Diffusion-controlled Wave Research Articles

Trinuclear Nickel Catalyst for Water Oxidation: Intramolecular Proton-Coupled Electron Transfer Triggered Trimetallic Cooperative O–O Bond Formation

Trinuclear Nickel Catalyst for Water Oxidation: Intramolecular Proton-Coupled Electron Transfer Triggered Trimetallic Cooperative O–O Bond Formation

Competition between Hydrogen Bonding and Proton Transfer during Specific Anion Recognition by Dihomooxacalix[4]arene Bidentate Ureas.

Competition between hydrogen bonding and proton transfer reactions was studied for systems composed of electrogenerated dianionic species from dinitrobenzene isomers and substituted dihomooxacalix[4]arene bidentate urea derivatives. To analyze this competition, a second-order ErCrCi mechanism was considered where the binding process is succeeded by proton transfer and the voltammetric responses depend on two dimensionless parameters: the first related to hydrogen bonding reactions, and the second one to proton transfer processes. Experimental results indicated that, upon an increase in the concentration of phenyl-substituted dihomooxacalix[4]arene bidentate urea, voltammetric responses evolve from diffusion-controlled waves (where the binding process is at chemical equilibrium) into irreversible kinetic responses associated with proton transfer. In particular, the 1,3-dinitrobenzene isomer showed a higher proton transfer rate constant (∼25 M(-1) s(-1)) compared to that of the 1,2-dinitrobenzene (∼5 M(-1) s(-1)), whereas the 1,4-dinitrobenzene did not show any proton transfer effect in the experimental conditions employed.

New theoretical insights into the competitive roles of electron transfers involving adsorbed and homogeneous phases

The traditional textbook view of adsorptive features in electrochemistry, viz., involving a bell-shaped wave prior to or after the diffusional wave is certainly right but concerns a very limited series of conditions in which adsorption kinetics are too slow vs. the scan rate. Laviron examined the converse situations in which extremely rapid adsorption kinetics make the voltammetric process follow the classical diffusional behavior though the effective electrochemical reactions proceed via adsorbed species. Thanks to a new simulation approach, implemented in KISSA©, the present work examines intermediate situations which could not be investigated since they do not lead to analytical formulations. Besides allowing investigating the transition between the two above limiting behaviors, it is established that during such transitions voltammograms display CE-type behaviors in which the rates of the pseudo-antecedent chemical steps feature those of adsorption. An electroactive adsorbed species is indeed involved in a dynamic steady state between its adsorption and its consumption by electron transfer at the electrode surface so that its current is independent of the potential. This is a general situation presently overlooked in electrochemical theories. For example, the same CE-like behavior is also shown to occur during electropolymerization of redox polymers though it now is hidden under the monomer diffusion-controlled wave.

Voltammetric quantification of tamoxifen

The electrochemical behaviour of tamoxifen has been investigated at gold electrode by cyclic voltammetry (CV), differential pulse voltammetry (DPV) and squarewave voltammetric techniques. The dependence of current and potential on pH, concentration, scan rate, nature of solvent, surfactants and different surfactant concentration is investigated. Tamoxifen is oxidized in a single two-electron, irreversible and diffusion-controlled wave. Linear calibration plots are obtained over the concentration range 1.0-5.0 and 1.0-6.0 µgmL(-1) in 1.0 M KCl and Britton Robinson buffers (pH 2.51) respectively. The procedure has been applied to the assay of the drug in tablet form with mean percentage recoveries of 99.98%. The suggested method can be successfully applied to the determination of tamoxifen in different drug formulations.

ChemInform Abstract: Kinetic Studies on the Chemical and Electrochemical Reduction of Some 4-Arylidene-5-pyrazolone Compounds.

Kinetics of chemical and electrochemical reduction of some 4-arylidene-5-pyrazolones were studied. The chemical reduction was performed using aluminum hydride. The electrochemical reduction in 0.1 mol dm−3 TEAP/DMF was studied by cyclic and convolution potential sweep voltammetry at HDME. The title compounds exhibit two diffusion-controlled irreversible mono electronic cv waves. Both waves follow EC kinetics and they are attributed to the reduction of the pyrazolin carbonyl group. The kinetics of the chemical and electrochemical reduction were proposed and discussed in terms of the molecular structure of the subject arylidenes.

