced in two separate one-electron steps. An adsorption peak accompanies the first step, while the second, below pH 3.5, is catalytic owing to chemical regeneration of the intermediate reduction product at the electrode surface.     

Electrochemical behaviour of heterocyclic amine derivatives in media of controlled proton availability: Part I. Reduction mechanism and electron spin resonance study of 2-phenyl 4-substituted quinolines in DMF

The electroreduction of some 4-substituted 2-phenylquinolines in dimethylformamide solutions was studied by dc and ac phase selective polarography, cyclic voltammetry, controlled-potential coulometry, ESR and UV spectrometry, in the presence and absence of protonating agents.In a protic medium 2-phenylquinolines are reduced via a bielectronic polarographic step to the corresponding 1,4-dihydroquinolines. In an aprotic medium two well-defined reduction steps are observed. The first is a reversible one-electron transfer producing a stable anion radical characterized by ESR spectroscopy. The second is an irreversible one-electron transfer producing the di-anion. A multistep scheme for the reduction process consistent with the experimental results is proposed.     

Anodic behaviour of 2-mercaptoethanol at a mercury electrode

The anodic processes which occur at the mercury electrode in 2-mercaptoethanol solutions are studied by various polarographic techniques (d.c., a.c. and differential pulse), controlled-potential coulometry, cyclic voltammetry and differential capacity curves. Two steps are distinguished in the process: a one-electron charge transfer and a dismutation step leading to the formation of a mercury(II) mercaptide. The final product is isolated and analyzed. Differential-pulse polarography can be used to determine ?10^?4 M 2-mercaptoethanol with a limit of M.     

Voltammetric study of the electrochemical reduction of lucigenin in aqueous medium

The electrochemical reduction of lucigenin (bis-N-methylacridinium nitrate) in aqueous solution was studied by d.c. Tast polarography at a dropping mercury electrode, by cyclic voltammetry at an amalgamated gold electrode and at a hanging mercury drop electrode, and by microcoulometry. The effects of pH, lucigenin concentration and temperature were studied, and special methods were applied to study the suspected adsorption and catalytic (regeneration) currents. A spectrophotometric study is also reported. It was found that lucigenin is reduced in two separate one-electron steps. An adsorption prewave accompanies the first step, while the second, below pH 3.5, is catalytic, owing to the chemical regeneration of the intermediate reduction product at the electrode surface.     

Polarographic Study of Lead-Tropolone Complexation

Abstract

A.C., D. C. and derivative pulse polarography of lead-tropolone complexes has been studied. It has been shown that the application of usual treatments for complexation in d. c. polarography are invalid in this system, where the irreversible wave in presence of the ligand is a composite step. It seems generally advisable to apply derivative pulse or a.c. polarography before using non-reversible d.c. steps for evaluation of K and p.     

Polarographic study of xylenol orange

In neutral medium the ring carbonyl group of xylenol orange is reduced in two one-electronsteps at a dropping mercury electrode. Step I is nearly reversible, but step II strongly irreversible. The half-wave potentials of both steps depend on pH, for the products of reduction, the dye radical formed in step I and the OH group produced in step II, behave as acids. From the E_1/2 vs. pH dependence, the pK of the dye radical is found to be 9.5, that of the hydroxy compound 5.6. On progressive acidification the height of step II continuously decreases, while that of step I increases. In solutions with pH<2, only a single (according to logarithmic analysis) one-electron, but overall (according to coulometry) two-electron step results. The ratio i^I/i^II for the two steps increases when larger amounts of alkali metal ions are added. The exaltation of the first wave at the expense of wave II is explained by an acid-dependent dismutation of the dye radical formed during the first one-electron step.     

Etude Polarographique De La Marcellomycine

Abstract

Polarographic study of marcellomycine.

d.c., a.c. and d.p. polarography and cyclic voltammetric techniques were used to investigate the electrochemical properties of marcellomycine. These are found to be quite similar to those of aclacinomycine or carminomycine, the main reversible two-electron wave being attributed tot he reduction of the quinonic structure. The molecule undergoes a degradation process starting in acidic medium and giving rise to the formation of two small irreversible waves at more negative potentials. This process is time, illumination and pH dependent. Important adsorption phenomena interfere in the investigated pH range but are lowered in very acidic medium. Quantitative measurements have been carried out at pH 1 and 6 in a 10^?3 to 10^?5 M concentration range using d.c. polarography. d.p. technique allows determinations in a restricted 10^?5 to 10^?4 M range. A detection limit of 1 · 10^?5 M has been fixed at pH 1.     

