A new model for snow avalanche dynamics based on non-Newtonian fluids

The purpose of this paper is to develop a model to describe the snow avalanche dynamics emphasizing the phenomenon of entrainment, the shape variation and the velocity profile thanks to the peculiar features of non-Newtonian fluids, in particular those showing shear thinning and Bingham-like constitutive behaviours. Two different approaches are proposed to simulate the avalanches numerically. The first considers the transformation of the avalanche domain into a simple shape domain that does not change in time. The second is based on the level set method, which is suitable for free boundary problems. Finally, the characteristics of the variation of the interface between avalanche and snowcover under a similarity hypothesis is put forward. The model is validated through the comparison with some experimental data.     

Ionic core-shell dendrimers with an octacationic core as noncovalent supports for homogeneous catalysts

Ionic core-shell dendrimers with an octacationic core have been applied as noncovalent supports for homogeneous catalysts. Catalytically active arylpalladium complexes, which bear a tethered sulfato group, were noncovalently attached to the ionic core-shell dendritic supports via a straightforward ion-exchange reaction under mild conditions. Diagnostic shifts in (1)H NMR and Overhauser contacts show that the sulfato groups of the catalysts are located close to the octacationic core of the dendritic support in the resulting assemblies. The location of the catalytic Pd(II) sites has been varied via two strategies: by increasing the dendrimer generation and/or by shortening of the sulfato tether. In addition, a metallodendritic assembly was prepared, which bears an alternative shell of apolar dodecyl groups. Both the dendrimer size and the nature of the dendritic shell have no influence on the binding properties of the dendritic supports, i.e., the octacationic dendrimers of generations 1-3 form discrete 1:8 assemblies with the arylpalladium complexes. The structural aspects and the nature of the metallodendritic assemblies have been studied by means of pulse gradient spin-echo NMR diffusion methods, Overhauser spectroscopy, and electron microscopy (TEM). These techniques showed that the dendritic supports and arylpalladium complexes are strongly associated in solution to give unimolecular assemblies of nanoscopic dimensions. Membrane dialysis can recover these metallodendritic assemblies due to their nanoscopic size. The catalytic performances of the metallodendritic assemblies are comparable, but slightly lower than the performance of the unsupported catalyst.     

On-line dynamic extraction and automated determination of readily bioavailable hexavalent chromium in solid substrates using micro-sequential injection bead-injection lab-on-valve hyphenated with electrothermal atomic absorption spectrometry

A novel and miniaturized micro-sequential injection bead-injection lab-on-valve (microSI-BI-LOV) fractionation system was developed for on-line microcolumn soil extraction under simulated environmental scenarios and accurate monitoring of the content of easily mobilisable hexavalent chromium in soil environments at the sub-low parts-per-million level. The flow system integrates dynamic leaching of hexavalent chromium using deionized water as recommended by the German Standard DIN 38414-S4 method; on-line pH adjustment of the extract by a 0.01 mol L(-1) Tris-HNO(3) buffer solution; isolation of the chromate leached from the matrix constituents onto a Q Sepharose strong anion-exchanger freshly packed into the microconduits of the microSI-assembly; air-segmented elution of the sorbed species by a 40 microL plug of 0.5 mol L(-1) NH(4)NO(3) (pH 8) eluent; and detection by electrothermal atomic absorption spectrometry (ETAAS). The effect of simulated acidic rain on the accessibility of chromate forms for plant uptake was also investigated. The proposed approach offers several advantages over conventional speciation/fractionation protocols in the batch mode, including immediate separation with concomitant preconcentration of the released chromate, minimization of Cr(vi) to Cr(iii) interconversion risks, enhanced accuracy, and non-existence of re-adsorption/re-distribution problems along with a detailed pattern of the kinetics of the leaching process. The reliability of the proposed method was evaluated via spiking of a moderately polluted agricultural soil material (San Joaquin Soil-Baseline Trace Element Concentrations) with water-soluble Cr(vi) salts at different concentration levels. The potential of the microSI-BI-LOV set-up with renewable surfaces for flame-AAS determination of high levels of readily bioavailable chromate in contaminated soils is also addressed.     

The mechanistic puzzle of transition-metal-catalyzed skeletal rearrangements of enynes

Three pathways actually compete in metal-catalyzed cyclizations of enynes in which the metal selectively activates the alkyne: an endocyclic process and two exo-cyclizations, one proceeding by anti attack of the alkene and a second one resulting in a syn addition. Although cyclobutenes may be formed in transition-metal-catalyzed cyclization of some enynes, particularly, 1,7-enynes, these compounds are not necessarily the intermediates in the skeletal rearrangement. Cyclobutenes are formed by ring expansion of syn-cyclopropyl metal-carbenes formed in the syn pathway.     

