Cyclic voltammetry of aromatic amines in aqueous and non-aqueous media

Cyclic voltammetry of some typical aromatic amines has been studied using the tubular graphite electrode in aqueous medium and a plane graphite electrode in non-aqueous medium. All aromatic amines, irrespective of the nature of the medium, undergo anodic oxidation through the formation of a monocation radical. In aqueous medium, the monocation radical undergoes, through its resonance structures, dimerisation involving tail-to-tail, head-to-tail and even head-to-head coupling. The products formed, being more easily oxidisable than the parent substance, undergo further oxidation at the same potential so that the overall oxidation involves a one-step (ie, a single wave), two-electron process. The N-alkyl anilines, however, oxidise in two steps instead of one. This is due to the fact that the electron-releasing alkyl groups attached to nitrogen disperse the positive charge on the cation radical and thus render it stable. The stabilised proportion of the cation radical undergoes further oxidation at a higher potential forming a dication. This accounts for the formation of the second wave. In non-aqueous medium, the monocation radical does not undergo dimerisation through coupling reactions. Retaining its identity, it undergoes further oxidation at a higher potential forming a dication. Thus, in non-aqueous medium, aromatic amines oxidise in two steps involving one electron in each step. The dication radical formed in aqueous or non-aqeous medium combines with a neutral molecule forming hydrazobenzene or its analogue as an intermediate product, the final product being azobenzene or its analogue.     

Electrochemical preparation and characterization of poly(1-amino-9,10-anthraquinone) films

Poly(1-amino-9,10-anthraquinone), PAAQ, films were prepared by electrochemical oxidation of the monomer, AAQ, in acetonitrile using LiClO₄ as supporting electrolyte. The influence of scan repetition, scan rate and monomer concentration on the formation of polymer film was studied. The electrochemical behavior of the formed polymer films was investigated in both non-aqueous and aqueous media. The prepared films were found to be more stable in organic solvents than in aqueous solutions. The investigated organic solvents are methanol, ethanol, acetone, carbon tetrachloride, benzene, and chloroform. The polymer film shows electrochemical response in both non-aqueous and aqueous media. In non-aqueous solutions it has a wide potential range of electroactivity (from −1.5 to +1.3 V). In aqueous media the polymer film shows electrochemical response in the potential range between −0.3 and +1.3 V only. The presence of quinone units suggests potential applications in diverse areas such as electrocatalytic processes and lithium ion batteries.     

Preparation and characterization of a 1,10-phenanthroline-modified glassy carbon electrode

This paper describes the grafting of 1,10-phenanthroline (P) molecules on the surface of a glassy carbon (GC) electrode. This modification was carried out in both aqueous and non-aqueous media. Britton Robinson (BR) was used in aqueous experiments at different pHs and tetrabutylammonium tetrafluoroborate (TBATFB), 0.1 M in acetonitrile (ACN) was used in non-aqueous experiments. Surface modification experiments were performed in the +1.2–2.7 V potential range with a scan rate of 100 mV/s and 30 cycles. The presence of P at the GC surface was characterized by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), contact angle measurement (CAM) and ellipsometry. The ability of the complex to modify surfaces was investigated with differential pulse voltammetry (DPV).     

An electrochemical study in aqueous solutions on the binding of dopamine to a sulfonated cyclodextrin host

Clear evidence for the formation of a weak inclusion complex between dopamine (DA) and a sulfonated β-CD host in aqueous solution was obtained using a combination of electrochemical approaches. Using cyclic voltammetry, a distinct increase in the oxidation potential of DA and a reduction in the peak oxidation current were observed on adding an excess concentration of the sulfonated β-CD to the electrolyte solution. Equally, a clear increase in the half-wave oxidation potential of DA was observed in the presence of the sulfonated β-CD using rotating disc voltammetry. The association constant, K, was computed as 331.3 ± 5.8, indicating the formation of a weak inclusion complex, while a 1:1 stoichiometry for the inclusion complex was deduced from a Job's plot analysis. The rate constant for the oxidation of DA was found to decrease on formation of the inclusion complex. This was attributed to higher reorganization energy for the oxidation of the included DA. These changes in the electrochemistry of DA were not observed when an excess of the smaller sulfonated α-CD was added to the electrolyte, indicating that these variations are not connected with simple electrostatic interactions between the protonated DA and the anionic sulfonated groups. It is proposed that the aromatic ring of the DA molecule includes within the cyclodextrin cavity, while the protonated amine group remains outside the cavity, bound electrostatically with the anionic sulfonated groups.     

