Current-time Curves Research Articles

Selective conversion of CO into methanol at ordinary temperature. Part 4. Activation by iron(II), iron(III), and chromium(III) complexes

The conversion of carbon monoxide into methanol has been performed at a platinum electrode modified with K2FeII[FeII(CN)6]. This reaction was activated by catalytic systems consisting of a primary alcohol and a series of iron(II), iron(III), or chromium(III) complexes. The current–time curves at a constant potential showed an oscillation, the period, induction time, and amplitude of which are a function of temperature, applied potential, and the nature and concentration of the metal complex or alcohol added. The pseudo-rate constant, the current efficiency, and the activation energy for the methanol formation have been determined. The current efficiency was almost 100% except in the presence of pentachlorochromate(III) and pentafluoroferrate(II), and the activation energy was 2.9–5.8 kcal mol–1 Infrared spectra of the metal complex isolated from the catalyst solution indicated that the reduction of CO takes place via methyl formate as an intermediate. These results are suggestive of a mechanism in which co-ordination of CO to the central metal in the form of methyl formate is a key step, and the diffusion of the co-ordinated species toward the modified electrode is rate determining.

A simple method of digitalization of polarographic curves. Description of the apparatus

The paper deals with the construction of a simple electronic equipment made from commercially available components, suitable for recording alternately the potential and the current data for a polarographic curve on a five-track perforated tape. The apparatus can operate in three modes making it possible either to record the polarographic curve or the current as a function of the time at constant polarization potential or the current-time curves on individual drops.

Irregular patterns of polarographic maxima in surfactant dispersions

Heterodispersions of fluid surfactants, such as unsaturated fatty acids and their esters, produce irregular oscillations of polarographic maxima of Hg(II) in seawater owing to random collisions of aggregates of variable size with the mercury electrode/solution interface. By recording the current-time curves at the potential of polarographic maximum in diluted heterodispersions it is possible to characterize single events of coalescence and relaxation of fluid aggregates into the adsorbed layer at the mercury electrode/solution interfaces. The technique developed enables a direct analysis of organic aggregates in natural waters.

Irregular patterns of polarographic maxima in surfactant dispersions

Heterodispersions of fluid surfactants, such as unsaturated fatty acids and their esters, produce irregular oscillations of polarographic maxima of Hg(II) in seawater owing to random collisions of aggregates of variable size with the mercury electrode/solution interface. By recording the current-time curves at the potential of polarographic maximum in diluted heterodispersions it is possible to characterize single events of coalescence and relaxation of fluid aggregates into the adsorbed layer at the mercury electrode/solution interfaces. The technique developed enables a direct analysis of organic aggregates in natural waters.

Influence of diffusion coefficient ratio [formula omitted] on potential-step chronoamperometric and linear voltammetric current at stationary planar electrodes in the case of a pseudo-first-order EC catalytic reaction scheme

Equations for limiting potential-step chronoamperometric and linear voltammetric currents at stationary planar electrodes for the case D O ≠ D R have been derived for a pseudo-first-order EC catalytic reaction scheme with reversible electrochemical and irreversible chemical reactions. The limiting potential-step chronoamperometric and linear voltammetric current curves have been calculated for various values of the diffusion coefficient ratio D O D R . The influence of the ratio D O D R on the theoretical currents has been examined. Theoretical possibilities to deduce both the kinetic parameter of the homogeneous chemical catalytic reaction and the ratio D O D R from the experimental limiting potential-step current—time curves or linear voltammetric current—potential curves have been discussed. A simple diagnostic criterion for linear voltammetry allowing evaluation of the chemical reaction rate constant and the ratio D O D R , has been proposed.

Voltammetric and chronoamperometric studies of tellurium electrodeposition of glassy carbon and gold electrodes

Electrodeposition of tellurium (IV) from acidic solutions was carried out on glassy carbon and gold electrodes by means of voltammetry and chronoamperometry. In both cases the anomalous recorded current-potential and current-time curves indicated the influence of adsorbed tellurium(0). Some considerations of the nucleation are discussed to explain the transient responses observed when double potential steps are applied. The semiconductor properties of tellurium when deposited as a thin layer may also be taken into account to explain the observed differences with the electroplating of a pure metal.