Determination of glucose in fermentation processes by means of an on-line coupled flow-injection system using enzyme sensors based on chemically modified electrodes

The application of an inexpensive, easy-to-make and disposable biosensor, based on a chemically modified graphite electrode with an adsorbed and cross-linked layer of glucose dehydrogenase, as a detector in a flow-injection system for on-line monitoring of glucose in fermenters is described. The performance of the sensor as a function of time is investigated, and used for assaying glucose in a wine fermentation process. It is concluded that the sensor operates satisfactorily during at least 3 days of continuous use. No interference from ethanol is encountered. With regard to practical applications, special emphasis is placed on the interfacing of the fermenter and the analytical system.     

Flow-injection enzymatic assays

The advantages to be gained by conducting enzymatic assays in flow-injection systems are discussed. Selected examples based on the use of soluble and soluble and immobilized enzyme preparations are given. This approach not only offers high sampling frequency and convenient solution handling in assays for enzymes and substrates but is also valuable for testing sensor performance and monitoring bioreactors.     

Homogeneous and heterogeneous systems : Flow injection analysis today and tomorrow

Chemical conversions by means of soluble and immobilized reagents including catalysts (enzymes) are reviewed, and estimates are made towards development of miniaturized devices which besides handling homogeneous samples will have the capability to process heterogeneous sample materials such as blood. Reference covering available FIA literature published in 1987 are listed in an Appendix.     

Simultaneous differential rate determination of iron(II) and iron(III) by flow-injection analysis

Flow-injection procedures for the simultaneous spectrophotometric determination iron(II) and iron(III), relying on the different kinetic-catalytic behaviour of iron(II) and iron(III) in the redox reaction between leucomalachite green and peroxodisulphate with and without the presence of the activator 1,10-phenanthroline, are described. Exploiting the fact that one of the chemical reactions is very rapid whereas the other one is comparatively slower, two experimental procedures are presented. In the first, two individual zones of sample solution are injected simultaneously into separate carrier streams of reagent in a two-line system. Taking advantage of the different residence times of the samples in the manifold lines, the resulting colour formation is measured by a single optical detector with two separate flow cells aligned within the same optical path. The second approach is based on the use of a single-line flow-injection system, exploiting the formation of a double peak as a result of injecting a large sample zone, sandwiched between reagent zones of appropriate composition. In this manner two time-resolved signals for the kinetically governed processes can be obtained and thus used for quantification of the individual species.     

1H and 19F PGSE diffusion studies on iridium PHOX complexes: counterion and solvent dependences

1H and (19)F pulsed gradient spin-echo (PGSE) diffusion studies on cationic mono- and trinuclear iridium complexes containing the PHOX chiral P,N-auxiliary (S)-4-tert-butyl-2-[2-(di-o-tolylphosphino)phenyl]-4,5-dihydrooxazole with the anions BF(4)(-), PF(6)(-), OTf(-), B(C(6)F(5))(4)(-), and BArF(-) in methanol, chloroform, methylene chloride, and 1,2-dichloroethane are reported. In chloroform, the anion and cation within each salt afford almost the same, relatively small, diffusion constant (D-value) suggesting strong ion-pairing. In methanol, the D-value for the cation is the same in the five mononuclear salts, suggesting that the cation is moving independently of the anion (no ion-pairing). In methylene chloride and 1,2-dichloroethane the diffusion data suggest a mixed picture for the five anions. While the smaller BF(4)(-), PF(6)(-), and OTf(-) anions do not affect the translation of the cations, the larger boron-based anions B(C(6)F(5))(4)(-) and BArF(-) clearly slow the motions of the cations. However, it would seem that for all five anions there is some--but not complete--ion pairing in these two solvents.     

Optosensing at active surfaces — a new detection principle in flow injection analysis

Reflectance spectrophotometry is applied to flow-injection measurements of pH and the assays of ammonia and urea with the aim of demonstrating the principle and testing the performance of optosensors integrated into microconduits. A novel injection approach, the split-loop technique, is applied. For pH measurements, detection is based on commercial non-bleeding acid-base indicator papers situated in the flow stream at the tip of the fibre optic. Measurements of pH in the range 4–10 are possible at a rate of 120 h^?1. Special attention is given to the physiological pH range; the standard deviation is 0.004 at pH 7.2. For the determinations of ammonia and urea (via urease), a bromothymol blue stream is used with a miniature gas-diffusion device.