Electrochemical assembly of [Ru(bpy)2tatp]2+ associated with surfactants on the MWNTs/GC electrode

The electrochemical assembly of [Ru(bpy)2tatp]2+ (where bpy = 2,2′-bipyridine, tatp = 1,4,8,9-tetra-aza-triphenylene) on the multi-walled carbon nanotubes-modified glassy carbon electrode (MWNTs/GC) in the presence of anionic and cationic surfactants has been investigated. A diffusion-controlled wave and three prewaves are exhibited on the differential pulse voltammogram of [Ru(bpy)2tatp]2+. The formal potential of the prewaves is found to be much negative than that of the diffusion-controlled wave. An appropriate amount of anionic surfactants including dihexadecyl phosphate (DHP) and deoxyribonucleic acid (DNA) can prompt the assembly of [Ru(bpy)2tatp]2+ on the MWNTs/GC electrode by using the method of repetitive voltammetric sweeping. In contrast, cationic surfactant such as hexadecyl trismethyl ammonium chrolide (HTAC) dispersed on the MWNTs surface is found to inhibit the assembly of [Ru(bpy)2tatp]2+. Meanwhile, the assembled principle of [Ru(bpy)2tatp]2+ on the MWNTs/GC electrode with the participation of surfactants is discussed in detail.

DNA-promoted electrochemical assembly of [Ru(bpy) 2dpp] 3+/2+ on the ITO electrode by introducing copper(II) ion

The electrochemical assembly of [Ru(bpy) 2dpp] 3+/2+ (where bpy = 2,2′-bipyridine, dpp = 2,3-bis (2-pyridyl) pyrazine) promoted by calf thymus DNA on an ITO electrode based on the introduction of copper(II) ion has been investigated. There exists a diffusion-controlled wave and two prewaves for the complex in the differential pulse voltammetric sweeping process. The formal potential of the high prewave shift ca. 0.530 V negatively compared with that of the diffusion-controlled wave. Dpp ligand with two vacant chelating N sites in the complex can bite Cu 2+ and the resultant heterometallic complex shows a weakened assembly in contrast to that of [Ru(bpy) 2dpp] 3+/2+ alone. Furthermore, double stranded DNA is able to accelerate the assembly of the ruthenium complex and heterometallic complex generated by chelating with Cu 2+ by using the ITO surface, the prompted strength of the latter is far stronger than the former. Their assembled mechanism enhanced by DNA is proposed.

Electrocatalytic oxidation of hydrazine on platinum electrodes in alkaline solutions

The mechanism and structure sensitivity of the electrocatalytic oxidation of hydrazine on platinum in alkaline solutions were investigated using cyclic voltammetry, steady-state current measurements, and on-line electrochemical mass spectrometry. The voltammetry of hydrazine oxidation on platinum in alkaline media is characterized by a single diffusion-controlled wave. The on-line electrochemical mass spectrometry measurements of hydrazine oxidation on Pt(1 1 1), Pt(1 0 0), and Pt(1 1 0) surfaces indicated the formation of molecular nitrogen. No oxygen-containing nitrogen compounds were detected under the given experimental conditions. A comparative analysis of voltammetric data for hydrazine oxidation in alkaline solutions on the three surfaces at low overpotentials points to a structure sensitivity of the reaction. The electrocatalytic activity of basal planes increases in the order Pt(1 1 0) > Pt(1 0 0) > Pt(1 1 1), as deduced from the onset of the oxidation wave. The structure of the electrocatalyst surface affects the mechanism of the reaction, although without affecting the selectivity. At low overpotentials the hydrazine oxidation on the Pt(1 1 0) and Pt(1 1 1) surfaces is limited by the rate of electrochemical steps, whereas on Pt(1 0 0) a chemical step that probably involves N 2H 2 adsorbed intermediate is the rate-determining step. Platinum is more active in hydrazine oxidation in alkaline solution than in acidic solutions. In contrast to hydrazine oxidation on platinum in acidic media, the stabilization of chemisorbed hydrazine does not occur to a significant extent in alkaline media.

