Interactions de la reduction du proton solvate et de l'oxygene dissous,sur une electrode de fer en solution sulfurique

Résumé A l'interface Fe/H₂SO₄, 1 N (aéré ou non), et dans le domaine de potentiel (–0·95, –1·2 V/E.S.S.), nous avons trouvé que le courant cathodique mesuré sur une électrode à disque tournant varie avec la vitesse de rotation suivant une loi de la forme:I=A+B ^1/2. A peut être identifié au courant de réduction du proton solvaté mais dépend fortement de la teneur en oxygène de l'électrolyte. La composante diffusionnelleB ^1/2 peut être identifiée à la réduction de l'oxygène dissous mais est très inférieure à celle relative à une surface uniformément réactive. Le blocage résultant est compatible avec l'analyse en fonction du potentiel de la corrélation entreA etB ^1/2 en supposant la réaction suivante: Fe H_adsFe H _ads ^* , où Fe H_ads est un hydrogène adsorbé faiblement lié de courte durée de vie (qq. s) et Fe H _ads ^* est un hydrogène adsorbé fortement lié de longue durée de vie (qq.h). Le déblocage résulte de la réaction chimique H_ads+1/4 O₂1/2 H₂O.Dans le cadre classique du mécanisme de dégagement de l'hydrogène en deux étapes, nous avons montré que notre modèle d'interdépendance implique que l'étape limitant la vitesse soit celle de Tafel à faible surtension et celle d'Horiuti pour des tensions cathodiques plus élevées.

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We found that at the 1 N Fe/H₂SO₄ (aerated or de-aerated) interface within the potential range (–0d95, –1.2 V/S.S.E.) the cathodic current measured on a rotating disc electrode varies with the rotation speed according to the relation:I=A+B^1/2. A can be assigned to the reduction of H⁺ but depends strongly on the oxygen concentration. On the other hand the diffusional componentB^1/2 can be assigned to the reduction of dissolved oxygen but is much lower than that relative to a uniform reactive surface. The resulting blocking is consistent with the analysis as a function of the potential of the correlation betweenA andB^1/2 by assuming the following reaction: Fe H_adsFe H _ads ^* . Fe H_ads and Fe H _ads ^* are adsorbed hydrogen low bonded with a short life time (a few s) and strong bonded with a long life time (a few h) respectively. The blocked surface is activated by the chemical reaction Fe H_ads+1/4 O₂1/2 H₂O+Fe.In the classic framework of the two-step hydrogen evolution mechanism, we demonstrated that our interdependence model implies that the rate determining step is the Tafel reaction at low overpotentials and the Horiuti reaction for the highest overpotentials.

     

Influence de la nature de l'electrolyte sur la reactivite electrochimique du chromate de baryum incorpore dans une electrode a pate de carbone

The electrochemical behaviour of baryum chromate is investigated by means of a carbon paste electrode with conducting binder. Current potential curves recorded, when electrolyte and binder are 1 M aqueous solutions of strong mineral acids, evidence only the reduction of chromium (VI) ions resulting from the more or less complete dissolution of the sample at the beginning of the experiment. The behaviour is different in the acids CH₃COOH and HCOOH. The curves then exhibit two peaks corresponding to the electrochemical transformations of chromium: the first is due, as in strong mineral acids, to the reduction of chromium (VI) ions, coming from the dissolution of the finest particles at the beginning of the experiment, into Cr³⁺ ions; the second one is characteristic of the progressive electrodissolution of the solid, via a chemical-electrochemical mechanism, during the potential sweep; its shape and position are greatly influenced by the morphology of the sample.     

Utilisation de radiotraceurs pour la mesure des nombres de transport ioniques en solution

A new technique of transference number measurements using labelled ions has been developed. Its validity has been tested by studying the concentration dependence of Na-22 transport number in aqueous solutions. Its large field of applications and its precision—0,2 per cent—makes this technique convenient to provide useful data in pure solvents or in aqueous solvent mixtures.     

Etude de la reduction sur electrode de mercure de l'acide β-bromopyruvique et de son ester ethylique

In aqueous or water/alcohol media, reductive cleavage of the carbonbromine bond of β-bromopyruvic acid 1 and its ethylester 2 occurs at the mercury electrode. In aqueous solution, the carbonyl group is strongly hydrated and cathodic current strength is small. In water/alcohol media, the electrode process is complicated by adsorption of bromide ions at the mercury electrode. When the electrode potential is in accordance with a positive value of the electrode charge a prewave appears corresponding to the adsorption of bromide ions. The electrocapillary curve is not affected by such an adsorption phenomena. The prewave does not represent the reduction of an organomercuric compound. No reaction occurs between 1 or 2 and the electrode material. When ethanol is added, the increase in adsorpion of bromide ions arises from the reduced solubility of bromide ions in the bulk of the solution.     

