Électrodialyse : convection naturelle verticale au niveau des membranes échangeuses d’ions polarisées

Les résultats d’un article précédent (1) concernant la détermination des concentrations aux interfaces d’une membrane échangeuse d’ions polarisée à l’aide des méthodes DICET et EQUI, sont interprétés dans le cadre des lois de la convection naturelle. Les couches de diffusion de chaque côté de la membrane semblent contrôlées comme pour les électrodes par une loi hydrodynamique de la forme : [math] De ce fait, il est possible de déterminer une densité de courant critique moyenne par extrapolation d’une relation reliant le gradient de concentration dans les films aux densités de courant, sans faire de mesure dans la zone de densité de courant sur-critique. D’autre part, un bon accord est obtenu entre différents calculs conduisant aux épaisseurs des couches de Nernst le long de la membrane

The rate of acid or base producing enzyme reactions: analytical treatments of the feed-back kinetics and experiments in solution

The representation of the pH-dependence, either linearized in H+, hyperbolic, or by a Gaussian law leads to analytical kinetii expressions in numerous homogeneous systems producing acidification (or alkalinization) of the medium.Calculations, made for zero or first order reactions, take into account the polyelectrolytic structure of the enzyme and the presence or the absence of buffering species. The calculations are based on the production by the reaction of a symbolic species, function of proton concentration.The results, collected in tables, cover a wide range from the most general case to the simplest one

Germination des semences, développement et croissance de quelques essences locales en zone forestière

Seed Germination, Growth and Development of some Local tree Species of the Forest Zone. Studies were conducted on eight indigenous multipurpose trees species : Alstonia boonei. De Wild, Ceiba pentandra (L.) Gearth., Cordia platythyrsa Bark., Milicia excelsa (Welw) ce Berg., Pycnanthus angolensis (Welw) Warb., Ricinodendron heudelotii (Baill.) Pierre ex Pax., Terminalia superba. Engl. et Diels et Triplochiton scleroxylon K. Schum. The objective of the study was to evaluate germination capacity, development and growth rate of the seedlings of the above mentionned species. The results showed that manual of seeds was clearly superior to the use of the other methods. For the initial growth, Ceiba pentandra and Ricinodendron heudelotii had the fastest growth. AH the species developped tap root system

Bacteriological analysis of water by potentiometric measurement of lipoic acid reduction: preliminary assays for selective detection of indicator organisms.

The practical task of adapting an original potentiometric technique to the bacteriological analysis of water is discussed. Various laboratory strains of organisms belonging to the usual aquatic flora were inoculated one by one in a minimal lactose broth supplied with lipoic (thioctic) acid. The time evolution of the redox potential of the cultures was followed during incubation by combined gold versus reference electrodes. When the incubation temperature was regulated at 36 degrees C, most organisms were able to grow and to reduce the coenzyme, generating changes in the redox potential of the culture. However, very few organisms developed significant reductive activity when the temperature was increased to 41 degrees C and when the broth was provided with sodium deoxycholate. Among the fecal coliform organisms, only Escherichia coli and Klebsiella pneumoniae exhibited early but reproducible potential-time responses. Positive potentiometric responses were also recorded with Acinetobacter calcoaceticus. E. coli showed rapid potentiometric signals as compared with K. pneumoniae. The time required for 100-mV shift of potential to be detected was related to the logarithm of the initial concentration of E. coli or K. pneumoniae in the culture broth. Experiments on natural surface water samples showed the the potentiometric method, associated with the selective incubation conditions, mainly detected E. coli among the bacterial flora of the tested environmental water. The calibration curve relating the time required for a 100-mV shift of potential to be detected to the number of fecal coliforms, as determined by control fecal coliform-selective plate counts, was consistent with the composite standard curve of detection times obtained with six different laboratory strains of E. coli.(ABSTRACT TRUNCATED AT 250 WORDS