Spectrophotometric determination of selenium(IV) with 5,5-dimethyl-1,3-cyclohexanedione

5,5-Dimethyl-1,3-cyclohexanedione (dimedone) reacts in acid aqueous solution with selenium(IV) to give a benzoxaselenol which has an absorption maximum at 313 nm with a molar absorptivity of 4.00 x 10(3) l.mole(-1).cm(-1). The compound is extractable into chloroform, to give a solution with an absorption maximum at 300 nm with a molar absorptivity of 3.77 x 10(3) l.mole(-1).cm(-1). The calibration graph is linear up to 30 ppm selenium, with a detection limit of 0.1 ppm in the final solutions. Of the various other ions tested, only iron(III) interferes at all concentrations but the addition of 1000 ppm fluoride will mask 50 ppm Fe(3+). The method has good reproducibility, with a relative standard deviation of 1.0% for pure solutions. The method has been applied to the analysis of fire-refined copper.     

Manganese-quinizarin-pyrazinecarboxylic acid mixed-ligand complex in aprotic media. Formation of a manganese(III)- semiquinone binuclear species

The monoanion of pyrazincarboxylic acid (PcA^–), the dianion of quinizarine (Qz⁼) and manganese(II) yield a soluble deep-blue complex in dimethylsulphoxide whose Mn^II:Qz⁼:PcA^– stoichiometry has been established as 1:1:1. This mixed-ligand complex is oxidized in two steps, each involving one equivalent of charge per complex present, as indicated by controlled-potential electrolyses done at +0.30V versus s.c.e. and +0.50 V versus s.c.e.. The association between the metal ion and the ligands prevail and the oxidations finally produce a dark-red complex which possesses the same stoichiometry as the original deep-blue species. In the latter the metal ion is present in oxidation state +3 with the quinizarine dianion as the corresponding semiquinone of the oxidized quinizarine. The monoanion of pyrazincarboxylic acid remains unchanged. Controlled-potential electrolysis at –0.20 V versus s.c.e. of a solution of the mixed-ligand complex indicates that it is binuclear, generating a Mn^II-Mn^IIImixed-valence species. The latter is, in turn, reduced at –1.60 V versus s.c.e. producing probably a mononuclear of manganese(II) species. If manganese(II) is combined with the semiquinone of quinizarine the metal ion exhibits themagnetic characteristic of manganese(III) and thesemiquinone is reduced to the quinizarine dianion, indicating that the mixed-ligand formed exhibits intramolecular charge-transfer. This is a good example of a binuclear species accumulating four oxidation equivalents after oxidation of both metal centers and the quinizarine ligands.     

Spectrophotometric determination of trace amounts of selenium with 4,5,6-triaminopyrimidine

4,5,6-Triaminopyrimidine reacts in acidic aqueous media with selenium (IV) to give a piazselenol which has an absorption maximum at 362 nm with a molar absorptivity of 1.72 x 10(4) 1.mole(-1).cm(-1). The compound is stable but not extractable into non-polar solvents. The calibration graph is linear up to 10 ppm of selenium, with a detection limit of 0.1 ppm in the sample solutions. Of the many different ions tested only iron (III) (in the presence of chloride) and tin (II) interfere. The method has good reproducibility, with a relative standard deviation of 1.5% for pure solutions. The application of this method to analysis of water and electrolytic copper is described.     

Redox chemistry of mononuclear manganese(II), binuclear manganese(III) and binuclear mixed manganese(II)-manganese(III) complexes with 3-aminopyrazine-2-carboxylate in dimethylsulphoxide

Summary Mn^II forms a yellow mononuclear species with the title ligand having a 12 stoichiometry and whose conditional stability constant is 8.9 × 10¹⁰ m ^–2. The c.v. of this complex shows an oxidation at +0.78V versus s.c.e. Controlled-potential electrolysis at +0.80V versus s.c.e. yields a binuclear species of Mn^III with a 12 metal:ligand stoichiometry.The addition of Mn^III(urea)₆(ClO₄)₃ to a solution of the ligand produces a mononuclear complex of Mn^III if the concentration of the metal ion is less than 1 mM. At higher concentrations a binuclear species is obtained. The latter is reduced in two steps, at +0.24 and –0.58 V versus s.c.e. Controlled-potential electrolysis at 0.0 V produces a dark green complex after the transfer of 0.5 equivalents of charge per mole of Mn. This binuclear L₂Mn^II-Mn^IIIL₂ mixed-valence complex can be obtained only by electrolysis of the binuclear L₂Mn^IIIMn^IIIL₂ species. Attempts to prepare the complex chemically were unsuccessful - the binuclear Mn^III species was obtained in every case.Author to whom all correspondence should be directed.     

