Contribution a l'etude du comportement electrochimique du nickel en milieu hydrazine

A potentiostatic set up with three electrodes was used to study the electrochemical behaviour of nickel electrodes in hydrazine. In acid medium it was possible to show a nickel corrosion. But in neutral saline medium no attack of the metal could be shown in hydrazine.     

Realisation et controle du fonctionnement d'une cellule pour l'electrosynthese en continu

An easy to build cell for organic electrosynthesis with flowing solution through porous electrodes (parallel grids) is described and tested at analytical scale. High selectivity for the electrode reaction and significant chemical yield are obtained with only one-flow through the working electrode despite a short time of electrode-solution contact (1-100 s). Along the same lines, a larger cell for macroscale electrosynthesis is feasible.     

Electrocristallisation et dissolution anodique d'un monocristal d'argent en milieu nitrate d'argent acidifie: aspects cinetiques et morphologiques

Silver electrodeposition and dissolution in acidified 1M AgNO₃ is investigated using several techniques: current-potential plots, scanning electron microscopy and impedance measurements down to low frequencies. For various NO^?₃ concentrations and single crystals of orientation (100), (110) or (111), it is observed that a low anodic or cathodic overpotential gives rise to a strong activation of the electrode surface which is blocked at the equilibrium. The deposit morphology depends on substrate orientation, current density and NO^?₃ concentration. Impedance diagrams are in agreement with a very fast electron transfer and reveal at low frequencies two time-constants independent on the value of anodic or cathodic current density, on NO^?₃ concentration and on substrate orientation. The results are discussed in terms of nucleation and propagation of edges.     

Calcul du profil de concentration et du facteur d'efficacite lors d'une reaction avec changement de volume

The influence of the stoichiometry of a reactive system involving a large number of components on the effectiveness factor of a catalyst has been studied. An adequate change of variables reduces the problem to solving a set of two non linear first-order differential equations describing the diffusion of a single component in the pores of the catalyst. The method is applied, as an example, to the oxidation of sulfur dioxide, in order to estimate errors predicted by the constant volume hypothesis, as functions of catalyst particle size, degree of conversion, and initial concentrations.     

Etude par resonance paramagnetique electronique de noirs de carbone traites par l'azodiisobutyronitrile en absence d'oxygene

Various carbon blacks were treated in absence of oxygen with azobisisobutyronitrile (AIBN). Samples of non-graphitized carbon blacks (Spheron 6, P33, Sterling FT) and graphitized carbon blacks (Graphon, P33 (HTT)) were studied by ESR before and after the AIBN treatment. The line width, g-factor, and the contributions of localized and conduction carrier spins were determined. The results obtained lead the authors to conclude that in the process of fixation of the products of the thermolysis of the AIBN no radicals are involved.     

Observations macroscopiques et microscopiques des produits d'insertion du graphite avec le trichlorure d'or

