Dissertação (Química)

Utilização de radiação de microondas na determinação espectrofotométrica de cromo com 4- (2- TIAZOLILAZO) - RESORCINOL (TAR).

2024-08-15T12:54:54Z

Utilização de radiação de microondas na determinação espectrofotométrica de cromo com 4- (2- TIAZOLILAZO) - RESORCINOL (TAR). Carvalho, Luciene Santos de Teixeira, Leonardo Sena Gomes In the present work it was developed an spectrophotometric method for fast determination of total chromium in the forra of Cr(III) through the formation of a colored compound with the 4-(2 -tiazolilazo resorcinol)-TAR reagent. The complexation reactions of chromium (III) with other organic reagents, in aqueous médium, is quite slow due to the formation of the hexaaquocomplex [CríIfeO^]3* that is inert. However, in this method the complexation ofCr(HI) was accelerated by the action ofthe microwave radiation that acts mainly in water molecules vibrating them and breaking the layer of solvency of the aquo-complex of chromium, allowing therefore its interaction with the complexant reagent Besides the irradiation, it was also used the surfactant cetyltrimetylamonium bromide(CTAB), for bettersolubilization ofthe compound in médium aqueous, because both the organic reagent and the compound which was formed are not much soluble in water. It was also verified that the presence of CTAB in the system promotes an increase sensitivity for the chromium spectrophotometric determination. The reaction between TAR and Cr(HI) occurs between pH 5,2 and 6,0, and the complex presents an absorption maximum at 545nm. The absortion signal maximum after 5min of 700W irradiation. The complex is stable for at least 2 hours. The developed method allowed the determination of chrome in the concentration range 1,63 x 10^ and 5,76 x 10'5 mol.LT1 and presented molar absortivity of 2,72 x 104 L.mol^cm1. The analytic curves were traced against a blank of the reagent and presented correlation coefficient of 0,9998 and the variation for the measurements estimated as was 1,1%. The addition ofsodium citrate and EDTA after the irradiation increases the selectivity ofthe reagent for determination of chromium. Synthetic samples, simulating steel-alloys composition, containing different proportions of Cr and the possible interferents (Cu, Mn, Ni, Mo, Co, W, V, Si, Al e Ti) were tested. in A tolerance limit was of± 3% established and it was verified that there are no interference of any ofthe species mentioned in the presence ofthe complexing agents. Iron interferes and it should be previously separated. The method was applied in the determination of chroraium in steel-alloys, steei-tools, river waters and residues of tanning industries and the obtained results were compared with the certified values. A application ofthe paired t-test (95% confidence levei) revealed that there are not significant differences among the obtained results and the certified values and those obtained by ICP-OES. Universidade Federal da Bahia Dissertação

Truncamento das funções de partição rovibracionais multiestruturais e seus efeitos na estimativa de constantes de velocidade térmica.

2023-10-28T05:04:42Z

Truncamento das funções de partição rovibracionais multiestruturais e seus efeitos na estimativa de constantes de velocidade térmica. Nascimento, Joel Leitão Alves, Tiago Vinicius Thermal rate constants for the hydrogen abstraction reaction of methyl pentanoate were calculated using the multistructural canonical variational theory state theory with small curvature tunneling. The conformational searches for the stationary points generated by these reactions were performed with an algorithm that combines systematic and stochastic searches in dual-level. At the high-level theory, MPWB1K/6-31+G(d,p), 23 conformers were found for the methyl pentanoate. The transition states of R1-R5 ranged from 27 to 64 structures. A systematic evaluation of the truncation of the rovibrational partition functions and their effects on thermal rate constants was carried out. Our results indicate that the truncation of the partition functions in a small set of conformations generates deviations in the rate coefficients ranging from 20 to 60%, compared to estimates of the rate coefficients with the consideration of all the structures of reagents and transition states. It is expected that the reliability and perspective of this master’s thesis will motivate and guide future investigations on reactions involving long-chain methyl esters with low computational costs. Universidade Federal da Bahia Dissertação

Desenvolvimento de procedimentos de decomposição em sistema fechado para análise elementar de queijos brancos.

