Etude cinétique de la complexation du cuivre en milieu hydrogénocarbonate par électrochimie

La détermination de la capacité complexante des eaux naturelles est généralement effectuée par titrage ampérométrique du cuivre à pH fixé. Beaucoup, sinon tous les tampons classiques utilisés perturbent les études de spéciation du cuivre par formation de complexes peu stables. Dans ce travail nous nous sommes intéressés à la complexation du cuivre par les ions carbonates généralement présents dans les eaux naturelles. Afin de mener à bien notre étude, nous avons mesuré, dans un milieu synthétique de force ionique et pH contrôlés (KNO3 0,02 M + NaHCO3 2,38.10-3 M) toutes les constantes des divers équilibres prévisibles : produits de solubilité de l'hydroxyde de cuivre Cu(OH)2 et de la malachite Cu2 (OH)2 CO3, constante de formation du carbonate de cuivre dissous CuCO3. A des valeurs de pH suffisamment faibles, la formation du complexe CUOH+ peut étre négligée. Les trois autres réactions ont été suivies race aux mesures combinées, d'une part du pH et, d'autre part, de l'activité en ion Cu2+ libre ou hydraté par ionométrie. Compte tenu des propriétés acido-basiques des diverses espèces, les mesures ont été effectuées à divers pH compris entre 5,5 et 8. Le complexe CuCO30étant l'espèce majoritaire dans ces solutions, sa labilité, pouvant perturber follement les déterminations des capacités complexantes par titrage ampérométrique, a été étudiée par diverses méthodes électrochimiques. La polarographie à tension sinusoïdale surimposée (AC) a indiqué une réduction rapide du cuivre (II) sur électrode de mercure. Deux vagues ont été obtenues en voltampérométrie sur électrode tournante à disque de platine (RDE), ce dédoublement étant d'origine cinétique compte tenu de l'évolution de ces vagues avec la température. Une estimation des valeurs des constantes de vitesse de formation kf et de dissociation kb, a permis de montrer la rapidité des réactions mises en jeu et la labilité du complexe CuCO30.The determination of the complexing capacity of natural waters by amperometric copper (II) titrations must be made in pH-buffered solutions. Most if not all of classical buffers influence copper speciation through the formation of weak complexes. This paper deals with an attempt to use the naturally occurring hydrogenocarbonate buffet. A detailed study of copper (II) chemistry in synthetic solutions at controlled ionic strength and pH (0.02 mol.l-1 KNO3, 2.38 mol.l-1 NaHCO3, controlled Pco2) was carried out. In these solutions copper (Il) chemistry may involve precipitation of copper hydroxide Cu(OH)2, copper hydroxycarbonate Cu2(OH)2 CO3 (malachite) and formation of the soluble complexes CuCO30 and CuOH+. At low enough values of pH CuOH+ formation may be neglected. The three other reactions are studied using copper ionometry with a specific electrode and equilibrium constants determined in the 5.5 to 8.0 pH range. As CuCO30 is a major species in these solutions its lability is of major concern for the validity of the amperometric determination of the complexing capacity by copper (II) titration and bas been studied by several electrochemical approaches. AC polarography indicated a rapid reduction of copper (II) on the mercury electrode. Two waves were obtained for the deposition of copper (Il) by voltamperometry with a rotated platinum disk electrode (RDE) presenting the characteristics of a kinetic control. However the values of the forward (kf) and backward (kb) reaction rate constants for CuCO30 formation or dissociation seem too high for a precise determination

Radicals, Metals and Magnetism

The interaction between unpaired electrons governs many physical properties of materials. Although in a fundamental sense the interaction is simple, a full understanding of the interaction in molecular systems is complicated by the presence of other bonding and non-bonding electrons. The resulting many body problem is very challenging. Nevertheless, much qualitative understanding can be obtained from applying simple molecular orbital theory and considering only the partly filled orbitals. The resulting model can be used to describe existing diradical and metal-radical systems and also has predicative value in the search for molecular magnets and design of nanoscale devices

Ground state order and spin-lattice coupling in tetrahedral spin systems Cu2Te2O5X2

High-resolution ac susceptibility and thermal conductivity measurement on Cu2Te2O5X2(X=Br,Cl) single crystals are reported. For Br-sample, sample dependence prevents to distinguish between possibilities of magnetically ordered and spin-singlet ground states. In Cl-sample a three-dimensional transition at 18.5 K is accompanied by almost isotropic behavior of susceptibility and almost switching behavior of thermal conductivity. Thermal conductivity studies suggest the presence of a tremendous spin-lattice coupling characterizing Cl- but not Br-sample. Below the transition Cl-sample is in a complex magnetic state involving AF order but also the elements consistent with the presence of a gap in the excitation spectrum.Comment: version accepted for publication in Phys.Rev.B-Rapid Communicatio

Adaptation de la mesure potentiométrique à l'estimation en continu de la teneur en cuivre de solutions aqueuses présentant de fortes variations physico-chimiques

