Prévision des courbes de dosages potentiométriques

The knowledge of polarisation curves can be used to explain the facts observed in the neighbourhood of the equivalent point in the course of potentiometric determinations as well as the anomalies presented by certain determination curves.It is also possible to interpret the role of a “potential mediator” during the determinations of oxido-reduction potentials.     

Interprétation des mesures potentiométriques au moyen des courbes de polarisation

The observed experimental results of the potentiometric determinations can be explained by means of the polarisation curves. Thus explanations are given of the following phenomena: Electro-activity and non-electro-activity of ions. Nature of the potential taken by an electrode. Time required to establish a stable potential. Significance of the limiting potentials and the limiting concentrations. Influence of oxygen on the value of the measured potentials and on the limit of electro-activity of the oxidants. Significance of the “irreversible” potentials. Formulas giving the potentials in the presence of oxygen. Utility of removing the oxygen from the solution in view of potentiometric determinations. It appears that for all the “slow” redox systems, which are very numerous in inorganic and organic chemistry, the knowledge of the polarisation curves is fundamental.     

Influence des réactions chimiques préalables sur la forme des courbes de polarisation

Equations have been established for polarographic waves observed while a chemical reaction slowly takes place before the exchange of electrons.The method of calculation is analogous to that already used by one of the authors in the case of polarisation curves relative to the reversible or irreversible systems.The variation of the two half-wave potentials and the limiting densities of current have been deduced. The results have been verified by means of the experimental results of R. brdicka concerning pyruvic and phenylglyoxylic acids.     

Introduction a l'étude des sels basiques insolubles: Étude des courbes de précipitation des sels basiques insolubles. Produits de solubilité des sels basiques et des hydroxydes

A critical study of curves of precipitation of basic salts by alkali hydroxides shows that the irregularities observed on these curves do not always permit the deduction of the constitution of the salt. They indicate nevertheless the appearance of a new phase and this observation explains, in particular, the important number of wrongly defined basic salts which have been observed.The theory of the titre curves leads to the proposal of formulas permitting the calculation of solubility products of basic salts and even those of hydroxides which were determined very accurately by other methods.     

Non-monotonie numérique de courbes de dispersion en élasticité linéaire

This study concerns the behaviour of dispersion curves stemming from a problem of guided waves in a plane elastic medium that is invariant by translation in one spatial direction and stratified with respect to the other one. We formulate an equivalent problem in a bounded domain using transparent boundary conditions. The dispersion curves that are shown are not monotonous, a phenomenon not yet been encountered but suspected.     

Utilisation des courbes de polarisation dans les dosages électrochimiques: Coulométrie á intensité constante

The conditions for coulometric determinations at constant intensity can be deduced from the polarisation curves; these are choice of current density and auxiliary systems, and possible accuracy of the determination. We have used for this study the results previously obtained for potentiometry at constant intensity.     

Modélisation des temps de réponse des capteurs chimiques

Modelling consists of considering the response and recovery times of a sensor to the target chemical species, as the sum of an ‘intrinsic’ response/recovery time, independent of the experimental conditions, and of an ‘extrinsic’ one, linked to the volume of the measurement cell and to the fluid flow. The experimental response/recovery times being almost always dominated by the extrinsic part, a dissymmetry between the response and recovery signals is observed for sensors whose response is not linear with the concentration of the target chemical species.     

Méthode de détermination des épaisseurs de films conducteurs par mesures de résistance de surface

To evaluate the thickness of the layers of a Ti/TiAl₃/Al system, experiments based on surface resistance measurements have been performed. The thickness of each layer is deduced from a comparison among calculated and measured values of resistance. We present here a solution for the simulation of surface resistance and the results obtained with this method on Ti/TiAl₃/Al systems.     

Influence de la cinétique des réactions chimiques sur les courbes de polarisation : Application à l'étude des complexes thiocyanés du titane(iv) et du titane(III)

The electrochemical oxidation-reduction of the system titanium(IV) — titanium(III) is slow at dropping mercury electrode in perchloric acid or hydrochloric acid, but becomes rapid in the presence of thiocyanate ions.On the basis of polarization curves, interpreted according to a preliminary theoretical study, the authors conclude that the rapid electrochemical reaction is the following: TiOH.SCN⁺² + H⁺ + e ? TiSCN⁺² + H₂OAn application of the findings to the polarographic determination of titanium is given.     

