Experimentos didáticos em química analítica envolvendo separação de fases e pré-concentração

The aim of this work was to propose two different didactic experiments, which can be used in practical classes of analytical chemistry courses. More flexible experiments related to the theme, giving some options to the instructor are proposed. In this way, the Experiment 1 was divided in two parts. In the first one, the visualization of two distinct phases separation is emphasized: the rich and the poor phases in surfactant. In the second part, the metal pre-concentration (Co as example) is emphasized. The Experiment 2 has three different parts. In the first one, the complex formation is pointed out, in the second one, the pH influence is shown and in the last one, the influence of the complexation time is demonstrated.

Determinação espectrofotométrica de alumínio em concentrados salinos utilizados em hemodiálise empregando pré-concentração em fluxo

An automatic flow injection procedure for spectrophotometric aluminium determination in purified water and solutions containing high salts concentrations used for hemodyalisis treatment was developed. The method was base on reaction of Al3+ with cianine eriochrome R (ECR) after preconcentration using the AG50W-X8 cationic-exchange resin. Elution was carried out using a 1 % (m/v) calcium chloride solution. The manifold comprised an automatic proporcional injector controlled by a computer equipped with an eletronic interface and software written in QuicBASIC 4.5 with facilities to control the injector and perform data acquisition. Samples with concentration ranging from 4.96 to 19.90 µg L-1 Al were analyzed and recoveries between 88 and 113% were obtained by using the standard addition method. Other profitable analytical characteristics such as a relative standard deviation 1.3 % (n = 10) for a typical sample 14.5 µg L-1 Al, a linear response ranging up to 60.0 µg L-1Al, and a sampling throughput of 10 determinations per hour were achieved. A detection limit of 4.2 µg L-1 Al was estimated as suggested by IUPAC.

Transferência de energia entre corantes catiônicos em sistemas homogêneos

In this work, the energy transfer by dipole-dipole interaction between cationic dyes in n-alcohols (methanol, ethanol, 1-propanol and 1-butanol) is studied by time resolved and steady state fluorescence measurements. The critical radii of energy transfer were determined by three independent methods; the spectral overlap, fluorescence decay profiles, and relative intensity measurements. In all solvents, R0 values of the dye pairs obtained from spectral overlap were between 40 to 90 Å. Steady state and time resolved fluorescence measurements resulted in values of R0 in the range of 50 - 80 Å, with good correlation of values.

Hidrogenação de compostos orgânicos utilizando método eletroquímico para geração de hidrogênio in situ: hidrogenação eletrocatalítica

Electrocatalytic hydrogenation (HEC) may be compared to catalytic hydrogenation (HC). The difference between these methods is the hydrogen source: HC needs a hydrogen gas supply; HEC needs a source of protons (solvent) to be reduced at a cathode surface. HEC has presented interesting advances in the last decades due to investigation of the influence of the supporting electrolyte, co-solvent, surfactant, presence of inert gas and the composition of the electrode on the reaction. Several classes of organic compounds have been hydrogenated through HEC: olefins, ketones, aldehydes, aromatics, polyaromatics and nitro-compounds. This paper shows some details about the HEC which may be regarded as a promising technique for the hydrogenation of organic compounds both in industrial processes and in laboratories.

Adaptação no método do peso da gota para determinação da tensão superficial: um método simplificado para a quantificação da CMC de surfactantes no ensino da química

The discussion based on surfactant cleaning action is commonly used in chemistry classrooms for the comprehension of theoretical concepts related to surface tension and micelle formation. The experimental quantification of surface tension of aqueous solutions of surfactants provides the practical instruments for this discussion. The present paper describes a simplification on the common apparatus employed for the drop-weight method, making the determination of surface tension accessible to any chemistry lab. The surface tension of various liquids and the critical micelle concentration, CMC, of three commercial surfactants were measured with this modified method, and proved to be consistent with literature values.