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.     

Utilisation des techniques dielectriques pour l'etude in situ de l'activation de la sphalerite par les ions cu2+

Dielectric methods permit to follow in situ the activation by Cu²⁺ ions of a ZnS pulp in ionic solution. The conductibility of the solid is strongly increased. The influence of size, ionic strength and copper concentration is examined.     

Etude de la reduction electrochimique d'alkyl-2 ethoxycarbonyl-2 cycloalcanones. Influence des conditions experimentales sur la stereochimie de la reduction

A few 2-alkyl-2-carbethoxy-cycloalkanones are selectively reduced with high yields into 2-alkyl-2- carbethoxy-cycloalkanols, electrochemically on mercury pool in hydroorganic medium. Carbon dioxide is bubbled with nitrogen through the solution to maintain the pH at about 9 to avoid side reactions.Cis and trans esters-alcohols were determinated by gas chromatography. The ratio is depending on several factors: the size of the cycle, the nature of the alkyl substituent, the nature of the cation of the supporting electrolyte, the nature of the cathode material and the temperature of the reaction. This last one is very important. At low temperature (?6°C) we have got preferentially the cis isomer and at high temperature (80°C) we have got almost exclusively the more thermodynamically stable trans isomer.To explain the mechanism we consider the presentation of the molecule on the electrode, the protonating effect of the medium and the thermodynamical equilibrium of the intermediate carbanion.     

Effet du pH en milieu basique sur la cinetique d''oxydation de l''hydrogene a une electrode de platine

The polarization curves of hydrogen oxidation in several alkalines media are determined by the rotating disc method. The electrode are made of smooth platinum or platinum covered by a thin layer of black platinum. For the total range of pH we show that the limiting process is always the diffusion of dissolved hydrogen to the electrode.     

Electropolymerisation de monomere 3-amino-1,2,4-triazole sur une electrode de cuivre et effet du polymere sur la corrosion de l'electrode en milieu NaCl 3%

Résumé L'electropolymérisation d'un nouveau polymére a partir de 3-amino-triazole (ATA) est réalisée. L'électrolyte support était de L'eau distillée avec un mélange de méthanol et de KOH. Le microscope électronique à balayage (MEB), la voltammétrie cyclique et le chronoampérométrie ont été utilisées pour caractériser le nouveau polymere. Ces mêmes techniques ont été employees pour étudier la nature du mécanisme d'inhibition de la corrosion du cuivre en milieu NaCl 3% par suite de la formation du polymere. L'électropolymérisation in situ du monomére donne lieu à un polymère isolant. Le traitement thermique du polymère améliore son homogénéité et son adhérence à l'électrode. Un mécanisme de la formation du poly-ATA est proposé. La formation du polymere aboutit à une diminution des courants cathodique et anodique. Les vitesses des réactions de réduction de l'óxygène et de dissolution du cuivre ont été remarquablement diminuées.

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The electrochemical polymerization of a new polymer is described. The monomer used was 3-amino-1,2,4-triazole (ATA) and the supporting medium was distilled water/methyl alcohol containing KOH. Scanning electronic microscopy (SEM), cyclic voltammetry and chronoamperometry were used, to characterize the new polymer. The same techniques were also used to study the nature of corrosion inhibition processes caused by the formation of the polymer on copper in NaCl 3%. The in-situ electropolymerization of the monomer gives an insulating polymer. Heat treatment of the polymer improves its homogeneity and adherence to the copper electrode on which it is formed. A mechanism for the formation of the poly-ATA is proposed. The formation of the polymer gives rise to a decrease in both anodic and cathodic currents. The rates of oxygen reduction and copper dissolution reactions are considerably decreased.

     

Comportement electrochimique et reactivite du systeme Ag(II)/Ag(I)-bipyridine dans le carbonate de propylene—III. Etude du mecanisme et de la cinetique de la reaction d'oxydation catalytique de la bipyridine

The electrochemical behaviour of the Ag(II)/Ag(I)-bipyridine system has been investigated in solutions containing bipyridine in excess with respect to the Ag⁺-bipy: 1–2 stoichiometry. The nature of the coupled chemical reaction indicates a chemical catalysis of an electrochemical reaction. The rate constant of the reaction of destruction of the intermediate complex (k₂ = 1.8 × 10^?3s^?1) has been calculated from measurements performed during electrolysis of Ag^I(bipy)⁺₂-bipy solutions.     

