Electro-oxidation of o-phenylenediamine at platinum electrodes in acetonitrile solutions

The study of o-phenylenediamine electro-oxidation at platinum electrodes in acetonitrile solutions under different experimental conditions is presented. Cyclic voltammograms show 4 oxidation peaks, which are assigned to o-phenylenediamine, o,o′-diamineazobenzene and protonated o-phenylenediamine anodic oxidation. An additional prepeak system is apparent at potentials less anodic than the first peak in successive scans. This prepeak system evidences the presence of two redox systems diphenylamine and H⁺, derived from the initial oxidation product o-phenylenediamine cation radical. The effect of base and acid addition is also studied. o,o′-Diamineazobenzene was identified as one of the principal soluble products in preparative scale electrolysis and a general mechanism for o-phenylenediamine oxidation is proposed.     

Electrochemical behaviour of isopropanol at platinum electrodes

The accessible potential range of isopropanol was found to be from +1.1 to -1.2 V vs. Ag/AgCl in non-aqueous 0.01 M LiCl, with acetone being formed at the anode and hydrogen at the cathode. Water was formed in a side-reaction, probably by ketal formation, the rate being proportional to the electrode area. Another side-reaction at the cathode produced an insoluble product after prolonged electrolysis. The coulometric current efficiency was nevertheless very high (99.6-99.97%) and almost independent of current density. The results indicate that the cathode reaction proceeded with somewhat less than theoretical current efficiency.     

Electrochemical oxidation of benzaldehyde and hydroxybenzaldehydes in acetonitrile on platinum and glassy carbon electrodes

The electrochemical oxidation of benzaldehyde and four hydroxybenzaldehydes was studied on platinum and glassy carbon electrodes in acetonitrile. A considerable difference was observed in electrooxidation performed on platinum and glassy carbon electrodes. All hydroxy derivatives fouled the glassy carbon electrode, but platinum was passivated only by the electrooxidation of 3-hydroxybenzaldehyde, highlighting the crucial role of the position of the substituent relative to the hydroxy group. On the glassy carbon electrode, the formation of the corresponding benzoyl radical could have taken place, which promoted the buildup of polymers on the electrode surface.     

Electrochemical reduction of sulphur dioxide on inert electrodes in acetonitrile solutions

The reduction of SO₂ on Pt and glassy carbon electrodes in acetonitrile-0.1 M LiBr solutions is studied by chronopotentiometric and chronovoltamperometric techniques in a wide range of SO₂ concentrations. It is found that the reduction process involves an irreversible adsorption of the reaction products followed by a fast formation of a weakly soluble dithionate film which passivates the electrode surface. At sweep rates >0.1 V s^?1 the electric charge associated with cathodic reaction corresponds to that of a monolayer. The dependencies of the peak current and the peak potential on the sweep rate are in agreement with the requirements of the adsorption pseudocapacitance theory. The strong effect of water on the height of the cathodic peaks is indicative of the depassivation of the electrode by some active species formed in the presence of water, SO₂ and LiBr by a slow reaction.     

Para-phenylenediamine oxidation at a platinum electrode in acetonitrile solutions

p-Phenylenediamine oxidation at platinum electrodes in acetonitrile solutions has been studied under a very wide range of experimental conditions. Chronopotentiometry, rotating disc electrode and cyclic voltammetry were used as electrochemical techniques. Coulometry at constant potential and product analysis were also performed.The electrochemical reaction appears as a fast and reversible one electron exchange per molecule of PPD. The electrode reaction is further complicated by follow-up chemical reactions giving unknown products in the bulk of the solution.The whole polarization curve under steady state conditions shows two waves, while under non-steady state conditions a small wave at intermediate potentials is also apparent.The reaction pathway for the first wave was interpreted as a non-conventional e.c.e. mechanism where the parent molecule acts as a base in the chemical step.These assumptions were confirmed through experiments performed with pyridine or water addition.     

Effect of solution pH on the electrochemical behavior of thiocarbamide at gold and platinum electrodes

The effect of solution pH on the electrochemical behavior of thiocarbamide at gold and platinum electrodes is studied by using cyclic voltammetry and quartz microgravimetry. It is shown that with the increasing of pH the half-wave potential of thiocarbamide oxidation shifts toward negative values in both cases. In the cathodic branches of the cyclic voltammograms, up to pH ≃ 4 the potential region of reduction of the thiocarbamide oxidation product remains practically unchanged; this allows concluding that the oxidation product is formamidine disulfide (at platinum) or its mixture with gold complex ions (at gold electrode). Further increasing of pH up to 9.5–10 results in the increase of the contribution of the reduction current of these ions to the cathodic signal. An explanation for this phenomenon is given. At the same time, new signals appear at E < −0.4 V; they may be interpreted as the current of elemental sulfur reduction. Increasing of pH over 10 leads to gold passivation by the elemental sulfur; this manifests itself as the electrode mass increase during the microgravimetry measurements. Introducing of sodium silicate (recommended in the literature as a solution stabilizer) to the solution does not eliminate the passivation of gold.     

