Catalyse redox homogene de reduction des p-toluenesulfonamides. Determination des parametres cinetiques et thermodynamiques

The redox catalysis of the reduction of tertiary p-toluenesulfonamides and gem-N-di-p-toluenesulfonamides by electrogenerated organic anion radicals, on mercury cathode, was studied by cyclic voltammetry and controlled-potential electrolysis in aprotic DMF. Tertiary tosylamides, not directly reducible by electrochemical means in presence of LiClO₄ as a supporting electrolyte, were cleaved by electrogenerated pyren anion radical. The new theoretical treatments, recently developed by Savéant and coworkers and extended to a SET-type mechanism and cyclic voltammetry results, allowed us to determine the rate controlling step of the catalytic process and to calculate the standard rate constant of the homogeneous electron transfer reaction and the standard potential in respect to the tosylamide anion radical formation.     

Determination des parametres des courbes de polarisation a l''aide d''un programme de calcul par regression non lineaire. Application a la corrosion d''un metal en milieu acide en presence d''oxygene dissous

A nonlinear regression method is used in the determination of parameters for potentiocinetic curves. This method is applied to the determination of exchange and corrosion currents in the case of nickel in aerated sulfuric acid-solution. This study has shown effects of the oxygen content on the parameters and on the intensity—potential curves form.     

Compensation de la chute ohmique par une methode analogique.: Application a la mesure des impedances electrochimiques et a la determination des courbes de polarisation

This method of ohmic drop compensation is carried out by making the difference between the potential of the working-electrode and a voltage proportional to the current passing through the cell. According to the chosen value for the proportionality coefficient, the electrolyte resistance R_e can be compensated partially, totally or even overcompensated, without any influence on the stability of compensation and regulation (potentiostatic or galvanostatic) together.Taking for example the Zn/Zn²⁺ interface, it is shown that not only the current-potential curve can be determined directly, but moreover impedances even low with respect to R_e can be measured with a good accuracy in the whole useful frequencies range.Therefore, the method allows to analyse fast as well as slow processes occurring in an electrochemical system.     

Comparaison des cinetiques de formation de la couche passivante de WO3 sur des electrodes monocristallines de tungstene orientees (100) et (110) en milieu H2SO4 1 M*

A detailed study of chronoamperometric curves, associated with a direct measurement of coverage through inhibiting properties of WO₃ allowed us to compare the formation processes of such oxide on (100) and (110) orientated tungsten single crystals in H₂SO₄ 1 M medium. This comparison shows that the chemical dissolution of WO₃ and the number of atoms ‘in depth’ of the inhibiting limiting layer does not depend on the crystallographic orientation of the metal. On the contrary the number of superficial atoms taken into account to form this layer and the kinetic parameters of last step of oxidation in the over all process are strongly dependent on the tungsten crystallographic orientation.     

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.     

Reduction electrochimique de composes organiques en presence de tensio-actifs—IV. Etudes polarographique et spectroscopique des nitroanilines

In order to elucidate the location of substrates in micelles we studied the polarographic and spectroscopic (ESR) behaviour of nitroanilines isomers as a function of pH in the presence of a neutral surfactant (BRIJ 35). In basic or neutral media, two polarogaphic waves are observed and the spectra of anion radicals can be recorded during “in situ” electrochemical reductions. In acidic aqueous solutions and without surfactant, o and p-nitroanilines give a six-electron wave whereas a four-electron wave is observed for m-nitroaniline. If BRIJ is added, the three isomers lead to four electron processes. Presence of surfactant causes a decreasing of the rate of the intermediate chemical step for the ECE mechanism. When the surfactant concentration reaches 0.1 M, electrolysis “in situ” (pH = 3.5) gives no ESR signal for o- and m-nitroaniline. Under the same conditions, the p-nitroaniline solution exhibits a well resolved spectrum of p-phenyl-enediamine cation radical. Formation of such a cationic intermediate during the reduction process is discussed and its stabilization is conceivable by means of a solvent “cage effect”.     

