Oxygen electrodes in fused salts: Investigation and mechanistic remarks on the system (Ni)CO2, O2/CO32− in molten (Na,K)NO3

A potentiometric investigation on the system (Ni)CO₂, O₂/CO₃^2? was carried out at 507–637 K in the (Na, K)NO₃ equimolar mixture containing carbonate ions in the range 10^?5≤[CO₃^2?]≤10^?2 mol kg^?1 and under a mixture of O₂ and CO₂ at variable partial pressure. The potential behaviour of the nickel electrodes was found largely dependent on the working temperature. At the highest tested temperature the system behaves irreversibly under potentiometric conditions and the potential was found to be independent of the oxygen concentration. At lower temperatures a large irreversibility of the system was still found, while a certain dependence (different from the theoretical one) of the potential on both oxygen and carbon dioxide partial pressure was experimentally demonstrated. The entire set of results was interpreted on the basis of the two following mechanistic models involving, in the potential determining step, solid species formed on the electrode surface by contact with the melt. Mechanism 1 (high temperature)

Mechanism 2 (low temperature)

     

Amperometric detection of sulfur-containing compounds in alkaline media

A copper-based chemically modified glassy carbon electrode (GC-Cu) was developed to be used as an amperometric sensor for electrochemically detecting several sulfur-containing compounds in alkaline media. Under optimised flow injection conditions the calibration curves for sulfite, sulfide, thiosulfate, cysteine, cystine, etc., were linear over three orders of magnitude of concentration. Detection limits were of the order of 0.04-1.5 microM. A simple and rapid method for determining sulfite in red and white wines by anion-exchange chromatography with electrochemical detection is described.     

CEMS studies of Sn−O thin films prepared by thermal evaporation

Thermal evaporated thin Sn?O films subjected to annealing treatments in air in the range 473–1173 K and in Ar in the range 473–773 K followed by annealings in air up to 1373 K were studied by CEMS (Conversion Electron Mössbauer Spectroscopy). Complementary Mössbauer and X-ray measurements were also performed on SnO powder that underwent the same series of annealings. The presence of the intermediate oxide Sn₃O₄ was detected. A temptative hyperfine characterization for the Sn²⁺ site in Sn₃O₄ is given.     

Evolution under thermal annealing of Mn-doped iron disilicides obtained by ball milling

Phase formation in the Mn doped $\upbeta $ -FeSi₂ system (Fe_1???x Mn _x Si₂, with 0.00 ≤?x?≤ 0.24) was studied using X-ray diffraction and Mössbauer spectroscopy. Samples were prepared by the simultaneous mill of pure Si, Mn and Fe under Ar atmosphere followed by an annealing at 1,123 K during 4 h at 1 × 10^???7 Torr. After milling, an admixture of $\upbeta $ -FeSi₂, $\upalpha $ -FeSi₂ and $\upvarepsilon $ -FeSi phases was present while $\upalpha $ -FeSi₂ disappeared after annealing, resulting $\upbeta $ -FeSi₂ the main phase. Depending on Mn concentration, small amounts of $\upvarepsilon $ -FeSi and Si segregation were also observed. A preferential substitution of Fe atoms by Mn ones in the Fe_II site of the $\upbeta $ -FeSi₂ regular lattice was inferred from the Mössbauer results.     

Peri- and enantioselectivity of thermal, scandium-, and [pybox/scandium]-catalyzed Diels-Alder and hetero-Diels-Alder reactions of methyl (E)-2-oxo-4-aryl-butenoates with cyclopentadiene

The cycloaddition between methyl (E)-2-oxo-4-aryl-3-butenoates (2 a-d) and cyclopentadiene, in addition to the expected normal Diels-Alder (DA) adducts endo-3 a-d and exo-4 a-d, gives the less expected endo-5 a-d products of the [4+2] hetero-Diels-Alder (HDA) reaction in which the alpha-ketoester behaves as a heterodiene. If a comparison is made between the thermal and the scandium(III) triflate-catalyzed conditions, the periselectivity changes and whereas under thermal conditions the main products are those from the DA reaction (3 a-d), in the presence of Sc(OTf)3 (OTf=triflate), the HDA products 5 a-d become largely predominant. The reactions are enantioselectively catalyzed by the scandium(III) triflate complex of (4'S,5'S)-2,6-bis[4'-(triisopropylsilyl)oxymethyl-5'-phenyl-1',3'-oxazolin-2'-yl]pyridine (1) and both the DA and the HDA products are obtained with excellent enantiomeric excess, up to >99% ee. The X-ray crystallographic structure determination of 5 c assigns it the 4R,4aS,7aR absolute configuration. The thermal retro-Claisen rearrangement of 3 c into (4R,4aS,7aR)-5 c allows the correlation of their absolute configuration, and 3 c has therefore the 2R,3R configuration. By analogy the same absolute configuration can be assigned to 3 a,b,d and 5 a,b,d, and the stereospecific thermal Claisen rearrangement of the optically active 5 a,b,d into 3 a,b,d completes the correlation between their absolute configuration. The [3,3]-sigmatropic rearrangements can be easily carried out under catalytic conditions with scandium(III) triflate, which promotes the equilibration between 3 a-d and 5 a-d, with a different degree of enantioselectivity characterizing the process starting from 3 a-d or 5 a-d. The unambiguous attributions of the configuration to the products allows us to propose a rationale of the stereochemical outcome of the catalyzed cycloaddition and to investigate the reaction mechanism of the competing DA and HDA reactions and shifts in products distribution by acid catalysis.     

Determination of Theophylline at a Cysteic Acid Modified Glassy Carbon Electrode

A glassy carbon electrode modified with electrodeposited cysteic acid is proposed for the determination of theophylline. Estimation of the linear range, calibration function and limit of detection, as well as determination in real samples such as medication tablets were performed.