Électroactivité des halogénures

The intensity-potential curves pertaining to the oxidation of iodides, bromides and chlorides at a polished platinum electrode, undergo deformations when the following factors are varied: initial concentration of halogen, pH, addition of ions that complex or reduce the halogen formed. Analysis of these deformations has led us to assume that the electrochemical behaviour of the three halides can be represented by a reaction of the type: 2 x₃^--2e ? 3 x₂ and not of the type: 2 x^-- 2e ? x₂ As far as bromides and chlorides are concerned the coulometric study agrees with this hypothesis     

L'analyse des roches dolomitiques par dosage thermogravimétrique simultané du calcium et du magnésium

Résumé La méthode que nous venons de décrire est simple, rapide et plus précise que la méthode complexométrique. Son emploi est général; on peut en effet l'utiliser même si l'on ne possède pas de thermobalance. Il suffit de porter le creuset contenant le précipité des carbonates dans un four à régulation et de le chauffer d'abord vers 500° puis vers 950° jusqu'à poids constant. Il faut s'assurer par plusieurs pesées successives que le poids constant à 500° est effectivement obtenu. De toute manière, il est toujours plus prudent de maintenir en thermostat à 500°–550° le creuset contenant le mélange CaCO₃+MgO pendant une demi-heure environ lorsque l'on enregistre la courbe de thermolyse. On est ainsi assuré d'observer un palier suffisamment net pour effectuer des mesures sur la courbe avec précision.D'autre part, l'application de la méthode n'est pas limitée au dosage du calcium et du magnésium dans les dolomies. On peut également l'employer pour doser ces deux métaux co-existant dans des proportions différentes de celles que l'on rencontre dans ces roches.

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Summary The method just described is simple, rapid and more accurate than the complexometric method. It is of general applicability, in fact it can be used even if a thermobalance is not available. It is sufficient to place the crucible containing the carbonate precipitate in a furnace that can be regulated and to heat it at first to around 500° and then to about 900° until constant weight is attained. Several successive weighings must be made to insure that constant weight at 500° has been really attained. Always it is better to keep the crucible containing the mixture of CaCO₃+MgO at 500 to 550° for about 30 minutes while recording the thermolysis curve. This guarantees obtaining a plateau adequately established to assure regions on the curve that are accurate. On the other hand, the application of the method is not limited to the determination of calcium and magnesium in dolomites. It may also be used to determine these two metals coexisting in ratios different from those encountered in these rocks.

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Zusammenfassung Die beschriebene Methode zur Bestimmung von Calcium und Magnesium in Dolomitgestein ist einfach, rasch und genauer als die komplexometrische. Sie ist allgemein anwendbar; man kann sie auch ohne Thermowaage benützen. Es genügt, den Tiegel mit dem Carbonatniederschlag in einem regulierbaren Ofen zuerst auf 500°, dann auf 950° bis zur Gewichtskonstanz zu erhitzen. Man muß sich nur durch mehrfache Wägungen davon überzeugen, daß bei 500° tatsächlich kein Gewichtsverlust mehr eintritt. Jedenfalls tut man gut daran, den Tiegel mit dem Gemisch aus Calciumcarbonat und Magnesiumoxyd etwa eine halbe Stunde im Thermostaten bei 500 bis 550° zu halten, wenn man die Thermolysekurve registriert. Man kann dann sicher sein, eine hinreichend saubere Stufe zu erhalten, um die Kurve genau ausmessen zu können.Anderseits ist die Methode nicht nur auf die Bestimmung des Calciums und Magnesiums in Dolomit beschränkt. Sie läßt sich auch dann anwenden, wenn diese beiden Elemente in anderen Mengenverhältnissen vorliegen als in Dolomit.

     

Chromatographie sur papier des Molybdates. Titrage direct des taches de Molybdène chromatographié, séparé du Chrome et du Vanadium

Résumé Dans les conditions opératoires précisées ici, le molybdène chromatographié sur papier Whatman n°4, se laisse doser parabsorptiométrie à ± 1% près, pour une quantité de l'ordre de 10 microgrammes.Letitrage direct de la même quantité de molybdène se fait à ± 3,5% près. Séparé du chrome et du vanadium par chromatographie, la précision du titrage après chromatographie reste du même ordre. La titration directe fait gagner trois heures sur l'absorptiométrie.

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Summary Under the conditions outlined here, molybdenum chromatographed on Whatman paper No. 4, may be determined by absorption, within about ± 1%, for an amount of the order of 10 micrograms. The direct determination of this same quantity of molybdenum by titration can be made within about ± 3%. Separated from chromium and vanadium by chromatography, the precision of the titration after chromatography remains of the same order. The direct titration gains three hours over absorption.

