José de Alencar Simoni

Interaction of cations with SH-modified silica gel: thermochemical study through calorimetric titration and direct extent of reaction determination

The reaction of silica gel with 3-(trimethoxysilyl)propane-1-thiol, MPTS, produced a thiol-modified silica gel, Sil–SH. This material removes silver, mercury, copper, zinc and nickel ions from aqueous solutions. Enthalpy changes that occur in the interactions between Sil–SH and these cations were obtained by a calorimetric titration technique, in which thermal effects and the quantities of cations that interact at each titration point were determined simultaneously. From this procedure, values of the maximum capacity of interaction (Ns), enthalpies of interaction at different coverages of the surface [Δint(i)Hm ] and integral enthalpy of interaction for a monolayer (ΔmonHm) have been established. It is shown that this method permits charting the basic sites created upon modification. From Δint(i)Hm values, the sequence of the interaction strength between the basic sites of the silica-attached MPTS and the cations is Hg2+>Ag+>Cu2+>Ni2+>Zn2+ , in agreement with the concept of hard and soft acids and bases. The influence of some parameters concerning the thermodynamics of solvation of the indicated transition metals has been also taken into account in the investigation of these interaction processes.

Microcalorimetric study of glucose degradation in some brazilian soils

Abstract Latosol soils, red, dark red, red-yellow, and red-yellow treated with vinasse, were studied by microcalorimetry. Microbial activity was stimulated by the addition of identical masses of glucose with ammonium sulphate, varying from 3.0 to 12.0mg per 1.5 g of soil (glucose being a limiting energy source) under controlled humidity at 298 and 306 K. After dosing all the soil samples (1.5 g) with 6.0mg of each nutrient and 0.8cm 3 of water, the power-time curves were recorded. These curves showed that red soil gave the largest power output (240μW) at a peak time of 35.2 h (the point of maximum amplitude of the experimental curve). Yellow soils, however, did not show any response to addition of nutrients. The power-time curves are all similar in shape, with the thermic effect and the peak time for both temperatures being directly dependent on the amount of nutrient. The peak time decreases with increasing temperature; values of 29.2–43.8 h at 298 K and 17.3–23.8 h at 306 K, were obtained.

Use of calorimetric titration to determine thermochemical data for interaction of cations with mercapto-modified silica gel

Abstract Interactions occurring between mercapto-modified silica gel and silver, mercury, copper and zinc ions in ethanolic solutions have been investigated by using calorimetric titrations with direct determination of the reaction extension. A calorimetric technique is described for the simultaneous determination of thermal effects and quantities of cations that interact at each titration point. From this procedure, values of maximum capacity of interaction ( N s ), differential interaction enthalpies at different coverages of the surface (Δ int(i) H m ), integral energy of interaction ( Q mon ) and molar integral enthalpy of interaction for a monolayer (Δ mon H m ) have been obtained. The maximum capacity of interaction values are compared with those previously published for analogous interaction processes in aqueous solutions. Thermochemical quantities are discussed in terms of evaluation criterion of the acid–base interactions, and generally support the conclusion that the Pearsons hypothesis of hard and soft acids and bases is appropriated to describe the behavior of the interaction of the above-mentioned cations.

The influence of the counter ion competition and nature of solvent on the adsorption of mercury halides on SH-modified silica gel

Abstract The adsorption behavior of thiol-modified silica gel, SiSH, towards mercury (II) chloride and bromide in water, ethanol, methanol and water–methanol solutions has been studied by a batch technique. The influence of the counter-ion competition and mixture of solvents were investigated. The Langmuir expression for adsorption isotherm was applied in order to determine the maximum adsorption capacity to form a monolayer, N mon , and the constant related to the adsorption intensity, b. In aqueous solutions there was a significant adsorption increase with the temperature and pronounced synergistic effects were observed. Adsorption measurements made in water(w)/methanol(MeOH) mixtures indicated that the synergism effect of mercury adsorption followed the order 75% (w)+25% (MeOH)>50% (w)+50% (MeOH)>25% (w)+75% (MeOH). From temperature effect on adsorption, exothermic enthalpy changes for a monolayer of anchored cations per gram of surface, Δ mon H m , are shown to be consistent with our previous calorimetric results for similar systems. The antagonistic effects for Δ mon H m results showed that the counter-ion competition is not favorable to the mercury/surface interaction strengths.