Determinação da concentração micelar crítica de ácidos húmicos por medidas de condutividade e espectroscopia

The aim of this study was the determination of the critical micelle concentration (CMC) of the sodium dodecyl sulfate (SDS) surfactant using spectroscopic and conductimetric determinations and to compare these methodologies in the determination of the CMC of different humic acids (HA). The CMC obtained by conductimetric determination was satisfactory. By spectroscopic determination two values of the CMC were obtained for HA. These values can be to due the intra and intermolecular interactions in the HA structure.

The role of fast atom bombardment mass spectroscopy (FABMS) in cluster characterization

Fast atom bombardment mass spectroscopy has been used to study a large number of cationic phosphine-containing transition-metal-gold clusters, which ranged in mass from 1000 to 4000. Many of these clusters have been previously characterized and were examined in order to test the usefulness of the FABMS technique. Results showed that FABMS is excellent in giving the correct molecular formula and when combined with NMR, IR, and microanalysis gave a reliable characterization for cationic clusters¹. Recently FABMS has become one of the techniques employed as routine in cluster characterization2,3 and also is an effective tool for the structure analysis of large biomolecules4. Some results in the present work reinforce the importance of these data in the characterization of clusters in the absence of crystals with quality for X-ray analysis.

Extração no ponto nuvem de compostos orgânicos homólogos e isômeros

In this work, the feasibility of employing micelle-mediated extraction for selective separation of homologous or isomeric organic compounds is demonstrated. Firstly, the main parameters controlling extraction performances, such as surfactant concentration and temperature were varied. A Scheffé-type experimental design was demonstrated as a novel and useful method to characterize the various experimental factors. At each point selected in the two-phase domain and for a given solute, extraction percentage (E%), concentration ratio, phase volume ratio, and equilibrium partition coefficient (K C) were determined. The values of E% and K C decrease in the following order: phenol > 1-phenylethanol ~ 2-phenylethanol > benzyl alcohol.

Decomposição térmica de complexos de Zn e Cd com isomaleonitriladitiolato (imnt)

Thermal decomposition of [Bu4N]2[Zn(imnt)2] and [M(NH3)2(imnt)] complexes with M = Zn and Cd, and imnt = (bis 1,1-dicyanoethylene-2,2 dithiolate) in inert atmosphere was investigated by thermogravimetric analysis (TG) and differential scanning calorimetry (DSC). Pyrolysis studies at different temperatures, 300, 400, 500, and 600 ºC, in N2 atmosphere were performed and the products were characterized by X-ray diffraction (XRD), infrared and Raman spectroscopy, and scanning electron microscopy (SEM). The products were identified as sulfide sub-micron particles, along with amorphous carbon. Particle sizes estimated by SEM were ca. 50 nm for the cationic complexes and 500 nm for the neutral complexes.

Cu and Co exchanged ZSM-5 zeolites: activity towards no reduction and hydrocarbon oxidation

|Cu x|[Si yAl]-MFI and |Co x|[Si yAl]-MFI catalysts were prepared by ion exchange from |Na|[Si yAl]-MFI zeolites (y = 12, 25 and 45). The activity of the catalysts was evaluated in the reduction of NO to N2 in an oxidative atmosphere using propane or methane as reducing agents. The Cu catalysts were only active with propane and they presented higher activity than the Co-based catalysts, the latter being active with both hydrocarbons. H2-TPR and DRS-UV/Vis data allowed correlation between the activity towards NO reduction and the presence of cationic charge-compensating species in the zeolite. It was also verified that the hydrocarbons are preferentially oxidised by O2, a reaction that occurs simultaneously with their oxidation with NO.

