Cathodic Potential Shift Research Articles

Electrodeposition of Zn–Co and Zn–Co–Fe alloys from acidic chloride electrolytes

The electrodeposition operating conditions for Zn–Co and Zn–Co–Fe alloys from chloride baths were studied. The electrodeposition was performed on a high strength steel substrate, under galvanostatic conditions, for a range of current densities at varying Co 2+ and Fe 2+ bath concentrations and at different temperatures. A transition current density was noticed above which a transition from normal to anomalous deposition took place. Below the transition current density electrodeposition of alloys with a higher amount of Co was obtained. Above the transition current density (i.e. in the anomalous range), both Zn–Co and Zn–Co–Fe alloys were deposited with a Co content lower than the composition reference line. This transition current density that resulted in normal to anomalous deposition was attributed to the shift in cathodic potential or polarization. However, it was found that under certain conditions the transition occurred from very high wt.% Co to almost equal to the amount of Co in the electrolyte. The increase of Fe 2+ ions in the electrolyte assisted in increasing the Co content and decreasing the Zn content in the deposits. In addition, the increase of Fe 2+ resulted in shifting the transition current density to a lower value. However, the cathodic current efficiency decreased with the addition of Fe 2+ ions in the electrolyte.

Tris( N-pyrrolidinyl)phosphine substituted diiron dithiolate related to iron-only hydrogenase active site: Synthesis, characterization and electrochemical properties

A tris( N-pyrrolidinyl)phosphine (P(NC 4H 8) 3) monosubstituted complex, [(μ-pdt)Fe 2(CO) 5P(NC 4H 8) 3] ( 2) was synthesized as a functional model of the hydrogen-producing capability of the iron hydrogenase active site. The structure was fully characterized by X-ray crystallography. IR and electrochemical studies have indicated that the P(NC 4H 8) 3 ligand has better electron-donating ability than that of those phosphine ligands, such as PMe 3, PTA (1,3,5-triaza-7-phosphaadamantane), PMe 2Ph PPh 3, and P(OEt) 3. The electrocatalytic activity of 2 was recorded in CH 3CN in the absence and presence of weak acid, HOAc. The cathodic shift of potential at −1.98 V and the dependence of current on acid concentration have indicated that complex 2 can catalyze the reduction of protons to hydrogen at its Fe 0Fe I level in the presence of HOAc. IR spectroelectrochemical experiments are conducted during the reduction of 2 under nitrogen and carbon monoxide, respectively. The formation of a bridging CO group during the reduction of 2 at −1.98 V has been identified using IR spectroelectrochemical techniques, and an electrocatalytic mechanism of 2 consistent with the spectroscopic and electrochemical results is proposed.

A study of quasi reversible nitro radical anion from β-nitrostyrene at wax-impregnated carbon paste electrode

A comprehensive study of the electrochemical reduction of Β-nitrostyrene and the corresponding heterocyclic analogue has been carried out in aprotic media using wax-impregnated carbon paste electrodes. Nitrostyrene exhibits quasi-reversible reduction process in aprotic medium at the waximpregnated carbon paste electrodes as compared to other electrodes reported in the literature. The nitroradical anion couple detected in the presence of tetrabutyl ammonium perchlorate is found to be stable only in aprotic media. Though, as reported, the pharmacological activity related to this nitro radical anion and its therapeutic value are related to the stability of the nitro radical anion, the stability itself depends on the electrode system employed. Added benzoic acid is found to bring about a positive shift in cathodic peak potential.

Electrochemical Characteristics of Poly(o-Aminobenzoic Acid) Modified Glassy-Carbon Electrode and Its Electrocatalytic Activity towards Oxidation of Epinephrine

Poly(o-aminobenzoic acid) (o-ABA) film is deposited on glassy-carbon electrode (GCE) by electropolymerization in pH 7.0 phosphate buffer solution (PBS). Electrochemical behavior of modified electrode is investigated by electrochemical impedance spectroscopy (EIS), different pulse voltammetry (DPV), and cyclic voltammetry (CV). The results indicate that there is a greater resistance during the electron transfer process in poly(o-ABA) film than in bare GCE for the redox of [Fe(CN)6]3−/[Fe(CN)6]4−. Further research indicates that epinephrine (EP) can be strongly absorbed on the surface of the poly(o-ABA) film-modified electrode. The modified electrode shows an excellent electrocatalytical activity on EP oxidation. The EP cathodic peak potential shifts negatively with a slope of −53.5 mV/pH, indicating that equal amounts of proton and electron are involved in the electrode reaction process. In pH 7.0 PBS, the peak current of EP and the concentration has a linear relationship from 0 to 65 µM by amperometric current-time curve.