Electroreduction and pulse-polarographic determination of nicotinamide in multivitamin tablets

The electroreduction of nicotinamide has been investigated by d.c., a.c. and pulse polarography, cyclic voltammetry and coulometry. A single well-defined polarographic wave is obtained from 2 M sulphuric acid and from 0.1 M sodium hydroxide. In both media, nicotinamide undergoes an irreversible 2-electron reduction; 3 hydrogen ions are consumed in acidic medium and two hydrogen ions in alkaline medium. The current is diffusion-controlled and proportional to the concentration in the range 0.6–120 μg ml^?1. Reduction mechanisms are proposed. A simple and rapid method for the determination of nicotinamide in multivitamin tablets by differential pulse polarography is described.     

The electrochemistry of benzo[b]quinolizinium ion

Summary In aprotic solvents, polarography and cyclic voltammetry indicate a two step reduction of acridizinium ion. The first reduction is a reversible one-electron step. The second involves two electrons and is irreversible. In situ ESR electrolysis at the potential of the first reduction gives no evidence of free radicals. Homogeneous chemical reduction of acridizinium by titration with sodium biphenylide indicates a one-electron reduction mechanism in solution. The absence of stable free radicals suggests a rapid post-reduction of the product.Addition of small amounts of water changes the aprotic solvent (mercury electrode) mechanism. The first wave becomes a two-electron irreversible wave at the expense of the second wave. In aqueous solvents two two-electron, irreversible reduction waves occur. Products of the second absorb and change the mechanism after the first CV scan.

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Elektrochemisches Verhalten des Benzo[b]chinolizinium-Ions

Zusammenfassung In aprotischen Lösungsmitteln wird durch Polarographie und cyclische Voltammetrie eine zweistufige Reduktion des Acridizinium-Ions angezeigt. Die erste Stufe ist reversibel und bezieht sich auf ein Elektron, die zweite Stufe umfaßt 2 Elektronen und ist irreversibel. Durch in situ ESR-Elektrolyse beim Potential der ersten Stufe konnte kein Beweis für freie Radikale gefunden werden. Homogene chemische Reduktion durch Titration mit Natriumbiphenylid deutet auf einen einelektronigen Reduktionsmechanismus hin. Durch Zugabe kleiner Wassermengen wird die erste Stufe auf Kosten der zweiten irreversibel und zweielektronig. In wäßrigen Lösungsmitteln treten 2 zweielektronige irreversible Reduktionsstufen auf.

     

An electrochemical study of (η-C5H5)2Ti(NCS)2

The reduction of titanocene dithiocyanate in various solvents has been examined using the techniques of polarography, voltammetry, controlled potential electrolysis and cyclic voltammetry. In THF and CH₂Cl₂, Cp₂Ti(NCS)₂ undergoes successive one-electron transfer reactions, and by using a combination of electrochemical and EPR techniques it has been possible to confirm the stability of [Cp₂Ti(NCS)₂]^? in these solvents. In DMF, however, an irreversible dimerization follows the first electron transfer.     

Polarography of Uranyl Complex with Chromotropic Acid

The polarography characteristic of uranyl ion in chromotropic acid solution was investigated systematically over the pH range 2.0 to 10.0 with ligand concentrations varying from 0.010 M to 0.200 M. At pH 5.5, the one-electron reversible reduction waves were obtained. The temperature coefficient of the half-wave potential was obtained to be ?0.32 mV per degree and the mean value of i_d/h^1/2 is 0.340 ± 0.003. The electrode reaction is UO₂(HA)₂^4? + c = UO₂(HA)^2? + HA^3? Where pH 5.5, an irreversible and diffusion-controlled wave was obtained. The diffusion coefficients and kinetic parameters of complex species were determined by deducing instantaneous equations.     

