L'electrolyse de l'alumine dans la cryolithe fondue. Etude du mecanisme de decharge cathodique a l'aide de l'electrode a disque tournant

The use of a rotating disk platinum electrode, on which aluminium has been previously deposited, has allowed us to determine the mechanism of the cathodic reaction during the electrolysis of cryolite—alumina melts. This mechanism requires three stages within the Nernst's diffusion layer. The first of these is the dissociation of the ion AlF^3?₆, at δ′ from the electrode, forming the ions Al³⁺ and F^?. The second one is the discharge of ions Al³⁺ diffusing towards the cathode and the third one, at δ″ from the electrode, is the reaction of the ions F^? with the ions AlF^?₄ to give again the ions AlF^3?₆. The ions Na⁺, which transport most of the current serve to maintain electro-neutrality.     

Contribution a l'etude des sulfures de cuivre non stoechiometriques

The anodic oxidation of cuprous sulphide has been studied using a carbon paste electrode. Typical voltammetric curves are obtained. At slow rates, the oxidation of Cu₂S in CuS and Cu²⁺ occurs via the formation of non stoichiometric compounds: Cu_1,92S, Cu_1,77S, Cu_1,60S and Cu_1,31S.     

Influence de l'antimoine et du cuivre sur la cementation du cobalt par le zinc

Zinc sulphate solution (160g/l Zn²⁺) used for zinc electrowinning is purified for cobalt by cementation with metallic zinc powder. Industrial practice shows that considerable acceleration of this operation is achieved by the presence of trace impurities in solution. Eventually, CuSO₄ and Sb₂O₃ are deliberately added to the solution. The aim of this work is to elucidate the mechanism of action of those impurities.Zn²⁺ ions present in the solution are responsible for the slow speed of cobalt deposition (160g/l Zn²⁺; 10 mg/l Co²⁺).The cathodic part of the cementation reaction was simulated on a flat electrode through potentiostatic deposits at ?730 mV/ENH (potential at zero current of metallic zinc in the solution). The deposits were studied by anodic dissolution, radioactive tracers, X-ray diffraction and fluorescence, atomic absorption spectrophotometry, optical and electronic microscopy (transmission and scanning). The results were compared with cementation on zinc plate and zinc powder.Antimony and copper are deposited together with cobalt and form alloys with reduced cobalt activity. Total voltage available to overcome the inhibitor effect of zinc ions is thus increased and the reduction of cobalt ions accelerated. Antimony, copper and cobalt triple alloys resist particularly well to corrosion with hydrogen evolution. Copper has a higher accelerating effect on cobalt cementation than antimony, but the latter stabilizes the deposit very effectively.The electrochemical methods used and the knowledge of the mechanism of action of the impurities open new trends for industrial practice.     

Contribution a l'etude de la reduction polarographique de l'anion Re3Cl123−

The polarographic reduction of Re₃Cl₁₂^3? anion gives a single polarographic wave corresponding to a two-electrons reversible process. Classical polarographic measurements and instantaneous current—time curves show that the limiting current is diffusion controlled. Plotting the half-wave potential vs pH and pCl, it appears that one chloride ion is released in the reduction of one atom of rhenium, and that H⁺ ion is not involved in the reduction. The overall electrochemical reaction is:

     

Cinetique de l'etablissement de l'equilibre d'une electrode semi-conductrice: cas de Ga(As1−xPx)

The variation of the capacity of interface of a semi-conductor electrode, related to the polarisation time is extended to the case of a Ga(As_1?xP_x) electrode, type n. The flat-band potential for a given pH depends neither on the polarisation time nor on the eventual illumination in the darkness. The number of ionised centers on the surface varies according to a kinetic of the first order with a rate constant of 3.7 10^?4 s^?1. Under illumination the steady state is almost instantaneous.     

Relation entre les proprietes acides de graphites naturels et leur activite catalytique dans l'isomerisation de l'oxyde de styrene

The surface properties of two natural graphites have been investigated by catalytic activity in the isomerisation of styrene oxide to phenylacetaldehyde and by programmed thermodecomposition (TDP) combined with mass spectrometry gas analysis. The oxygen and water vapour influence has been studied in both cases.The two catalysts are respectively a natural Ceylon graphite (GC solid) with a 3.0 m²/g BET area and a natural Madagascar graphite (GM solid) which has been ground and has a 10.6 m²/g BET area (Table 1).The catalytic isomerisation runs were carried out at 305°C and the results are summarised in Table 2 for different gas mixture compositions. Oxygen directly favours the conversion (TTG) while water vapour mainly influences the selectivity (S).The thermodecomposition products of these solids are mainly CO and CO₂ as shown on Fig. 1. However the relative percentage of CO₂ which is related to the acidic properties of graphites (24) is very different, 16% of the BET area for the GC solid, but only 2% for the GM solid (Table 3). When the catalyst CC is pretreated at 300°C in oxygen instead of nitrogen, the TDP spectrum remains the same but the number of sites remarkably increases as shown in Table 4. Moreover the comparison of Tables 2 and 4 shows a parallelism between isomerisation activity and acidic properties. For this reason, we further investigated the influence of gaseous atmosphere by studying the TDP of the two graphites, first degassed at 850°C and then pretreated at 300°C with air or nitrogen in presence (or not) of water vapour. Important modifications are observed in the spectra (Figs. 2 and 3). Oxygen increases the total number of acid sites. The effect of water vapour is weaker. However, for the GM solid it increases the relative percentage of strong acid sites, i.e. desorbing at the lowest temperatures. For the GC solid, the effect is opposite.These results suggest that only a fraction of the acid functional groups is active for isomerisation, specifically those desorbing CO₂ near the reaction temperature. This assumption is also supported by the fact that a used catalyst presents such groups, while a fresh one does not, as shown on Fig. 4, which means that catalytic sites have been developed during reaction. Their number is increased when oxygen is present in the gaseous phase, and their stability is modified by the presence of water vapour.     