Réactions colorées de stéroĭdes

A number of colour reactions for the identification and determination of steroids are described.     

Dérivées de fonctionnelles intégrales dans des perturbations de fissures exprimées par intégrales de frontière

This note presents, in the framework of three-dimensional linear elastodynamics in the time domain, a method for evaluating sensitivities of integral functionals to crack shapes, based on the adjoint state approach and resulting in a sensitivity formula expressed in terms of surface integrals (on the external boundary and the crack surface) and contour integrals (involving the direct and adjoint stress intensity factor distributions on the crack front). This method is well-suited to boundary element treatments of e.g. crack reconstruction inverse problems.     

Détermination des cinétiques de croissance et de dissolution du perborate de sodium tetrahydrate en lit fluidisé

The authors present the results of an experimental study on the crystallization of tetrahydrate sodium perborate in a fluidized bed. The influences of supersaturation, seed crystal size, temperature and solution velocity on the overall growth rate were examined. The overall dissolution rate determined under comparable hydrodynamical conditions for various seed crystal sizes and temperatures is also reported.The growth rate is not very sensitive to the variations in crystal size, the temperature and the hydrodynamical conditions, but it depends on the supersaturation and the presence of some soluble impurities.The comparison between growth and dissolution rates shows that under actual working conditions the diffusional mass transfer step is not the limiting step of the growth mechanism.     

Microdosages des composés caractéristiques de la fermentation butanediolique

Compounds characteristic of the butanediol fermentation are 2–3 butanediol, acetoin and diacetyl; tlie determination of each of these compounds is based on a determination of dimethylglyoxime by nickel (II) (as in the classical methods, which are time-consuming and not very sensitive). The method now deseribed uses the soluble red complex given by dimethylglyoxime with niekel (IV). By this means 4 colorimetric methods have been established which, respcetively, permit: 1. The determination in 5 minutes, to about 1 %, of 120 to 600 μg, of dimethylglyoxime in a 10 ml sample. 2. The determination in 30 minutes, to about 1 %, of 90 to 450 μg, of diacetylin a sample of 10, 50 or 100 ml. 3. The determination in 50 minutes, to about 1 %, of 90 to 450 μg, of acetoin in a sample of 10, 50 or 100 ml. 4. The determination in 70 minutes, to about 3 %, of 100–500 μg, of butanediol in a 10 ml sample. These methods, based on specific reactions of the substances being determined, (in contradistinction to the numerous methods recently proposed for the determination of butanediol) are applicable even when the substances are accompanied by 1000 times their weight of glucose, glycerol, etc.     

Caractérisation des propriétés électriques des parois de pores d’une membrane

The characterisation of the surface electrical properties of membrane materials is critical for understanding and predicting the filtration performances of membranes. In this paper, four simple experimental methods – streaming potential, electro-osmosis, pore conductivity and membrane potential –, allowing the characterisation of the charge state of membrane pore walls are presented. The four experimental parameters provide information about the sign of the electrical net charge. Examples illustrating the influence of the pH, electrolyte concentration and nature on the four experimental parameters are given. The zeta potential can be then deduced from these measurements by means of a model.     

Sur le noircissement des précipités de sels d'argent

Sans résumé     

Etude des réactions de l'acétylformazyle et de ses dérivés avec les ions minéraux

Résumé L'étude systématique de l'acétylformazyle et de ses dérivés pour l'analyse à la touche à fourni des réactions sensibles permettant la recherche du mercure avec la thiosemicarbazone, du cuivre et du palladium avec l'oxime.

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Summary The systematic study of acetylformazyl and its derivatives for spot test analysis has yielded sensitive reactions which permit the detection of mercury by means of the thiosemicarbazone, and of copper and palladium with the oxime.

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Zusammenfassung Die systematische Untersuchung von Acetylformazyl und seinen Derivaten für die Zwecke der Tüpfelanalyse führte zu empfindlichen Reaktionen zur Prüfung auf Quecksilber mit dem Thiosemicarbazon, auf Kupfer und Palladium mit dem Oxim.