Ferrocene substituted N, N′-bis(3-aminopropyl)oxamide: A new electrochemical sensor for copper(II) and fluoride ions in the biological pH range

A new pH responsive ferrocene functionalized receptor N, N′-bis(3-ferrocenylmethyl aminopropyl)oxamide (R) was synthesized and characterized. The electrochemical responses of the receptor R as a function of pH have been determined using cyclic voltammetric technique. Increase in H + ion concentration in the medium increases the redox potential of the Fc/Fc + redox couple of the receptor. Addition of metal ions like Mn(II), Ni(II) and Zn(II) show anodic shift of the Fc/Fc + redox couple. The presence of Cu(II) ion shows cathodic shift of the Fc/Fc + redox couple at acidic pH values. The potential shifts between the free receptor and the receptor with anions (F −, Cl − and Br −) at different pH were determined. Among the halide anions, fluoride ion shows appreciable amount of cathodic potential shift.

Adsorption of crown ethers on single and polycrystalline platinum-electrodes

For the functionalisation of electrode surfaces by organic molecules, platinum substrates offer the advantage of forming stable adsorption bonds to the molecules. The chemical stability and surface coverage of several adsorbed crown ethers, 18-crown-6 (18C6), benzo-18-crown-6 (B18C6) and 4′-amino-dibenzo-18-crown-6 (4′-ADB18C6) and for competitive purposes methoxy-benzene, have been studied at single- and polycrystalline platinum surfaces in 0.5 M sulphuric acid with differential electrochemical mass spectrometry (DEMS). The crown ethers adsorb chemically intact in a limited potential region between 200 and 800 mV vs. RHE. They undergo reductive cleaving of the ether ring moieties during cathodic potential shifts down to −100 mV. By 13CO co-adsorption and subsequent oxidative desorption, the stability and reversibility of the adsorption of the crown ethers was investigated. STM images of 4′-ADB18C6 reveal a disordered adsorbate layer.

Highly sensitive voltammetric biosensor for nitric oxide based on its high affinity with hemoglobin

Although heme protein-based, amperometric nitric oxide (NO) biosensors have been well documented in previous studies, most have been conducted in anaerobic conditions. Herein we report a novel hemoglobin-based NO biosensor that is not only very sensitive but also usable in air. The heme protein was entrapped in a sodium montmorillonite film, which was immobilized at a pyrolytic graphite electrode surface. Film-entrapped hemoglobin can directly exchange electrons with the electrode, and this process has proven to favor the catalytic reduction of oxygen. In addition, NO induced a cathodic potential shift of the catalytic reduction peak of oxygen. This potential shift was proportional to the logarithm of NO concentration ranging from 4.0 × 10 −11 to 5.0 × 10 −6 mol/L. The detection limit has been estimated to be 20 pM, approximately four orders lower than previously reported amperometric detectors.

Photoinduced and chemical oxidation of coordinated imine to amide in isomeric osmium(II) complexes of N-arylpyridine-2-carboxaldimines. Synthesis, characterization, electron transfer properties, and structural studies.