Etude cinetique des etapes initiales de l'electrocristallisation de quelques metaux bivalents (Cu,Cd Pb)— I. Mise en evidence de la participation des anions par les methodes de double impulsion

Galvanostatic and potentiostatic double pulse methods allowed shorttime range (70 ns–100 μs) kinetic measurements, when the contribution of the fast step M/M(I) to the total current was preponderant. The kinetics of the interfacial exchange processes appeared to be during the first instants, independent of the concentration of metal ions in solution, but essentially a function of the nature of the supporting electrolyte anion and even, in the case of the CIO^?4 anion, of its concentration. Analysis of double pulse potentiostatic transients I (√t, which were linear up to values of t between 25 and 100 μs, depending on the systems studied, confirmed the anionic contribution of interfacial exchanges and revealed the essential part played by the anion adsorbability.     

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.     

Reduction electrochimique du nitrobenzene,en milieu micellaire aqueux,a divers pH. Influence de la nature des tensio-actifs

The electrochemical reduction of nitrobenzene has been studied in the presence of some anionic, cationic and non-ionic surfactants using a glassy carbon electrode or a dropping mercury electrode (polarography). The results are depending upon the electrode nature, the surfactant polarity and the pH of the solutions.On glassy carbon electrode and with cationic surfactants, the reduction is always a one step process for the totality of the studied pH scale (2–12). With the anionic and non-ionic surfactants, the potential is shifted towards the more negative values for a one step process or for a two step process, according to the pH value.For the polarographical studies, the electrochemical reduction of nitrobenzene is generally retarded by the presence of monomer surfactant molecules adsorbed on the mercury electrode, principally in acid media. In micellar solutions, the CTAB decreases the protonation rate of the nitrobenzene radical anion and facilitates the reduction by a dianion way. The anionic and non-ionic surfactants increase the radical anion stability and slow down the rate of the radical anion reduction.     

Determination conductometrique de la stoechiometrie des complexes donneur-accepteur dans les systemes en evolution lente—application au systeme pyridine + pchloranile

The stoechiometry of electron donor-acceptor complexes, turning slowly into ionic species in polar organic media, can be determined by means of the kinetic analysis of the conductance increase of the medium. This method is working as a complement to that of Gutmann and Keyzer and is tested with the E.D.A. systems pyridine + pchloranil and phenothiazine + pchloranil.     

Molecules ethyleniques et acetyleniques conjuguees VI. En evidence de la formation intermediaire d'allenol-1 au cours de la reduction polarographique en milieu acide,de cetones α-acetyleniques α′-pyridiniques

During the polarographic reduction, in acid medium, of α- and γ-pyridylphenylpropynones 2α and 2γ, the intermediate formation of unstable allenols-1 was shown. These compounds are fast isomerised into ethylenic ketones. In the case of 2α, the isomerisation rate is 0.167 s^?1 at 21°C (HCl 0.6 N; EtOH 30%); it is 48 times faster than the isomerisation rate into saturated ketone of the corresponding enol. The stability of such allenol-1 compounds is due to the vicinity of the pyridinium ion.     

Mécanisme de la réduction électrochimique en milieu non aqueux de materiaux cathodiques utilises dans les piles au lithium. V. Utilisation des hexacyanoferrates de fer (II et III) comme materiaux cathodiques reversibles

Résumé On présente les résultats obtenus concernant la réduction électrochimique dans le mélange solvant 80% en volume de 1,2-diméthoxyéthane-20% en volume de carbonate de propylène rendu conducteur par LiClO₄ 1M, d'électrodes membranaires constituées d'hexacyanoferrates der fer (II) et (III). Les hexacyanoferrates étudiés ont pour formule approximative: (a) KFE^IIFe ₃ ^III [Fe^II(CN)₆]₃, (b) Fe ₃ ^III [Fe^II(CN)₆]₃, et (c) KFe^III [Fe^II(CN)₆]. Leur réduction électrochimique s'effectue vers 3 V par rapport au système Li⁺/Li avec un rendement voisin de 90%. On montre, par voltampérométrie, que cette réduction est reversible et qu'il est meme possible d'oxyder partiellement les trois hexacyanoferrates.Des cycles de charge-décharge effectués sur des électrodes membranaires des différents produits confirment la réversibilité des divers systèmes électrochimiques engagés. Ainsi, pour des décharges du produit (a) jusqu'à 2.5 V et des charges jusqu'à 4V le rendement électrochimique évolue de 94% au premier cycle à 50% au 300 ème cycle. On interprète ces résultats, en supposant que le Fe(III) contenu dans les composés est susceptible de se réduire à l'état (II) sans destruction de la structure cristalline avec probablement insertion de cations Li⁺ en provenance de l'électrolyte selon: Fe _cristal ³⁺ + Li⁺+eFe²⁺Li _cristal ⁺ . Il est donc possible d'utiliser ces hexacyanoferrates comme matières actives dans des générateurs secondaires à électrolyte non aqueux et notamment à électrode négative de lithium.