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Zusammenfassung Bei Einhaltung der beschriebenen Versuchsbedingungen läßt sich Molybdän nach chromatographischer Adsorption auf Whatman-Papier Nr. 4 in der Größenordnung von etwa 10 mit ± 1% Genauigkeit durch Absorptiometrie bestimmen. Die direkte Titration gleicher Molybdänmengen läßt sich mit einer Genauigkeit von ± 3,5% ausführen. Nach chromatographischer Trennung von Chrom und Vanadium ergibt die Titration die gleiche Genauigkeit. Somit lassen sich durch direkte Titration gegenüber der Absorptionsmessung drei Stunden Zeit gewinnen.

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Cette recherche a bénéficié d'un subside du Fonds National de la Recherche Scientifique. Nous en témoignons ici notre sincère gratitude.

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Avec 3 figures.

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Travail exécuté en vue de l'obtention du grade de Licencié en Sciences Chimiques (Faculté des Sciences, Université Libre de Bruxelles), juillet 1952.     

Paramètres photophysiques de désactivation de l'état premier singulet excité et de l'état triplet des molécules de durène protoné et deutérié dans le méthyl-3 pentane

An extensive photophysical study of protonated and deuterated durene (1,2,4,5-tetramethyl benzene) molecules in 3-methyl pentane solution at 77.4 and 300 K is reported. A time-correlated single photon counting system is also described. The real singlet radiative lifetime is obtained through the measured fluorescence lifetime coupled to the fluorescence quantum yield and compared to the theoretical radiative lifetime. It is proposed that the geometry of the relaxed excited singlet state differs a little from the gound state geometry. It is also shown that deuteration of the durene molecule affects the radiative properties much more than the radiationless properties of this molecule.     

Utilité de la définition de brönsted des acides et des bases en chimie analytique

Brönsted's definition of acids and bases presents the following advantages: One may reason along the lines of the acid-base couple and the proton exchange just as done in the case of an oxidation-reduction couple and the electron exchange. The force of the acid is connected with that of the corresponding base.The acid or basic character of certain compounds is evidenced by their tendency to exchange protons, something that does not necessitate the use of “hydrolytic” phenomena.Properties of “strong acids” and “strong bases” are explained.The formulas of the pH of acid solutions, basic solutions, or salt solutions are unified and their use simplified.In acidimetry the reasoning is simplified, and complex problems may be approached with more facility.In the case of non-aqeous solvents that offer great possibilities in acidimetry spbrönsted's theory permits predictions about limitations in the acidityscale. It furnishes an explanation of phenomena in inert solvents.     

Réactivité des N1-tosylhydrazonates vis-à-vis du réactif de Lawesson,du cyanamide et des hydrazides : synthèse de thiadiazaphospholines et d’aminotriazoles tosylés

Reactivity of N¹-tosylhydrazonates towards Lawesson’s reagent, cyanamide and hydrazides: synthesis of tosylated thiadiazaphospholines and aminotriazoles. The reaction of N¹-tosylhydrazonates with the Lawesson reagent, cyanamide and hydrazides leads respectively to tosylated thiadiazaphospholines, C-aminotriazoles and N-aminotriazoles in good yields. The structure of all obtained products was determined on the basis of their IR and NMR data. To cite this article: M.B. Mosbah et al., C. R. Chimie 6 (2003).     

Amélioration des comportements thermique et mécanique des structures vitreuses par addition d'oxyde métallique et traitements thermiques

Improvement of mechanical and thermal properties of glass structures by a metallic oxide addition and suitable heat treatments. The study of glass-ceramic materials based on the system SiO₂-Li₂O-Al₂O₃, using differential thermal analysis (DTA), X-ray diffraction and scanning electron microscopy revealed the nucleation and growth, in the glass structure, of crystalline phases with nature and distribution very sensitively controlled by the heat-treatment program. These phases have a strong effect on the physical and mechanical properties of glass-ceramic materials when their volume fractions reach values higher than 15 %. They also tend to lower the coefficient of thermal expansion and consequently improve the resistance to thermal shock of these materials without reducing considerably the transparence which is characterized by the refractive index and the absorption coefficient. Fracture surfaces produced by thermal and mechanical shocks of glass-ceramic materials put in evidence the role of the morphology and of the crystalline phase distribution on the resistance to abrupt crack propagation.     