Adsorção de oligonucleotídeos em nanoemulsões obtidas por emulsificação espontânea

This paper describes the adsorption of an oligothymidylate (pdT16) on nanoemulsions obtained by spontaneous emulsification procedures. Formulations were composed by medium chain triglycerides, egg lecithin, glycerol, water (NE) and stearylamine (NE SA). After optimization of operating conditions, the mean droplet size was smaller than 255 nm. Adsorption isotherms showed a higher amount of pdT16 adsorbed on cationic NE SA (60 mg/g) compared to NE (20 mg/g). pdT16 adsorption was also evidenced by the inversion of the zeta-potential of NE SA (from +50 to -30 mV) and the morphology of oil droplets examined through transmission electron microscopy. The overall results showed the role of electrostatic interactions on the adsorption of pdT16 on the oil/water interface of nanoemulsions.

Estudo comparativo da eficiência da difenilcarbazida e do óleo de coco saponificado microemulsionados na inibição da corrosão de aço carbono

The effectiveness of microemulsions (ME) of saponified coconut oil (OCS-ME) and diphenylcarbazide (DC-ME) on a carbon steel corrosion inhibition process was evaluated using an electrochemical method of polarization resistance. The ME was prepared with OCS, butanol, kerosene and saline solutions. OCS-ME and DC-ME showed highest inhibitions effects (77% and 92%, respectively) at lower concentrations (0.5% and 0.48 - 0.50%, respectively). The surfactant OCS (in H2O) showed lower efficiency (63% at 0.20 - 0.25% concentration). The greatest inhibitory effect of DC-ME could be correlated with the chemical structure and the rich O/W ME system, which are very important for adsorption phenomena in interfacial ME systems.

Síntese da hidroxiapatita e refinamento estrutural por difração de raios-x

A sample of hydroxyapatite was synthesized and its crystalline structure was analyzed by X-ray diffraction by means of the Rietveld method. Two functions were used to fit the peak profiles, modified Voigt (TCHZ) and Pearson VII. The occupational factors and lattice parameters obtained by both models show that the sample does not contain relevant cationic substitutions. The interatomic distances from Ca1 to oxygens O1, O2 and O3 were adequate for a pure hydroxyapatite without defect at site Ca1. Besides, the use of multiple lines in planes (300) and (002) associated with the model Pearson VII resulted in good agreement with the TCHZ model with respect to the size-strain effectswith an ellipsoidal shape of crystallites. In conclusion, the procedures adopted in the synthesis of hydroxyapatite produced a pure and crystalline material. The experimental results of transmission electron microscopy confirmed the predicted shape of crystals.

Cloud point extraction for the determination of copper in environmental samples by flame atomic absorption spectrometry

A simple cloud point extraction procedure is presented for the preconcentration of copper in various samples. After complexation by 4-hydroxy-2-mercapto-6-propylpyrimidine (PTU), copper ions are quantitatively extracted into the phase rich in Triton X-114 after centrifugation. Methanol acidified with 0.5 mol L-1 HNO3 was added to the surfactant-rich phase prior to its analysis by flame atomic absorption spectrometry (FAAS). Analytical parameters including concentrations for PTU, Triton X-114 and HNO3, bath temperature, centrifugation rate and time were optimized. The influences of the matrix ions on the recoveries of copper ions were investigated. The detection limits (3SDb/m, n=4) of 1.6 ng mL-1 along with enrichment factors of 30 for Cu were achieved. The proposed procedure was applied to the analysis of environmental samples.

Can affinity interactions influence the partitioning of glucose-6-phosphate dehydrogenase in two-phase aqueous micellar systems?

In this work, we provide an investigation of the role and strength of affinity interactions on the partitioning of the glucose-6-phosphate dehydrogenase in aqueous two-phase micellar systems. These systems are constituted of micellar surfactant solutions and offer both hydrophobic and hydrophilic environments, providing selectivity to biomolecules. We studied G6PD partitioning in systems composed of the nonionic surfactants, separately, in the presence and absence of affinity ligands. We observed that G6PD partitions to the micelle-poor phase, owing to the strength of excluded-volume interactions in these systems that drive the protein to the micelle-poor phase, where there is more free volume available.