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The results of the electrochemical reduction of membrane electrodes of Fe(II) and Fe(III) hexacyanoferrates in 80 vol% 1,2-dimethoxyethane, 20 vol% propylene carbonate with 1M LiClO₄ are presented. The hexacyanoferrates studied were of the following formula: (a) KFe^IIFe^III[Fe^II(CN)₆]₃, (b) Fe ₄ ^III [Fe^II(CN)₆]₃, and (c) KFe_III [Fe^II(CN)₆]. Reduction occurred at about 3V with 90% efficiency. It is shown by voltammetry that this reduction is reversible and that it is possible to partially oxidize the three hexacyanoferrates. The charge-discharge cycles carried out on the electrode membranes of different composition confirmed this reversibility for various electrochemical systems. For (a), the cycle: discharge at 2.5 V and charge at 4 V gave an efficiency ranging from 94% on the first cycle to 50% on the 300th. These results are interpreted by supposing that the Fe(III) is easily reduced to Fe(II) without the destruction of the crystalline net, but with the probable insertion of Li⁺ cations from the solutions: Fe _crystal ³⁺ + Li⁺ + eFe²⁺Li^crystal/+. It is possible to use these hexacyanoferrates as active constituents in secondary cells with non-aqueous electrolytes, particularly with a negative lithium electrode.

     

ETude cinetique des systemes Cd---Cd(II) et Fe(II)---Fe(II) en milieu aqueux par trace d'oscillogrammes lineaires au moyen de la methode de double impulsion potentiostatique

The principle and construction of a quadratic function generator are described. The voltage output, supplying the oscilloscope time-base, gives a linear i(√t) oscillogram and allows the transfer cd i_a to be read on the oscilloscope screen.

The kinetic parameters of the Cd---Cd(II0 and Fe(II)---fe(III) sytsems are determined in this way. In these two examples, the second Fick equation is solved numerically in order to establish the validity of the formula used to calculate T₀, the time corresponding to the cd i_a.

Zusammenfassung

Man beschreibt die Einstellung von einem Quadratischfunktiongenerator. Wenn die Klemmenspannung dieses Generators mit der Zeitablenkung des Oszillographes angeschaltet wird, erreicht man ein lineares Oszillogramm i(√t); die Durchtrittstromdichte i_a wird sofort auf dem Oszilloskopschrim gelesen.

Mann kann mit dieser Methode die elektrolytischen Konstanten der Reaktionen Cd---Cd(II) und Fe(II)---Fe(III) bestimmen. Jedenfalls integriert man die Zweite Diffusion differentialgleichung zahlenmässig und so, kann man die Richtigkeit der die Zeit gebenden Formel, welcher Zeit die Ordinate i_a auf die Linie i(√t) entspricht, bestätigen.     

Reduction electrocatalytique de l'oxygene sur electrodes solides d'oxydes mixtes contenant des ions manganese. I. Cas des manganites de cuivre CuxMn3−xO4 (0<x<1.8) en milieu légèrement acide

Résumé L'oxygène peut être réduit électrochimiquement, en milieu légèrement acide (pH=6.6) sur les manganites de cuivre, Cu _x Mn_3–x O₄, de type spinelle. La réduction se fait probablement sur les sites actifs créés par les ions manganèse tétravalents, Mn⁴⁺, jouant le rôle d'accepteur des électrons vis à vis du solide et celui de donneur vis à vis des molécules ou atomes d'oxyène adsorbés. La vitesse de la réduction électrochimique de l'oxygène dépend de la concentration des sites actifs superficiels qui varie en fonction des proportions de cuivre ajoutées. La nature et la concentration des anions de l'électrolyte support influence la vitesse de réduction.