La mesure en continu, sans prélèvement et sous conditions physico-chimiques variables, de la teneur en cuivre d'une solution aqueuse, constitue le cadre de ce travail. La méthode d'estimation proposée est basée sur la potentiométrie sélective à l'aide d'une électrode spécifique au cuivre. Le comportement de l'électrode est modélisé par une loi non linéaire s'inspirant de la relation de Nernst généralisée. Le modèle intègre les paramètres température, conductivité et pH de la solution. Il rend possible une estimation de la concentration totale de cuivre même en présence de complexations hydroxyles. La phase de modélisation est présentée en détail. Les performances de l'estimateur sont évaluées puis discutées.Ion Selective electrodes (ISEs) offer an attractive solution for continuously evaluating the content of certain ionic species in aqueous media. Manufacturers propose a wide range of electrodes specific to heavy metals (Cu2+, Pb2+ …). Because they eliminate the need for sampling, are of reasonable size and have few electronic parts, ISEs seem highly appropriate for continuous monitoring in urban purification systems. Measurements obtained by these sensors in controlled media in the laboratory are usually precise, reliable and reproducible. However, it is not so with complex and uncontrolled media. This work falls within the general scope of the continuous measurement of heavy metals in wastewater. More particularly, it is devoted to the description of the behaviour of a copper-selective electrode (ISECu) in a medium presenting wide physicochemical variations.Experimental set-upIn order to study ISE behaviour, we developed an experimental platform that allowed us to reproduce in a reactor the physicochemical variations observed in wastewater, particularly with regards to salinity and acidity. The reactor was fitted with a measuring set consisting of five electrodes that measured the following parameters: pH (ref. integrated Ag/Agcl), redox (red), ISECu (ECu), temperature (T) and conductivity (s). A computer system carried out the acquisition of the five signals with a 10-second sampling period. The species concentration in the reactor was determined by calculating the weight of the solutions extracted from or injected into the reactor. Controlling the temperature of the system was undertaken using a cryostat. Sequential tests allowed the pH, redox potential and conductivity of the medium to be varied and were carried out by successive injections of different chemical products. The response times of the conductivity probe and of the pH and redox electrodes are shown here; the short response time of the sensors (20 to 30 s) and the strong correlation between the measured pH and redox are noted. ISE modellingThe model used to explain the ISE response is based on a generalization of Nernst's Law that takes into account the temperature and the activity of the free ions (Cu2+). Taking into consideration chemical equilibria and mass equations allowed us to link the activity of the free copper ions to the total injected copper concentration |Cu2+|tot and to the pH. Redox, strongly correlated to pH, was ignored in the mathematical model. Since hydroxyl complexation is the major complexation reaction (compared to other copper-binding ligands), the potential measured with the ISE took the following form:ECu=b0+b1T.log[(ϒ2|Cu2+|tot) / (1+b2ϒ210pH+b3ϒ2102pH) + b4]The activity coefficient ϒ2 of the Cu2+ ions was calculated from the ionic strength (I) of the solution, using the Debye-Hückel approximation. Ionic strength was derived from conductivity corrected to 25 °C. In wastewater, the ranges of the physicochemical parameters were as follows: T from 5 to 35°C; pH from 4 to 9; Omega from 500 to 2000 mS/cm; redox from 400 to -400 mV/ENH; and copper concentrations 10-3 mol/dm3.In order to identify the bi coefficients of the model, we established an experimental plan comprising 108 measurement points that covered, with a minimal number of experiments, the ranges of variations of the parameters of influence. A dispersion diagram of measured and modelled values gave a linear adjustment coefficient close to 0.99 and a standard deviation of 8.8 mV, which corresponds to a 0.34 decadal standard error in the concentration estimate. With a temperature of 25 °C, the model has a sensitivity of -26.4 mV/decade, very close to the theoretical slope of an electrode sensitive to divalent ions.ISE measurement of the copper concentration with large pH variations pH is the parameter which exerts the greatest influence on ISE response, which is why tests simulating copper pollution with large variations of pH were carried out. These tests enabled us to evaluate the performances of the model in terms of the estimation of copper content. Four solutions of total copper concentration equal to 10-6, 10-5, 10-4, 10-3 mol/dm3 respectively, were used. Their temperature was 25 °C and their conductivity was fixed at approximately 500 mS/cm. We varied the pH of each solution between 4 and 10. For the four tests, we show the estimate of the copper concentration obtained with our model starting from the potential measured by the ISE.In the case of strong copper pollution (10-3 mol/dm3), the model yields an overestimated concentration below pH 7 with a decadal error of less than 0.5. Above pH 7, the concentration is underestimated while maintaining a decadal error of less than 0.5. At pH 7, a 0.04-decade minimal error is found. For pollution equal to or less than 10-4 mol/dm3, the model gives good results in an acid or neutral medium with a decadal error usually less than 0.3. In an alkaline medium, concentration is overestimated. In this case the error increases in a roughly linear manner with the pH and the co-logarithm of copper concentration. From the results of these tests, we defined a valid domain of ISE copper concentration measurement using our model. In conclusion, the suggested method, although not very accurate, could be used as an indicator of the copper concentration level in wastewater. The ISE-response correction model is currently being tested under operational conditions at a water treatment plant in Nancy-Maxéville (France)

