Spectrofluorimetric determination of thiabendazole in a micellar medium

A sensitive, selective Spectrofluorimetric method has been developed for the determination of thiabendazole in a sodium dodecylsulphate micellar medium. The method features a linear determination range of 0.020-15 g and a detection limit of 0.2 ng/ml, with a relative standard deviation of less than 3%. By using a straightforward extraction procedure with ethyl acetate and hydrochloric acid, the method provided average thiabendazole recoveries above 96% from apple, pear and potato samples.     

Experimental evidence for the existence of non-exo-anomeric conformations in branched oligosaccharides: NMR analysis of the structure and dynamics of aminoglycosides of the neomycin family

It is commonly known that the exo-anomeric effect is a major factor governing the conformational behavior of naturally occurring oligosaccharides. Conformational flexibility in these molecules mainly concerns the aglycon psi angle since phi is restricted by this stereo-electronic effect. In fact, to the best of our knowledge no case of a natural glycoside adopting a non-exo-anomeric conformation in solution has yet been reported. With respect to the flexibility among naturally occurring carbohydrates, branched type oligosaccharides including sugar residues glycosidated at contiguous positions (such as blood type carbohydrate antigens Lewis X) have been considered as the paradigm of rigid saccharides--the rigidity being enhanced by van der Waals interactions. Herein, we demonstrate unambiguously that both common beliefs are not to be generalized. For example in neomycin B, a branched oligosaccharide antibiotic, a large number of non-exo-anomeric conformations was detected in solution for the first time in naturally occurring sugars. This unusual behavior is attributed to branching. Here, polar contacts between non-vicinal sugar units lead to an enhanced flexibility of the ribose glycosidic torsion phi. The influence of sugar flexibility on RNA recognition will also be discussed.     

Valence Ionization Energies of Hydrocarbons

Experimental valence ionisation energies of 143 hydrocarbons, C_nH_m, have been determined from their He (IIα) (40.80 eV) and He (Iα) (21.22 eV) excited photoelectron spectra. Ionization energies, usually up to 26 eV are given in tabular form for 108 hydrocarbons with n≤6, together with their (tentative) assignment. The ionization energies up to approximately 25 eV, of 35 selected hydrocarbons with 7≤n≤10, are presented by means of their He (IIα) photoelectron spectra.     

Kinetic analysis of thermal decomposition of praseodymium(III) nitrate hexahydrate

The kinetics of thermal decomposition of praseodymium(III) nitrate hexahydrate was studied by using isothermal and dynamic thermogravimetric techniques. Kinetic analysis of the isothermal data with respect to various solid-state reaction models showed that the reaction is best described by phase boundary-controlled and random nucleation models. Kinetic analysis of the dynamic TG curves was discussed and a critical comparison was made of two integral methods, that of Coats and Redfern and that of Ozawa. The results showed that the Ozawa method gives a better correlation, and the results are in good agreement with those obtained under isothermal thermogravimetric conditions.

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Zusammenfassung Mittels isothermer und dynamischer thermogravimetrischer Methoden wurde die Kinetik der thermischen Zersetzung des Hexahydrates von Praseodymnitrat untersucht. Eine kinetische Auswertung der isothermen Meßdaten unter Anwendung verschiedener Feststoffreaktionsmodelle ergab, daß die Reaktion am besten durch ein phasengrenzenkontrolliertes Randomkeimbildungsmodell beschrieben werden kann. Die kinetische Auswertung der dynamischen TG-Kurven wurde diskutiert und ein kritischer Vergleich zwischen zwei Integriermethoden, der von Coats und Redfern und der von Ozawa, angestellt. Die Betrachtungen ergaben, daß die Methode von Ozawa eine bessere Korrelation liefert und daß die Resultate gut mit denen der isothermen thermogravimetrischen Messungen übereinstimmen.

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On the Concentration Dependence of the Cluster Fractal Dimension in Colloidal Aggregation

We have undertaken the task to calculate, by means of extensive numerical simulations and by different procedures, the cluster fractal dimension (d) of colloidal aggregates at different initial colloid concentrations. Our first approach consists in obtaining d from the slope of the log-log plots of the radius of gyration versus size of all the clusters formed during the aggregation time. In this way, for diffusion-limited colloidal aggregation, we have found a square root type of increase of the fractal dimension with concentration, from its zero-concentration value: d = d⁰ _f + a , with d⁰ _f = 1.80 ± 0.01, a = 0.91 ± 0.03 and = 0.51 ± 0.02, and where is the volume fraction of the colloidal particles. In our second procedure, we get the d via the particle-particle correlation function g_cluster(r) and the structure function S_cluster(q) of individual clusters. We first show that the stretched exponential law g_cluster(r) = Ar^{d –3}e^–(r/) gives an excellent fit to the cutoff of the g(r). Here, A, a and are parameters characteristic of each of the clusters. From the corresponding fits we then obtain the cluster fractal dimension. In the case of the structure function S_cluster (q), using its Fourier transform relation with g_cluster(r) and introducing the stretched exponential law, it is exhibited that at high q values it presents a length scale for which it is linear in a log-log plot versus q, and the value of the d extracted from this plot coincides with the d of the stretched exponential law. The concentration dependence of this new estimate of d, using the correlation functions for individual clusters, agrees perfectly well with that from the radius of gyration versus size. It is however shown that the structure factor S(q) of the whole system (related to the normalized scattering intensity) is not the correct function to use when trying to obtain a cluster fractal dimension in concentrated suspensions. The log-log plot of S(q) vs. q proportions a value higher than the true value. Nevertheless, it is also shown that the true value can be obtained from the initial slope of the particle-particle correlation function g(r), of the whole system. A recipe is given on how to obtain approximately this g(r) from a knowledge of the S(q), up to a certain maximum q value.     

