Cinética não-isotérmica na análise térmica de sólidos

In this work, a computer program called Thermal Kinetics was implemented to simulate thermal analysis experiments by numerical integration of the kinetics equations. The computer program was tested in two different examples: non-isothermal transformation of a Cu-Al alloy and non-isothermal decomposition of calcium oxalate monohydrated. In spite of the rather crude approximations of the model, the simulated profiles are very similar to the experimental curves. Both, the dalpha /dt and the dalpha /dT profiles reproduce the experimental transition temperatures with an error smaller than 25%.

Filmes de óxidos anódicos de nióbio: efeito eletrocrômico e cinética da reação de eletro-intercalação

The aim of this work is to study the electrochromism and the reaction kinetics of lithium electrointercalation in anodic niobium oxide films. The oxide grown in an acid environment by application of an alternating potential shows interference colour (iridescence) and when reduced in lithium perclorate/PC solution, the intercalation of Li+ ions and electrons causes a reversible colour change (electrochromism), characterized here by electrochemical and optical measurements. A model where the reaction kinetics is dominated by diffusion of ionic pairs (Li+, e-) in the oxide film permitted the reproduction of current and absorbance temporal dependence, confirming the relationship between the electrochromic and electrochemical reactions. From the results obtained, a relation was established where the colour change is associated to the reduction of Nb+5 to Nb+4 ions with simultaneous cations injection.

Ozonólise das ligninas organossolve e kraft eucalipto. Parte II: cinética nos meios ácido e básico

Organosolv and kraft lignins were treated with ozone both in basic and acid media and the reaction was studied kinetically. In contrast to reported studies, ozone was more efective in basic medium. Kraft lignin was degraded faster than organosolv lignin in both media but in the basic medium the rate of reaction was very much faster than in the acid one: for kraft lignin, the observed degradation was 93% for 2 min of reaction in the basic medium and 56% for 10 min of reaction in the acid medium; for organosolv lignin, 47% and 25%, respectively, in the same times. Higher phenolic hydroxyl groups contents increase the reaction rate.

Determinação de potássio em méis após precipitação com tetrafenilborato de sódio e separação em coluna de troca-iônica

A new method for determination of potassium in honey samples of different colors was developed as an alternative method for determination of this metal. Analysis of genuine honeys attested by the qualities and quantities tests officially adopted in Brazil, showed that the concentration of potassium ranged from 181 to 315 mg/kg for light honeys, from 393 to 570 mg/kg for medium honeys and from 791 to 915 mg/kg for dark honeys. Recoveries making use of spikes of potassium added to the honey samples and to the deionized distilled water showed results close by hundred percent at pH <= 2,0 under temperature bellow 20°C.

Cinética e mecanismo da reação de auto-oxidação de S(IV) catalisada por íons metálicos de transição

The oxidation process of sulfur (IV) species (SO2, HSO3- e SO32-) by oxygen, catalysed by trace metal ion and complexes, can play an important role in atmospheric, analytical and bioinorganic chemistry. An overview of the most important reactions in these fields is presented. A fascinating redox cycling of the metal ions and complexes during such autoxidation process was revealed by the combination of kinetics and coordination chemistry studies.

O emprego de lipases como agentes de resolução cinética de enantiômeros em síntese orgânica: aspectos gerais sobre a influência do solvente

In organic synthesis, lipases are the most frequently used biocatalysts. They are efficient stereoselective catalysts in the kinetic resolution of a wide variety of chiral compounds. The discovery that enzymes possess catalytic activity in organic solvents has made it possible to address the question of reaction medium influence on enzymatic specificity. Perhaps the most exciting and significant development in this emerging area is the discovery that enzyme specificity, in particular enantioselectivity, can be affected by changing from one organic solvent to another. This article discusses the scope and possible mechanistic models of this phenomenon in hydrolases, specially lipases, as well as directions of future research in the area.

Estudo da influência do diluente reativo PGE na cinética de cura de resina epóxi utilizada em compósitos estruturais

Phenil glycidyl ether (PGE), a monofunctional diluent, has been used in epoxy resins formulations in order to increase the toughness of the epoxy molded composite. In a systematic study concerning its influence in the cure kinetics of the epoxy resin, it was used in concentrations of 2,5; 5,0; 10 and 20% in relation to a diglycidyl ether bisphenol-A (DGEBA)/diamino diphenil-sulfone (DDS) base matrix. Dynamic and isothermal scanning analysis were carried out using a differential scanning calorimety (DSC) equipment. For all the concentrations of PGE, a n order kinetics was observed, with n varing between 0,35 -- 0,91 as a function of the increase in the PGE concentration.

Morfologia e cristalinidade de hidroxicarbonato de zinco obtido via precipitação homogênea: influência dos ânions cloreto e nitrato

The effects of chloride and nitrate anions and their respective concentrations, as well as urea presence, on solid phase morphologies were investigated. Zinc hydroxide carbonate was prepared by aging diluted zinc salt solution in presence of urea at 90ºC. Samples were identified by X-ray powder diffractograms showing the characteristic patterns of hydrozincite. The crystallinity was correlated with the concentrations of reagents. Spherulitic-type aggregates and single acicular particles were obtained from diluted chloride and nitrate solutions while porous aggregates of uniform size were formed from solutions with high chloride and urea concentrations.