Cotton Seed Oil Oxidation Catalysed by Amino Acids and Albumin in Aqueous and Non-Aqueous Media

The oxidation of refined cotton seed oil catalysed by various α-amino acids and albumin have been studied in aqueous and non-aqueous media. Cysteine, phenylalanine and albumin possessed pro-oxidant effect in cotton seed oil in aqueous and non-aqueous media. Serine exhibited pro-oxidant activity in aqueous media and minor anti-oxidant effect in non-aqueous media. The effectiveness of the amino acids on cotton seed oil oxidation was in the following descending order in both aqueous and non-aqueous media: cysteine > phenylalanine > serine. The pro-oxidant effect in aqueous media might be due to the predominant presence of the protonated amino nitrogen. Whilst, amino acid-metal complex might be responsible for the pro-oxidant effect in non-aqueous media.     

Particle size and zeta potential of carbon black in liquid media

Zeta potential is a parameter that can be used for studying and predicting colloid stability and surface morphology. Even though zeta potential measurements have been applied to both aqueous and non-aqueous suspensions, theories for colloidal behavior and zeta potential in non-aqueous media are not as well-established as those in aqueous environments. There are few reports on systematic studies of zeta potential of colloids in liquid media. In the present study, zeta potential and particle size measurements of two carbon black powder samples in various media were performed. Combined with particle size characterization, the experimental zeta potential values were used to discuss charge transfer and colloid stability of carbon black in suspension.     

Electrochemical oxidation of catechol in the presence of an aromatic amine in aqueous media

The electrochemical oxidation of catechol has been studied in the presence of p-nitroaniline as a nucleophile in aqueous media at the surface of glassy carbon electrode, using cyclic voltammetry. The products of electrosynthesis have been purified and characterized by Fourier-transform infrared (FT-IR), ¹H nuclear magnetic resonance (NMR), ¹³C NMR, and distortionless enhancement by polarization transfer (DEPT), and the mechanism of anodic oxidation was deduced from voltammetric and spectroscopic data.     

Voltammetric Response and Determination of an Anti-Inflammatory Drug at a Cationic Surfactant-Modified Glassy Carbon Electrode

Electrochemical oxidation of thiosalicylic acid in the presence of cationic surfactant, cetyl trimethylammonium bromide (CTAB), at a glassy carbon electrode was investigated. The electrochemical response of a modified sensor towards thiosalicylic acid determination was studied by the means of cyclic voltammetry, linear sweep voltammetry and differential pulse voltammetry (DPV). The liquid phase oxidation of thiosalicylic acid in the presence of CTAB leads to a notable enhancement in the peak current and a lowering of the peak potential. The electrochemical process was observed to be adsorption-controlled, irreversible and involves oxidation of one electron. Effects of anodic peak potential (E _p), anodic peak current (I _pa) and heterogeneous rate constant (k ⁰) were calculated. The linear response was obtained in the range of 1.0 µM–1.0 mM with a detection limit of 113 nM.     

The inclusion of Mo, Nb and Ta in Pt and PtRu carbon supported electrocatalysts in the quest for improved CO tolerant PEMFC anodes

The effect of the inclusion of Mo, Nb and Ta in Pt and PtRu carbon supported anode electrocatalysts on CO tolerance in proton exchange membrane fuel cells (PEMFC) has been investigated by cyclic voltammetry and fuel cell tests. CO stripping voltammetry on binary Pt_xM/C (M: Mo, Nb, Ta) reveals partial oxidation of the CO adlayer at low potential, with PtMo (4:1)/C exhibiting the lowest value. At 80 °C, the operating temperature of the fuel cell, CO oxidation was observed at potentials close to 0 V versus the reversible hydrogen electrode (RHE). No significant difference for CO electro-oxidation at the lower potential limit, compared to PtRu/C, was observed for PtRuM_y/C (M: Mo, Nb). Fuel cell tests demonstrated that while all the prepared catalysts exhibited enhanced performance compared to Pt/C, only the addition of a relatively small amount of Mo to PtRu results in an electrocatalyst with a higher activity, in the presence of carbon monoxide, to PtRu/C, the current catalyst of choice for PEM fuel cell applications.     

