The combined effect of photodegradation and stress cracking in polystyrene

This work aims to analyze the effects of photodegradation on the stress cracking resistance of polystyrene. Injection moulded samples were exposed to the ultraviolet light for various times in the laboratory prior to solvent contact. The bars were then stressed in a tensile testing machine under the presence of butanol. During this period the stress relaxation was monitored and the ultimate properties were evaluated after selected periods of stress cracking. Complementary tests were done by size exclusion chromatography and by scanning electron microscopy. The results indicated that butanol causes significant modification in polystyrene, with extensive surface crazing as well as reduction in mechanical properties. This is intensified under higher mechanical stress. The previous degraded samples showed a higher level of stress relaxation and a greater loss in tensile strength in comparison to the undegraded ones. The synergist action of photodegradation and stress cracking in polystyrene may be a consequence of the chemical changes caused by oxidation like the formation of polar chemical groups and the reduction in molecular weight.     

Nickel determination in osteoblast-like cell culture medium by adsorptive cathodic stripping voltammetry with a mercury microelectrode

The purpose of this study is to investigate the influence of nickel, which is an alloying element in commonly used metallic biomaterials, on the biomaterials mineralization process. An electrochemical method was developed to quantify this metal ion in osteoblast-like cell culture medium (OST) by performing adsorptive cathodic stripping voltammetry (CSV) with dimethylglyoxime (DMG) at a mercury film microelectrode (MFM). The optimized analytical conditions and the square-wave CSV parameters for the analysis are: DMG concentration: 5.00 × 10⁻⁴ mol L⁻¹; ammonium chloride buffer: 0.10 mol L⁻¹ (pH 9.2); frequency: 50 Hz, amplitude 20 mV; step: 2 mV; adsorption time: 10 s, deposition potential: −0.70 V and reduction potential: −1.20 V. The limit of detection was 7.70 × 10⁻⁹ mol L⁻¹ for an adsorption time of 10 s. The results achieved by CSV using the MFM were compared to those obtained by atomic absorption spectrometry (AAS) to ensure the reliability of the electrochemical method. The mineralization process was evaluated by biochemical and histochemical assays.     

Thermodynamic properties of quinoxaline-1,4-dioxide derivatives: a combined experimental and computational study

The mean (N-O) bond dissociation enthalpies were derived for three 2-methyl-3-(R)-quinoxaline 1,4-dioxide (1) derivatives, with R = methyl (1a), ethoxycarbonyl (1b), and benzyl (1c). The standard molar enthalpies of formation in the gaseous state at T = 298.15 K for the three 1 derivatives were determined from the enthalpies of combustion of the crystalline solids and their enthalpies of sublimation. In parallel, accurate density functional theory-based calculations were carried out in order to estimate the gas-phase enthalpies of formation for the corresponding quinoxaline derivatives. Also, theoretical calculations were used to obtain the first and second N-O dissociation enthalpies. These dissociation enthalpies are in excellent agreement with the experimental results herewith reported.     

Microdetermination of phosphate with EDTA

An indirect microdetermination of phosphate via EDTA titration is described, which can be applied to minerals, soils, fertilizers, biological samples, drugs and organo-phosphorus compounds. The method is based on the precipitation of phosphate as the very insoluble silver orthophosphate (K_sp = 1.3 × 10^?20), dissolution of this precipitate in a solution of potassium cyanonickelate and titration of the nickel displaced by silver. The phosphate content is obtained indirectly by multiplying the number of ml of the titrant by a factor. The method takes about an hour after the phosphate is brought into solution. The accuracy is about 1% for samples containing 5–50 mg PO₄^3? and about 3–5% for samples with 100 to 5000 μg PO₄^3?.     

Di­chloro­bis(1‐propyl­imidazolidine‐2‐thione‐κS)­cobalt(II)

The crystal structure of the title compound, [CoCl₂(C₆H₁₂N₂S)₂], consists of monomer units of a Co^II atom coordinated to two 1‐propyl­imidazolidine‐2‐thione ligands and to two chloride ions. The heterocyclic thione ligand is monodentate and coordinated to the metal through the thione S atom. The environment around the Co^II atom is a slightly distorted tetrahedron. The Co—S bond lengths are 2.341 (2) and 2.330 (2) Å, and the Co—Cl bond lengths are 2.234 (2) and 2.238 (2) Å. The most important point of distortion is the S—Co—S bond angle of only 97.83 (8)°. Intramolecular classical hydrogen bonds are found between the chloride ions and the N—H groups. Additionally, intra‐ and intermolecular non‐classical hydrogen bonds are found.     