Some contributions to the theory of cyclic quartic extensions of the rationals

K is a cyclic quartic extension of Q iff $\text{K = Q((rd + p d}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext><mtext>)</mtext><mtext>1</mtext><mtext>2</mtext>}}\text{)}$, where d > 1, p and r are rational integers, d squarefree, for which p² + q² = r²d for some integer q. Following a paper of A. A. Albert we show that the absolute discriminant, $\text{d(}\text{K}\text{Q}\text{)}$, of the general cyclic quartic extension is given by $\text{d(}\text{K}\text{Q}\text{) = (W}{}^2\text{d}{}^2\text{)}$ for an explicitly computable rational integer W. We next find that the relative discriminant, $\text{d(}\text{K}\text{F}\text{)}$, is given by $\text{d(}\text{K}\text{F}\text{) = (W d}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)}$, where $\text{F = Q (d}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)}$ is K′s uniquely determined quadratic subfield. We use this last result in conjunction with Corollary 3, page 359, of Narkiewicz's “Elementary and Analytic Theory of Algebraic Numbers” (PWN-Polish Scientific Publishers, 1974) to establish the following Theorem 1: If the (wide) class number of$\text{F = Q(d}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)}$is odd then every cyclic quartic extensionKofQcontainingFhas a relative integral basis overF. We give a second, more organic, proof of Theorem 1 which also allows us to prove the following converse result, namely Theorem 2: Suppose the quadratic fieldFis contained in some cyclic quartic extension ofQand suppose thatFhas even (wide) class number. There then is a cyclic quartic extensionKofQcontainingFsuch thatKhas no relative integral basis overF.     

Differentiation of Spanish brandies according to their metal content

Eleven metals, namely, aluminium, calcium, cadmium, copper, iron, lead, magnesium, manganese, potassium, sodium and zinc were determined in twenty samples of Sherry brandies and twelve samples of Penedés brandies by applying atomic spectrometry techniques. Flame atomic absorption spectrometry was used for quantitating calcium, copper, iron, magnesium, manganese and zinc; atomic emission spectrometry to determine potassium and sodium; and graphite furnace atomic absorption spectrometry to analyse aluminium, cadmium and lead. A chemometric approach was followed to study the discrimination between brandies from Sherry or Penedés according to the metal profile.     

Column preconcentration of organotin with tropolone-immobilized and their determination by electrothermal atomization absorption spectrometry

A method for the determination of total organic tin from marine water samples by electrothermal atomization absorption spectrometry (ETAAS) is described. Samples are previously preconcentrated with a chelating molecule (tropolone) impregnated on a macroporous polymer (Amberlite XAD-2). The graphite furnace programme and preconcentration parameters were optimized. Calibration and addition graphs were performed. Sensitivity obtained with this procedure was 13 ng l⁻¹. Relative standard deviation was always >10% and analytical recovery were satisfactory, 100%. Some possible interferences were investigated, having no problems with this factor. This procedure allows the distinction between organotin compounds and inorganic tin IV, since the latter is not retained on the column.     

Gravimetric determination of uranyl ion with 2-hydroxy-1-naphthaldehyde /2H-1N/

The precipitation of uranyl ion with 2-hydroxy-1-naphthaldehyde /2H–1N=HL/ was studied. The solid complex /orange crystals/ was characterized by IR, UV-Vis spectra. Uranium was determined as U₃O₈ after calcination of the complex at 850°C /37.78% U experimental, 36.64% U calculated for C₂₂H₁₄O₆U, UO₂L₂/. Using a statistical experimental design, the best conditions for quantitative precipitation were obtained. A gravimetric method for the determination of UO ₂ ²⁺ is proposed by weighing the complex after drying at 110°C.On leave from Instituto de Química, U.N.A.M.     

Plasma chemistry of NO in complex gas mixtures excited with a surfatron launcher

The plasma chemistry of NO has been investigated in gas mixtures with oxygen and/or hydrocarbon and Ar as carrier gas. Surface wave discharges operating at microwave frequencies have been used for this study. The different plasma reactions have been analyzed for a pressure range between 30 and 75 Torr. Differences in product concentration and/or reaction yields smaller than 10% were found as a function of this parameter. The following gas mixtures have been considered for investigation: Ar/NO, Ar/NO/O2, Ar/NO/CH4, Ar/CH4/O2, Ar/NO/CH4/O2. It is found that NO decomposes into N2 and O2, whereas other products such as CO, H2, and H2O are also formed when CH4 and O2 are present in the reaction mixture. Depending on the working conditions, other minority products such as HCN, CO2, and C2 or higher hydrocarbons have been also detected. The reaction of an Ar/NO plasma with deposits of solid carbon has also been studied. The experiments have provided useful information with respect to the possible removal of soot particles by this type of plasma. It has been shown that carbon deposits are progressively burned off by interaction with the plasma, and practically 100% decomposition of NO was found. Plasma intermediate species have been studied by optical emission spectroscopy (OES). Bands and/or peaks due to N2*, NO*, OH*, C2*, CN*, CH*, or H* were detected with different relative intensities depending on the gas mixture. From the analysis of both the reaction products and efficiency and the type of intermediate species detected by OES, different plasma reactions and processes are proposed to describe the plasma chemistry of NO in each particular mixture of gases. The results obtained provide interesting insights about the plasma removal of NO in real gas exhausts.