A fragmentation model with neighborhood interaction

Several models of fragmentation have been studied that suppose random fracture forces. In this article, we did a numerical study on a dynamic model for fragmentation in which the fracture forces are generated by neighboring fragments and are proportional to the size of the common boundary between two fragments. The following assumptions were also considered: the material defects are represented by a random distribution of point flaws; the total mass is conserved; and the iterative fracture of each fragment is randomly stopped by a condition that considers a constant probability and a minimal fragment size. The motivation for this model was to determine under what circumstances a continuous fragmentation model with fracture forces defined by the neighbors’ interaction produces results that are in agreement with the experimental evidence. The main result of this work establishes that the fragment size distribution follows a power-law for fragments of greater area than the minimal fragment size m_fs. The visualizations present complex fracture patterns that resemble real systems.     

Simultaneous kinetic determination of chlorpyrifos and carbaryl based on differential degradation processes in alkaline oxidative medium

A kinetic method for the simultaneous determination of chlorpyrifos and carbaryl is described. The method is based on the degradation of these compounds in alkaline hydrogen peroxide medium by measuring the absorbance at two different reaction times and at two different wavelengths. A homemade stopped-flow system coupled with a diode-array detector and the proportional equation method was used. Resolution was achieved over chlorpyrifos: carbaryl ratios ranging from 61 to 14, with relative errors of 4 and 3%, respectively. The method was successfully applied to the analysis of a commercial formulation.     

Polynomials on the Cauchy circle

Summary. Let be a complex polynomial of degree with and Cauchy radius 1 about the origin. We discuss the order of magnitude of the minimal number such that Previous estimates of are improved to . Some other related properties of these polynomials are also exhibited. Received March 3, 1993     

P-Containing ORMOSILS for bone reconstruction

PDMS–CaO–SiO₂–phosphatoethylsilane (PES) ORMOSILS with different P content were prepared as monoliths. The use of a phosphorous source such as diethyl phosphatoethyl triethoxysilane (DEPETES) allowed the introduction at a molecular level of P into an ORMOSIL network. The effect of the amount of P on the in vitro bioactivity of these sol–gel derived ORMOSILS (denoted as Type A) was studied. The in vitro bioactivity of these new ORMOSILS was evaluated by soaking them in a simulated body fluid (SBF) and characterising the new calcium phosphate layer formed over the monolith surfaces. In an attempt to increase the homogeneity of the organic content throughout the hybrid network, the precursor monomer of PDMS (dimethyl diethoxysilane [DDS]) has been used to produce the polymerisation in situ. This polymerisation took place at the same time as that of the hydrolysis and condensation of TEOS and the calcium source. The in vitro bioactivity of these P-free materials in SBF was investigated. The DDS content required to produce monoliths that form a calcium phosphate layer over their surface when soaked in SBF was found to be between 0.5 and 1.5 (DDS:TEOS in molar ratio). Larger amounts of DDS led to phase separation and non-bioactive materials.     

Stability constants of complexes formed by some metal ions with 2-(phenyliminomethyl)pyridine and 2-(o-hydroxyphenyliminomethyl)pyridine

The stability constants of some metal complexes of these two reagents have been determined.     

Study and determination of the pesticide Imidacloprid by square wave adsorptive stripping voltammetry

The kinetics of the reaction between double stranded DNA (dsDNA) and formamide is monitored at the single DNA molecule level. We find that stretching of the DNA leads to an accelerated reaction rate and to a shift in the final equilibrium concentrations. The larger the stretching force, the faster the reaction and the larger the denatured fraction of the product DNA. The single molecule kinetics is obtained from the change in the contour length of the DNA which, in turn, is measured using optical tweezers on a microbead-single DNA molecule-cover slip construct.     

Spectrofluorimetric determination of BrO 3 − by oxidation of 1,2,4-trihydroxyanthraquinone-3-carboxylic acid

Summary A method is described for the fluorimetric determination of BrO₃, at the nanogram level, based on the oxidation of 1,2,4-tri-hydroxyanthraquinone-3-carboxylic acid in sulfuric acid medium. The fluorescence emission is monitored at 585 nm (wavelength of excitation 521 nm). The method shows a detection limit of 5 ng/ml, being the most sensitive of all the established methods for the determination of this anion by photometry and fluorimetry.

---

Spektrofluorimetrische Bestimmung von Bromat durch Oxydation von 1,2,4-Trihydroxyanthrachinon-3-carbonsäure

Zusammenfassung Die fluorimetrische Bestimmung von Bromat im Nanogrammbereich auf der Grundlage der Oxydation von 1,2,4-Trihydroxyanthrachinon-3-carbonsäure in schwefelsaurer Lösung wurde beschrieben. Die Fluoreszenz wird bei 585 nm gemessen (Anregung bei 521 nm). Die Erfassungsgrenze liegt bei 5 ng/ml. Das Verfahren ist somit das empfindlichste unter allen bekannten Methoden zur photometrischen bzw. fluorimetrischen Bestimmung dieses Anions.

     

Derivative spectrophotometric determination of sulphonamides by the Bratton-Marshall reaction

A modified method for the determination of sulphonamides, based on the Bratton-Marshall reaction with derivative spectrophotometry, is proposed. Diverse sulphonamides are determined with excellent precision by using first- to fourth-order derivative spectra. The method was applied for the determination of sulphonamides in urine, honey and pharmaceutical formulations without pretreatment of the samples.     

A new spectrophotometric method for quantitative multicomponent analysis resolution of mixtures of salicylic and salicyluric acids

A new spectrophotometric method for resolving binary mixtures is proposed. The method is based on use of the first derivative of the ratios of spectra. The absorption spectrum of the mixture is obtained and the amplitudes at appropriate wavelengths are divided by the corresponding amplitudes in the absorption spectrum of a standard solution of one of the components, and the first derivative of the ratio spectrum is obtained. The concentration of the other component is then determined from a calibration graph. The method has been applied for resolving binary mixtures of salicylic and salicyluric acids. Calibration graphs for 2.6-52 ppm salicylic acid and for 2.1-42 ppm salicyluric acid were established by measuring the analytical signals at the maximum at 241.5 nm (for salicylic acid) and from the peak at 258 nm to the trough at 247 nm (for salicyluric acid) in the first derivative ratio spectra.