Anodic oxidation of cyanide and cyanate ions on a platinum electrode

The oxidation processes of cyanide and cyanate ions on a Pt electrode in aqueous and methanol solutions were studied by infrared spectroscopy. In aqueous solution, the cyanide ion was oxidized to cyanate and successively to carbon dioxide. The reaction proceeded on an oxidized platinum surface. In methanol solution, HNCO is the main product during anodization.     

Baylis-Hillman mechanism: a new interpretation in aprotic solvents

Using reaction rate data collected in aprotic solvents, we have determined that the Baylis-Hillman rate-determining step is second order in aldehyde and first order in DABCO and acrylate. On the basis of these data, we have proposed a new mechanism involving a hemiacetal intermediate. The proposed mechanism was then supported using two different kinetic isotope experiments.     

Behaviour of aminoacids and aliphatic aldehydes in dipolar aprotic solvents: Formation of oxazolidinones—behaviour of aminoacids and aliphatic aldehydes in dipolar aprotic solvents

Reactions of aliphatic branched aldehydes with proline in dimethyl sulfoxide or acetonitrile solution afford oxazolidin-5-ones with high diastereoselection. Linear aldehydes afford aldolic/crotonic condensation products; with short reaction times, the presence of Oxazolidinones can be detected in the pmr spectra. Acyclic aminoacids and branched aldehydes yield a reaction mixture the pmr and ir spectra of which give evidence for iminic-oxazolidinone equilibria. The structure of (2R,5S)-2-trichloromethyl-1-aza-3-oxabiciclo-[3.3.0]octan-4-one has been confirmed by X-ray diffraction analysis.     

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.     

Anodic stripping voltammetry and chronopotentiometry of copper at a rotating carbosital disk electrode

The use of a new carbon material — carbosital — for electrodes is reviewed. The behaviour of copper deposited on the carbosital electrode surface in anodic stripping voltammetry and chronopotentiometry is discussed. In anodic stripping voltammetry with a rotating carbosital disk electrode, the peak current and the number of coulombs involved in stripping copper are directly proportional to the square root of the electrode rotation rate during preelectroiysis; the peak current is directly proportional to the potential scan rate during stripping. For anodic stripping voltammetry and anodic stripping chronopotentiometry, linear calibration graphs are obtained in the range 1 X 10^-3–1 x 10^-6 M copper(II). The method is applicable to analysis of high-purity cadmium for copper.     

Solvatochromic equilibria of cobalt(II) chloride in binary mixtures of dipolar aprotic solvents with water

The visible absorption spectra of CoCl₂ in binary mixtures of acetone, dmf, dma and dmso with H₂O show the existence of configuration equilibria between tetrahedral (T) and octahedral (O) species. Linear correlations between the values of log K (K = [O]/[T]) and the mole fraction of H₂O, _w, have been observed in all the systems under study; their slopes decrease linearly with an increase of the donor number, DN, of the organic component of the mixture. The formation of octahedral species is enthalpy favoured but entropy disfavoured. It was found that both H and S values change linearly with _w and the slopes of these relationships depend clearly on the DN values.     

The study of the electrochemical oxidation mechanism of diphenylamine derivatives in dipolar aprotic solvents: III. Effect of acids and bases of different strengths

The effect of addition of different acids and bases in a wide range of concentration on the voltammetric oxidation of both Variamine blue (VB) and 4-aminodiphenylamine (4-ADPA) was examined in acetonitrile (AcN). The oxidation potential values of either wave of both depolarizers were found to depend on the acid or base strength of the additive—relative to the depolarizer and solvent—and on their concentration in solution. Addition of strong acid as CH₃SO₃H resulted in a single oxidation wave at potentials either equal to or more positive than that of the original second wave. Relatively weak acids as picric and pyridinium ion only shifted the oxidation potential of the first wave to more positive values without affecting the general oxidation pattern.On the other hand, strong bases like ethanolamine, resulted in the formation of a preoxidation wave at more negative potential when present in low concentration and a single wave at identical negative potential at high enough concentrations. Bases of comparable strengths, as that of the depolarizer, like pyridine do not form prewaves but only shifted the final single oxidation wave to more negative potential values. Weaker bases like water and ethanol mainly shifted the oxidation potential of the second wave to less positive values.     

Ion-pair effects on the electroreduction and electrochromic properties of ortho-chloranil in dipolar aprotic solvents

The use of ortho-chloranil (o-CA) as an electrochromic material requires three conditions to operate properly: (1) the reversibility of the electronic transfer of the reaction

; (2) the formation of a visible-light absorbing charge-transfer complex between o-Ca⁻ and the counterion M⁺, and (3) the use of an insoluble salt o-CA⁻M⁺ as a film adherent to the electrode surface. The behavior of the redox o-CA/o-CA⁻ couple has been investigated in different solvents and in the presence of several alkaline and alkaline earth cations (M^z+). The reversibility of the electron transfer and the formation of an adhesive salt layer on the electrode are strongly related to the counter ion M^z+. The ion-pair formation depends on the solvent and particularly on its weak electron donor character. The best electrochromic properties of o-CA were obtained in 0.1 M NaClO₄ acetonitrile solutions.     