Studies concerning the growth of cadmium dendrites. II. Morphology in acidic media

The morphology of cadmium deposits formed during potentiostatic deposition onto etched cadmium substrates in a range of cadmium sulphate solutions with 0.5 mol dm−3 sulphuric acid as supporting electrolyte has been investigated. The deposit morphology and induction time was found to be both concentration and overpotential dependent. At 10−1 mol dm−3 CdSO4 for an overpotential range −20 to −80 mV, large crystalline aggregates were observed and large dendrites resulted after longer deposition times. The induction time was less than 1 min and the current time curves linear, indicating instantaneous rather than progressive initiation. At 10−2 mol dm−3 CdSO4 the morphology varied from fine, 2D-fern dendrites atη=−75 mV to needle dendrites atη=−150 mV. The morphology at 10−3 mol dm−3 CdSO4 closely resembled that at 10−2 mol dm−3 CdSO4 but, showed finer structural detail with less filling in of the main skeletal structure. The induction time was an order of magnitude greater at 10−3 mol dm−3 CdSO4 than at 10−2 mol dm−3 CdSO4, and the time taken to grow dendrites of the same length was also increased.

A Potential-step Chronoamperometric Study of Ion Transfer at the Water/Nitrobenzene Interface

Abstract Potential-step chronoamperometry, cyclic voltammetry, and chronopotentiometry have been applied to the study of the transfer of the tetramethylammonium (TMA+) ion across the interface between a 0.1 mol dm−3 lithium chloride aqueous solution and a 0.1 mol dm−3 tetrabutylammonium tetraphenylborate nitrobenzene solution. An analysis of the current-time and current-potential curves obtained by means of potential-step chronoamperometry indicates that the transfer of the TMA+ ion across the water/nitrobenzene interface is a reversible diffusion-controlled process, which can be explained by the theory of the chronoamperometry of a reversible charge transfer at the electrode surface. The values of the diffusion coefficient of the TMA+ ion in the aqueous phase and of the reversible half-wave potential of the TMA+ ion transfer, as determined by means of potential-step chronoamperometry, agreed well with those determined by means of cyclic voltammetry and chronopotentiometry.

Chronoamperometry of polymer-modified electrodes Charge transport by diffusion and migration

A theoretical analysis of charge-transport phenomena due to both diffusion and electrostatic migration in a polymer-modified electrode is presented for chronoamperometry. Concentration distributions and current-time curves were derived for the simple case of a constant electric field in the polymer film.

Chronoamperometry of polymer-modified electrodes: Charge transport by diffusion and migration

A theoretical analysis of charge-transport phenomena due to both diffusion and electrostatic migration in a polymer-modified electrode is presented for chronoamperometry. Concentration distributions and current-time curves were derived for the simple case of a constant electric field in the polymer film.

Comparative studies of the electrochemical behaviour of aluminium in 1 M H 2SO 4/water and in 1 M H 2SO 4/dimethylformamide (anhydrous)

Anodic current potential curves of Al (99.999%) in H 3O/1 M H 2SO 4 are quite different from that in anhydrous DMF/1 M H 2SO 4. Al is active in DMF/H 2SO 4, and at a potential of 0.4 V (SCE) gives a constant current-time curve, where in H 2O/H 2SO 4 the current decreases in the same time to a low value. In DMF, it decreases also at a later step. This happens when the solution is saturated with anhydrous Al 2(SO 4) 3 (0.18 g l −1). Comparison of cut-off curves after polarization to 0.4 and 3 V (SCE) shows that the curves are similar, but the potential decreases more slowly in DMF/H 2SO 4 than in H 2O/H 2SO 4. This proves that the space charge of Al with a Al 2(SO 4) 3 layer in anhydrous DMF has a much higher half-time of potential decay than that of Al with an oxide layer in water. This can be explained on the basis that the oxide layer in water has incorporated water molecules and perhaps H + or OH −, whereas the anhydrous Al 2(SO 4) 3 layer is free from solvent molecules, and the ionic conductivity of Al 2(SO 4) 3 itself is very low. The high space charge in DMF/H 2SO 4 could also be shown by potentiostatic measurements using an interrupter. The different behaviour of Al, when passivated by salt layers on the one hand or by oxide layers on the other, is discussed.