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Oxygen is electrochemically reducible in slightly acidic medium (pH=6.6) on copper manganites, Cu _x Mn_3–x O₄, of the spinel type. The reduction is likely to occur on active sites created by the tetravalent Mn⁴⁺ ions. These ions act as acceptors of electrons from the solid and as donors to the adsorbed-oxygen molecules or atoms. The rate of the electrochemical reduction depends on the surface concentration of active sites, which itself is a function of copper content. The nature and, concentration of the anions of the supporting electrolyte influence the rate of reduction.

     

Comportement electrochimique d''oxocarbones et d''aci-reductones en relation avec le transfert d''electrons dans les processus biologiques—IV: incidence des equilibres d''hydratation, de dimerisation et de dismutation sur la reversibilite du couple ninhydrine/derive dihydro

This voltammetric and potentiometric study concerns the reduction pre-waves of ninhydrin 1, the one electron cathodic and anodic waves of the pinacol reduction product 3 and the anodic (two electron) of the dihydro derivative, aci-reductone 2.

The rate of the electronic transfer depends on the disproportionation and dimerisation reactions of the semiquinone radical 1^·. In dilute solution, the redox system 1/2 is reversible; the value of ^′° (+ 30 mV) stands between those of the dehydro ascorbic acid/ascorbic acid and the rhodizonic acid/tetrahydroxyquinone systems previously studied. This value explains the catalytic action of the system 1/2 in the oxydation of thiol.     

Théorie et application de la distribution normale de θ1/3 aux temps de séjour des fluides dans les réservoirs agités en régime continu

Statistical models of residence time distributions in ideal stirred tanks in series are well approximated by a normal distribution of θ^1/2, θ^1/2 or θ according as the number of tanks is > 15, > 50 and very large respectively. For one stirred tank, the normal distribution of θ^1/3 approximates well the residence time distribution for both ideal and nonideal mixing. For ideal mixing, the mean x_m and standard deviation σ of the θ^1/2 distribution assume values of 8/9 and 1/3 respectively. For nonideal mixing, x_m is < 8/9 while σ is > 1/3. At constant fluid viscosities, when the rotational speed of the agitator increases, x_m increases gradually to reach its upper limiting value of 8/9 whilst σ decreases to reach its lower limiting value of 1/3. At fixed rotational speeds, if the viscosity increases, x_m decreases whilst σ increases. A simple mixing efficiency criteria derived from the normal distribution of θ^1/3 is proposed and is compatible with Danckwerts' segregation concept.     

Evolution de l'impédance d'un générateur en fonction de son état de charge: application au système V2O5/LiClO4+C.P./Li

Résumé La durée de vie en cyclage d'un accumulateur constitue l'un des critères principaux de l'intérêt que l'on peut lui porter. Il est donc très utile de pouvoir l'évaluer rapidement. Nous avons étudié l'évolution, en fonction de son état de charge, de l'impédance d'un générateur au lithium soumis à des cycles décharge-charge dans le but d'analyser, dans un premier temps, les variations des grandeurs caractéristiques de cette impédance en espérant apporter, par cette technique de mesure, des éléments d'évaluation de sa durée de vie.

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Battery systems based on a lithium anode and a lithium intercalation cathode show considérable promise for high density storage batteries. Studies of rechargeability of such systems are very important but, at the same time, are very tedious and lack rapidity.This paper describes the evolution of the a.c. impedance of the cell Li/LiClO₄ (1 mol dm^–3) in propylene carbonate/V₂O₅ measured at room temperature as a function of frequency from 10^–3 Hz to 10⁶ Hz at various states of charge when an alternative discharge-rest-charge-rest procedure is applied to the system.It is hoped that such a procedure may successfully be employed to rapidly estimate the life duration of batteries.