Electroreduction of molybdenum(VI) at a prehydrogenated platinum electrode

The use of hydrogenated platinum electrodes allows observation of the electroreduction of some oxygenated ions, which is otherwise masked by the reduction of the hydrogen ion. The present paper deals with the reduction of molybdenum(VI) at a prehydrogenated platinum electrode in acid solutions. The experimental conditions for the electrode hydrogenation process are the following: 90 min at a cathodic current density of about 7 A/cm(2) for microelectrodes with an area of 0.02-0.03 cm(2); about 120 min at a current density of 1.5-2 A/cm(2) for microelectrodes with an area of 0.25-0.35 cm(2). The reduction of molybdenum(VI) in 0.8-1.6M H(2)SO(4) occurs in two consecutive steps: the more cathodic wave [Mo(V) to Mo(III)] is for the most part masked by the reduction of the solvent; the less cathodic wave [Mo(VI) to Mo(V)] takes place at E(1 2 ) values of about +0.07 V, is well shaped, diffusion-controlled and usable for the determination of molybdenum down to 4 x 10(-5)M or 6 x 10(-5)M if a rotating disk electrode is used. Interferences from diverse ions have been studied. A generalization of the effect of electrode hydrogenation on the reduction of those oxygenated ions so far studied [i.e., vanadium(IV), uranium(VI) and molybdenum(VI)] is presented.     

A new electrocatalytic mechanism for the oxidation of phenols at platinum electrodes

Electrochemical oxidation of phenolic compounds generally produces unstable phenoxy radicals that readily polymerize to passivate the surface of solid electrodes. In this study, the electrocatalytic oxidation of phenol in the presence and absence of methanol was investigated by cyclic voltammetry on a platinum electrode. The cyclic voltammogram of phenol in a mixture of phosphate buffer/methanol solution showed well-defined peaks at ∼600 mV vs. Ag/AgCl reference electrode, which surprising, gradually increased with repetitive scanning, stabilizing after 50 cycles. This unexpected behavior is in contrast to previous studies involving phenolic compounds, which always show a decrease in intensity during continuous potential scanning. Scanning electrochemical spectroscopy (SEM) was further used to investigate the changes in the surface morphology of the Pt electrode after electrodeposition. A new electrocatalytic mechanism for phenol oxidation on the surface of a Pt electrode is suggested in the presence of methanol. The proposed mechanism is based on the formation of a film of Pt oxide/hydroxides onto which the phenol and the products of its electrochemical oxidation are further deposited. The mechanism was also studied using more complex phenolic compounds including resveratrol, quercetin and bisphenol A. The results emphasized the effect of aryl substituents on the electrochemistry of this particular class of compounds.     

甲醇在Pt-Sn/GC上的电催化氧化

直接甲醇燃料电池（DMFC）以环境友好的甲醇为燃料,具有广泛的应用前景。目前使用的阳极催化剂主要是铂族贵金属,成本高,寿命和活性较差,因此,研究出高活性催化剂仍是目前的热点。通常方法是向Pt中添加一种或多种亲氧金属如Ru或Sn等,制成二元或多元铂合金。本文采用循环伏安法在玻碳电极上修饰铂锡合金,金属用量少,操作方便,相比Pt／GC,对甲醇的氧化呈现了较高的催化活性。     

Response mechanism of platinum electrode to uncoupled ions(I)

The transient response mechanism of the platinum electrode to the uncoupled ions may be interpreted with the mixed phase formation (MPF) model of the transient response of precipitate-based ion-selective electrodes to interfering ions for K_xy ≪ 1. It is discovered that the peak height of the transient signal is related to the solubility of M(OH)₂ and hydration heat of M²⁺. The relation between the positive peak height of transient signal of pb²⁺ or cd²⁺ and lga_m obey the Nernst equation, while that of Ca²⁺ or Mg²⁺ does not. The equilibrium potential is not of Nernst response for all ions. Project supported by the National Natural Science Foundation of China.     

Stability of fluorenylemthoxycarbonylamino groups in peptide synthesis. Cleavage by hydrogenolysis and by dipolar aprotic solvents

$\text{N}$-Fluorenylmethoxycarbonyl derivatives of amines are unexpectedly cleaved by catalytic hydrogenation with t$\text{1}\text{2}$ 3–33 h. under various conditions. They are also cleaved on standing in the solvents dimethylformamide, dimethylacetamine, and $\text{N}$-methylpyrrolidone, but much more slowly.     

