Mecanismes de reduction electrochimique de complexes acetylacetonates de metaux de transition

Electroreduction mechanisms of Mn, Fe, Co, Ni, Cu, Ce acetylacetonato complexes have been established in aprotic medium, on mercury electrode, by polarography, cyclic voltammetry and potentiostatic coulometry followed by identification of the reduction products. Two reaction schemes are proposed: (1) E.C. mechanism:

(2) Reversible monoelectronic reduction:

After this reversible reduction, Fe(tfac)₃ leads to iron (0) in a further reduction step. The diffusion coefficients, the Stokes-Einstein radii and the available apparent rate constants of the electrochemical steps are given.     

Contribution a l'etude de quelques equilibres et de cinetiques de reductions d'oxydes de manganese

This work concerns the reduction kinetics of certain manganese oxides in an acid medium. An attempt was made to evaluate these reduction mechanisms with polarization curves.

We have shown that polarization curves may be obtained only in zones of thermodynamic instability of the oxides, since the stability zones show high overvoltages with respect to the equilibrium potential.

The experimental work confirmed that in these zones, e.g. β-Mn₂O₃, there are very pronounced electrochemical equilibria. Furthermore “pseudo-equilibria” are shown to exist within the instability zones. These may explain the lowering of the above mentioned overvoltages; polarization curves in this region can be drawn.     

Etude de la reduction sur electrode de mercure de thioethers et de sulfoniums derives de l'acide β-mercaptopyruvique

Thioethers with general formula RSCH₂COCOOR′ where R is paratolyl, methyl or ethyl, and sulfonium salts like Br^?,(CH₃)₂$\text{S}\text{+}$-CH₂COCOOR′ which proceed from β-mercaptopyruvic acid or its ethylester, have been investigated polarographically and by means of controlled potential electrolysis, in aqueous or aquo—alcoholic media.Reduction of sulfonium salts at the mercury electrode is in accordance with the general rule a reductive cleavage of the substituting group RS— occurs. Behaviour of thioethers is not uniform. When R is aryl, cleavage of the carbone—sulphur bond still occurs; when R is alkyl, the carbonyl group undergoes the reduction and the substituted secondary alcohol is formed.     

Role de la temperature de cuisson sur les proprietes de l''anhydrite

The paper briefly reviews the effects of the calcination temperature on some anhydrite properties. The different treatments modify: specific area, granulometry, exoemissivity and dissolution kinetics. Some of these evolutions can be interpreted by a variation of the defects due to the grain sintering.

Résumé

Dans cet article, les auteurs examinent l'effet de la température de cuisson de l'anhydrite sur certaines de ses propriétés. Ces différents traitements modifient notamment: la surface spécifique, la granulométrie, le pouvoir exoemissif et la cinétique de dissolution. Certaines de ces évolutions peuvent s'interpréter par une variation des défauts dûe au frittage des grains.     

Cinetique de la reduction de l'acetate de CoIII dans l'acide acetique

We studied, through visible spectrophotometry (470–820 nm), the acetate reduction kinetics of Co(III) in acetic acid between 35°C and 80°C. We demonstrated, with the presence of a 670 nm isobestic point, two Co(III) complexes: Co(III)α and Co(III)β. Spectral deconvolution allowed access to various absorbencies and suggests the following mechanism: The rate constants and activation energies of the three elementary steps of the reaction were obtained. Determination of Co(III)α and Co(III)β spectra allowed Co(III)α to be identified to the Co(III)s complex already known.     

Coupure cathodique de liaisons carbone-oxygene—III Reduction de pinacols sur electrode de mercure—une possibilite de rupture electrochimique de liaisons carbone-carbone

Pinacols (or α-glycols) may be reduced electrochemically if they have at least one electroactive group (an “electrophore”) α to the alcoholic function. In protic media a cleavage of the CO bonds and the central CC bond occurs, and its has not been possible to separate the steps. Strong bases cleave the cental CC bond of pinacols with formation of the anion radical of the ketone and in aprotic media the electrolytically generated base is probably responsible for the splitting of the CC bond.The results are discussed in light of the previously obtained results on the reduction of alcohols and a demonstrated electrolytical cleavage of certain hydrocarbons.     

Chaleurs de dissolution et de protonation de la piperazine et de ses derives dans divers melanges eau-ethanol

The thermodynamic functions ΔG⁰_n, ΔH⁰_n and ΔS⁰_n for the protonation of piperazine (Pz), 2-methylpiperazine (2MPz), N-methyl-piperazine (NMPz) and N-phenyl-piperazine (NPPz) have been determined by potentiometric and calorimetric procedures at 25°C in mixtures of water-ethanol with 73·4 and 85·4% ethanol by weight (I = 0·1M NaClO₄). The values obtained, also the heats of dissolution of these diamines in the similar mixtures with 0%, 52%, 73·4%, 85·4% and 100% ethanol are advanced as evidence for the important solvation properties of the nitrogen doner.     

La flottation de spheres et de cylindres

A study is made of the maximum weight of spherical and cylindrical particles which can be supported by a pendant bubble and at an infinite fluid/liquid interface. The maximum weight increases with the radius, the contact angle (measured through the lower fluid), the density difference between the two fluids and the interfacial tension. It follows that to ensure good flotation suface active agents must be added to increase the contact angle. The maximum particle weight which can be supported at an infinite interface is always greater than that which can be transported there by the pendant bubble.     