The Stress Corrosion Cracking of Deformed Austenitic Stainless Steel in an Acidified Chloride Solution

Abstract The critical electrochemical parameters and their relation to SCC were examined in untransformed 301 stainless steel in an acidified concentrated NaCl plus Na2SO4 solution. Potential-time curves showed that a substantial period of time was necessary before the abrupt shift to the rest potential. During this period, there was a copious evolution of hydrogen which ceased abruptly coincident with the potential shift. A critical cracking potential was clearly identified near −375 mV, SCE. There was a true incubation period for crack initiation even after the potential had attained the corrosion potential. Current-time curves clearly indicated the formation of a pseudo-passive film where the current dropped from an initially high value to close to zero. This film, the precursor to SCC, was relatively thick and on examination after removal revealed Cr enrichment, Fe and Ni depletion, and chlorine buildup. Electron diffraction identified the film as a chromium oxyhydroxide. It is concluded that several ...

Catalytic prewaves of cobalt(II) in presence of dimethyldithiocarbamate

Two well-defined prewaves of cobalt(II) in the presence of dimethyldithiocarbamate (Me 2DTC) in a medium containing ammonia (borax-NH 4Cl and NH 4OH-NH 4Cl buffer) are observed. The effect of gelatine, Me 2DTC, cobalt(II), ammonium hydroxide and ammonium chloride concentration, mercury reservoir height, buffer capacity, pH and temperature on the prewaves are described. In addition, the characteristics of the current-time and electrocapillary curves are shown. This allows to propose a mechanism where the cobalt(II) prewaves are produced by cobalt(II) complexes with Me 2DTC. In these complexes, the cobalt(II) and Me 2DTC are in the relationship 1:1 and 1:2 for the first and second prewave respectively. A value of the rate constant for the formation of the first prewave catalytic complex of 6.0 x 10 6 lmol −1 s −1 has been found at 20°C.

The riboflavin—dihydroriboflavin system in acidic medium on mercury: Part II. A polarographic investigation

Polarographic current—potential characteristics and current—time curves for the reduction of riboflavin (RF) to dihydroriboflavin (DRF) in 0.01 M HClO 4 + 0.09 M NaClO 4 have been examined in detail. It has thus been shown that the RF adsorption prewave is due to the formation of two overlapping adsorbed monolayers of DRF molecules. Lateral interactions of RF molecules between themselves, of DRF molecules between themselves and of RF with DRF molecules in the first adsorbed monolayer are weak, whereas vertical interactions between overlapping DRF molecules are relatively strong. This explains the abrupt increase in the slope of the RF adsorption prewave and its shift towards more positive potentials as soon as the second adsorbed monolayer of DRF starts to form. The so-called “normal” wave for RF reduction has a half-wave potential E II 1 2 = −0.180 V/SCE, practically coinciding with the formal potential of the RF/DRF couple, and a slope corresponding to a reversible two-electron reduction unaffected by semiquinone formation. The shape of polarographic mean current vs. potential curves has been accounted for through an approximate solution of the corresponding diffusional problem.

404 - Potential induced compaction of adsorbed polyelectrolytes at electrode surfaces: Part II. A model for the π-state of DNA

At distinct potentials conformational transitions of adsorbed polynucleotides, especially double- and single-stranded DNA on the h.m.d.e. occur, recorded by a.c. polarographic current-time curves at the potential of peak 2. From the dependences of such fading reactions on potential, temperature, a.c. frequency, a.c. amplitude, ionic strength, pH value, complex formation and other effects, a coulombic model for DNA adsorption and compaction is established on the basis of current polyelectrolyte theory. The following steps are essential: 1. partial dehydration of adsorbed segments; 2. partial transition B → A form; 3. enhancement of fluctuation in base pairing; 4. compaction of neutralized DNA (π-state) by enhancement of cations in the vicinity of the electrode; 5. dissociation field effect on condensed counter ions in the attached parts of DNA; 6. direct and indirect (hopping mechanism) electron uptake, especially at lower pH values A.c-polarography (after Breyer) indicates certain transitions of tertiary structures, not only of adsorbed polynucleotides but also polyamino acids.