Electrochemical behavior of tartaric acid at solid electrodes in aqueous and mixed solutions

Electrochemical behavior of D-tartaric acid at the Pt-, Cu-, Ta-, Cu-Hg-electrodes and the Cd-alloys with Sn, Ni, Hg, and In in aqueous and mixed solutions (water mixtures with acetone, ethanol, dimethylformamide, and pyridine) is studied by voltammetry. It is shown that the tartaric acid voltammograms always show waves related to the hydrogen ion discharge. A method recently suggested by authors allowed calculating the dissociation degrees at different stages, the concentrations of ionized and not ionized forms in dilute solutions of stereoisomers: DL-tartaric, L-tartaric, D-tartaric, and meso-tartaric acids. Simple empiric equations are suggested for the approximate calculating of some dissociation parameters for these acids.     

Electrochemical behaviour of platinum at polymer-modified glassy carbon electrodes

In this paper, the preparations and voltammetric characteristics of chitosan-modified glassy carbon (Ct-MGC) and platinum electrodes are studied. Ct-MGC can be used for pre-concentration and quantification of trace amounts of platinum in solution. At low pH medium, the complex of Pt with protonated group-NH³⁺ in the chitosan molecule has been confirmed by FT-IR spectra studies.     

Electrochemical behavior of gold electrodes modified with self-assembled monolayers with an acidic end group for selective detection of dopamine

Gold electrodes modified with monolayers with ionic end groups of different acidity and structure have been evaluated for the detection of dopamine. It has been demonstrated that the selectivity of monolayers depends not only on ionic charge but also on the nature of the ionic end group and defects in the monolayer. Catalytic effects due to the electrocatalytic oxidation of ascorbic acid by dopamine are present at the monolayer modified gold electrode. The angle resolved XPS (X-ray photoelectron spectroscopy) experiments demonstrate that the sulfonate group do not compete with thiol on binding the gold electrode. The capacitance measurements demonstrate that the capacitance depends on the length, end group type and defects present in monolayers.     

Electrochemical behavior of marcellomycin at lipid-modified carbon-paste electrodes

A lipid-modified carbon-paste electrode is prepared by mixing phospholipids with the carbon-paste matrix. The resulting electrodes have polar head-groups which allow interactions with positively charged drugs and improved preconcentration/extraction steps. The accumulation of the antitumor drug, marcellomycin, is enhanced in the presence of lipids, giving an 8-fold enhancement of current. The electrode response has been optimized with respect to paste composition, nature of the lipid, pH, temperature, and stirring time. A mechanism for marcellomycin accumulation is proposed, based on electrochemical and UV-visible spectrometric measurements as a function of pH. The electrode response is linearly related to the marcellomycin concentration within the range 1 x 10(-8)-6 x 10(-6)M. Known amounts of marcellomycin added to a urine sample diluted sixfold with water have been measured by use of the medium-exchange technique.     

Studies of gold,platinum and palladium indicating electrodes in strongly oxidizing aqueous solutions

A comparison of the behavior of gold, platinum and palladium indicating electrodes in various strongly oxidizing solutions with the behavior observed when these electrodes are oxidized anodically reveals that the same reaction are occurring at the electrodes These reactions are attributed to oxide (or hydroxide) film formation because the potentials at which these various reactions occur agree reasonably well with the potential of known oxide-metal transitions The present study emphasizes the transient potential-time response of an indicating electrode when immersed in a strongly oxidizing solution It is concluded that all “inert” indicating electrodes undergo stepwise chemical oxidation in strongly oxidizing media.     

Electrochemical and surface characterization of 4-aminothiophenol adsorption at polycrystalline platinum electrodes