Proprietes nucleophiles de surfaces de silicate tricalcique

It has been experimentaly established that surface centres of tricalcium silicate, which play a fundamental role in the hydration mechanism, present a highly nucleophilic character. Use of tetracyanoaethylen (TCNE) has shown the electron transfer from the surface centres towards adsorbed molecules with TCNE ions formation.Study of these ions' formation has been performed by thermoluminescence, electron spin resonance and microfluorescence measurements.Moreover, the effect of natural ageing and thermal treatment of tricalcium silicate on the nucleophilic character of the surface centres has also been studied.     

Verification de la linearite et de la stationnarite des systemes electrochimiques lors de la mesure de leur impedance par une methode de bruit

A transfer-function analysis employing white-noise excitation is used to measure the impedance of some electrochemical systems (platinium disc electrode in a K₃Fe(CN)₆/K₄Fe(CN)₆ solution, Leclanché cell). The coherence function is calculated to determine the validity of transfer-function measurement and its is shown that this method provides an internal check on the validity of the measurements.     

Etude de la validite de la loi de Fick pour le transfer de matiere dans un fluide viscoelastique

An experimental study has been realized in a viscoelastic fluid to know if the Fick's law can be applied. This study also permitted to give some precisions about the differencies observed between the mass transfer measurements and the theorie results from the Ostwald's law.     

Etude de la resistivité de corps pulvérulents à base de carbone en fonction de la compression

Electrical resistance measurements were performed on different varieties of carbon blacks (Sterling Fr, MT, S, Black Pearls 2) under compression up to 660 kgf/cm² to determine the real resistivity of the carbon material. The general shape of the curves log p= - s log P are in good agreement with those reported by Mrozowski. Two theories have been proposed to explain the mechanism of the current flow through the particles. Holm's Contacts and the Tunnel effect between particles. The results for thermal blacks seem to agree with the first interpretation while the behaviour of the Channel blacks rather follows the second.     

Comportement electrochimique de la galene (PbS) dans les conditions de pH proches de celles de la flottation

The electrochemical behaviour of lead sulphide mineral (galena) has been investigated by cyclic voltammetry and controlled potential electrolyses. In the pH range 5–9, the oxidation leads—at + 0.5 V vs sce—to the formation of elemental sulphur and lead species: Pb²⁺, Pb(OH)₂ or Pb(OH)Cl depending on the pH, and on the nature of the electrolyte.

The reactions involved in the overall anodic/cathodic cycle have been interpreted; at pH 9, galena is reduced at − 1.3 V, to lead metal and HS⁻ ions.     

Mecanismes de colmatage et performances comparées de quatre membranes d'alumine: Cas de l'ultrafiltration de gelatine

This paper deals with gelatin ultrafiltration using alumina membranes of various pore sizes (0.2, 0.8, 1.4 et 2.0 μm). Pressure, tangential velocity and concentration effects are successively examined. From both a quantitative analysis of experimental data (pure solvent fluxes with clean or prefouled membranes, ultrafiltration fluxes, percent protein rejection) and the application of traditional model equations (filtration theory, film model), fundamental hypotheses are suggested concerning membrane functional characteristics. In particular, it is how fouling mechanisms and protein deposit structures are related to membrane characteristics. Finally, a comparison of membrane performances leads to the conclusion that the best fluxes and percent protein rejections are obtained using the 0.2 μm pore size membrane.     

Utilisation de la reaction de reduction de l'oxygene pour determiner sur electrode tournante la reactivite de quelques alliages dentaires semi-precieux

The reactivity of three dental alloys in artificial saliva has been studied through the diffusion current of oxygen reduction. In order to account for the fluctuations of the potential which can occur in vivo, we have studied the effect of a transient anodic polarization. The less reactive alloy after the anodic treatment appears to be the alloy which contains 10% of Pt and also much less Cu than the other alloys. This conclusion is supported by optical and electron miscroscopic investigation of the samples, which shows that for alloys containing little or no Pt, even with a high Au content, there is a drastic modification of the substrate by the formation of a thick anodic layer which is not sufficiently protective.     

Traitement anodique de suspensions de poudre de graphite sur des électrodes de métal déployé

Résumé L'article présente une étude expérimentale de l'oxydation anodique de suspensions de particules de graphite dans un réacteur fermé agité puis dans un réacteur ouvert dont l'amenée de courant anodique est un empilement de grilles de métal déployé traversé par la suspension. L'influence de plusieurs paramètres est examinée.

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This paper reports an experimental study of the anodic oxidation of suspensions of graphite particles in two types of cell, a closed stirred cell and a flow-through cell. In the latter case the anodic current feeder is a grid or a stack of grids of expanded metal; the suspension flows through this current feeder. The influence of various parameters is investigated.

     

Potentiels de charge nulle d'electrodes monocristallines de cuivre d'orientation (111) et (100) au contact de solutions aqueuses de perchlorate de potassium

The potentials of zero charge (pzc) of (111) and (100) copper planes polarized in aqueous KClO₄ solutions, are determined by using differential capacity—potential curves C(E) obtained during a linear potential sweep. The values obtained: E_{σ = o}[Cu(111)] = (−0.85 ± 0.01) V/(mse) and E_{σ = 0}[Cu(100)] = (−1.19 ± 0.01) V/(mse) are clearly more negative than the averaged results reported in literature. The comparison between the pzc change (ΔE_{σ = o} = E_{σ = o}(111) − E_{σ = o}(100)) for copper (ΔE_{σ = o} = 0.34 V) and that for other metals of IB group (ie gold and silver) shows that the influence of crystalline anisotropy on the metal—solution interface is much more important in the case of copper.