Remarques sur les définitions des acides et des bases

In the present state of knowledge it is not possible to account for the reactions used in analytical chemistry by setting up a single table of the “acids” and “bases” defined according to LEWIS.The most general definition which can be usefully adopted involves taking into account the exchange of “particles” (electrons, protons, ions, molecules) in the reactions. It follows from this that there arc as many types of reactions as there are “particles”.In accordance with the brönsted theory, in the case where the “particle” exchanged is the proton, an acid-base reaction is involved.     

Microlensing photométrique et détermination des masses stellaires

Microlensing was suggested for stellar mass determination of nearby stars by several authors (e.g. Paczyński). There are two aspects in gravitational microlensing: photometry and astrometry. Here only the photometric aspect, i.e. magnification of a background source by a stellar lens, will be considered. The first study in this domain was done by Paczyński. An attempt to investigate some observational constraints (Alard et al.) takes its origin in this study. It will be shown here that blending of the source by the lens not only induces a degeneracy which has been pointed out by Woźniak and Paczyński and can be removed by measuring the flux of the source outside the microlensing event, but also strongly reduces the photometric cross section as well as the duration of microlensing events. The expected number of events decreases strongly with blending. Blending effects can be reduced by selecting faint lens candidates. Unfortunately however, it is difficult to monitor a sufficient number of lens candidates per night, and restricting oneself to nearby and high proper motion objects would lead to a very small number of lens candidates. In the case of short duration events, the light curve must be sampled at short time intervals. When sampling daily, the condition is very strong and the expected rate of events very small. Observing from space would increase significantly the rate, but this gain is still insufficient. We conclude that the event rate will be very small and that the project does not seem to be feasible, at least under current observing conditions and even in the near future.     

Préparation,caractérisation et réactivité de l’acide 1-vanado-11-molybdo-phosphorique supporté sur des matériaux silicatés mésoporeux dans l’oxydation du propène

The H₄PMo₁₁VO₄₀ heteropolyacid (HPA) was supported at 30 wt.% by the dry impregnation method on HMS, CMI-1 and SBA-15 mesoporous materials. The state of the HPA and those of the supports were examined by nitrogen physisorption, X-ray diffraction, (DR) FT–IR and X-ray photoelectron spectroscopies, thermal analysis (TG–ATD) and scanning electron microscopy (SEM). The effect of support on the catalytic behavior of H₄PMo₁₁VO₄₀ was studied in the propene oxidation at 350 °C. It was shown that the presence of H₄PMo₁1VO₄₀, modifies the textural properties of mesoporous materials (decrease of surface area) without destroying their structure. The interaction support–heteropolyacid leads to the formation of (SiOH²⁺)(H₃PMo₁₁VO₄₀^?) surface species more stable than H₄PMo₁₁VO₄₀ species and that appear to be the active sites in the propene oxidation.     

Le microdosage de l'azote amidé etα aminé par la méthode aux acides chlorhydrique et nitrique concentrés

Résumé L'étude de la réaction du mélange acides chlorhydrique et nitrique concentrés sur les acides-monoaminés monocarboxylés a montré que celle-ci s'effectue d'après le schéma: R-CHNH₂-COOH + HCl + HNO₃ = R-CHCl-COOH + N₂O + 2 H₂O.L'étude quantitative de cette réaction a, en particulier, montré que le volume de gaz recueilli correspond rigoureusement à l'azote aminé de la molécule. Cette constatation nous a permis, lorsque la molécule n'est pas susceptible de donner des réactions secondaires avec l'acide nitrique, de développer à l'échelle micro une méthode simple et précise de dosage de l'azote aminé des acides-aminés et des acides-aminosulfonique ainsi que de l'azote amidé des amides et des sulfonamides.

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Summary The study of the reaction of a mixture of concentrated hydrochloric acid and nitric acid on the mono-carboxylated monamines has shown that it proceeds: R—CHNH₂—COOH + HCl + HNO₃ = R—CHCl—COOH + N₂O + 2 H₂O.The quantitative study of this reaction has proved, in particular, that the volume of gas collected corresponds rigorously to the amine nitrogen of the molecule. This finding has made it possible for the authors, provided the molecule is not susceptible to yielding secondary reactions with nitric acid, to develop on the microscale, a simple and precise method of determining the amine nitrogen of amino acids and aminosulfonic acids, as well as the amide nitrogen of amides and sulfonamides.