The intercalation of gold trichloride has been studied in the classical manner by the action of the vapour on graphite in a sealed tube[12]. Tables 1 and 5 summarize the results obtained with various samples of natural and artificial graphite and with carbon materials graphitized at high temperatures. Under optimal reaction conditions (245°C), a blue, richest ($\text{Δm}\text{mo}\text{= 200\%}$) product is obtained, of formula C_12.6AuCl₃: gold trichloride intercalates in the pure state (Table 2). Comparison of the results obtained by thermogravimetry (Fig. 1), dialatométry (Fig. 2) and X-ray analysis (Fig. 3) indicates that this final product C_12.6AuCl₃ is of first stage with an interplanar distance of 6.80Å (Table 3). Study of the relative intensities[17] of the (001) reflections allows stating that the gold trichloride intercalates are in the form of planar molecules Au₂Cl₆ (Table 4). Macroscopic observation of the pyrographite samples shows that the insertion occurs in an “all or nothing” manner[18]: during the reaction the layers are gradually deformed in the neighbourhood of the “invasion front” (Fig. 4) and when the intercalation is terminated, a pleating of the layers appears very clearly at the surface (Figs. 5 and 6). These dislocations visible on the edges of the samples (Fig. 7) are a manifestation of the “separation work” necessary for the intercalation which practically excludes the possibility of reagent layers inserted with a lower ratio than in C_12.6AuCl₃.Graphite and its intercalation compounds lend themselves only with difficulty to a microstructural study by visualization of the lattice planes (instability of the products under an electron beam, basal carbon planes invariably parallel to the grid support).Carbon blacks (Sterling MTG 3000°C) are small circular rods (θ < 1μ) graphitized throughout their mass (Fig. 8) which give with gold trichloride, rich products ($\text{Δm}\text{mo}\text{= 140\%}$), almost of the second stage. With these carbons, it was not possible to intercalate ferric or chromic trichloride[21]. The lattice distances measured on micrographs of the intercalated products (Fig. 9) allowed to identify the most contrasted fringes as direct representations of gold trichloride layers inserted between those of the carbon (the finer fringes). The different types of layers observed along the segment AB (Fig. 9) indicate a succession of different stages from A to B (1st-1st-2nd-3rd-4th-3rd-5th) which indicates the inhomogeneity of the products studied[10]. These results seem to corroborate the “statistical definition” of stage identifiable in the combination of graphite with the halogenides[8,19,22].     

Comportement electrochimique d'oxocarbones et d'aci-reductones en relation avec le transport d'electrons dans les processus biologiques—V. Etude du couple redox indantrione-1,2,3/dihydroxy-2,3 indenone-1 en solvant non aqueux

The reduction of indantrione-1,2,3 and the oxydation of dihydroxy-2,3 indenone-1 has been investigated on mercury electrode in dimethylformamide. According to the acidity of the medium, either two successive one-electron reversible waves or a single two-electron wave are observed. In the second situation, the proton transfert reaction which are coupled to the electron transfer steps are elucidated. The radical anions produced by electrochemical reduction form associations with the sodium cation allowing isolation of its organic salt.     

Comportement electrochimique d'oxocarbones et d'aci-reductones en relation avec le transport d'electrons dans les processus biologiques—VII. Reduction de l'acide rhodizonique en milieu aqueux acide

The hypothesis of an ECE1 mechanism may justify the electrochemical behavior observed for the reduction of dihydrated rhodizonic acid. In a first step, RDZ, 2H₂O is irreversibly reduced via a two-electron process without preceding dehydration. The following chemical step is an acid-catalysed dehydration of the monohydrated intermediate species and produces 1,4-tetrahydroxybenzoquinone (THQ) which is reduced via a two-electron process at a potential less negative than RDZ, 2H₂O. According to the hypothesis of an ECE1 mechanism a value of k = k′ (H⁺) with k′ = 145 ± 7 s^?1 is found for the rate constant of the chemical step. The monohydrated intermediate species is reducible via a two-electron process, and without preceding dehydration, at a potential more negative than RDZ, 2H₂O.     

Etude comparative des processus electrochimiques d'oxydation de la N-methylphenothiazine et du ferrocene en milieu micellaire aqueux

A comparative study of oxidative electrochemistry of N-methylphenothiazine (MeP) and ferrocene in aqueous micellar systems was made by the use of cyclic voltametry and voltamperometry on rotating disk electrode. In acid media MeP becomes oxidized as ferrocene in neutral medium, in a reversible one-electron process, whose rate (at the potential of the current plateau) is limited essentially by the diffusion of micelle particules containing the electroactive compound. Levich criteria (at the limiting current) indicate that other phenomena must occur: partial blocking of anode or (and) hypothetical C.E. mechanism. The stability of the cation radicals MeP⁺ obtained by anodic oxidation in micellar medium was studied by chronoabsorptometry and with a ring-disk electrode. Micelles seem to have little influence on the rate of disappearance of MeP⁺ by the reactive species of the solution.     