DNA-enhanced assembly of [Ru(bpy) 2ITATP] 3+/2+ on an ITO electrode

A novel technique for controllable assembly of [Ru(bpy) 2ITATP] 3+/2+ (where bpy = 2,2′-bipyridine, ITATP = isatino[1,2-b]-1,4,8,9-tetraazatriphenylene) on an ITO electrode in the absence and presence of calf thymus DNA is proposed. The [Ru(bpy) 2ITATP] 3+/2+ and double stranded DNA is assembled onto the ITO electrode using repetitive voltammetric sweeping. The assembly is confirmed by ex situ cyclic voltammetry and the fluorescence microscopy. A pair of diffusion-controlled waves and prewaves for [Ru(bpy) 2ITATP] 3+/2+ is observed in the voltammetric sweeping process. The formal potential of the prewaves is found to be much negative than that of the diffusion-controlled waves. The controllable assembly of [Ru(bpy) 2ITATP] 3+/2+ on the ITO surface is accelerated by DNA and affected by ionic strength. With this DNA-prompted electrochemical technique, a multifunctional biomolecular film containing surface-confined redox center of controllable thickness is fabricated.

DNA-promoted electrochemical assembly of [Ru(bpy) 2IP] 3+/2+ at an ITO electrode

A controllable assembly technique of [Ru(bpy) 2IP] 3+/2+ (where bpy = 2,2′-bipyridine and IP = imidazo[4,5,f][1,10]phenanthroline) promoted by calf thymus DNA at an ITO electrode is proposed. The stable assembled layer containing [Ru(bpy) 2IP] 3+/2+ and double stranded DNA is obtained on the ITO electrode using repetitive voltammetric sweeping, confirmed by ex situ voltammetry, X-ray photoelectron spectroscopy (XPS) and the inverted fluorescence microscopy. There exist two pairs of diffusion-controlled waves and two pairs of prewaves for [Ru(bpy) 2IP] 2+ in the voltammetric sweeping process. The half-wave potentials of the prewaves are far more negative than those of the diffusion-controlled waves. These experimental results suggest that double stranded DNA is enable to accelerate and increase the controllable assembly of Ru(bpy) 2IP] 3+/2+ by using the ITO surface. The fluorescence microscopy imaging reveals that [Ru(bpy) 2IP] 3+/2+ has the ability to bind with double strand DNA. The fluorescence intensity of [Ru(bpy) 2IP] 3+/2+ with DNA is stronger than that without DNA.

Voltammetric Determination of Benazepril and Ramipril in Dosage Forms and Biological Fluids through Nitrosation

A simple and highly sensitive voltammetric method was developed for the determination of benazepril (I) and ramipril (II). The compounds were treated with nitrous acid, and the cathodic current produced by the resulting nitroso derivatives was measured. The voltammetric behavior was studied by adopting direct current (DCt), differential pulse (DPP), and alternating current (ACt) polarography. Both compounds produced well-defined, diffusion-controlled cathodic waves over the whole pH range in Britton-Robinson buffers (BRb). At pH 3 and 5, the values of diffusion-current constants (Id), were 5.90 +/- 0.40 and 6.66 +/- 0.61 for I and II, respectively. The current concentration plots for I were rectilinear over the range of 1.5-40 and 0.1-30 microg/mL in the DCt and DPP modes, respectively; for II, the range was 2-30 and 0.1-20 microg/mL in the DCt and DPP modes, respectively. The minimum detectabilities (S/N = 2) were 0.015 microg/mL (about 3.25 x 10(-8)M) and 0.012 microg/mL (about 2.88 x 10(-8)M) for I and II, respectively, adopting the DPP mode. Results obtained for the proposed method when applied to the determination of both compounds in dosage forms were in good agreement with those obtained using reference methods. Hydrochlorthiazide, which is frequently co-formulated with these drugs, did not interfere with the assay. The method was also applied to the determination of benazepril in spiked human urine and plasma. The percentage recoveries adopting the DPP mode were 96.2 +/- 1.21 and 95.7 +/- 1.61, respectively.