The reaction of N-arylpyridine-2-carboxaldimine [C(5)H(4)NC(H)NC(6)H(4)R] (HL) with ammonium hexabromoosmate (NH(4))(2)[OsBr(6)] in boiling 2-methoxyethanol afforded a violet solution from which two geometrical isomers of [OsBr(2)(HL)(2)] (1 and 2) were isolated. These are characterized by analytical and spectroscopic data. (1)H NMR spectral data were used for the identification of the isomers. The blue-violet isomer, 1 (designated as ctc), has a 2-fold symmetry axis and gave rise to resonances for only one coordinated HL. The geometry of the ctc-isomer was, however, revealed from the X-ray structure determination of a representative example. The red-violet isomer (2, designated as ccc), on the other hand, is unsymmetrical and gave rise to a large number of proton resonances. The isomeric complexes, [OsBr(2)(HL)(2)], showed intense MLCT transitions in the visible region. This transition, in the ccc-isomer, is slightly (10 nm) red shifted in comparison to the ctc-isomer. These diimine complexes showed one metal based reversible oxidation assignable to the Os(III)/Os(II) process followed by two irreversible oxidations at more anodic potentials (>1.4 V). In addition to these, the complexes also showed two irreversible ligand reductions at high cathodic potentials (<-1.4 V). An unusual type of photochemical transformation of the azomethine function of coordinated HL in osmium compounds 1 is studied. When an air equilibrated acetonitrile solution of 1 was exposed to a xenon lamp, it underwent oxidation affording the mixed ligand, amido complexes of general formula [OsBr(2)(HL)(LO)], 3 (LO = C(5)H(4)NC(O)-N-C(6)H(4)R), in an excellent yield (>95%). This transformation (1 --> 3) was achieved chemically when H(2)O(2) was used as an oxidant. Notably, the chemical oxidation with H(2)O(2) also led to the formation of a tetravalent complex, [OsBr(2)(LO)(2)], 4, as a minor product. Compound 3 was characterized by various spectroscopic and analytical techniques. The room temperature magnetic moment of 3 corresponds to a t(2)(5) configuration for the osmium(III) center. EPR spectra of the amido complexes were recorded at 77 K in 1:1 dichloromethane-toluene glass, and they were anisotropic in nature. FAB mass spectra of 3 displayed intense peaks due to parent molecular ions. For example, the complex [OsBr(2)(HL(1))(L(1)O)], 3a, showed a strong peak at m/z 729 amu. The electronic spectrum of compound 3 consisted of a broad LMCT transition (ca. 525 nm; epsilon, 3000 M(-1) cm(-1)). The cyclic voltammogram of compound 3 consisted of two responses, one each on the positive and negative side of SCE, corresponding to Os(IV)/Os(III) (ca. 0.8V) and Os(III)/Os(II) (ca. -0.3V) couples, respectively. There has been a large cathodic shift of potential for the Os(III)/Os(II) couple in 3 in comparison to that in the parent complex, 1. The diamido compound [OsBr(2)(LO)(2)], 4, is diamagnetic and insoluble in common solvents. The X-ray structure determination of a representative sample, 4a, is reported. The molecule contains a C(2)-symmetry axis with bromide ions in relative cis positions. The Os-N(amide) bond lengths are considerably shorter than the Os-N(pyridine) lengths. All other bond lengths and angles fall within the expected range.

Substituent Effects on the Electrochemistry and Photostability of Model Compounds of Calcium Channel Antagonist Drugs

We have synthesized three nitrocompounds in order to mimic the model structure of some relevant drugs such as nifedipine, nitrendipine, nisoldipine, nimodipine, and related ones, in order to investigate the effect of structural change on its electrochemical and photodegradation properties. We have found that ortho substitution with a nitro group produces a distortion of the coplanar arrangement, thus decreasing the resonance interaction between the nitro group and the aromatic system. A consequence of this fact is the observed cathodic peak potential shift toward negative values in the ortho-substituted isomer. It is observed that the position of the nitro substitution also produced differences in the photostability of the studied compounds. Stability studies of the compounds in buffered solutions (pH 7) stored in room light conditions show that only the ortho derivative was unstable in the time scale of the experiment (2 h). The instability of the ortho derivative may be explained if we consider that in this isomer, the photoexcited state of the nitro group is not stabilized by resonance as a consequence of the loss of coplanarity between the nitro group and the aromatic ring. Using molecular modeling, we have found that the configuration of minimal energy shows torsion angles of 70, 7.5, and 2.2° for ortho- meta- and para-derivatives, respectively, confirming the lost of coplanarity between the nitro group and the aromatic ring in the ortho derivative. © 2001 The Electrochemical Society. All rights reserved.

In Situ FTIR Spectroelectrochemical Investigations on Cu2+-ZSM-5 Modified Electrodes. Reactivity of the Electrogenerated Species Towards NO in Aqueous Solution

In situ FTIR spectroelectrochemical experiments have been carried out on Cu2+-ZSM-5 with 40 and 100 % ion exchange level with Nafion films coated on glassy carbon electrodes, and on the application of these electrodes to the interaction of NO with the electrogenerated species in aqueous solution. Voltammetric results revealed that the reduction of Cu2+–exchanged zeolites proceeds by two successive quasireversible one-electron steps. The first electron gain resulted in the formation of Cu+-ZSM-5. Cathodic peak potential shifts observed in NO-saturated aqueous solution indicated electrogenerated Cu+-ZSM-5–NO interactions. FTIR measurements showed that the Cu+ ions in the electroreduced Cu2+-ZSM-5 form with NO, among others products, Cu+-ZSM-5-nitrosylic complexes.

