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.     

Etude electrochimique des complexes formes par dissolution de CrBr3, 6H2O dans le N,N-dimethylformamide et de leurs equilibres en solution. Influence de l'addition d'ions X− (X = Cl,Br)

The identification of several complex cations formed during the solubilization of hexahydrated chromium bromide in DMF and their evolution during time has been investigated from the electrochemical reduction curves and the visible absorption spectra. With chloride anions in the solution, a rapid insertion of the chloride ions into the internal chromium complexation sphere can take place, which leads to formation of [Cr(DMF)₄Cl₂]⁺ and then [Cr(DMF)₃Cl₃] from [Cr(DMF)₅Br]²⁺ first formed.     

Reduction electrochimique des cations complexes formes par dissolution dans le N,N-dimethylformamide des sels de chrome du type CrX3, 6H2O (X = Cl,Br, ClO4)

The electrochemical reduction of different cations formed by the solubilization of hexahydrated chromium salts in DMF was studied by polarography and cyclic voltammetry on stationary mercury dropping electrode, with tetrabutylammonium tetrafluoroborate 0.2 M as an indifferent electrolyte.The nature of the cations formed by water—DMF ligand exchange in the chromium complexation sphere depends on the nature of X(X = Cl, Br, ClO₄).The reduction potentials and some electrochemicals kinetics parameters of [Cr(DMF)₆]³⁺, [Cr(H₂O)₆]³⁺, [Cr(DMF)₅Br]²⁺, [Cr(DMF)₄Cl₂]⁺ and [Cr(DMF)₃Cl₃] were determined as well as exchange reactions between these electroactive species which may be simultaneously present either in solution or at the electrode.     

Obtention de sulfures de cuivre non stoechiometriques,par voie electrolytique,analyse des solides par diffraction de rayons X

The anodic oxidation of cuprous sulphide has been studied in acidic medium using different methods: electrolyses of suspensions of cuprous sulphide and studies by means of a carbon paste electrode. At low current densities, the oxidation proceeds via the formation of four non stoichiometric copper sulphides: Cu_1.92S; Cu_1.77S; Cu_1.60S and Cu_1.31S.Some of these compounds have been isolated from controlled-potential electrolyses and characterized by X-ray diffraction measurements.     

Etude du mecanisme d'oxydation des noirs de carbone par l'ozone en milieu aqueux

The kinetic study of the oxidation of carbon blacks in an aqueous medium shows two distinct and simultaneous reactions: formation of degradation products which are partially oxidized to carbon dioxide, and a direct oxidation into carbon dioxide. The ratio of oxidized carbon in both reactions is constant and characteristic for a given carbon black. It is shown that degradation products arise from less organized areas of the particles. Determination of crystallinity of the carbon blacks by X-ray diffraction indicates a degree of crystalline disorganization directly related to the interplanatary d spacing. The relation established between the fraction degraded by oxidation and the disorganization factor leads to a classification of the carbon blacks according to the degree of perfection of their interplanar d spacing distribution pattern.     

Etude electrochimique de l''iode et de ses derives en solution dans la pyridine variation des proprietes d''oxydo-reduction en fonction du pH

The electrochemical behaviour of iodine and redox derivatives in solution in pyridine was studied by the use of voltammetric method. By controlling the pH value, it is possible to establish the normal apparent thermodynamic diagram of stability of iodine in this solvent. In acid media, the four chemical species I⁻, I⁻₃, I₂ and I⁺ are stable; their respective stability constants are evaluated. In basic media, one observes the disproportionation reaction of iodine, at first into triodide and into a base complexing with iodine in (I⁺), then into iodide and iodate. A salt of positive univalent iodine was isolated by neutralizing I⁺ with an excess of tetraethylammonium hydroxide.     

Etude de la reaction d'oxydo-réduction de l'aluminium dans des bains de chlorures alcalins fondus

Résumé Dans le présent travail nous étudions la réaction d'oxydo-réduction de l'aluminium dans l'eutectique fondu NaCl–KCl–LiCl à 723 K en se penchant plus particulièrement sur le problème de la détermination de la valeur du nombre,n, d'électrons échangés au cours de cette réaction. Deux types d'expériences sont réalisées, tout d'abord des déterminations de l'activité des ions aluminium par mesure de fém, puis des mesures de cinétique à l'aide de la méthode chronopotentiométrique sur une électrode d'argent. Nous avons constaté que l'analyse des courbes provenant de ces méthodes nécessite la connaissance des coefficients d'activité ainsi que la variation du coefficient de diffusion, en fonction de temps pour en déduire une valeur den. La prise en compte de ces paramètres au cours de l'analyse des chronopotentiogrammes obtenus, conduit à des valeurs den voisines de 3 conformément à la réaction d'oxydo-réduction de l'aluminium.

