Deposição eletrolítica catódica e anódica simultâneas para minimizar interferências de cobre e chumbo na determinação espectrofotométrica de cádmio em água e alimentos via reação com Verde de Malaquita e iodeto

Simultaneous electrolytic deposition is proposed for minimization of Cu2+ and Pb2+ interferences on automated determination of Cd2+ by the Malachite Green-iodide reaction. During electrolysis of sample in a cell with two Pt electrodes and a medium adjusted to 5% (v/v) HNO3 + 0.1% (v/v) H2SO4 + 0.5 mol L-1 NaCl, Cu2+ is deposited as Cu on the cathode, Pb2+ is deposited as PbO2 on the anode while Cd2+ is kept in solution. With 60 s electrolysis time and 0.25 A current, Pb2+ and Cu2+ levels up to 50 and 250 mg L-1 respectively, can be tolerated without interference. With on-line extraction of Cd2+ in anionic resin minicolumn, calibration graph in the 5.00 - 50.0 µg Cd L-1 range is obtained, corresponding to twenty measurements per hour, 0.7 mg Malachite Green and 500 mg KI and 5 mL sample consumed per determination. Results of the determination of Cd in certified reference materials, vegetables and tap water were in agreement with certified values and with those obtained by GFAAS at 95% confidence level. The detection limit is 0.23 µg Cd L-1 and the RSD for typical samples containing 13.0 µg Cd L-1 was 3.85 % (n= 12).

A quimiluminescência como ferramenta analítica: do mecanismo a aplicações da reação do luminol em métodos cinéticos de análise

Relevant aspects of proposed mechanisms of the chemiluminescent reaction of luminol are presented and commented to emphasize its perspectives for kinetic analysis. A careful search for analytical applications of this reaction is discussed in order to point out new trends of the studies. Kinetic analysis using the luminol reaction is proposed to be a very attractive due to the good performance of the reaction in analytical applications and the positive characteristics of kinetic analysis, such as low cost and sensibility. It is pointed out that kinetic analysis using the chemiluminescent reaction of luminol should be encouraged.

Reação de bis-inserção de 1,2-difenilacetileno na ligação Pd-C de ciclometalados

The present paper deals with the bis-insertion reactions of 1,2-diphenylacetylene into Pd-C bond of the cyclopalladated complexes [Pd(dmba)(µ-NCO)]2 (1) and [Pd(dmba)(MeCN)2](NO3) (2) (dmba = N,N-dimethylbenzylamine, MeCN = acetonitrile). Two new complexes [Pd{PhC=CPh-CPh=CPhC6H4CH2N(CH 3)2}(NCO)] (3) and [Pd{PhC=CPh-CPh=CPhC6H4CH2N(CH 3)2}(NO3 )] (4) were obtained and characterized by IR and NMR spectroscopy and elemental analysis.

Catalisadores Ni/Al2O3 promovidos com molibdênio para a reação de reforma a vapor de metano

Mo-promoted Ni/Al2O3 catalysts for the methane steam reforming reaction were studied in this work. The Ni/Al2O3 catalysts were prepared by precipitation and molibdenum was added by impregnation up to 2%wt. The solids were tested using a micro-reactor under two H2Ov/C conditions and were characterized by ICP-OES, XRD, N2 adsoption, H2 chemisorption and TPR. NiO and NiAl2O4 phases were observed and the metallic area decreased with the increase of the Mo content. From the catalytic tests high stability was verified for H2Ov/C=4.0. On the other hand, only the catalyst containing 0,05% Mo stayed stable during 30 hours of the test at H2Ov/C=2.0.

Híbridos inorgânico-orgânicos derivados da reação de filossicatos com organossilanos

Silylation reactions involving hydroxylated surfaces are an important route for synthesis of new materials that could present selected properties, for application in different areas such as catalysis, chromatography, adsorption and electrochemistry. An overview of many synthetic routes, comprising organosilanes to yield phyllosilicates is now presented.

Qual o sítio de reação? Um experimento computacional

A computational quantum chemistry experiment is described on the determination of the most reactive atom in a molecule for a reaction. The reaction studied was the S N2 of 4-(dimethylamino)pyridine and methyl iodide. Several indexes (HOMO coefficent, (c), charges, (q), nucleophilic softness, (s+), and Fukui index, (f+)) were employed to verify which correctly describe what nitrogen will react. The calculations were made by AM1 and HF/STO-3G methods. The correct reactivity order is only reproduced by s+ and f+. The lack of agreement of FMO based indexes was discussed.

Efeito do teor metálico em catalisadores Co/Al2O3 aplicados à reação de reforma a vapor de etanol

The development of cobalt catalysts to produce hydrogen from ethanol is the goal of this investigation. Co/Al2O3 catalysts were prepared by impregnation and characterized by atomic absorption, nitrogen adsorption, X-ray diffraction, Raman spectroscopy, temperature programmed reduction and carbon analysis. The catalysts contained Co3O4 oxide and Co3+ and Co2+ species interacting with alumina. The cobalt load affects the crystal size and the crystalline structure and higher Co loads influence the reaction mechanism, changing the selectivity of the catalysts, decreasing the amount of CO produced and avoiding the formation of products catalyzed by the support. The ethanol conversion was 50-70% with 10-<1% of CO in the hydrogen.