The Effect of Blocking Additives on the Electrodeposition of Cadmium

Voltammetric and current efficiency studies were made on the effect of benzyl and substituted benzyl alcohols on the competitive deposition of cadmium and evolution of hydrogen. The adsorbed alcohol additives acted as general blocking agents blocking the evolution of hydrogen. In the weakly complexing sulfate system these additives formed complexes with the Cd(II), and in the chloride system they caused a cathodic shift of the cadmium reduction potential, indicating a shift in the distribution of the chloride complex ions to a higher concentration of the more highly complexed species. Electrogenerated hydroxyl ions precipitated cadmium hydroxide in longer chain alcohol films. It was shown that in the presence of these alcohol additives in the chloride system in which a portion of the Cd‐bearing ions are negatively charged, the addition of quaternary ammonium ions, which are known phase‐transfer catalysts, accelerated the rate of reduction of cadmium(II), indicating that ion pairing occurred. On the other hand, these additives either inhibited or did not affect the rate of reduction of cadmium(II) in the sulfate systems in which the cadmium ions are present as positively or neutrally charged species. The fact that the ion pairing and acceleration occurs with the negative chloride complex, even though this complex is present in very small concentrations in the bulk of the solution, indicates that the presence of the low‐dielectric‐constant alcohol at the electrode interface has increased the concentration of these negative complexes in the neighborhood of the electrode.

Trends in the spectral and redox potential data of mononuclear iron(III)(S= 5 2 ) phenolate complexes

A series of high-spin octahedral iron(III) complexes of Schiff bases derived from salicyladehyde and aromatic amines has been synthesised. The ligands were selected to encompass various co-ordination spheres FeN2O4, FeN2O2O′2, FeN3O3 and FeN4O2(where O represents a phenolic oxygen, N an aliphatic or aromatic nitrogen and O′ a carboxylate oxygen) to provide generalizations regarding the overall co-ordination environment of the iron centre in a closely related group of complexes. They reveal the effect of stereochemical and/or donor atom variations on the UV/VIS and EPR spectra and FeIII–FeII redox potentials. Information on the iron(III) site symmetry has been obtained by EPR measurements. The optical spectra are largely determined by transitions originating in the iron–salicylaldiminate chromophore. The ligand-to-metal charge-transfer bands systematically shift to higher energy as the number of phenolate-containing donor sites increases. This blue shift is also reflected in more negative FeIII–FeII redox potentials. The order of increasing cathodic potential shift with respect to the co-ordination sphere is N2O4 > N2O2O′2 > N3O3 > N4O2. This is a reflection of the decreased Lewis acidity of the iron(III) centre due to the increase in basicity of the donor atom. A linear spectroelectrochemical correlation has been obtained between the phenolate-to-iron(III) charge-transfer band energy and the FeIII–FeII redox potential. Based on this correlation, trends in the redox potentials of iron tyrosinate proteins are discussed.

Electrochemical reduction of N-aryl- and N-arylsulphonylbenzoquinoneimines in acetonitrile—II. Electron affinity and relationship between oxidation and reduction potentials

Electroreduction potentials of the first, one-electron, reversible stage in acetonitrile have been used to determine numeric values of electron affinity ( E A) for N-arylquinoneimines (I), N-arylsulphonylquinoneimines(II), and N-arylthioquinoneimines(III). It has been found that polarographic half-wave potentials correlate themselves linearly with E ItA values can be determined by means of charge transfer spectroscopy. A common straight line correlation can be constructed for E 1 2 and E itA values of benzoquinones and benzoquinoneimines of various structure. It makes it possible to use this common equation to determine unknown E itA values for various compounds using polarographic data. N-arylquinoneimines(I) tend to be oxidized on a glassy carbon electrode in the far anodic potential range (1.3-1.9V). Substituent effects on electro-oxidation have been determined, and a relationship between reduction and oxidation potential difference and long wave spectral energy has been found. E A of quinoneimines have been determined, and various quinoneimines have been ranged according to their electron acceptor properties. Replacement of one oxygen atom in the benzoquinone molecule by an N-arylamino fragment results in a cathodic potential shift of 220 mV, while replacement of this atom by an N-arylsulphonylamino fragment shifts the potential anodically by 370 mV. The corresponding shifts for diimines are redoubled. Electron acceptor properties of the compound influence the reversibility of the first electroreduction stage as well.