Photomicrocalorimetry: Photosubstitution of carbonyl by phosphites in [Mn(η5-C5H4CH3)(CO)3]

Abstract A photomicrocalorimeter is described for studies of reactions involving oxygen and moisture-sensitive compounds in organic solvents. This calorimeter was used to investigate the thermochemistry of photochemical substitutions of a carbonyl in [Mn(η 5 C 5 H 4 CH 3 )(CO) 3 ] by P(OR) 3 (R  Ph or 1 Pr). The measured reaction enthalpies in heptane (λ = 400 nm), ΔH r = −15.7 ± 0.5 and −18.6 ± 0.9 kJ mol −1 , respectively, were identified with the bond dissociation enthalpy differences D (MnCO)  D [MnP(OPh) 3 ] and D (MnCO)  D [Mn-P(O i Pr) 3 ] in solution. These results are discussed in terms of the basicity of the phosphates, spectroscopic data of the complexes (CO stretching frequencies), and the available thermochemical data for other transition metal-phosphorus(III) ligand interactions.

Refletindo sobre o caso celobar

By mid 2003, the Brazilian people accompanied astonished, in the press, the news about the death of more than 20 persons due to ingestion of a pharmaceutical product containing a suspension of barium sulfate (Celobar®) commonly used as a radiological contrast. Analysis of the product indicated the presence of barium carbonate (about 13% weight/weight) which reacts easily with the hydrochloric acid in the stomach liberating barium ions, a severe poison. In this article, we briefly discuss the possible economic, personal and technical causes that led to this disaster.

Avaliação da capacidade tamponante - um experimento participativo

The aim of this work is to show an experiment from which students can learn some of the main characteristics of buffer solutions. A mixture of some acid-base indicators, named as Yamadas indicator, can be used to estimate pH values in an acid-base titration of a buffer, with good approximation. In the experiment it is also possible to verify the relationship between the buffer capacity and the concentrations and the molar ratio of the components of a NH3 / NH4+ buffer solution. The shortage of experiments associated with the relative small importance given to many aspects of buffer solutions, is now explored with simplicity. In the proposed experiments, students prepare buffer solutions by themselves, calculate the pH, understand how acid-base indicators act and learn how buffer solutions work through graph constructed by sharing experimental data.

Um experimento com propostas múltiplas para um laboratório de química geral

This article suggests a sequence of experiments on the preparation, analysis and some photochemical aspects of potassium tris (oxalato) ferrate(III) trihydrate. The sequence of experiments could be carried out in four or five 4-hour laboratory periods. The new part of this article is related to the kinetics studies involving the ambient illumination as well as the use of the cellophane paper of different colors as light filters. The aspects such as quantum yield, light absorption and photochemical reactions are explored in order to illustrate the relationships between the exposure time, light intensity and wavelength range on the photochemical reactions.

AN ISOTHERMAL LABYRINTH FLOW CALORIMETER WITH THERMISTORS

An isothermal labyrinth flow calorimeter was constructed using thermistors and low-cost materials. Electrical calibrations data, time constant and other characteristics are presented. For testing the system, the molar heat capacity of copper and aluminum were determined by drop calorimetry; both showed a difference of < 1% from the literature values.

Montagem de uma disciplina experimental: contribuição para a química geral

The purpose of this paper is to present a classification criterion for general chemistry experiments departing from a survey of 35 years of the experimental general chemistry discipline at the Chemistry Institute - UNICAMP. It is shown that by considering the most remarkable characteristics of an experiment and classifying it under Conceptual, Fundamental, Methodological or Quantitative criterion, teachers could better develop the discipline plan and prevent a subject superposition. Taking into account the system Fe3+/SCN - , the article also exemplify how is possible to modify an experimental approach and to conceive other procedures with different didactic purposes.