Microemulsions as a medium in chemical kinetics,II. The I− + S2O8= and crystal Violet + OH− reactions in different surfactant/oil/water microemulsions

A kinetic study of the reactions I^? + S₂O₈⁼ and CV⁺ + OH^? (CV = 4, 4′, 4″-tris(dimethylamino)triphenylmethyl chloride or crystal violet) in different water in oil microemulsions is reported. The dependence of the rate constants on the nature of the surfactant as well as on the molar ratio R = [H₂O]/[Surfactant] has been investigated. The results are interpreted on the basis of the electrostatic interactions between the reactants and the surfactant polar heads and considering the water properties inside the aqueous core of the droplets.     

Sequential speciation of selenium by flow injection cathodic stripping voltammetry

A semi-automatic continuous method for the determination of Se(IV) based on flow-injection cathodic stripping voltammetry (FICSV) is reported. The flow injection approach incorporates a thin mercury film on glassy carbon as the working electrode, on which Se(IV) is deposited at an applied potential of 0.0 V. A cathodic scan (from 0.0 to –0.9 V) is applied and the Se is stripped at –0.54 V, providing a current intensity proportional to the Se(IV) concentration in the sample. This method features a linear determination range between 0.5 and 30 ng/ml (r²=0.998, RSD=3.6%). The non-interference levels (foreign species to analyte ratio) are 2.5:1 for Cu(II), 7.5:1 for Pb(II), 35:1 for Cd(II), 250:1 for Zn(II) and 500:1 for Fe(III). After developing the method for Se(IV), the speciation of this element has been performed by sequential injection of the dissolved sample into a carrier which may or may not have been previously reduced off-line thus determining the sum (Se(IV)+Se(VI)) or only Se(IV), respectively. The method has been applied to selenium speciation in water samples.     

A potentiometric study on the complex equilibria between nickel(II) and 2-mercapto-2-phenylacetic acid

Complex formation between nickel(II) and 2-mercapto-2-phenylacetic acid (H₂MPA) has been studied at 25°C and 1 mol dm^?3 ClO₄^? in     

Spectrophotometric and potentiometric determination of the protonation constants of dithiocarbazates and studies on some of their metallic chelates

The protonation constants of 2-methyldithiocarbazate, 3,3-dimethyldithiocarbazate and 3-methyl-3-phenyldithiocarbazate, have been determined potentiometrically (I = 1.00M) and spectrophotometrically (I = 0.01M) at 25 degrees in aqueous solution. The analytical properties of the reagents have been studied, and also the compositions of some of their insoluble metallic chelates.     

The nitrosation of Amino Acids

With a view to clarifying analogies and differences between the mechanisms involved in the nitrosation of amino acids and secondary amines, we studied the kinetics of the nitrosation of five imino acids (azetidine-2-carboxylic acid, pyrrolidine-2-carboxylic acid, piperidine-2-carboxylic acid, piperidine-3-carboxylic acid, and piperidine-4-carboxylic acid) and of the ethyl esters of three of them. Reaction kinetics were determined by the initial rate method, by spectrophotometric monitoring of the concentration of nitroso amino acid formed. The presence of the ? COO^? group in the amino acids opens a new mechanistic route for the nitrosation of the secondary amino group: a nitrosyl carboxylate formed initially acts as an internal nitrosating agent, resulting in intramolecular migration of ? N ? O from the carboxylate group to the secondary amino group. The observed order of the α?, β?, and γ-amino acids as regards the ease of N-nitrosation by this route is explained in terms of the relative energies of (a) the equatorial and axial orientations of the C_ring? C_carboxyl bond, and (b) the chair and boat forms of the piperidine ring. © 1994 John Wiley & Sons, Inc.     

Incidence of element distribution on the ultrasonic field of segmented annular arrays

Segmented annular arrays are sometimes used for 3D ultrasonic imaging. However, owing to their geometrical complexity, the acoustic field generated by this type of aperture has not been adequately described. In this work, a method based on the array factor approach is used to describe the field radiated by sector annular arrays. This approach allows one to analyse the influence on the field of several aperture parameters, such as the number of elements per annulus, size and spatial distribution of elements, etc. In addition, strategies to reduce grating lobes are presented.     

Signal-to-noise ratio enhancement based on the whitening transformation of colored structural noise

In the ultrasonic testing and evaluation of highly scattering materials (i.e. non-homogeneous media such as composites, layered and clad materials) structural noise is an important limitation to the visibility of flaw echoes. This noise cannot be reduced by conventional linear filtering or by time-averaging techniques. In order to enhance the defect-to-background noise ratio (SNR), many different algorithms have been developed over the years. This work analyzes three new strategies for SNR enhancement based on the whitening transformation of the colored structural noise. By using this transformation, the small spectral differences between noise and flaw echoes are exploited, thereby allowing an improvement in the visibility of the flaw.     

Achieving matrix consistency in AHP through linearization

Matrices used in the analytic hierarchy process (AHP) compile expert knowledge as pairwise comparisons among various criteria and alternatives in decision-making problems. Many items are usually considered in the same comparison process and so judgment is not completely consistent – and sometimes the level of consistency may be unacceptable. Different methods have been used in the literature to achieve consistency for an inconsistent matrix. In this paper we use a linearization technique that provides the closest consistent matrix to a given inconsistent matrix using orthogonal projection in a linear space. As a result, consistency can be achieved in a closed form. This is simpler and cheaper than for methods relying on optimisation, which are iterative by nature. We apply the process to a real-world decision-making problem in an important industrial context, namely, management of water supply systems regarding leakage policies – an aspect of water management to which great sums of money are devoted every year worldwide.     

A sequential-injection system for spectrophotometric determination of p-aminobenzoic acid in sunscreens

A sequential injection method is proposed for spectrophotometric determination of p-aminobenzoic acid (PABA) in cosmetic formulations. The method is based on diazotization of the analyte, coupling with 8-hydroxyquinoline, and the subsequent formation of a colored product. The experimental conditions used (coupling reagent, sandwich arrangement, volumes aspirated, propulsion flow rate, reaction coil length) were studied. Response of the sequential injection method were linearly dependent on concentrations up to 25 micro g mL(-1) and the detection limit was 2 micro g mL(-1). Throughput was 51 measurements per hour and a complete cycle, including three measurement per sample and a washing step, resulted in a sample throughput of 13.8 h(-1). Sunscreen formulations containing PABA were analyzed by the proposed method and validated by use of an HPLC procedure. The two methods gave comparable results. Yields ranged between 97-103% and good precision (RSD between 2 and 6%) was obtained. The accuracy, speed, and selectivity of the method make it suitable for safe application for quality-control of final products.     

Municipal Water Demand Forecasting: Tools for Intervention Time Series

This article introduces some approaches to common issues arising in real cases of water demand prediction. Occurrences of negative data gathered by the network metering system and demand changes due to closure of valves or changes in consumer behavior are considered. Artificial neural networks (ANNs) have a principal role modeling both circumstances. First, we propose the use of ANNs as a tool to reconstruct any anomalous time series information. Next, we use what we call interrupted neural networks (I-NN) as an alternative to more classical intervention ARIMA models. Besides, the use of hybrid models that combine not only the modeling ability of ARIMA to cope with the time series linear part, but also to explain nonlinearities found in their residuals, is proposed. These models have shown promising results when tested on a real database and represent a boost to the use and the applicability of ANNs.