Stereochimie de l'electroreduction de bromocyclopropanes—III. Diastereoselectivite de la reduction electrochimique de gem-dibromocyclopropanes

Cleavage of a single carbon bromine bond occurs in the two electrons reduction of 2,2 disubstituted 1,1 dibromo cyclopropanes.Analysis of the diastereoisomeric mixture of monobromo compounds obtained at the term of electrolysis at a mercury cathode in a hydroethanolic media allows to determine which of the two bromine atoms is preferentially cleaved.

When R² = Ph, R¹ = H or CH₃ only a low stereoselectivity is generally observed. Nevertheless, yields up to 84% of the cis isomer are achieved when R¹ = H.Best results are obtained when R² is a carboxy group (CO₂H, CO₂Me or CO₂Et) especially when the reduction is carried out in the presence of quaternary ammonium cations. The cis isomer is always preferentially formed; yields up to 92% when R² = CO₂H and R¹ = Ph, and up to 99% when R² = CO₂Et and R¹ = CH₃ can be achieved.Influence of electrolyses conditions (working potential, nature of the supporting electrolyte) on the stereochemical course of the reduction is investigated.     

Mesure de pO2− au moyen d'une electrode a membrane de zircone stabilisee et determination potentiometrique de constantes d'equilibre d'echange de O2− dans le melange equimolaire NaCl-KCl fondu

A galvanic cell with a Ag/AgCl reference electrode and a pO^2? indicating electrode—made of a solid-state membrane in zirconia stabilized by calcia and filled by a mixture of Ni + NiO—has been constituted for working in the molten mixture NaCl-KCl (1:1), from 700 to near 1000°C. The response of the zirconia electrode in the fused salt has been tested by known variations of pO^2?, due to coulometric titrations of free oxide ions, then of hydroxide ions, by means of metallic ions (Ni²⁺, Cu⁺) forming scarcely soluble metallic oxides. The results of measurements were in a good accordance with the variation laws theoretically established, giving the proof that measurements of pO^2?, between about 2 and 12, are possible in the electrolytic and temperature range considered, by means of this system. An alteration of the electrode can arise in media with too low a pO^2? value.Equilibrium constants have been determined: solubility products (K_s) of the oxides NiO and Cu₂O, constant values of the equilibria 2OH^? ? H₂O (g) + O^2? (K₁) and H₂O (g) + 2Cl^? ? O^2? + 2HCl (g) (K₂); as functions of the absolute temperature T/°K and in the molar fraction scale (partial pressure of the gazeous substances in atm):

The solubility product values (at 727°C) for a few metallic oxides have been calculated from the free enthalpy values of formation and experimental values of the activity coefficients of the metallic ions and O^2? dissolved in NaCl-KCl.Limitations to the ability of the membrane electrode for measuring pO^2? have been established by an analysis based on the chemical and electrochemical behaviour of zirconia. A potential-pO^2? diagram shows the area inside which one has to work.     

Comportement electrochimique de l'ethylxanthate sur differents types d'electrodes: platine,or, galene,mercure

The electrochemical behaviour of ethylxanthate (C₂H₅OCS^?₂) has been studied by cyclic voltammetry, polarography, and controlled-potential electrolysis, in the pH range 9–10.The oxidation product, at platinum, gold and galena electrodes was found to be dixanthogen (C₂H₅OCS₂)₂; the mechanism of the reaction is complicated by the adsorption of xanthate and dixanthogen.With the dropping mercury electrode, an anodic wave was found at ?0,3 V vs sce; the electrochemical reaction involves the oxidation of mercury in the presence of xanthate, and the formation of a mercuric compound: Hg(C₂H₅OCS₂)₂. Strong adsorption of C₂H₅OCS^?₂ and Hg(C₂H₅OCS₂)₂ occurs.     

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.     