Study of the Correlation of the Cyclic Voltammetric Responses of a Nonionic Surfactant Containing an Anthraquinone Group with the Dissolved States

Cyclic voltammetric behavior of a nonionic surfactant, α-(anthraquinonyloxyhexyl)-ω-hydroxy-oligo(ethylene oxide), ACPEG, has been studied in detail in aqueous media with NaCl as the supporting electrolyte. Attempts have been made to correlate the electrochemical responses with the dissolved states of the surfactant. The shapes of the cyclic voltammograms at a glassy carbon electrode have proved to be fairly dependent on the concentration of ACPEG and on the redox state of the anthraquinone group. Below the critical micelle concentration (cmc), the cyclic voltammogram (v = 10 mV/s) corresponds to the surfactant molecules adsorbed onto the electrode surface. The adsorption of ACPEG molecules at the electrode surface is very weak in nature owing to its low surface activity. A high cmc value and weak adsorption allow diffusion of the monomeric species prior to the reduction to contribute to the total current even below the cmc. At concentrations higher than the cmc, the diffusion-controlled waves are superim...

Anodic Polarographic Behavior and Determination of Aztreonam in Dosage Forms and Biological Fluids

ABSTRACT The anodic polarographic behavior of aztreonam was studied in different supporting electrolytes. In 0.1 M hydrochloric acid, a single, well-defined, diffusion-controlled anodic wave was produced. The current-concentration relationship was found to be rectilinear over the ranges 2x10−5 to 2x10−3M and 1x10−5 to 1.6x10−1M in the direct current (DC) and differential pulse polarography (DPP) modes, respectively, with a minimum detectability of 4x10−6 M using the latter technique. The mechanism of the electrode reaction was verified. The proposed method was applied successfully to the determination of aztreonam in pure form, in dosage forms and in biological fluids. L-arginine which is frequently co-formulated with aztreonam did not affect the polarographic signal of aztreonam to any significant extent.

Electrochemical Studies of a Redox-Active Surfactant. Correlation between Electrochemical Responses and Dissolved States

Electrochemical studies of a nonionic surfactant, α-(ferrocenylundecyl)-ω-hydroxy-oligo(ethylene oxide) (FPEG), have been conducted in 0.16 M NaCl aqueous solution at a glassy carbon electrode. The correlation between electrochemical responses and dissolved states has been examined in detail. Cyclic voltammograms (v = 10 mV/s) of the FPEG solutions change with the concentration; below the critical micelle concentration (cmc), the voltammetric shape is of a typical surface-confined species, while above the cmc the diffusion-controlled waves are superimposed at the cathodic side of the surface waves, and far above the cmc the voltammetric shape is of a typical diffusion-controlled species. FPEG in the aqueous solutions adsorbs onto the glassy carbon electrode surface to form a monolayer because of its surface activity. The adsorption isotherm has been represented by the Frumkin isotherm, which includes the repulsive interaction between the adsorbed FPEG molecules. Above the cmc, FPEG forms micelles. Chronoa...

Voltammetric reduction of polyprotic acids at the platinum microelectrode: dependence on supporting electrolyte

The voltammetric behavior of weak and strong polyprotic acids was studied at platinum microelectrodes under steady-state conditions in solutions of very low ionic strength, including those without added supporting electrolyte. Three diprotic acids (sulfuric, oxalic, and malonic), one triprotic acid (phosphoric acid), and one tetraprotic acid (pyrophosphoric acid) were chosen for the investigation. The reduction of hydrogen ion in solutions of strong diprotic acid (sulfuric acid) with no supporting electrolyte results in a voltammetric wave 1.5 times higher than the diffusional wave obtained with excess electrolyte. This agrees well with the theoretical prediction for one-electron reduction of monovalent cation accompanied by divalent anion. The voltammetric reduction waves of oxalic, malonic, and phosphoric acids in solutions without supporting electrolyte were twice as high as the diffusion-controlled waves with excess electrolyte. This is what theory predicts for a monoprotic strong acid. The strong (H +) and weak (HA − for oxalic and malonic, H 2A − for phosphoric, and H 2A 2− for pyrophosphoric acid) forms of acids are reduced in one (oxalic and malonic) or two (phosphoric and pyrophosphoric) voltammetric waves. The influence of the concentration of supporting electrolyte on the height of reduction waves of acids was examined for two kinds of supporting electrolyte, with mono- and divalent cations, over a wide range of their concentrations. The experimental results are compared with theoretical predictions. The diffusion coefficients of H(COO) − 2 and HCH 2(COO) − 2 ions were calculated from the steady-state diffusion-controlled currents.