404 — Potential induced compaction of adsorbed polyelectrolytes at electrode surfaces Part II. A model for the π-state of DNA,

At distinct potentials conformational transitions of adsorbed polynucleotides, especially double- and single-stranded DNA on the h.m.d.e. occur, recorded by a.c. polarographic current-time curves at the potential of peak 2. From the dependences of such fading reactions on potential, temperature, a.c. frequency, a.c. amplitude, ionic strength, pH value, complex formation and other effects, a coulombic model for DNA adsorption and compaction is established on the basis of current polyelectrolyte theory. The following steps are essential: o (1) partial dehydration of adsorbed segments; (2) partial transition B→A form; (3) enhancement of fluctuation in base pairing; (4) compaction of neutralized DNA ( π -state) by enhancement of cations in the vicinity of the electrode; (5) dissociation field effect on condensed counter ions in the attached parts of DNA; (6) direct and indirect (hopping mechanism) electron uptake, especialy at lower pH values. A.c. -polarography (after Breyer) indicates certain transitions of tertiary structures, not only of adsorbed polynucleotides but also polyamino acids.

347 — Development of an electrochemical catalyst-binder electrode for glucose measurement

First results about glucose measurement with a catalyst-binder electrode in phosphate buffer solution at 37°C are shown. A new type of compact, electrochemical, glucose sensor was developed, which consists of three electrodes: the working electrode, a 0.5 mm diameter glass-sheathed platinum wire, was covered with a mixture of catalyst and binder, and two Ag¦AgCl electrodes are used as the reference and the rejuvenating electrode respectively. Mixtures of the catalyst platinum black, palladium black and platinum black adsorbed at charcoal with the binders PMMA and PVA were investigated for various mixing proportions. The glucose concentration dependence of the sensor behaviour was studied by means of cyclic voltammetry, potentiostatic measurements of current-time curves and non-equilibrium measurements with a periodic alternation of rejuvenation and measurement. For the lastmentioned measuring method a special programmable control unit was designed. The calibration curves for the determination of glucose measured by using the control unit exhibit a nearly linear dependence up to a concentration of 44.4 mM.

Development of an electrochemical catalyst-binder electrode for glucose measurement

First results about glucose measurement with a catalyst-binder electrode in phosphate buffer solution at 37°C are shown. A new type of compact, electrochemical, glucose sensor was developed, which consists of three electrodes: the working electrode, a 0.5 mm diameter glass-sheathed platinum wire, was covered with a mixture of catalyst and binder, and two Ag¦AgCl electrodes are used as the reference and the rejuvenating electrode respectively. Mixtures of the catalyst platinum black, palladium black and platinum black adsorbed at charcoal with the binders PMMA and PVA were investigated for various mixing proportions. The glucose concentration dependence of the sensor behaviour was studied by means of cyclic voltammetry, potentiostatic measurements of current-time curves and non-equilibrium measurements with a periodic alternation of rejuvenation and measurement. For the lastmentioned measuring method a special programmable control unit was designed. The calibration curves for the determination of glucose measured by using the control unit exhibit a nearly linear dependence up to a concentration of 44.4 m M .

Electrochemistry in fluosol-43, a perfluorinated blood substitute: The influence of pluronic f-68 surfactant on some polarographic electrode processes

The inhibition of thaUium(I), lead(II) and cadmium(II) reductions at the dropping mercury electrode is studied in Fluosol-43 and explained as a result of surfactant adsorption. The adsorption of PIuronic F-68 on the DME, examined by electrocapillary, tensammetric and current—time curves, is diffusion-controlled. The similarity established between aqueous solutions of Pluronic F-68 at concentrations near the critical micelle concentration and Fluosol-43 confirms that the adsorption depends only on the monomer form of the surfactant. The magnitude of the inhibitory effect, evaluated from charge-transfer coefficients and rate constants, decreases in the order CD 2+ > Pb 2+ > Tl +; for the same cation, inhibition depends on the nature of the anion of the supporting electrolyte, the adsorbability of which decreases in the sequence ClO 4 - > NO 3 - > SO 4 2-.

Electrochemical formation and reduction of silver oxides in alkaline media

The anodic oxidation of silver electrodes in NaOH solution and the reduction of the silver oxides formed were studied by potential step chronoamperometry. Oxidation of Ag to Ag 2O is a diffusion-controlled reaction, the diffusion control being established in the solid phase. Oxidation of Ag 2O to AgO proceeds via a nucleation and growth-controlled process. The amount of AgO decreased with increasing step height. The current—time curves for this reaction have been analysed with the Kolmogoroff—Avrami equation. Reduction of AgO to Ag 2O occurs initially on the outside of the electrode, and the rate of the reaction is limited by diffusion of ions across the thickening layer of Ag 2O. Reduction of Ag 2O to Ag proceeds via a nucleation and growth reaction.