Determination potentiometrique des constantes de stabilite des complexes entre l'argent(I) et quelques heterocycles azotes en milieu neutre et acide dans le carbonate de propylene

The formation of AgL⁺_n (n = 1–4) (L = pyridine), AgL′⁺ and AgL′₂⁺ [L′ = 2,2′ bipyridine, 2,2′ biquinoline, 2-(2-pyridyl)benzimidazole] complexes has been studied using potentiometry with a silver electrode. The acidity constants of the ligands have been measured potentiometrically with the hydrogen electrode. The silver distribution between the Ag⁺ and AgL_n⁺ species has been computed by means of the so obtained β_n. In acidic medium, the metallic complexes are destroyed by the ligand protonation.     

Reduction electrochimique a l'electrode de mercure de composes α-hydroxycarbonyles en solution aqueuse α — I. Glyoxals RCOCH(OH)2

Reduction at the mercury electrode in aqueous solution of the unhydrated form of glyoxals exhibits evidence of a reversible system: RCOCHO + 2e + 2H⁺ ?har2; RC(OH)CHOH with RCH₃ or C₆H₅.For the hydrated form C₆H₅COCH(OH)₂, investigation indicated occurrence of two types of irreversible processes. In neutral media transfer of two electrons involve either the hydrogenolysis of the COH bond, yielding hydroxyacétophenone: C₆H₅COCH₂OH, or alternatively the reduction of the keto grouping yielding mandelaldehyde: C₆H₅CHOHCHO.The COH cleavage is exclusive in acidic media and for aliphatic glyoxals RCOCH(OH)₂.     

Contribution a l'etude des complexes du cation cadmium (II) avec l'acide laćtique et l'acide malonique en milieu Na+(ClO−4)3M A 25°C

The potentiometric study of neutralisation at 25°C, in Na⁺ (ClO^?₄)3M medium, with NaOH as titrant, of solutions: Cd²⁺ cation-lactic acid (between neutralisation rate values 0 to 1); and of Cd²⁺ cation-malonic acid (between neutralisation rate values 1–2), allows consideration of the formation of mononuclear complexes with lactic acid, and mononuclear and polynuclear complexes with malonic acid.The interpretation of experimental data ph = f(x)_R induces one to suppose the existence of the following species: lactic acid: Cd(Lact)⁺, Cd(Lact)₂; malonic acid; Cd(Mal); Cd(Mal)^2?₂; Cd(Mal)^4?₃; Cd₂(Mal)₂; Cd₂(Mal)^2?₃.The constants of formation of those complexes have been determined by non-linear regression.     

Comportement electochimique de complexes molybdene—cysteine,catalyseurs de reduction de l'azote en ammoniac

The binuclear molybdenum(V)—cysteine complex: Mo₂O₄(cys)^2-₂ is unstable in alkaline solutions. Polarographic and spectrophotometric studies indicated that the complex disproprotionates at pH > 9 into molybdenum(III) and molybdenum(VI). Chemical and electrochemical reduction of Mo₂O₄(cys)^2-₂ has been studied in ammonia buffer. The compound is directly reduced to a molybdenum(III)—cysteine complex, without formation of molybdenum(IV).     

Etude des reactions anodiques des ions oxydes dissous dans l'eutectique LiCl-KCl sur electrodes de graphite par des methodes electrochimiques impulsionnelles

The oxide anodic reaction on a graphite electrode in fused LiClKCl eutectic has been investigated by use of chronopotentiometry, chronoamperometry and cyclic voltammetry. The analysis of the anode response shows the existence of two steps; the first is due to adsorbed oxide ions, whereas a second part of the signal corresponds to the diffusion of electroactive species towards the electrode. The amount of oxide ions γ_eq adsorbed on the graphite anode and the diffusion coefficent D of the electroactive species are calculated: γ_eq ? 8.10^?8 moles cm^?2 for a concentration of oxide ions in the liquid phase C_O^2? = 4,96.10^?5 moles cm^?3; D_O^2? = 2.10^?5 cm² s^?1 at 442°C.A rapid determination of the oxide ions content in the fused LiClKCl eutectic is proposed for a range of relatively high concentration values (C_O^2? > 20 ppm).     

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 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.     

Mecanisme de la reduction electrochimique d'indenones-2,3 disubstituees

At a mercury cathode, in aqueous alcoholic media, the electroreduction at low potential of 2–3 disubstituted indenones can lead to the parent-indanones. The stereochemical analysis of the results needs to be acquainted with the electrochemical mechanism; undoubted assumption of the latter is difficult, as regards inhibition that occurs at some experimental conditions, due to the diol resulting to the first 1e reduction step.In acidic media, according to the sequence H⁺eeH⁺, the polarogram of very diluted solutions shows a single 2e wave; for more concentrated samples, the polarographic curve separates in two waves, the heights of which are equivalent. At any case, the enolic form of the indanone is obtained at the electrode.In basic media, pH being less than 12, the reduction leads directly to the indanone. The mechanism eH⁺eH⁺ (single 2e wave) predominates. As pH increases, it is, little by little, supplanted by the sequence eeH⁺H⁺ (two distinct 1e waves) but this has been undoubtedly established only in the case of 2–3 dimethyl-indénone.     

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.