Analysis of time-dependent kinetics for the oxidation of methanol on a platinum electrode and reaction mechanism

The reaction kinetics for the oxidation of methanol on a platinum electrode have been examined under precisely controlled conditions. The Tafel relations at constant surface coverages of the strongly adsorbed species show the existence of two potential regions where the predominant reaction path is different. The surface reaction of the strongly adsorbed species with OH(a) is rate determining at E > ca. 0.55 V, while the oxidative adsorption of methanol to form a reactive intermediate becomes the rate-determining step at E < ca. 0.55 V. In the latter potential region, the strongly adsorbed species is not oxidized so that its accumulation on the surface decreases the rate of the oxidative adsorption and thereby the total oxidation rate.     

Electrochemical studies of sulfonates in non-aqueous solvents: Part III. Trifluoromethanesulfonic acid as a strong acid in dipolar aprotic solvents and acetic acid

Dissociation of trifluoromethanesulfonic acid in non-aqueous solvents has been investigated mainly by conductometry and compared with that of p-toluenesulfonic, methanesulfonic, fluorosulfuric and perchloric acids. The solvents used are dimethyl sulfoxide, N,N-dimethylacetamide, N,N-dimethylformamide, n-butyronitrile, propylene carbonate, acetonitrile, nitromethane, methyl isobutyl ketone and acetic acid. Trifluoromethanesulfonic acid is completely dissociated in rather basic solvents such as dimethyl sulfoxide, dimethylacetamide and dimethylformamide, while it is associated to some extent in the other solvents, especially in methyl isobutyl ketone and acetic acid. The strength of trifluoromethanesulfonic acid is almost the same as that of perchloric acid in all the solvents. Dissociation constants of the other acids are also obtained.     

Electroreduction of peroxodisulfate anion at platinum rotating disc electrode in the cyclic voltammetry mode

Electroreduction of peroxodisulfate anion at smooth polycrystalline and platinized (at different deposition potentials) platinum in perchloric acid and sulfuric acid solutions is studied by rotating disc electrode and cyclic voltammetry techniques. Dependences of the process rate on the electrode rotating velocity, the potential scan rate, the anodic limit of the scanning, the peroxodisulfate anion concentration in the solution and the platinizing conditions are found. The suggestion on the complications in the peroxodisulfate anion reduction caused by adsorbate formation is corroborated, at least, for certain potential region. The reaction structure sensitivity is evidenced, which makes it possible to use the reaction for characterization of the platinized Pt surface structure. The comparing of obtained results with literature data concerning smooth platinum and the single-crystal platinum basis faces allows concluding that the peroxodisulfate anion reduction maximal rate in sulfuric-acid solutions occurs at the potentials close to those observed for the (110) face. When the platinized Pt surface roughness factor exceeds ~30, the peroxodisulfate anion reduction reaction proceeds under the inner-diffusion limitation control. The platinized-Pt rotating disc electrode can serve as model tool in the studying of properties of disperse material microdeposits.     

Electroreduction of uranium (VI) at a platinum electrode and its analytical applications

Under normal conditions, the reduction of uranium(VI) at a platinum electrode, in acid solutions, is masked by the reduction of the hydrogen ion. If the working electrode is subjected to hydrogen evolution (at a current density of about 7 A cm (2) for 90-120 min) the H(ads) on the platinum surface, acting as a bridge in the electron transfer, shifts the reduction wave of uranium(VI), in 1M sulphuric acid solutions, to potentials (E(1 2 ) congruent with - 0.03 V) less negative than that of the hydrogen discharge (about -0.25 V). The wave corresponding to the reduction of uranium(VI) to uranium(IV) is well shaped, diffusion-controlled, and can be used for the determination of uranium down to 2 x 10(-5)M or 3 x 10(-6)M if a rotating electrode is used. Interferences arise from those ions with similar E(1 2 ) [i.e., Cu(II) and Bi(III)], or from those such as permanganate and dichromate, which oxidize the H(ads) on the platinum electrode. Because of the time required for the electrode pretreatment, the determination is time-consuming but in some respects it appears a useful improvement over the DME.     

Electrochemical oxidation of isoniazid catalyzed by (FcM)TMA at the platinum electrode and its practical analytical application

The electrocatalytic oxidation of isoniazid (INH) by (ferrocenylmethyl)trimethylammonium [(FcM)TMA] at the platinum electrode in 0.10 M Na₂SO₄ aqueous solution was studied by cyclic voltammetry (CV). Although INH itself showed a very poor electrochemical response at the platinum electrode, the response could be greatly enhanced by using (FcM)TMA as a mediator, which enables a sensitive electrochemical determination of the substrate INH. The reaction rate constant for catalytic oxidation reaction was evaluated as (3.98±0.10)×10³ M⁻¹ s⁻¹ by using chronoamperometry (CA). Experimental conditions such as supporting electrolyte and its concentration, solution pH, and the concentrations of the catalyst (FcM)TMA and the substrate INH were investigated to maximize the current efficiency of the electrocatalytic oxidation. The method can be used for the sensitive practical determination of INH, and also opens an avenue for using (FcM)TMA as a mediator in electroanalytical determination which is very simple, cheap, and rapid. Furthermore, no sample pretreatment or time-consuming extraction steps are required prior to the analysis.