Etude des phenomenes electrochimiques et des transports de matiere d'un systeme metal electrolytique: cas d'un disque tournant en cuivre dans des solutions aqueuses d'acide chlorhydrique

In the first two areas of the curves I = F(E) corresponding to Cu/CuCl_x^1?x and Cu/CuCl/CuCl_x^1?x redox systems we study the diffusion of Cl^?, Cu⁺ Cu²⁺ and CuCl_x^1?x species: in acidic chloride media (0,1 Ml^?1 ?HCl ? 3 Ml^?1) the diffusion of Cl^? does not exist because the amount of ions consumed is negligible compared to the large quantities contained in the solution, these HCl solutions behave like supporting electrolyte. Cu⁺ and Cu²⁺ does not exist in the diffusion layer on account of the high Cl^? concentration. At all events the diffusion of CuCl_x^1?x complex is the rate determining step. Experimental current is a pure diffusion current because the transference number tCuCl_x^1?x is negligible compared to one and the charge transfer step is very fast. The electrode surface seems to be uniformly reactive although for the Cu/CuCl/CuCl_x^1?x system we can assume there may be a partially blocked electrode surface constituted by independent sites.     

Cinetique du couple redox ferro-ferricyanure a des electrodes d'oxyde et de graphite. Effets de la nature de l'electrode et du cation alcalin

At an electrode made of graphite or of the mixed oxide La_0.8Sr_0.2CoO₃, it is shown with the rotating disk method that the limited current of oxidation of ferrocyanide is governed by the diffusion of a participating species only with K ⁺ and Cs ⁺. Conversely, with Na ⁺ and Li ⁺, it is governed by the diffusion and by a heterogenous reaction which takes place before the charge-transfer. This step is apparently catalysed by the surface of the oxide. By scanning potential cyclic voltametry, it is shown that, with Na ⁺ and Li ⁺, the oxidation reaction is semifast, though it is an order of magnitude faster with K ⁺. By conductibility measurements, we have verified that the number of ion pairs formed is larger with K ⁺ than with Na ⁺ and Li ⁺. A mechanism in term of “contact” and “non contact” ion pairs is proposed.     

Etude des cinetiques successives d'insertion et de desorption du brome dans un pyrocarbone

A single sample of pyrocarbon was submitted to 15 successive sorptiondesorption cycles in a bromine vapor atmosphere, at various temperatures and pressures. The sample mass and diamagnetic anisotropy were simultaneously measured in situ as a function of time. It is concluded that the initial and final states of the evolution of a system may be varied in a rather broad range without impairing the validity of Pacault's rule. Apparent activation energies of 1–2 kcal/mole (sorption) and 15–25 kcal/mole (desorption) are derived from the results. A quantitative interpretation of the successive sorptions kinetics may be achieved with a simple diffusion process along the basal planes of the graphitic crystallites. Some empirical correlations between bromine content and diamagnetism during the sorption or desorption processes are found but their physical meaning is not yet clear.     

Etude electrochimique de l'iode et du brome en presence de cryptands

In presence of cryptands ([2.2.2], [2.2.1] and [2.1.1]) halogens X₂(bromide and iodine) give rise to the formation of an halonium ion complex in chloroformic solution. Preliminary voltammetric studies of halide and halogen alone allow to measure the stability constant of the trimeric X₃^? species in this solvent. By calculation it is then possible to determine the constant ratio: [cryptand —I⁺] [I₂]^?1 [cryptand]^?1 for iodine only, which is supposed to be a ratio fo the kinetic constants of formation and decomplexation of the halonium complexe.     

Mise en oeuvre des methodes d'impusions galvanostatiques rapides: determination de la resistance de transfert de charge et de la capacite differentielle d'une electrode

Reading within a very short time the voltage response to galvanostatic pulses requires the use of a microelectrode having a suitable geometry and an area not exceeding a few mm². The comparison of different methods (single, double pulse, charge step-decay) applied to the H⁺/H system and to some Me^z+/Me systems show that the double pulse method is the only one to yield the determination of the R_t and C_d characteristic parameters 60–80 ns after the beginning of polarization.     