Aplicação da calorimetria exploratória diferencial no estudo da cinética de transição alfa -> delta HMX

Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) or octogen is a white crystalline substance which occurs in four polymorphous forms. It is used in a wide variety of military and industrial formulations owing to its suitable properties. Researchers have demonstrated the usefulness of this energetic material in explosive components. In the present work we apply differential scanning calorimetry (DSC) to measure the a ® d solid-solid phase transition energy of HMX. The results obtained by Kissinger's and Ozawa's methods were 487 and 495 kJ/mol, respectively.

Influência do agente precipitante na preparação do óxido de nióbio (V) hidratado pelo método da precipitação em solução homogênea

This work reports the preparation, characterization and study of the ion exchange behavior of hydrous niobium oxide prepared by a homogeneous precipitation method. The precipitating agent was obtained in aqueous solution by thermal decomposition of urea or ammonium carbonate. The compounds were chemically and physically characterized by X-ray diffractometry, thermal analysis (TG/DTG), surface area measurements and ion exchange behavior with sodium. The materials prepared with ammonium carbonate presented a higher degree of crystallinity and better ion exchange capacity with sodium than materials prepared with urea. In the homogeneous precipitation method, materials were obtained with specific surface area of 123 - 224 m² g-1. A variation of the preparation process produced hydrous niobium oxide with a different degree of hydration and specific surface area. This provided materials with different physico-chemical properties.

Composição química da precipitação atmosférica no sul do Brasil: estudo preliminar

The present work is a preliminary study of total and wet precipitation in the Candiota region, RS. The samples were collected from January to June 2001 at four different sites. The following variables were analysed: pH, conductivity, alkalinity, Cl-, NO3-, F-, SO4(2-), Na+, K+, Mg2+, NH4+, Ca2+ and Zn, Cu, Fe, Al, Mn. The results showed slightly acidic precipitation and higher concentrations of NH4+, Na+, Cl- and SO4(2-). Factor analysis applied to the variables studied allowed identifying the major sources. Na+, Cl- and Mg2+ have their origin in sea salts and NH4+, Ca2+, K+, SO4(2-) and NO3- are from local anthropogenic sources.

Utilização de um eletrodo de grafite-epóxi recoberto com [Zn(FEN)3][tetratris(4-clorofenil) borato]2 sensível a zinco(II) em meio 1,10-fenantrolina como eletrodo indicador em titulações potenciométricas de precipitação

The construction and analytical evaluation of a coated graphite-epoxy electrode sensitive to the zinc-1,10-phenantroline complex based on the [Zn(fen)3][tetrakis(4-chlorophenyl)borate]2 incorporated into a poly(vinylchloride) (PVC) matrix are described. A thin membrane film of this ion-pair, dibutylphthalate (DBPh) and PVC were deposited directly onto an electrically conductive graphite-epoxy support located inside a Perspex® tube. The best PVC polymeric membrane contains 65% (m/m) DBPh, 30% (m/m) PVC and 5% (m/m) of the ion-pair. This electrode shows a response of 19.5 mV dec-1 over the zinc(II) concentration range of 1.0 x 10-5 to 1.0 x 10-3 mol L-1 in 1,10-phenantroline medium, at pH 6.0. The response time was less than 20 seconds and the lifetime of this electrode was more than four months (over 1200 determinations by each polymeric membrane). It was successfully used as an indicator electrode in the potentiometric precipitation titration of zinc(II) ions.

Cinética e caracterização físico-química do fermentado do pseudofruto do caju (Anacardium occidentale L.)

The production of cashew apple wine has the purpose of minimizing the wastage in the Brazilian cashew production. Knowing that the cashew apple fermentation produces a good cashew wine, a study of alcoholic fermentation kinetics of the cashew apple and the physico-chemical characterization of the product were made. The cashew wine was produced in an stirred batch reactor. The results of the physico-chemical analysis of volatiles, residual sugars, total acidity and pH of cashew wine showed that their concentrations were within the standard limits established by the Brazilian legislation for fruit wines.

Adsorção de íons dicromato nos compósitos celulose/ZrO2.nH2O preparados pelos métodos da precipitação convencional e em solução homogênea

This article describes the preparation and characterization of the cellulose/ hydrated zirconium oxide composites prepared by conventional precipitation (PC) and homogeneous solution precipitation (PSH) methods. The composite obtained by the PC method was prepared by using an ammonia solution as the precipitating agent, while the composite obtained by the PSH method was prepared by using urea as the precipitating agent. The adsorption of dichromate ions on the composites was studied using factorial design 2³. The variables were: initial concentration, agitation time and mass of the composite. The data obtained agree better for the composite obtained by the PC method.

Estudo do equilíbrio e cinética da biossorção do pb2+ por saccharomyces cerevisiae

The biosorption, based on the use of biomass for removal of ions is distinguished as an innovative and promising technology when compared with the traditional methods. In this context, the aim of the present work is to use Saccharomyces cerevisiae as biosorbent for the retention of Pb2+ metal ions. Factorial design was used for evaluation of the process. The observed equilibrium data were well described by Langmuir and Freundlich adsorption isotherms. The maximum adsorption capacity was 1486.88 mg/g. The results indicated that Saccharomyces cerevisiae is suitable for biosorption of Pb2+ metal ions.