Structural distortions of the [Fe 4S 4] 2+ core of [Fe 4S 4(S-t-C 4H 9) 4] 2− in different crystalline environments and detection and instability of oxidized ([Fe 4S 4] 3+) clusters

The structural and redox chemistry of the clusters [Fe 4S 4(SR) 4] 2− with R = t-alkyl have been investigated for the purpose of determining the structures of the same cluster in different environments and the stability of the oxidized species [Fe 4S 4(SR) 4] 1−. (Me 3NCH 2Ph) 2[Fe 4S 4(S-t-Bu) 4] crystallizes in the monoclinic space group P2 1/c with no imposed symmetry. (Et 4N) 2[Fe 4S 4(S-t-Bu) 4] crystallizes in the tetragonal space group I 4 2m with D 2d symmetry imposed on the cluster. The [Fe 4S 4] 2+ cluster cores in both compounds exhibit compressed tetragonal structures with different extents of distortion from T d symmetry. These structures are compared to those of other [Fe 4S 4] 2+ clusters by means of core shape parameters. Clusters with R = t-alkyl (t-Bu, C(CH 3) 2CH 2OH, C(CH 3) 2CH 2NHPh) in DMF exhibit, in addition to the usual 2−/3− and 3−/4− redox reactions common to all [Fe 4S 4(SR) 4] 2− species, discrete one-electron oxidations near −0.1 V vs. s.c.e. Cyclic voltammetry of [Fe 4S 4(S-t-Bu) 4] 2− reveals an essentially reversible 1−/2− couple with E 1 2 = −0.12 V, supporting the authenticity of clusters containing an oxidized ([Fe 4S 4] 3+) core. This couple cannot be electrochemically resolved from multi-electron oxidation in the case of clusters in DMF with other types of R substituents, a behavior apparently due to cathodic potential shifts by t-alkyl groups. Stability of oxidized clusters is low, and [Fe 4S 4(S-t-Bu) 4] 1− could not be generated in appreciable concentrations at longer times by coulometric or chemical oxidation. The relative stabilities of analogue and protein [Fe 4S 4] 3+ clusters are discussed. Preparations of four new [Fe 4S 4(SR) 4] 2− cluster salts are described including water-soluble (Et 4N) 2[Fe 4S 4(SC(CH 3) 2CH 2OH) 4].

Specific adsorption of formate ions at the mercury—aqueous solution interface

Adsorption on polarized Hg electrode of formate ions from aqueous solutions of pure HCOONa using differential capacity measurements was studied. Anion specific adsorption was indicated by the cathodic shift of potential of zero charge with increase in bulk-electrolyte activity. Variation of charge due to surface excess of sodium ions (Γ + Z + F) and of charge due to specifically adsorbed formate ions ( q 1), with electrode charge ( q M ) indicated super-equivalent adsorption of formate ions at all positive charges and for all concentrations studied. Esin—Markov coefficient was found to be unreliable criterion of the occurence of specific adsorption. Logarithmic form of constant charge adsorption isotherms were found to be followed. The plots of φ M − 2 vs q 1 at constant q M were not linear and resembled those for F −, BF 4 − and CH 3COO − ions; which is attributed to relatively weak specific adsorption of anions.

On the origin of voltammetric pre-peaks and cathodic potential shifts during anodic substitution reactions

During voltammetric pyridination of 9-phenylanthracene a pre-peak was observed, the height of which was in direct relation to the stoichiometric ratio of 9-phenylanthracene and pyridine. In order to determine whether or not the latter result could be taken as evidence for a nucleophile assisted electron transfer, a “non-assisted” mechanism was simulated. It was found that an ecec mechanism gave substantial cathodic shifts of the peak potential for oxidation of a substrate in presence of a reactant, and if the rate constants were sufficiently high not only a cathodic shift, but also a pre-peak was observed during simulation. Thus, the experimental observations cannot be used as evidence for an assisted mechanism.

Corrosion Inhibition in HCl Using Methyl Pyridines

The inhibiting effect of pyridine and seven of its methyl derivatives on the corrosion of iron in O2‐free, at 35°C was determined. Corrosion rates were measured by colorimetric analysis for iron with mercaptoacetic acid. The relative inhibitor efficiency and cathodic potential shift increased with increasing inhibitor concentration and, for a given concentration, with increasing electron density at the nitrogen atom. Both increased slightly with decreasing concentration, but a temperature increase of 10°C had no effect on either. The proposition that chemisorption is involved in corrosion inhibition is supported by these experiments. As a supplementary item the great importance of careful purification of the organic compounds used in such researches as this is documented.