Spectrophotometric determination of BrO3− using the 1,2,4-trihydroxyanthraquinone-3-carboxylic acid as reagent

The reaction between the 1,2,4-trihydroxyanthraquinone-3-carboxylic acid and BrO₃^? in hydrochloric acid medium has been studied spectrophotometrically. The red color of the oxidation product serves as basis for a new method for determination of traces of BrO₃^?, measuring the absorbance at 520 nm. The influence of foreign ions on the determination, the stoichiometry of the reaction, and the nature of the oxidation product, have been reported.     

Synthèse et étude structurale du N-méthyl-aza-10-bicyclo[4,3,1]decane-8-spiro-5′-hydantoïne

We describe for the first time the synthesis of the spirohydantoine derivative of the N-methylhnomogranatane (N-methyl-10-azabicyclo-[4,3,1]decane) with a spiranic center at C-8. We have assigned the configuration α to this center. The conformation and associations of this molecule in the solid state have been studied and compared with those of the lower homologues. Some Mannich bases have also been prepared.     

Multichannel integrating flame photometer

A new high-speed optical chopping and demodulation system is described for use in flame photometry. Methods are described which allow integration of signal and noise over many optical pulses of flame energy. The method has been used to develop a direct-reading instrument for sodium, potassium and calcium. With this method, 4 signal channels are integrated simultaneously. With the integration technique, ordinary flame noise is significantly reduced. By using an internal standard, all signal channels are referenced to known energy levels regardless of flame variations.     

A chemometric sensor for determining sulphaguanidine residues in honey samples

The inclusion complex of sulphaguanidine (SGN) in β-cyclodextrin has been investigated. To avoid the problem of the low solubility of β-cyclodextrin in water, solutions of β-cyclodextrin in urea have been used. A 1:1 stoichiometry and an association constant of 450 M⁻¹ have been established for the complex. A new spectrofluorimetric method has been developed for the determination of SGN residues in honey samples. This sulphonamide is widely employed for honey treatment. The method for the determination is based on second-order multivariate calibration, applying parallel factor analysis (PARAFAC). No previous separation or samples pre-treatment were required. The calibration solutions were prepared in water, with concentrations in the range from 0.02 to 0.20 μg mL⁻¹ for SGN. The use of the second-order calibration method in the standard addition mode, using the excitation-emission matrices (EEMs) as analytical signal, allowed its determination in honey samples, even in the presence of interferences, with satisfactory results. The proposed procedure was validated by comparing the obtained results with a HPLC method, with satisfactory results for the assayed method.     

An electronic tongue using potentiometric all-solid-state PVC-membrane sensors for the simultaneous quantification of ammonium and potassium ions in water

The simultaneous determination of NH₄⁺ and K⁺ in solution has been attempted using a potentiometric sensor array and multivariate calibration. The sensors used are rather non-specific and of all-solid-state type, employing polymeric (PVC) membranes. The subsequent data processing is based on the use of a multilayer artificial neural network (ANN). This approach is given the name "electronic tongue" because it mimics the sense of taste in animals. The sensors incorporate, as recognition elements, neutral carriers belonging to the family of the ionophoric antibiotics. In this work the ANN type is optimized by studying its topology, the training algorithm, and the transfer functions. Also, different pretreatments of the starting data are evaluated. The chosen ANN is formed by 8 input neurons, 20 neurons in the hidden layer and 2 neurons in the output layer. The transfer function selected for the hidden layer was sigmoidal and linear for the output layer. It is also recommended to scale the starting data before training. A correct fit for the test data set is obtained when it is trained with the Bayesian regularization algorithm. The viability for the determination of ammonium and potassium ions in synthetic samples was evaluated; cumulative prediction errors of approximately 1% (relative values) were obtained. These results were comparable with those obtained with a generalized regression ANN as a reference algorithm. In a final application, results close to the expected values were obtained for the two considered ions, with concentrations between 0 and 40 mmol L^–1.     

Synthesis, structure, spectroscopy and redox chemistry of square-planar nickel(II) complexes with tetradentate o-phenylenedioxamidates and related ligands