Convolution—deconvolution cyclic voltammetric equilibria study on cadmium(II)- L-histidinate in aqueous solutions

POLAG computer programme was employed for processing convoluted—deconvoluted cyclic voltammetric data to study cadmium(II)- L-histidinate system. This was performed in 0.1 mol/dm 3 NAClO 4 aqueous solution at different pH's at 298K. The results show that the reduction of cadmium(II) and its complexes proceeds via a reversible and diffusion-controlled wave of two electrons at the entire range of pH. The system was studied at two ranges of pH (6.73–7.44 and 8.53–8.92). It was evident that the system at the first range of pH is well described by the presence of a mixture of binary complexes, [Cd(HisO.H)], [Cd(HisO.H 2) 2] and a ternary one [Cd(HisO.H)(HisO.H 2)]. For the second pH range, it was revealed that the most likely model corresponds to the presence of a mixture of [Cd(HisO) 2] and [Cd(OH)] species. The overall stability constants were computed. The structure of the detected complexes was discussed on the basis of ligating sites of histidine.

Polarographic determination of metyrosine through treatment with nitrous acid.

A simple and sensitive polarographic method is described for the determination of metyrosine through treatment with nitrous acid. The different experimental parameters affecting the derivatization process, as well as the polarographic analysis were studied. The derivatization product was found to be reducible at the dropping mercury electrode over the whole pH range in Britton Robinson buffers. At pH 5, a well-defined diffusion-controlled cathodic wave was produced. The limiting current versus the concentration plot was linear over the range 8-80 mumol/l in the direct current mode with a detection limit of 0.2 mumol/l. The method was then applied to the determination of metyrosine capsules, and the results obtained were in good agreement with those given by the USP method.

Voltammetric reduction of 2-(4?-N,N-dimethylaminostyryl)-1-ethyl pyridinium iodide inDMF

The voltammetric reduction mechanism of 2-(4′-dimethylaminostyryl)-1-ethyl pyridinium iodide at the hanging dropping mercury electrode has been studied in N,N-dimethylformamide solution containing 0.1 mol dm−3 tetraethylammonium perchlorate. The depolarizer is reduced via a single diffusion-controlled irreversible two-electron cyclic voltammetric wave. The wave is attributed to the reduction of the azomethine bond of the pyridinium nucleus. Cyclic voltammetric studies indicate that the cv wave follows an ECE mechanism. The chemical reaction is proposed to be a protonation.

Synthesis and physico-chemical studies on transition metal complexes of symmetric bis-Schiff base ligands

A new series of symmetric bis-Schiff bases derived from 1,2-diaminoethane with bis-salicylaldehyde (H2L1) and/or bis-o-hydroxyacetophenone (H2L2) have been prepared. The metal(II) complexes of these ligands were isolated and characterized by elemental analyses, magnetic moments and spectral (visible, i.r.) measurements. I.r. spectra show that the ligands behave as dianionic or neutral tetradentate. Replacing the imine C—H with a bulky methyl group leads to a steric interaction with the methylene protons of the chelate ring. The (NiL2) is diamagnetic four coordinate square planer. The electronic spectrum shows that [Cu2(H2L1)Cl4(OH2)4] complex is tetragonally distorted octahedral. The polarographic reduction of Schiff bases was investigated in Britton-Robinson buffer solutions at pH 3–12. The polarograms at dme indicated that depolarizers, (H2L1 & H2L2) are reduced through 8-electron irreversible diffusion-controlled wave. The mechanistic pathway of the electrode reaction is discussed.

Electrochemical behavior and simultaneous determination of copper(II) and cobalt(II) at a dropping mercury electrode in the presence of phenyl-2-picolylketone-2-pyridylhydrazone

Differential pulse polarography has been investigated for the simultaneous determination of copper and cobalt in the presence of phenyl-2-picolylketone-2-pyridylhydrazone, PPKPyH, as a complexing agent at pH 4.2; μ = 0.1. Well-defined diffusion-controlled waves were obtained for both systems. Reversible and irreversible waves were observed for copper and cobalt systems, respectively. A method is proposed for the simultaneous determination of these metals when they are present together in pure solution as the difference in their E 1/2 values is sufficient for this purpose. The method has also been applied to the simultaneous determination of these metal ions in some synthetic samples.