Polarographic Properties and Electrochemical Reduction of 1,2-Dimethyl-3-indolyl Heteroaryl Ketones

The reduction of a series of 1,2-dimethyl-3-indolyl heteroaryl ketones, where the heteroaryl components are 2-benzoxazolyl, 2-behzpthiuzolyl. 1-methyl-2-benz-imidazolyl, 4- and 2-pyridyl, in aprotic and prone solution are described by means of DC. and AC. polarography, cyclic voltammetry and coulometry. In acetonitrile a two-step reduction mechanism with a reversible first and an irreversible second one-electron transfer has been observed, confirmed by AC.-polarographic and cyclovoltammetric measurements. The reduction potentials, between ?1.377 and ?1.787 V for the first step, and between ?1.937 and ?2.250 V (vs. Ag/AgCl) for the second, could not be correlated with the LUMO energies from HMO calculations which was explained as being due to a distorted configuration of the π systems. In protic media (ethanol/water) reduction occurs in an irreversible one-step mechanism. Measurements of the pH dependence of the half-wave potential in the pH* range 0–12 and the evaluated transfer coefficients show that the reduction step represents the transfer of two electrons and two protons yielding secondary alcohols and no dimerization products under any condition. These findings are supported by coulometric and macroscale electrolysis of the ketones; the products isolated were characterized by IR. and ¹H-NMR. data.     

524—NAD/NADH as a model redox system: Mechanism,mediation, modification by the environment

The biologically important redox couple, β-nicotinamide adenine dinucleotide/1,4,β-dihydronicotinamide adenine dinucleotide, provides a grossly reversible prototype system for an overall electrode reaction consisting of two successive one-electron (1 e^?) transfer steps coupled with (a) dimerization of an intermediate free radical product, (b) protonation—deprotonation of an intermediate product, (c) other chemical reactions, (d) adsorption of reactant, intermediate and product species, and (e) mediation by electrode surface species. Cathodic reduction of NAD⁺ proceeds through two 1 e^? steps well separated in potential; protonation of the free radical produced on the first step occurs prior to the second electron-transfer; a first-order chemical reaction coupled to the latter may involve rearrangement of an initial dihydro product to 1,4-NADH (and some 1,6-NADH). In the apparently single stage 2 e^? anodic oxidation of NADH, the initial step is an irreversible heterogeneous electron transfer, which proceeds to at least some extent through mediator redox systems located close to the electrode surface; the resulting cation radical, NADH^+?, loses a proton (first order reaction) to form a neutral radical, NAD^?, which may participate in a second heterogeneous electron transfer (ECE mechanism) or may react with NAD^+? (disproportionation mechanism DISP 1 or half-regeneration mechanism) to yield NAD⁺.     

卟啉配位剂meso－四（对－羧基苯基）卟啉（H_2TCPP）极谱行为的研究

采用直流极谱和循环伏安等多种手段研究了卟啉配位剂（Ｈ_２ＴＣＰＰ）在汞电极上的极谱性质。在碱性介质中，Ｈ_２ＴＣＰＰ的电极反应具有以下三个特性：（１）第一步为受扩散控制的２ｅ并有２Ｈ~＋参与的可逆还原过程，第二和第三步为不可逆各为２ｅ的还原反应；（２）Ｈ_２ＴＣＰＰ在汞电极上存在强吸附性；（３）第一步电极反应产物可发生随后的化学反应，即前两步还原经由ＥＣＥ机理进行。     

Electrochemical behaviour of tris (2,2′-bipyridine) osmium(II) complex in dimethylformamide

The electrochemical behaviour of Os(bpy)₃²⁺ (bpy=2,2′-bipyridine) has been investigated in N,N-dimethylformamide by utilizing predominantly the techniques of polarography and cyclic voltammetry. The study has been carried out at different temperatures in the range ?20 to +30° C. The number of reduction waves observed depends markedly on temperature. For intermediate temperatures, the complex exhibits six reduction waves, the maximum number of waves observed as a function of temperature.The first three reduction processes, corresponding to the first three reduction waves, are one-electron, diffusion-controlled reversible processes in all conditions. Conversely, process four is consistent with one-electron reversible transfer only at the lowest temperature. In fact, for higher temperatures the liberation of bpy, preferentially as a radical anion rather than a neutral molecule, occurs. In the latter case, the liberated neutral bpy molecule can be reduced by one-electron transfer. Process five is due to the reduction of species formed by chemical reaction in the preceding electrode process, i.e. the bpy radical anion and Os(bpy)₂^1?. Process six is consistent with the addition of five electrons to the starting complex, followed by the liberation and successive reduction of the bpy radical anion.     