Redox chemistry of 1,2-dihydroxynaphthalene, 1,2-naphthosemiquinone and 1,2-naphthoquinone and their complexes with manganese(II) and manganese(III)

Cyclic voltammetry and controlled-potential electrolyses were carried out on solutions of 1,2-naphthoquinone (1,2-NQ), 1,2-dihydroxynaphthalene [1,2-Di(OH)-Cat] and their reduction and oxidation products in dimethyl sulfoxide. A study of the interaction of these species with MnII and MnIII has been undertaken. The complexes MnIII(Cat2–)33–, MnII(Cat2–)22– and MnII-(Cat2–), where Cat2– represents the dianion of 1,2-dihydroxynaphthalene, were obtained in solution and the species MnIISQ has been detected only on the surface of the electrode. An attempt to obtain the latter complex quantitatively caused intramolecular charge-transfer yielding the more stable MnIII-catechol complex. The MnIII–semiquinone complex is unstable and the ligand is easily oxidized. The charge-transfer reaction between MnIII(Cat2–)33– and MnII(Cat2–)22– is observed at –0.54/–0.52 V versus s.c.e. whereas the MnII complexes are reduced beyond the electrochemical window (at a potential more negative than –2.00 V versus s.c.e). On the other hand, all these species are destroyed when the coordinated ligand is oxidized. U.v.-vis. spectroscopy and magnetic susceptibility measurements were performed on the compounds in solution in order to characterize and to confirm the oxidation states of the metal ion in the species.     

Electropolymerization of 2,3‐diaminophenol

The optima conditions to electrosynthesize poly(2,3‐diaminophenol) by electro‐oxidation of the monomer were determined, and the electrodeposits obtained characterized by electrochemical methods, UV‐vis, FTIR, conductivity and viscosity measurements. The influence of parameters such as electrolytical medium and electrochemical conditions on the electro‐oxidation of 2,3‐diaminophenol were also investigated. It has been established that appropriate deposits are obtained only when very anhydrous acetonitrile is used as solvent. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1698–1703, 2000     

A Two-Steps-Ahead Estimator for Bubble Entropy

Aims: Bubble entropy (bEn) is an entropy metric with a limited dependence on parameters. bEn does not directly quantify the conditional entropy of the series, but it assesses the change in entropy of the ordering of portions of its samples of length m, when adding an extra element. The analytical formulation of bEn for autoregressive (AR) processes shows that, for this class of processes, the relation between the first autocorrelation coefficient and bEn changes for odd and even values of m. While this is not an issue, per se, it triggered ideas for further investigation. Methods: Using theoretical considerations on the expected values for AR processes, we examined a two-steps-ahead estimator of bEn, which considered the cost of ordering two additional samples. We first compared it with the original bEn estimator on a simulated series. Then, we tested it on real heart rate variability (HRV) data. Results: The experiments showed that both examined alternatives showed comparable discriminating power. However, for values of 10<m<20, where the statistical significance of the method was increased and improved as m increased, the two-steps-ahead estimator presented slightly higher statistical significance and more regular behavior, even if the dependence on parameter m was still minimal. We also investigated a new normalization factor for bEn, which ensures that bEn=1 when white Gaussian noise (WGN) is given as the input. Conclusions: The research improved our understanding of bubble entropy, in particular in the context of HRV analysis, and we investigated interesting details regarding the definition of the estimator.     

Generalized Moment Theory and Bayesian Robustness Analysis for Hierarchical Mixture Models

In applications of Bayesian analysis one problem that arises is the evaluation of the sensitivity, or robustness, of the adopted inferential procedure with respect to the components of the formulated statistical model. In particular, it is of interest to study robustness with respect to the prior, when this latter cannot be uniquely elicitated, but a whole class Γ of probability measures, agreeing with the available information, can be identified. In this situation, the analysis of robustness consists of finding the extrema of posterior functionals under Γ. In this paper, we provide a theoretical framework for the treatment of a global robustness problem in the context of hierarchical mixture modeling, where the mixing distribution is a random probability whose law belongs to a generalized moment class Γ. Under suitable conditions on the functions describing the problem, the solution of this latter coincides with the solution of a linear semi-infinite programming problem.