Spectrophotometric analysis of mixtures of two components with extensively or completely overlapping spectra by the H-point standard additions method

Summary This paper discusses the requirements and scope of application of the H-Point Standard Additions Method (HPSAM) for resolving binary mixtures. Various mixtures of dyes with extensively or completely overlapping spectra were assayed with accurate, precise results. The method thus developed was also applied to the spectrophotometric resolution of binary mixtures of phenol and o-cresol, the absorption bands of which lie in similar positions and feature similar absorptivities.     

Polynomial first integrals of quadratic vector fields

We classify all quadratic polynomial differential systems having a polynomial first integral, and provide explicit normal forms for such systems and for their first integrals.     

On some open problems in planar differential systems and Hilbert’s 16th problem

This review paper contains a brief summary of topics and concepts related with some open problems of planar differential systems. Most of them are related with 16th Hilbert problem which refers to the existence of a uniform upper bound on the number of limit cycles of a polynomial system in function of its degree. These open problems are proposed as open questions throughout the text. Finally, an extensive bibliography, which does not intend to be exhaustive, is also given.     

Thermal Stability of Sol-Gel Hydrotalcites

Sol-Gel Hydrotalcites were synthesized using magnesium ethoxide and variable aluminum sources: aluminum acetilacetonate, aluminum chloride, aluminum nitrate and aluminum sulfate; in all cases, the gelation was done at pH 10. X-ray diffraction studies show that the crystallinity depends on the aluminum precursor used. The crystallinity was found in the order aluminum acetylcetonate>aluminum chloride>aluminum nitrate>aluminum sulfate. The precursor determines the sintering behavior as well.     

Characterization of UO 2 2+ exchanged Y zeolite

The present study discusses the incorporation of uranyl ion into Y-zeolite framework. The UO ₂ ²⁺ sorption was measured by neutron activation analyses. The Y-zeolite framework distorts in response to the cations present in the structure. Hence, depending on the amount and the location of the exchanged cations, the features of the X-ray diffraction pattern may vary. From the Rietveld analysis of these patterns, the positions occupied by the UO ₂ ²⁺ cations in the zeolite network were determined.     

An extension of Ekeland's variational principle to locally complete spaces

We prove an extension of Ekeland's variational principle to locally complete spaces which uses subadditive, strictly increasing continuous functions as perturbations.