Torque magnetometry study of magnetically ordered state and spin reorientation in the quasi-one-dimensional S=1/2S=1/2 Heisenberg antiferromagnet CuSb2_2O6_6

We present an experimental study of macroscopic and microscopic magnetic anisotropy of a spin tetramer system \cso using torque magnetometry and ESR spectroscopy. Large rotation of macroscopic magnetic axes with temperature observed from torque magnetometry agrees reasonably well with the rotation of the $\mathbf{g}$ tensor above $T \gtrsim 50$~K. Below 50~K, the $\mathbf{g}$ tensor is temperature independent, while macroscopic magnetic axes continue to rotate. Additionally, the susceptibility anisotropy has a temperature dependence which cannot be reconciled with the isotropic Heisenberg model of interactions between spins. ESR linewidth analysis shows that anisotropic exchange interaction must be present in \csos. These findings strongly support the presence of anisotropic exchange interactions in the Hamiltonian of the studied system. Below $T_N=8$~K, the system enters a long - range antiferromagnetically ordered state with easy axis along the $^*$ direction. Small but significant rotation of magnetic axes is also observed in the antiferromagnetically ordered state suggesting strong spin-lattice coupling in this system.Comment: 10 pages, 10 figure

Mise en place syntectonique des minéralisations cuprifères du gîte d'Ifri (district du Haut Seksaoua, Haut Atlas occidental, Maroc)---Syntectonic emplacement of the Ifri copper mineralization (High Seksaoua, western High Atlas, Morocco)

International audienceL'étude structurale et métallogénique (gîtologique) du gisement de cuivre d'Ifri (Haut Atlas occidental, Maroc) montre qu'une grande partie de la minéralisation est liée à un stockwerk, dont la formation est contrôlée par un cisaillement ductile tardi-hercynien à vergence NNW. Ce résultat significatif remet en question le caractère syngénétique jusqu'alors évoqué pour la minéralisation et permet ainsi de proposer un nouveau guide d'exploration pour ce type de gisement. Structural and metallogenic studies of the Ifri copper deposit (western High Atlas, Morocco) demonstrate that a great part of the mineralization was linked to a stockwork developed during a Late-Variscan NNW-verging shearing event. With this significant result, the until now accepted syngenetic character for this mineralization has to be reconsidered and allows to suggest a new guide of prospection for this kind of deposit

Le magmatisme de la région de Kwyjibo, Province\ud du Grenville (Canada) : intérêt pour les\ud minéralisations de type fer-oxydes associées

The granitic plutons located north of the Kwyjibo property in Quebec’s Grenville Province are of\ud Mesoproterozoic age and belong to the granitic Canatiche Complex . The rocks in these plutons are calc-alkalic, K-rich,\ud and meta- to peraluminous. They belong to the magnetite series and their trace element characteristics link them to\ud intraplate granites. They were emplaced in an anorogenic, subvolcanic environment, but they subsequently underwent\ud significant ductile deformation. The magnetite, copper, and fluorite showings on the Kwyjibo property are polyphased\ud and premetamorphic; their formation began with the emplacement of hydraulic, magnetite-bearing breccias, followed by\ud impregnations and veins of chalcopyrite, pyrite, and fluorite, and ended with a late phase of mineralization, during\ud which uraninite, rare earths, and hematite were emplaced along brittle structures. The plutons belong to two families:\ud biotite-amphibole granites and leucogranites. The biotite-amphibole granites are rich in iron and represent a potential\ud heat and metal source for the first, iron oxide phase of mineralization. The leucogranites show a primary enrichment in\ud REE (rare-earth elements), F, and U, carried mainly in Y-, U-, and REE-bearing niobotitanates. They are metamict and\ud underwent a postmagmatic alteration that remobilized the uranium and the rare earths. The leucogranites could also be\ud a source of rare earths and uranium for the latest mineralizing events

Micromechanical investigation of the influence of defects in high cycle fatigue

This study aims to analyse the influence of geometrical defects (notches and holes) on the high cycle fatigue behaviour of an electrolytic copper based on finite element simulations of 2D polycrystalline aggregates. In order to investigate the role of each source of anisotropy on the mechanical response at the grain scale, three different material constitutive models are assigned successively to the grains: isotropic elasticity, cubic elasticity and crystal plasticity in addition to the cubic elasticity. The significant influence of the elastic anisotropy on the mechanical response of the grains is highlighted. When considering smooth microstructures, the crystal plasticity have has a slight effect in comparison with the cubic elasticity influence. However, in the case of notched microstructures, it has been shown that the influence of the plasticity is no more negligible. Finally, the predictions of three fatigue criteria are analysed. Their ability to predict the defect size effect on the fatigue strength is evaluated thanks to a comparison with experimental data from the literature