The formation of a self-assembled monolayer (SAM) of 4-aminothiophenol (4-ATP) on polycrystalline platinum electrodes has been characterized by surface analysis and electrochemistry techniques. The 4-ATP monolayer was characterized by cyclic voltammetry (CV), linear sweep voltammetry, Raman spectroscopy, reflection-absorption infrared (RAIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). CV was used to study the dependence of the adsorption time and 4-ATP solution concentration on the relative degree of coverage of 4-ATP monolayers on polycrystalline Pt electrodes. The adsorption time range probed was 24-72 h. The optimal concentration of 4-ATP needed to obtain the highest surface at the lowest adsorption time was 10 mM. RAIR and Raman spectroscopy for 4-ATP-modified platinum electrodes showed the characteristic adsorption bands for 4-ATP, such as nuNH, nuCH(arom), and nuCS(arom), indicating the adsorption on the platinum surface. The XPS spectra for the modified Pt surface presented the binding energy peaks of sulfur and nitrogen. High energy resolution XPS studies, RAIR, and Raman spectrum for platinum electrodes modified with 4-ATP indicate that the molecules are sulfur-bonded to the platinum surface. The formation of a S-Pt bond suggests that ATP adsorption leads to an amino-terminated electrode surface. The thickness of the monolayer was evaluated via angle-resolved XPS (AR-XPS) analyses, giving a value of 8 A. As evidence of the terminal amino group on the electrode surface, the chemical derivatization of the 4-ATP SAM was done with 16-Br hexadecanoic acid. This surface reaction was followed by RAIR spectroscopy.     

Electrochemical generation of ferrate in acidic media at boron-doped diamond electrodes

An extremely strong oxidant, ferrate (Fe(VI) or FeO4(2-), has been produced electrochemically in an acidic aqueous medium for the first time.     

Electrochemical characterisation and anodic stripping voltammetry at mesoporous platinum rotating disc electrodes

Using the technique of liquid crystal templating a rotating disc electrode (RDE) was modified with a high surface area mesoporous platinum film. The surface area of the electrode was characterised by acid voltammetry, and found to be very high (ca. 86 cm(2)). Acid characterisation of the electrode produced distorted voltammograms was interpreted as being due to the extremely large surface area which produced a combination of effects such as localised pH change within the pore environment and also ohmic drop effects. Acid voltammetry in the presence of two different types of surfactant, namely Tween 20 and Triton X-100, suggested antifouling properties associated with the mesoporous deposit. Further analysis of the modified electrode using a redox couple in solution showed typical RDE behaviour although extra capacitive currents were observed due to the large surface area of the electrode. The phenomenon of underpotential deposition was exploited for the purpose of anodic stripping voltammetry and results were compared with data collected for microelectrodes. Underpotential deposition of metal ions at the mesoporous RDE was found to be similar to that at conventional platinum electrodes and mesoporous microelectrodes although the rate of surface coverage was found to be slower at a mesoporous RDE. It was found that a mesoporous RDE forms a suitable system for quantification of silver ions in solution.     

Electrochemical behavior of hydrazine at mechanically renewed solid electrodes

The electrooxidation of hydrazine was studied at metallic wire electrodes made of Co, Ni, Ag, Cu, and a graphite-epoxy composite by anodic and cyclic voltammetry with a linear potential sweep in alkaline-supporting electrolytes. Electrode working surfaces were regenerated by mechanically cutting a thin 0.5-μm layer in situ before each polarization cycle. The effects of the electrode material and the renewal of its surface on the parameters of anodic voltammograms of hydrazine were demonstrated. Hydrazine anodic peaks obtained at clean surfaces of electrodes fabricated from Ni, Ag, and a graphite-epoxy composite and also peaks at the potentials of the oxide formation on Ni and Cu electrodes can be used as analytical signals for the voltammetric determination of hydrazine.     

Electrochemical behavior of polyaniline films in acetonitrile

The electro-oxidation and electroreduction behavior of polyaniline films in acetonitrile in the absence and presence of added acid or base were investigated using cyclic voltammetry. A general mechanism is proposed that enables the interpretation of the electroactivity loss and the catalytic and autocatalytic properties of the film. The electroactivity loss was found to be a reversible phenomenon which is accelerated in alkaline solutions through deprotonation. Electroactivity was recovered when the electroinactive films were reduced in an acidic solution. The films that lose their electroactivity on electro-oxidation were found to be conducting, and various cation radicals in the structure are believed to be responsible for this nonprotonic conductivity. The proton content of the polyaniline film was found to be crucial in determining its electrochemical and physical properties. Autocatalysis was detected when protons were produced electrolytically in situ during electropolymerization.     

Anion re-adsorption and displacement at platinum single crystal electrodes in CO-containing solutions

Bulk CO oxidation experiments have been carried out in sulphuric and perchloric acid solutions on Pt(1 1 1) and Pt(1 1 0) electrodes under hanging meniscus rotating disk electrode (HMRDE) configuration. The comparison between the two different electrolytic media reveals an important influence of the anion in the oxidation kinetics. Once the adsorbed CO layer has been oxidized after the ignition peak, anions are re-adsorbed on the electrode surface and the presence of these anions affects the stationary currents measured at positive potentials. In the negative-going sweep, adsorbed anions are displaced from the surface when the CO oxidation rate is lower than the corresponding CO adsorption rate. The charge associated to this displacement has been measured at high scan rates and is in agreement with that expected from the CO displacement experiments performed at low potentials.