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Zusammenfassung Das Studium der Umsetzung eines Gemisches von konzentrierter Salzsäure und Salpetersäure mit-Monoaminomonocarbonsäuren ergab die Erkenntnis folgenden Reaktionsablaufes: R—CHNH₂—COOH + HCl + HNO₃ = R—CHCl—COOH + N₂O + 2 H₂O. Die quantitative Untersuchung dieser Reaktion ergab im besonderen, daß das erhaltene Gasvolumen dem vorhandenen Aminostickstoff genau entspricht. Sofern die Substanz keiner Sekundärreaktion mit Salpetersäure unterliegt, ermöglichte diese Feststellung die Ausarbeitung einer einfachen und genauen Mikrobestimmung des Aminostickstoffes in-Aminosäuren und-Aminosulfonsäuren und ebenfalls des Amidostickstoffes in Amiden und Sulfonamiden.

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Actuellement chargé de cours à l'Institut Agronomique de l'Etat à Gembloux.

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Avec 1 figure.     

Instabilité et décrochage tournant en grille d'aubes

Rotating stall is a fundamental hydrodynamic instability which is encountered in most hydraulic and aeraulic turbomachinery. It appears to exist only with cascades that considerably slow down the fluid flow and occurs in the form of rotating structures in which the flow rate may be negative. The paper analyses this instability and its transitions in a simple two-dimensional configuration using a multi-domain numerical scheme and solver. The space-time variation of the flow rates occurring for an angle of attack increasing by steps shows first standing waves, then intermittent and permanent travelling waves of limited amplitude, corresponding to blade stall. At higher angles of attack, a high amplitude wave propagates a single structure (cell) with a negative flow rate, corresponding to stalling of the cascade.     

Étude des propriétés des mélanges pebd régénéré / pebd vierge

This study is a contribution to the valorization of recycled low-density polyéthyléne (LDPE). First the characterization of five recycled LDPE produced from wastes coming from different sources was performed. The physical properties (density, melt flow index), chemical structure (Fourier transform infra-red spectroscopy) and mechanical properties (tensile strength and hardness) were investigated. The effect of the ratio of virgin LDPE on these physical and mechanical properties was studied in the case of the blends of recycled LDPE / virgin LDPE.     

Réaction d’isomérisation de l’acide urocanique et de ses dérivés: stabilité comparée des isomères Z et E

Thermal and photochemical isomérisation of urocanic acid and its derivatives (O- and/or N-alkylated) have been studied. The data show the wavelength and the molar extinction coefficient-dependent photochemistry of E urocanic acid and its derivatives. Thermal isomérisation of these compounds is difficult and undergoes a “one-way” Z → E reaction. Thus, E isomers are thermodynamically more stable than the Z ones. Under basic conditions, derivatives having an unsubstitued imidazole ring (NH function free) exist in their anionic forms, which lowers the energy barrier without changing the isomérisation pathway.     

Profils d'activités enzymatiques des genres Agrobacterium,alcaligenes, alteromonas,flavobacterium et pseudomonas

Profiles of enzymatic activities were studied using 19 chromogenic substrates for 22 species (211 strains) belonging to the genera Agrobacterium, Alcaligenes, Alteromonas, Flavobacterium and Pseudomonas. The observed patterns of reactions may be useful as an aid in identification of these species and for epidemiological studies.     

Préparation et structure des germanates métalliques. I

Résumé L'auteur reprend et complète la méthode de dosage microgravimétrique du germanium à l'état de germanimolybdate de tétraphénylarsonium (F _Ge=0,02137). Dans une deuxième partie, il décrit une méthode de préparation des heptagermanates alcalins et de métagermanates de métaux bivalents à partir des solutions d'oxyde GeO₂ et d'acétates métalliques. Les courbes de thermolyse et les spectres d'absorption infrarouge sont présentés. Aucun orthogermanate ne précipite des solutions aqueuses.

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Summary The author resumes and completes the method for the microgravimetric determination of germanium as the germanium molybdate of tetraphenylarsonium (F _Ge=0.02137). In a second part he describes a method for preparing some heptagermanates of divalent metals from solutions of the oxide GeO₂ and metal acetates. The thermolysis curves and the infrared absorption curves are given. No orthogermanate precipitates from aqueous solutions.

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Zusammenfassung Der Verfasser berichtet über die mikrogravimetrische Bestimmung des Germaniums als Tetraphenylarsonium-Germanium-Molybdat (F _Ge=0,02137). Im zweiten Teil der Arbeit wird die Herstellung von Alkaliheptagermanat sowie von Metagermanaten bivalenter Metalle aus Germaniumoxidlösungen und Metallacetaten beschrieben. Die Thermolysekurven und IR-Absorptionsspektren werden vorgelegt. Aus wäßrigen Lösungen fällt kein Orthogermanat aus.

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Mémoire en l'honneur du professeurA. A. Benedetti-Pichler.