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A study of the redox reaction of aluminium in a NaCl–KCl–LiCl melt at 723 K has been carried out with particular emphasis on the problem of determination of the number of electrons,n, exchanged during this reaction. Two types of experiments were performed, initially the determination of the aluminium ion activity by e.m.f. measurement and, secondly, investigation of the reaction kinetics using chronopotentiometry on a silver electrode. For analysis of the curves resulting from this method, a knowledge of the activity coefficients and of the variation of the diffusion coefficient with time is needed in order to derive a value forn. Taking into account these parameters, a value ofn approximately equal to 3 was obtained for the aluminium redox reaction.

     

Comportement electrochimique de l'uranium,du fer et du vanadium,dans l'acide orthophosphorique et dans le melange H3 PO4 5M—HF 1M. Etude cinetique de la reduction de l'uranium (VI) par le fer (II)

The electrochemical behaviour of uranium, iron and vanadium in 0,1 M–10 M phosphoric acid solutions, has been investigated by polarography, cyclic voltammetry and controlled-potential electrolyses. In concentrated phosphoric acid solutions (|H₃ PO₄| > 3 M), uranium (VI) can be reduced by iron (II). Kinetic measurements of the reduction in 5 M H₃PO₄ and 5 M H₃PO₄—1 M HF mixtures have been carried out.     

Etude par chronopotentiometrie des reactions anodiques sur electrodes de carbone dans les bains cryolithe—alumine

The anodic behaviour of carbon electrodes in cryolithe-alumina melts has been investigated by use of galvanostatic pulses in a wide range of intensity i (2.3–30 A/cm²). The corresponding transition times τ vary from 0.07 to 10 ms. The classical law describing the galvanostatic process (Sand law) is not obeyed; the experimental quantity i τ ^{$\text{1}\text{2}$}/C^o is always much smaller than the theoretical previsions. In addition, by operating in air-tight apparatus it is shown that a lowering of the pressure leads to a decrease of the transition time. These observations are interpreted in term of variation of the active surface of the electrode. A large part of these variations are attributed to the presence of gaseous compounds which remain on the electrode during the pulse of current. The consequences of this hypothesis on the derivation of the transition time are calculated. A semi-empirical treatment is proposed to take account of this process.     

Cinetique de graphitisation d'un coke de brai et d'un pyrocarbone suivie in situ par diffraction de rayons X

An X-ray spectrometer has been adapted to allow the recording of relatively fast transformations “in situ”. The experimental set-up was used to measure the changes in d₀₀₂ occurring with time for two kinds of carbon samples (a pitch coke sample and a pyrocarbon) while the sample was maintained at a constant temperature HTT. Methods to smooth out the scatter are applied to the graphitization kinetic curves and the dangers involved in using these methods are shown and discussed. Thus it turned out not yet possible to follow “in situ” small d₀₀₂ variations with a precision needed to establish the presence of step processes in graphitization.     

Etude de la formation du monocarboaluminate de calcium hydrate au contact d''un granulat calcaire dans une pate de ciment portland

Calcite reacts with Portland cement paste. Calcium hydrocarboaluminate C₃A.CaCO₃.11H₂O is formed by reaction between hydrated calcium aluminate C₃A.1/2CO₂.12H₂O and carbonate ions produced by dissolution of calcium carbonate. Investigations by X ray diffraction and S. E. M. show that, due to the low mobility of dissolved ions, these reactions are limited to the interfacial zone, and the reaction rate depends on W/C.

The effective contact surface between paste and aggregate is growing because of these reactions and the mechanical properties of the bond are improved.