Etude theorique de mecanismes de regeneration du deuxieme ordre en voltammetrie en couche mince ou en surface—application a l'etude de la regeneration catalytique de l'iode par les iodates

A theoretical study of current-potential curves in thin layer or in surface potential sweep voltammetry is presented for second order regeneration mechanisms. The theory is applied to the study of the catalytic reduction of iodate by thin layer potential sweep voltammetry. A trace of iodine is added to the iodate solution and the electroreduction of iodine to iodide initiates the catalytic reduction of iodate. The rate constant of the reaction of iodate with iodide which is determined is in good accord with values given in the literature.     

Separation isotopique d'azote 15—III. influence du degre d'oxydation des gaz de reflux pour le systeme NO,NO2-HNO3

The authors present a spectrophotometer built to measure NO₂ concentration in nitrogen oxides mixture. The coefficient of oxydation of these vapours, at fixed pressure and temperature, is quite linear with the NO₂ concentration in the gas.In reduction of nitric acid with SO₂, a constant coefficient of oxydation of nitrogen oxides has been obtained using the spectrophotometer as the controller in the regulation loop. The influence of this parameter on the isotopic separation of nitrogen 15 is described; reflux gases with a coefficient of oxydation of 25 per cent is recommended for the first stage of a separation process using a 57% weight nitric acid feed.     

Etude du comportement d'un réacteur en régime continu à haute température

The authors discuss the features of a continuous reactor designed to operate at high temperatures (3000–4000°K). Assessment criteria have been provided by the study of the kinetics of a standard reaction-dissociation of carbon dioxide. Thermal energy was obtained using a plasma torch. The longitudinal and radial pressure, temperature and concentration profiles, under various flow and initial temperature conditions, have been determined. The measurements to establish these profiles were obtained by using special probes. With the aid of pyrometric sighting evidence of a clearly defined isothermal core, at the centre of the reaction zone, has been provided. A comparison between computed and experimental conversion figures has been found quite acceptable. The authors propose a carbon dioxide disappearance rate correlation; this equation agrees with those determined using shock wave tubes, although the method of heat generation is entirely different in each case.     

Etude de la reduction de l'oxygene sur les phtalocyanines monomeres et polymeres—III. Phtalocyanines monomeres de fer en couche mince sur l'or

Two kinds of iron phthalocyanine electrodes are prepared by vacuum deposition on gold substrates. The show remarkable differences of activity concerning O₂ reduction, and are investigated by X-ray, optical spectroscopy and X-ray photoelectron spectroscopy (ESCA) techniques. As predicted in a previous paper [3], the iron ion in the active form, is presenting a high spin d⁵ configuration.     

Etude electrochimique a l'etat solide du couple anthraquinone—9.10—anthraquinol,en tant que matiere active d'electrode de generateur secondaire

The—9.10 anthraquinone—anthraquinol couple has the proper characteristics to be used, in the solid state, in negative electrode reactions of secondary electrochemical generators in the presence of an aqueous electrolyte.Our research work covered the conditions of electrochemical formation of anthraquinol, of little stability in the presence of oxygen, within the electrode and starting with anthraquinone. The electrochemical characteristics of such electrodes were then measured.The equilibrium potential of the electrochemical system was about 80 mV/enh in a normal sulphuric medium and the actual capacities recorded were measured with respect to different operation conditions of the electrodes. The variation of these characteristics, according to electrode composition, medium acidity and temperature, were studied.This research led to the development of an experimental battery supplying a voltage of approx 0,5 V and having a mass energy ratio of about 25 Wh/k of electrode.     

Etude du mecanisme et de la cinetique de la reduction des ions uranyles dans des solutions d'acide phosphorique

The electrochemical reduction of uranyl ions in 0,1-9 M phosphoric acid has been investigated by polarography, cyclic voltammetry, chronopotentiometriy and potentiostatic coulometry.In concentrated phosphoric acid solutions (H₃PO₄ < 1 M), an adsorption pre-wave appears in addition to the main wave of diffusion. The electrochemical reaction involves a single electron. Kinetic usually corresponds to a quasi-reversible system, but it evolves according to the variation of U(VI) and H₃PO₄ concentrations. So, when the concentration of U(VI) increases and/or that of H₃PO₄ reduces, the system becomes reversible.