A series of four-coordinate square-planar nickel(II) complexes of o-phenylenebis(N'-methyloxamidate)(L1) and related o-phenylene(N'-methyloxamidate)oxamate (L2) and o-phenylenebis(oxamate)(L3) tetradentate ligands have been synthesized and characterized structurally, spectroscopically and electrochemically. The parent nickel(II)-L1 complex presents an intense MLCT band in the UV region (lambda max = 357 nm) and a distinctive 1 s --> 4p CT satellite in the Ni K-edge XANES spectrum (E = 8339.2 eV). These features together with the short Ni-N(amidate) bond lengths (1.85-1.93 A) as revealed by the analysis of the Ni K-edge EXAFS spectrum and confirmed by single-crystal X-ray diffraction are typical of square-planar low spin (S = 0) Ni(II) ions. The dianionic nickel(II) complexes, [Ni(II)L(i)](2-)(i = -3), experience two redox processes in acetonitrile at 25 degrees C. The first redox process, at moderately low potentials (E1 = 0.12-0.52 V vs. SCE), is a reversible one-electron metal-centered oxidation to the corresponding monoanionic nickel(III) complexes, [Ni(III)L(i)]-. The second redox process, at relatively high potentials (E2 = 0.86-1.04 V vs. SCE), is a quasireversible to irreversible one-electron oxidation largely centered on the o-benzenediamidate fragment of the non-innocent ligand, yielding the corresponding neutral nickel(iii) complexes with a o-benzosemiquinonediimine pi-cation radical ligand, [Ni(III)(L(i))*+]. The singly and doubly oxidized species of the parent nickel(II)-L1 complex have been prepared by chemical oxidation and characterized spectroscopically in acetonitrile at -40 degrees C. The stable singly oxidized nickel(III)-L1 species presents an intense LMCT band in the NIR region (lambda max = 910 nm) and a rhombic X-band EPR spectrum (g1 = 2.193, g2 = 2.080 and g3 = 2.006) characteristic of square-planar low spin (S = 1/2) Ni(III) ions. The unstable double oxidized nickel(III)-L1 pi-cation radical species exhibits a rather intense visible band (lambda max = 645 nm) that is tentatively assigned as a MLCT transition from the Ni(III)-benzosemiquinone type ground state to the Ni(IV) excited state.     

A spectrophotometric method to determine the siderophore production by strains of fluorescent Pseudomonas in the presence of copper and iron

A simple procedure to determine levels of siderophore production by strains of Pseudomonas, particularly the Avm strain is described. Bacterial cells were incubated for 24 h in iron-rich (RM) and iron-limiting conditions (RM-Fe) with and without 6 and 60 μM of CuSO₄. Cells grown under iron-limiting conditions developed a green color even in the presence of Cu. The spent media supernatants from the Avm cells grown in RM-Fe medium showed a maximum peak of absorbance at 400 nm, which suggest that this strain produced a single type of siderophore. The presence of 60 μM of CuSO₄ in the cultures did not interfere with the detection of siderophores in the spent media. Clear supernatants obtained from cultures of 10 fluorescent Pseudomonas were diluted 1 to 10 in deionized water and the absorption at 400 nm was determined. The results demonstrated the clear discriminating capacity of this highly practical procedure to categorize a great number of fluorescent Pseudomonas strains by the range of siderophore production.     

Poly(ethylene terephthalate) copolymers containing 5‐nitroisophthalic units. III. Methanolytic degradation

The methanolysis of poly(ethylene terephthalate) (PET) copolymers containing 5‐nitroisophthalic units was investigated. Random copolyesters containing 10 and 30 mol % of such units were prepared via a two‐step melt copolycondensation of bis(2‐hydroxyethyl) terephthalate (BHET) and bis(2‐hydroxyethyl) 5‐nitroisophthalate (BHENI) in the presence of tetrabutyl titanate as a catalyst. First, the susceptibility of these two comonomers toward methanolysis was evaluated, and their reaction rates were estimated with high‐performance liquid chromatography. BHENI appeared to be much more reactive than both BHET and bis(2‐hydroxyethyl) isophthalate. The methanolysis of PET and the copolyesters was carried out at 100 °C, and the degradation process was followed by changes in the weight and viscosity, gel permeation chromatography, differential scanning calorimetry, and ¹H and ¹³C NMR spectroscopy. The copolyesters degraded faster than PET, and the rate of degradation increased with the content of nitrated units. The products resulting from methanolysis were concluded to be dimethyl terephthalate, dimethyl 5‐nitroisophthalate, ethylene glycol, and small, soluble oligomers. For both PET and the copolyesters, an increase in crystallinity was observed during the degradation process, indicating that methanolysis preferentially occurred in the amorphous phase. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 76–87, 2002     

Putting error bars on the ab initio theoretical estimates of the magnetic coupling constants: the parent compounds of superconducting cuprates as a case study

The influence of the basis set size and computational method in the calculation of the magnetic coupling constant J is evaluated using a series of cuprate superconductor parent compounds as a case study. The variational DDCI method and an iterative modification, the IDDCI method, are tested, as well as the perturbative CASPT2 method, with two different reference wave functions. Results show that the DDCI magnetic coupling constant is in rather good agreement with the experiment, although it shows a moderate basis set dependency. The IDDCI results are less dependent on the size of the basis set, but slightly overestimate the magnetic coupling constant. CASPT2 results are nearly independent of the chosen basis set. With a minimal active space values are obtained that are about 20% smaller than the DDCI results. The experimental coupling constant can be reproduced when an extended reference wave function is used.