Mechanistic investigation of N-thioamidoimidates: Oxidation by heterogeneous and homogeneous electron transfers

Electrochemical study of N-thioamidoimidates 1 is carried out in non-aqueous solvent at platinum electrodes by means of cyclic voltammetry at different scan rates. A mechanistic investigation shows that the oxidation of the NH bond of these substrates leads, during macroscale electrolysis, to oxadiazole-3-thione 2 with satisfactory yields according to an ECE mechanism. On the other hand, the rate of deprotonation k_H and the standard oxidation potential of the N-thioamidoimidate cation radical/N-thioamidoimidate couple were derived directly and indirectly by the use of the homogenous redox catalysis. The perfect agreement of the predicted kinetic with the experiment data both under homogeneous and heterogeneous electron transfer provides further support for the ECE mechanism.     

Electrochemical characterization of single-walled carbon nanotubes for electrochemical double layer capacitors using non-aqueous electrolyte

Single-walled carbon nanotubes (SWCNTs) were investigated by cyclic voltammetry and electrochemical impedance spectroscopy in a non-aqueous electrolyte, 1 M Et₄NBF₄ in acetonitrile, suitable for supercapacitors. Further, in situ dilatometry and in situ conductance measurements were performed on single electrodes and the results compared to an activated carbon, YP17. Both materials show capacitive behavior characteristic of high surface area electrodes for supercapacitors, with the maximum full cell gravimetric capacitance being 34 F/g for YP17 and 20 F/g for SWCNTs at 2.5 V with respect to the total active electrode mass. The electronic resistance of SWCNTs and activated carbon decreases significantly during charging, showing similarities of the two materials during electrochemical doping. The SWCNT electrode expands irreversibly during the first electrochemical potential sweep as verified by in situ dilatometry, indicative of at least partial debundling of the SWCNTs. A reversible periodic swelling and shrinking during cycling is observed for both materials, with the magnitude of expansion depending on the type of ions forming the double layer.     

Electrode kinetics in non-aqueous vanadium acetylacetonate redox flow batteries

Three electrode materials (glassy carbon, gold, and platinum) were investigated for application in a non-aqueous single-metal redox flow battery based on vanadium (III) acetylacetonate, supported by tetraethylammonium tetrafluoroborate in acetonitrile. Redox couples associated with the one-electron disproportionation of V(acac)₃ were observed in voltammetry for each metal tested. An elementary kinetic model was created and used to determine rates for oxidation or reduction of the vanadium complex. The oxidation rates for V(acac)₃ were mass-transfer limited on all electrode materials, suggesting reversible kinetics. For the V(acac)₃ reduction reaction, exchange-current densities of 1.3, 3.8, and 8.4 A m⁻² were observed on glassy carbon, platinum, and gold electrodes, respectively.     

Electrochemical modification of glassy carbon electrode by poly-4-nitroaniline and its application for determination of copper(II)

Electrochemical modification of glassy carbon (GC) electrode by poly-4-nitroaniline (P4NA), electrochemical reduction of P4NA and applicability of electrode modified in this way for determination of copper(II) (Cu(II)) is reported in this study. Electrochemical surface modification was performed by cyclic voltammetry in the potential range between +0.9 V and +1.4 V vs. Ag/Ag⁺ (in 10 mM AgNO₃) at the scan rate of 100 mV/s by 100 cycles in non-aqueous media. In order to provide electrochemical reduction of nitro groups on the P4NA-modified GC electrode surface (P4NA/GC), the cyclic voltammograms inducing/evidencing the reduction of nitro groups were performed in the potential range between −0.1 V and −0.8 V vs. Ag/AgCl/(sat.KCl) at the scan rate of 100 mV/s. The reduced P4NA/GC surfaces (Reduced-P4NA/GC) were treated with aqueous solution of nitrilotriacetic acid. The sensitivity of GC electrode modified in described way towards Cu(II) was investigated in Britton-Robinson buffer solution, pH 5.0. The potentiometric generic pulse technique was applied as innovative electrochemical method for detection of analytical signal. It was shown that GC electrodes modified in here described way will be suitable for the determination of Cu(II) in technological waste water and/or some other solutions containing Cu(II) ions.     