Determination of Methylmercury by Phase-Sensitive A. C. Polarography

Abstract

The polarographic reduction of methylmercury in aqueous medium takes place in two steps, each of which involves the transfer of a single electron. The first reduction step proceeds reversibly both in direct current and in sinusoidal alternating current. Since the polarograms corresponding to this step are well defined, they can be successfully used to determine methylmercuric compounds by phase-sensitive sinusoidal a.c. polarography, in the concentration range 1 × 10^?7 ?1 × 10^?4 mole.dm.^?3     

Electrochemical behavior and voltammetric determination of the herbicide metribuzin at mercury electrodes

The electrochemical behavior of the herbicide metribuzin (4-amino-6-tert-butyl-4,5-dihydro-3-methylthio-1,2,4-triazin-5-one) at mercury electrodes was studied in aqueous solutions by direct current (DC) and tast polarography, differential pulse (DPV) and cyclic voltammetry (CV), and controlled-potential coulometry. The electrolysis products were separated and identified by chromatographic techniques combined with mass spectrometric detection. The reduction process in acid media includes two irreversible steps. In the first four-electron step the N–NH₂ and the 1,6-azomethine bonds are reduced. The second step leads to the formation of 5-tert-butyl-2,3,4,5-tetrahydroimidazol-4-one at the mercury-pool electrode. The first reduction step combined with adsorptive accumulation of the herbicide molecule at the mercury electrode surface was used for its determination by differential pulse adsorptive stripping voltammetry (DPAdSV). Calibration curves were linear in the range 1–30 μg L^–1 with a detection limit of 0.27 μg L^–1 (1 nmol L^–1) under the conditions used (buffer pH 4.5, E_acc = –0.45 V relative to Ag/AgCl and t_acc = 10 s). Preconcentration on solid-phase extraction columns (SPE-phenyl) was used for the determination of very small amounts of metribuzin in river water samples. Recovery was approximately 97%. The reproducibility of the analytical procedure including SPE treatment and DPV determination was expressed as relative standard deviations of 2.53 and 3.66% for 2 and 6 μg L^–1 metribuzin, respectively.     

A.C. polarographic determination of unsaturation

A wide range of olefins can be determined by a.c. polarography of their addition compounds with mercuric acetate. The first step in the reduction of these compounds is a reversible, one-electron process to form the organo-mercury radical and gives a well defined a.c. peak. The optimum conditions for the determination of a representative selection of olefins has been established.     

Coupling real-time surface-enhanced raman spectroscopy with linear-sweep voltammetry : Application to elucidation of electrochemical mechanisms

The broad-based utilization of real-time surface-enhanced Raman spectra, obtained by using an intensified diode-array detector, to examine interfacial redox transformations under cyclic voltammetric conditions is exemplified for the oxidation at gold electrodes of irreversibly adsorbed benzidine and the reduction of adsorbed azoxybenzene. The first reaction, involving a pair of irreversible one-electron oxidation steps, enabled a quantitative comparison to be made between the extent of oxidation of the adsorbed layer during the potential sweep and the simultaneous spectral response. Although relatively slow (10–50 mV s^?1) sweeps are required in order to yield spectral sequences having a suitably narrow (?50 mV) potential interval, the spectral data acquisition requires only a single potential perturbation. This “single-shot” spectroelectrochemical approach, desirable for the examination of irreversible interfacial systems, differs from conventional time-resolved methods that involve signal integration over numerous repetitive perturbations.     

Estimation of tungstate ion by tensammetry

It is shown that tungstate ion can be estimated quantitatively by the same experimental technique as that used for alternating current polarography. The underlying process differs, however, fundamentally from polarography in that no electron transfer takes place at the electrode and thus no d.c. polarographic steps arc obtained. The alternating current is produced solely by adsorption/ desorption processes at the electrode/solution interface, a phenomenon to which the name “tensammetry” has been given.The current-voltage curve for sodium tungstatc at pH 1.8 shows two -well defined tensammetric waves which can be used for purposes of quantitative estimation. No waves are obtained at pH values higher than 6, showing that the compound responsible for the production of the waves is not the simple WO₄^-4ion.