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 du mecanisme de l'inhibition du zinc en milieu HCl n par les sels de phosphonium

The different steps of mechanism for zinc corrosion inhibition in a 1N HCl solution with phosphonium salts Ph_{4 ? y} P⁺nBu_y, X^? (0 ? y ? 4) are clarified according to the nature of the substituting groups.The secondary type of inhibition, involving the reduction products of these salts, ie phosphine does not appear when y is greater than 1. In this case, the electrostatic adsorption and chemisorption are the predominant steps.     

Etude de l'effet retardateur du zinc sur l'hydratation de la pate de ciment Portland

The retarding effect of zinc on the hydration of C₃S and C₃A, the two principal Portland cement components, has been investigated by X - ray diffraction. The results show that the C₃S retardation is more important than that of C₃A. This retardation is due to the precipitation of an amorphous layer of zinc hydroxide around the anhydrous grains. The effect of this coating depends on its permeability. The hydration reaction starts again through the transformation of the zinc hydroxide into the crystalline calcium zinc hydroxide Ca Zn₂ (OH)₆, 2H₂ O.     

Electrochimie dans la cryolithe fondue—I influence de la teneur en fluorures et en alumine sur le potentiel du systeme A1(III+)/A1(0)

The influence of additions of NaF and AlF₃ on the potential of an aluminium electrode has been studied in molten cryolite. The redox system Al(III+)/Al(0) is reversible, and the value K = 0,03 (ionic fractions) for the dissociation equilibrium (AlF^3?₆ ? AlF^?₄ + 2F^?) has been deduced from potentiometric measurements. Dissolution of Al₂O₃ probably leads to AlOF^2?₃ species.     

Convection naturelle a l'interieur d'un diedre. Etude experimentale du transfert de masse a la paroi

The authors review the electrochemical technique utilised and present the experimental correlations between the global average Sherwood and Rayleigh numbers. They compare these results with those obtained for a flat plate under the same condition.     

Electrochimie dans la cryolithe fondue—II oxydation electrochimique de l'aluminium “dissous”

It is shown by voltammetry that the species often called “dissolved aluminium” which comes from the interaction of aluminium with molten cryolite is electrochemically oxidized at several electrodes (platinum, nickel, copper, graphite, vitrous carbon). The diffusion plateau is used to measure the “solubility” of aluminium as a function of temperature and composition (0·11 weight per cent at 1015°C, is cryolite saturated with alumina). An application to the efficiency of alumina electrolysis is proposed.     

Application de la thermoluminescence a l'etude de l'hydratation du silicate bicalcique

Hydraulic properties differences between β and γ dicalcium silicate varieties are well known. In this work, a water vapour chemisorption on the surface of both solid phases is shown by thermoluminescence and by quadrupole mass spectrometry.The similar behaviour of β and γ dicalcium silicate varieties in those experiments means that the fundamental difference between their hydraulic reactivities is not due to this hydration step.     

Possibilites de la diffusion centrale dans l'examen d'etats alteres du ciment

After a review of the idea of specific surface in relation to x-ray diffuse scattering, we examine in a comparative way the data gathered on normal cements and on cements having been subjected to different types of modifications. We show that these modifications are very evident on pure kinds (of cement), but are much more uncertain on concrete.     

Etude electrochimique de l'oxydation simultanee de la galene (PbS) et de l'ethylxanthate—condition de formation du xanthate de plomb en surface de la galene

The electrochemical oxidation of galena (PbS) has been investigated in the pH range 9–10, by controlled potential electrolysis using a special bed electrode. In the absence of xanthate, the oxidation of PbS leads to the formation of elemental sulphur, thiosulphate and lead species (Pb(OH)₂). Simultaneous oxidation of galena and xanthate has also been studied and the products characterized by their uv-visible spectrum. The oxidation produces lead xanthate and dixanthogen. It has been established that lead xanthate comes from an ion exchange reaction between excess xanthate in solution and lead species resulting from the lead sulphide oxidation.     

Le mecanisme du developpement de l'oxyde poreux d'aluminium—I. Sur la possibilite du contact direct aluminium-solution electrolytique au cours du processus de formation—transformation de la couche barriere

Experimental evidence on the electrolyte permeability of the barrier layer during the anodic oxydation of Al in H₂SO₄ are reported. On this basis, a new theoretic mecanism of anodic oxidation process is proposed. The current efficiency of the oxydation process η_ox (determined by weighing the oxide) depends linearly on the barrier layer voltage, V, η_ox = V/2V^x, were V^x is the critical oxidation voltage.     

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.     

Generalisation de la theorie de l'electrode dite “electrode a disque-anneau’, dans un fluide d'otswald. Calcul des facteurs de collection

Supposing that in the diffusional boundary layer, the tangential component V_x and radial component V_r of the relative fluid velocity are respectively written in the form V_xS₀x^by and V_ra₀r^by (S₀, a₀ are independent coefficient of the normal coordinate y, tengential coordinate x, radial coordinate r;b is a numerical exponent that is expressed in terms of the behaviour law and of the flow), the authors calculate a analytical expression for the collection factor N defined as the quotient, with the sign inverted, of the limiting diffusional flux on the downstream active surface by the limiting diffusional flux on the upstream active surface. This factor N is dependent only on geometrical characteristics of system and on exponent b.     

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.     

Calcul du flux limite de diffusion sur une microelectrode de section circulaire—equivalence avec une electrode de section rectangulaire. Verification experimentale dans le cas du disque tournant en regime laminaire

The authors recalculate the dimensions of the rectangular surface equivalent to a circular micro-electrode. Theoretical results are in agreement with practice for a diameter inferior to or equal to 4 mm.     

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.     

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.     

Existence de quatre types de sites reactionnels dans l'oxydation du graphite

We have studied the temperature programmed decomposition of graphite-oxygen surface complexes using a gas flow apparatus under atmospheric pressure with infrared CO and CO₂ detection. We have shown that the oxygen is chemisorbed on four types of sites forming surface complexes which on decomposition give mainly CO and some CO₂. The first two types of sites, A and B, are formed by ‘labile’ carbon atoms created during the degassing carried out prior to each experiment. These two types of sites disappear without reconstitution upon desorption of the complexes. The other two, C and D, are formed by edge carbon atoms, normally linked to other atoms in the graphite lattice. The C type sites form by oxidation only at temperatures below about 950°C and give rise to hexagonal pits with sides oriented in the (101?0) crystallographic direction (‘arm-chair’ configuration). The D type sites form at temperatures above about 950°C and give rise to hexagonal pits oriented in the (11?20) crystallographic direction (‘zig-zag’ configuration). Water inhibits the formation of oxygen surface complexes on the C sites and it may be considered that it is essentially on these sites that the water is chemisorbed.     

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.