Catechol as an electrochemical indicator for voltammetric determination of <Emphasis Type="Italic">N</Emphasis>-acetyl-<Emphasis Type="SmallCaps">l</Emphasis>-cysteine in aqueous media at the surface of carbon paste electrode

The utilization of catechol as an electrochemical indicator in the presence of N-Acetyl-l-cysteine (NAC) at a carbon paste electrode (CPE) has been investigated in aqueous media using cyclic voltammetry (CV), differential pulse voltammetry (DPV), and double-step potential chronoamperometry methods. The results show that NAC participates in Michael type addition reaction with electrogenerated quinone from electrooxidation of catechol at CPE to form the corresponding thioquinone derivative. The reoxidation of the adduct leads to increase in the oxidative current which is proportional to the concentration of NAC. Therefore, in the optimum condition (pH = 6.00) by CV, the oxidation of NAC occurs at a potential about 400 mV versus Ag|AgCl|KCl_sat in the presence of catechol at the surface of CPE. The practical utility of the method showed that low detection limit and high sensitivity for voltammetric determination of NAC. The proposed method is useful for the routine analysis of NAC in pharmaceutical formulations.     

Electrochemical characterization of nanoporous honeycomb diamond electrodes in non-aqueous electrolytes

Oxygen plasma-etched nano-honeycomb diamond thin film electrodes were examined for electrochemical capacitor applications in non-aqueous electrolytes. As-deposited and nano-honeycomb diamond electrodes in 0.5 M TEABF₄/PC both exhibited a wide potential window (approx. 7.3 V), similar to that of glassy carbon electrodes. For as-deposited diamond, the impedance behavior was found to be similar for non-aqueous and aqueous electrolytes, and the double-layer capacitance was found to be 21.8 μF cm⁻², almost the same as that obtained in aqueous electrolytes. For the honeycomb diamond electrodes, however, the impedance behavior observed in non-aqueous electrolytes was significantly different from that in aqueous electrolyte and indicated that the ac signal cannot penetrate to the bottom of the honeycomb pores in the non-aqueous electrolytes due to low conductivity, and that not all the surface may contribute to the double-layer capacitance. This result was verified by mathematical simulation.     

Electrocatalytic oxidation of thiocyanate, l-cysteine and 2-mercaptoethanol by self-assembled monolayer of cobalt tetraethoxy thiophene phthalocyanine

Catalytic activity of a self-assembled monolayer (SAM) of cobalt tetra ethoxythiophene phthalocyanine (CoTEThPc-SAM) complex towards oxidation of thiocyanate (SCN⁻), l-cysteine (CYS) and 2-mercaptoethanol (2-ME) is reported. The oxidation of thiocyanate occurs via a two electron transfer, whereas l-cysteine and 2-ME require 1 electron. The oxidation of thiocyanate is catalysed by ring based processes, while l-cysteine is catalysed by both Co^III/Co^II process and by ring based processes. 2-ME is catalysed by Co^III/Co^II process. The oxidation of thiocyanate on CoTEThPc was performed in acid media instead of basic media commonly employed. The reaction order was found to be unity for all the analytes, showing that only one molecule of analyte interacts with one molecule of the catalyst during the rate determining step.     