Résumé

Les granulats calcaires réagissent avec la pâte de ciment Portland. Les ions carbonates libérés au cours de la dissolution du carbonate de calcium se combinent avec l'aluminate de calcium C₄A.1/2CO₂.12H₂O pour former le monocarboaluminate C₃A.CaCO₃.11H₂O. Des études par diffractométrie de rayons X et M. E. B. montrent que, du fait de la faible mobilité des ions dissous, ces réactions se produisent dans la zone interfaciale et que leur vitesse dépend de la quantité de l'eau de gâchage. Du fait de cette attaque, la surface réelle de contact entre le granulat et la pâte augmente, ce qui améliore la résistance mécanique de l'ensemble.     

Etude, par voie electrochimique, de l''influence du fer (II) sur la reduction de cetones en solution dans le N, N-dimethylformamide (DMF)

The electrochemical reduction, in DMF, of a series of aromatic ketones has been studied in the presence of iron salts. With these ketones, the first reduction peak of the ketone is always preceeded by another peak which appears at a less negative potential. This peak is attributed to a fast chemical reaction between Fe(II) and the anion radical of the ketone, which shifts the electrochemical peak potential. With fluorenone, several species are formed corresponding to 1/1, 3/4, 1/2, 1/4 iron/ketone ratios. For the other ketones, the 1/1 species is mainly formed.     

Etude de la reversibilite en solution aqueuse de couples constitues par des aci-reductones cycliques et leurs derives dehydro

The oxidation products of inositol, rhodizonic and croconic acids

and ninhydrin

undergo two electrode processes: an irreversible process concerns the α unsaturated carbonyl group; a reversible process occurs on the β unsaturated carbonyl group after the hydration equilibrium has been shifted.The high polarity of the middle carbonyl group causes the easy reductibility of dehydrated ninhydrin and the stability of hydrated and radical species.From the difference ΔE _{$\text{sol1}\text{2}$} between the value of half-wave potential of the cathodic reversible wave and that of the irreversible wave, it can be deduced an approximate value of $\text{Δ}\text{G}$°* for the dehydration governing step in irreversible process.     

Etude de la polymerisation electrochimique de triphenylamines substituees par le groupe hydroxy-1 ethyle. Preparation d'electrodes modifiees par des films de polymeres redox

Upon anodic oxidation of 4-(1-hydroxyethyl) triphenylamine 1 and 4,4′-di(1-hydroxyethyl)tri-phenylamine 2, a redox polymer grows on the electrode, and another one precipitates in the solution. The mechanism of polymerization of these carbinols is discussed on the basis of electrochemical, infrared and mass studies. The polymerization, initiated by oxidation, proceeds by acid catalysis. The redox polymer contains benzidine and triphenylamine moieties.Study of electrodes modified by electropolymerization of 1 shows that the rate of film electrochemical reactions is controlled by the rate of diffusion of the electrochemical charge through the film.     

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 cinétique de la réaction de carburation du molybdène par le méthane

The kinetics of carburization of molybdenum by methane has been studied in a fluidized bed differential reactor. Scanning electron microphotographs of the reacting solids, at various conversion levels, have been analyzed. It has been shown that pore formation resulted in considerable increase in reaction rate. Initial rates are analyzed in terms of growth of superficial carbide nucleus. The rate of growth of the germs was found to decrease with time of reaction. A model is proposed to explain this decrease.     

Interpretation des courbes de polarisation cathodique du cuivre dans les solutions chlorurees acides desaerees: etude simultanee de la corrosion du cuivre et de la reduction des ions H+

During cathodic polarization of copper in deaerated acidic chloride solutions, two simultaneous processes occur: the dissolution of the metal and the reduction of the H⁺ ions. One can explain the steady state curves lg I = f(E) by means of the theory of adsorbed species. Each interpretation differs according to the nature of the adsorbed species (Cu H⁺_ads for the first one, Cu H_ads for the second one). They are made up of three consecutive reactions involving a rate determining step (R.D.S.) and another step during which the desorption of the adsorbed species brings out the two simultaneous experimental phenomena.     

Observation directe au microscope electronique par transmission de la liaison pate de ciment-granulats dans des mortiers de calcite et de quartz

A component mainly composed of hydrated silicate of calcium or of a by-product, making very close binding, exists at the proximate contact between the aggregates of calcite or quartz surrounded by a hardened cement paste or C₃S. This component, observed by electron microscopy, doesn't offer the same appearance close to two minerals which leads one to think that chemical binding strength does exist, above all with calcite.