Methode particuliere d'electrolyse pour la preparation en continu de la N-amino methyl-2 indoline par reduction de la N-nitroso methyl-2 indoline

A particular method of continuous electrolysis through porous electrodes is described for the preparation of N-amino 2-methyl indoline by electroreduction of N-nitroso 2-methyl indoline.The process takes into account the instability of (i) the starting material in very acidic medium (reaction 2), (ii) the formed hydrazine in the presence of the nitroso compound (reaction 3).The results are compared with those obtained in less acidic medium (acetic buffer) where the compounds are more stable.The analytical study is carried out on a small porous electrode (Ø 2 cm) and extended to the preparative scale on larger cathodes (Ø 5 and 10 cm).     

Etude de la reduction de l'oxygene sur les phtalocyanines monomeres et polymeres—II: Polyphtalocyanines de fer impregnees sur noir d'acetylene Y

The electrocatalytic activity of different iron polyphthalocyanines directly impregnated on carbon blacks during their synthesis is observed as a function of the synthesis parameters. The electrodes are formed by electrophoresis. A reaction mechanism function of the electrolytic medium and of the former treatménts of the electrode is proposed.     

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.     

Contribution de la voltammetrie a l'etude du comportement electrochimique sur electrode de mercure,de certains ortho- et para-nitrophenylazobenzenes

The electroreduction mechanism of ortho- and para-nitrophenylazo-para-cresols, in basic media, at a mercury electrode is studied specially by the means of cyclic voltammetry. It is possible to explain the formation, during preparative runs, of nitrogen heterocyclic compounds of the type benzotriazole N-oxyde easily reducible to the parent benzotriazole, in the case of the ortho isomer, and of an azoxy derivative in the case of the para isomer.     

Influence de l'adsorption du depolarisant ou d'un produit de la reaction electrochimique sur les courants polarographiques—XVIII. Etude de la dimerisation de radicaux mercuroorganiques par voltammetrie a variation lineaire de tension

Compounds with the general formula ArHgX, adsorbed on a mercury electrode are reduced to free radicals, which dimerize on the surface of the electrode. A study by linear potential sweep voltammetry shows that the dimerization involves two free radicals and establishes the validity of the theory elaborated previously for that type of reaction.     

Etude de la reduction de l'oxygene sur les phtalocyanines monomeres et polymeres—I. principes fondamentaux,choix de l'ion central

The catalytic activity of compounds of the phthalocyanine type can be predicted by means of present theories of catalysis. Study of the activity of iron, cobalt, nickel and copper phthalocyanines shows that the Fe(II) compounds are the most active, the order being PcFe > PcCo > PcNi > PcCu. Molecular orbital theory can be used to relate the order of activity to the filling of the d-levels. The electrocatalysis of oxygen reduction has been experimentally examined.     

Etude voltamperometrique de la n-acetyl l-tyrosine amide sur electrode a pate de graphite en milleux aqueux I—comportement dans les domaines de balayage superieurs a 0,5 v/enh

Graphite paste electrode allows to determine elementary processes of the electrochemical oxidation in aqueous media of an electrochemical probe such as: N-acetyl L-tyrosine amide. Mathematical analysis of voltammograms gives the following EC mechanism: R?C₆H₅OH?R?C₆H₅O^. + H⁺ + e 2 R?C₆H₅O^. → R?C₆H₅O⁺ + R?C₆H₅O^?, R?C₆H₅O^? + H⁺ → R?C₆H₅OH, R?C₆H₅O⁺ → [R?C₆H₄O]^.. + H⁺, n[R?C₆H₄O]^.. → ?[R?C₆H₄O]^?_n.     

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.     

Influence de la structure sur les propriétés mécaniques dynamiques de quelques polytéréphtalates de polyméthylèneglycol. Relaxations β et γ

The influence of structure, cristallinity and orientation on the β- and γ-relaxation mechanisms of several poly(α,ω-alkandiolterephthalates) has been investigated. The γ-relaxation consists of two main components involving molecular motions in the amorphous phase, and motions of CH₂ groups (glycolic residue) in the gauche or trans form. A third component involves movements of bigger groups. The β-relaxation at low frequencies can be compared with the dilatometric glass transition.