Preparation of polypyrrole/ferrocyanide films modified carbon paste electrode and its application on the electrocatalytic determination of ascorbic acid

Functionalized polypyrrole film were prepared by incorporation of (Fe(CN)₆)⁴⁻ as doping anion, during the electropolymerization of pyrrole onto a carbon paste electrode (CPE) in aqueous solution by using potentiostatic method. The electrochemical behavior of the (Fe(CN)₆)³⁻/(Fe(CN)₆)⁴⁻ redox couple in polypyrrole was studied by cyclic voltammetry and double step potential chronoamperometry methods. In this study, an obvious surface redox reaction was observed and dependence of this reaction on the solution pH was illustrated. The electrocatalytic ability of polypyrrole/ferrocyanide films modified carbon paste electrode (Ppy/FCNMCPEs) was demonstrated by oxidation of ascorbic acid. It has been found that under optimum condition (pH 7.00), the oxidation of ascorbic acid at the surface of such electrode occurs at a potential about 540 mV less positive than unmodified carbon paste electrode. The kinetic parameters such as electron transfer coefficient, α and catalytic reaction rate constant, k_h′, were also determined by using various electrochemical approaches.The catalytic oxidation peak current showed a linear dependent on the ascorbic acid concentration and a linear calibration curve was obtained in the range of 4.5×10⁻⁴ to 9.62×10⁻³ M of ascorbic acid with a correlation coefficient of 0.9999. The detection limit (2σ) was determined as 5.82×10⁻⁵ M.     

Liquid-liquid interfacial processes at hydrophobic silica carbon composite electrodes: ion transfer at water-nitrobenzene, water-o-nitrophenyloctylether, and at water-o-nitrophenylphenylether interfaces

Ion transfer processes across liquid-liquid phase boundaries of the type aqueous solution-polar organic solvent supported on a hydrophobic silica carbon composite are studied by cyclic voltammetry and differential pulse voltammetry. The organic phase consists of a redox probe (ferrocene, t-butylferrocene, or decamethylferrocene) dissolved in a polar hydrophobic solvent (nitrobenzene, o-nitrophenyloctylether, or o-nitrophenylphenylether). The organic phase was immobilised in a ceramic carbon material composed of a hydrophobic silicate prepared via a sol-gel process from a methyltrimethoxysilane based sol and carbon particles. When immersed into aqueous electrolyte, ion transfer processes can be monitored as a function of potential. The contributions of solvent, electrolyte, and redox probe to the transition from anion transfer to cation transfer are discussed. Effects due to the presence of a high surface area microporous solid matrix are considered.     

Synthesis, electrochemical and spectroscopic investigations of New N-BEDOT derivatives containing anil substituted carbazole subunits

A conjugated oligomer containing central bi-ethylenedioxythiophene (BEDOT) unit and two terminal anil-substituted carbazole units has been characterized by cyclic voltammetry and spectroelectrochemical methods (UV-vis-NIR and Raman). It shows two oxidation waves at −0.01 V and +0.24 V (versus Fc/Fc⁺), which can be ascribed to two successive oxidative dopings of the molecule. The molecule is transmissive sharp yellow when fully reduced, transmissive green upon oxidation up to the end of the first oxidative process, and dark blue upon full oxidation. UV-vis-NIR and Raman spectroelectrochemical investigations show that the colour changes within the potential of the first oxidation wave are associated with the oxidation of both carbazole and ethylenedioxythiophene subunits whereas the electrochromic effect in the potential range of the second oxidation wave involves predominantly the oxidation of the ethylenedioxythiophene subunits.     

Electrocatalytic oxidation of methanol and ethanol at carbon ceramic electrode modified with platinum nanoparticles

The use of carbon ceramic electrode (CCE) modified with platinum particles was studied for the electrocatalytic oxidation of methanol and ethanol by cyclic voltammetry and chronoamperometry. After preparation of a carbon ceramic as an electrode matrix by sol–gel technique, its surface was potentiostatically coated with Pt nanoparticles at −0.2 V vs. SCE in an aqueous solution of 0.1 M H₂SO₄ containing 0.002 M H₂PtCl₆. The electrocatalyst was characterized by XRD, SEM and cyclic voltammetry. The effective parameters on electrocatalytic oxidation of the alcohols, i.e. the amount Pt loadings, medium temperature and working potential limit in anodic direction were investigated and the results were discussed. This modified electrode showed an enhanced current density over the other Pt-modified electrodes making it more attractive for fuel cell applications.