Micro-determination of potassium with EDTA

Summary A micro-determination method for potassium with EDTA is described. The perchlorate precipitate is reduced to chloride by heating the perchlorate with ammonium chloride, and the chloride is precipitated as silver chloride. This precipitate is dissolved in an ammoniacal solution of potassium tetracyanonickelate. By dissolution, two atoms of silver (or chlorine) displace one of nickel. The freed nickel is titrated with, EDTA and potassium is thus determined indirectly. The method is much more rapid than the classical gravimetric procedure, and the results are satisfactory.

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Zusammenfassung Eine Mikromethode zur Bestimmung von Kalium mit ÄDTA wird beschrieben. Der Perchloratniederschlag wird durch Erhitzen mit Ammoniumchlorid reduziert und das Chlorid mit Silber gefällt. Dieser Niederschlag wird in einer ammoniakalischen Lösung von Kaliumtetracyanonickelat aufgelöst, wobei zwei Atome Silber ein Atom Nickel verdrängen. Das freigesetzte Nickel wird mit ÄDTA titriert und so das Kalium indirekt bestimmt. Das Verfahren ist rascher als die klassische gravimetrische Methode. Seine Resultate sind befriedigend.

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Résumé On décrit une méthode de microdosage du potassium par l'EDTA. En chauffant le perchlorate avec du chlorure d'ammonium, on réduit le précipité de perchlorate en chlorure que l'on précipite à l'état de chlorure d'argent. On dissout ce précipité dans une solution ammoniacale de nickel-II tétracyanure de potassium. La mise en solution s'accompagne du déplacement d'un atome de nickel par deux atomes d'argent. On titre le nickel libéré par l'EDTA et l'on dose ainsi le potassium indirectement. La méthode est beaucoup plus rapide que le procédé gravimétrique habituel et les résultats sont satisfaisants.

     

Convergence of consistent and inconsistent schemes for fractional diffusion problems with boundaries

This paper provides iterative construction of a common solution associated with a class of equilibrium problems and split convex feasibility problems. In particular, we are interested in the equilibrium problems defined with respect to the pseudomonotone and Lipschitz-type continuous equilibrium problem together with the generalized split null point problems in real Hilbert spaces. We propose an iterative algorithm that combines the hybrid extragradient method with the inertial acceleration method. The analysis of the proposed algorithm comprises theoretical results concerning strong convergence under suitable set of constraints and numerical results concerning the viability of the proposed algorithm with respect to various real-world applications.

     

α-Sober spaces via the orthogonal closure operator

Each ordinal equipped with the upper topology is a T ₀-space. It is well known that for =2 the reflective hull of in Top₀ is the subcategory of sober spaces. Here, we define -sober space for each 2 in such a way that the reflective hull of in Top₀ is the subcategory of -sober spaces. Moreover, we obtain an order-preserving bijective correspondence between a proper class of ordinals and the corresponding (epi)reflective hulls. Our main tool is the concept of orthogonal closure operator, first introduced in [12].The author acknowledges financial support from Instituto Politécnico de Viseu and from Centro de Matemática da Universidade de Coimbra.     

Highly Efficient and Practical Syntheses of Lavendamycin Methyl Ester and Related Novel Quinolindiones

The novel 7-(N-formyl-, 7-(N-acetyl-, and 7-(N-isobutyrylamino)-2-methylquinoline-5,8-diones were synthesized in excellent overall yields in three steps via the nitration of the commercially available 8-hydroxy-2-methylquinoline followed by a reduction-acylation step and then oxidation. Acid hydrolysis of 7-(N-acetylamino)-2-methylquinoline-5,8-dione (14a) afforded the novel 7-aminoquinoline-5,8-dione 7 in excellent yields. Due to our efficient preparation of dione 14a, we now report a short and practical method for the total synthesis of the potent antitumor agent lavendamycin methyl ester (1b) with an excellent overall yield.