A reação de metátese de olefinas: reorganização e ciclização de compostos orgânicos

The olefin metathesis reaction allows the exchange of complex alkyl units between two olefins, with the formation of a new olefinic link and a sub-product olefin usually ethylene. This reaction has found extensive application in the last ten years with the development of the Grubbs and Schrock catalysts, in total synthesis of complex organic molecules, as opposed to the very important use in the petrochemical industry with relatively simple molecules. This review intends to trace a historical and mechanistic pathway from industry to academy, before illustrating the more recent advances.

Parâmetros termodinâmicos da reação de adsorção de boro em solos tropicais altamente intemperizados

The driving force of boron adsorption on some tropical soils was evaluated by means of thermodynamic parameters. The batch method was employed, and the reaction was monitored at different pH values. The Langmuir equation successfully fitted the experimental results and provided reasonable isotherm parameters. Boron adsorption increased as a function of the pH of the soil solution and the concentration of added boron. The reaction was favorable and proceeded spontaneously, being strongly exoergic, as indicated by a negative free energy (deltaG) and a separation factor (K R) < 1. The boron adsorption phenomenon and the soil-solution interface were thermodinamically described using a theoretical model.

Síntese de aliloxi-orto-iodobenzamida derivada de d-glicose e reação de ciclização radicalar mediada por hidreto de tri-n-butilestanho

Methyl 6-O-allyl-2,3-di-O-benzyl-4-deoxy-4-(2 -iodobenzoylamine)-alpha-D-glucopyranoside was synthesized in nine conventional steps from methyl alpha-D-glucopyranoside. Its Bu3SnH-mediated aryl radical cyclization provided a benzomacrolactam, resulting from 11-endo aryl radical cyclization and the reduced uncyclized product methyl 6-O-allyl-4-benzoylamine-2,3-di-O-benzyl-4-deoxy- alpha-D-glucopyranoside. The structures of the three new products were supported by ¹H and 13C NMR spectroscopy and DEPT, COSY and HMQC experiments.

A reação de ciclização de prins: uma estratégia eficiente para síntese estereosseletiva de anéis tetraidropirânicos substituídos

The Prins cyclization reaction has significantly advanced in the last years as demonstrated by a number of applications described in the literature. The objective of this report is to introduce this powerful synthetic methodology to the undergraduate and graduated student, since it is rarely presented in an organic synthesis formal course. This reaction is, in some cases, the methodology of choice for the preparation of natural products or drugs that present the tetrahydropyrane moiety in their structures. In this report we show some aspects of this reaction, including mechanism, scope and limitations.

Efeito dos ácidos hexenurônicos e da lignina no desempenho da ozonólize, em diferentes pHs da reação

The effect of pH on the performance of the ozonolysis stage in pulp production was evaluated for conventional and acid treated brown and oxygen delignified eucalyptus kraft pulps. The objective was to determine separately the effects of lignin and hexenuronic acid on the performance of the ozonolysis stage. The reaction of ozone with hexenuronic acid is less sensitive to pH than the reaction of ozone with lignin. The efficiency and the selectivity of the reaction of ozone with pulp decreases after removal of hexenuronic acids. Increasing up to 7.0 the pH during the ozonolysis is viable in the sequence Z/D(EOP)D, resulting in savings of H2SO4 (8,5 kg/tsa) and NaOH (5 kg/tsa), but is not recommended in the sequence Z/ED(PO).

Avaliação da reação foto-fenton na decomposição de resíduos de carrapaticida

Experimental procedures based on factorial design and surface response methodology were applied to establishe experimental conditions for the decomposition of a 1:400 (v/v) Supocade® (chlorfenvinphos 13.8% and cypermethrin 2.6%) solution, used to control cattle ticks. Experiments exploring photo-oxidative reactions were performed with and without UV radiation, fixing exposition time and pesticide volume, and varying the oxidant mixture. The use of 3.6 mmol L-1 Fe2+ plus 1.9 mol L-1 H2O2 plus UV radiation provided destruction of 94% of the original carbon content and reduction of aromatic, aliphatic and carbinolic compounds, evaluated by determination of residual carbon content by ICP OES and NMR analysis.

Mecanismo e aplicações da reação de fenton assistida por compostos fenólicos redutores de ferro

The mechanism and applications of the Fenton reaction assisted by iron-reducing phenolic compounds (IRPC) is reviewed. The presence of IRPC leads to the formation of a larger number of free radicals. The relationship between the redox potential and the IRPC structure is discussed. The effect of humic substances in the degradation of xenobiotics is also included, since these substances are able to reduce metallic ions. The natural occurrence of Fe3+/H2O2/IRPC in wood biodegradation processes, as well as their application is also discussed. The review concludes with the advantages of the Fe3+/H2O2/IRPC systems and some considerations for further process optimization and their applications at industrial levels.

Estudo do efeito de tratamentos térmicos em catalisadores de PtRu/C frente à reação de oxidação de hidrogênio na presença de CO

In this work the effects of time and temperature of thermal treatments under reducing atmosphere (H2) on PtRu/C catalysts for the hydrogen oxidation reaction (HOR) in the presence of CO on a proton exchange membrane fuel cell (PEMFC) single cells have been studied. It can be seen that the increase of the treatment temperature leads to an increasing sintering of the catalyst particles with reduction of the active area, although the catalyst treated at 550 ºC presents more CO tolerance for the HOR.