2023-10-28T05:06:22Z

Desenvolvimento de procedimentos de decomposição em sistema fechado para análise elementar de queijos brancos. Santos, Allana Sobrinho dos Korn, Maria das Gracas Andrade Cheese is the most consumed dairy product, and its popularity is attributed to its flavor, versatility of use, price, wide variety, and nutritional value. Recently, consumers have preferred white cheeses over yellow ones because of the lower caloric content. However, studies on determining its essential elements are still scarce in Brazil. In this sense, this work aims to propose analytical strategies for determining the total content of Ca, Cu, K, P, Na, and Zn in white cheese samples using inductively coupled plasma optical emission spectrometry (ICP OES) in order to contribute to the establishment of their nutritional contents. The following acid decomposition procedures were proposed: i) by conductive heating in a closed digester block; ii) assisted by microwave radiation. In both, the decomposition parameters, HNO3 concentration, decomposition time, and temperature were optimized, employing two-level full factorial designs, ensuring maximum digestion efficiency. For this purpose, 6 mL of HNO3 was added to samples of white cheese (m ≈ 0.250 g) in the presence of 2 mL of 30% H2O2 (w w-1). The decomposition efficiency was evaluated by visual appearance and dissolved carbon content, in addition to using multiple responses. The optimal condition for decomposition in the digester block was established: HNO3 concentration of 2.0 mol L-1 and heating at a temperature of 180 ºC for 80 minutes. For the microwave oven procedure, the optimal condition was: HNO3 concentration of 2.0 mol L-1 and irradiation for 30 minutes at a temperature of 140 ºC. Under the optimized conditions, the DOC contents determined in digested with the digester block and microwave oven were 1.35 ± 0.11 and 1.04 ± 0.06 g L-1. The proposed procedures were validated by determining the figures of merit as recommended by ANVISA. They showed good accuracy with concordant results (p<0.05) with the certificates in the certified reference material (SRM NIST 1549a – Powdered milk). Furthermore, good precision was found (RSD < 10%), with acceptable linear working ranges and good linearity (R≥ 0.999), and no matrix effect. Limits of detection ranged from 2 (P) to 16 μg g-1 (Ca) (MW procedure) and 0,3 (Cu) to 14 μg g-1 (Na) (closed block). The validated procedures were applied to analyze nine white cheese samples of different types (buffalo mozzarella, minas fresh cheese, ricotta, curd, and cottage cheese). It was observed that the highest concentrations were for Ca, followed by P, Na, and K. The lowest concentrations were observed for the microelement Cu, followed by Zn. It was found that the type of cheese influenced the variation in nutrient content. Furthermore, for all types of cheese, a discrepancy (p<0.05) was observed between concentrations for at least one analyte, confirming the lack of uniformity in nutritional composition. The recommended daily intake (RDI) evaluation showed that all samples of buffalo mozzarella cheese and Minas fresh cheese analyzed could be considered sources of Ca, P, and Zn. Regarding sodium, only the ricotta of F brand had concentrations below the recommended by law (>80 g 30 g-1 ). Additionally, these contents were significantly higher than those declared on the labels. Universidade Federal da Bahia Dissertação

Estudo computacional das interações intermoleculares do tipo O-H...π em pequenos complexos com água

2023-10-21T05:05:12Z

Estudo computacional das interações intermoleculares do tipo O-H...π em pequenos complexos com água Oliveira, Murillo Halo Queiroz de Moreno, Roberto Rivelino de Melo The study of weakly bound intermolecular systems involving polycyclic aromatic hydrocarbons (PAH`s) and water agglomerates has opened new perspectives for a better understanding of the mechanisms of formation of aggregates in interstellar regions. In the special, for the understanding of the initial process of ice formation at cold molecular clouds. In this dissertation, we investigate three intermolecular complexes of water with aromatic molecules: (i) benzene-water 1:1, (ii) naphathalene-water dimer; and (iii) acenaphthylene-water trimer. We use as a theoretical procedure ab initio methods and density functional theory (DFT). Initially, to establish an appropriate methodology for the problem, we carried out one benchmark for the benzene-water system. In this case, we employ different density functionals containing dispersion corrections (DFT-D) e with non-local exchange energy (HMDFT), combined with atomic bases containing polarization and diffuse functions. Our results were compared with these obtained via perturbative methods. Subsequently, we studied geometric, energetic, vibrational aspects and kinect parameters of the naphathalene-water dimer and acenaphthylene-water trimer with DFT-D and HMDFT. Our targets have consisted of investigating the structural mobility of small water aggregates in the presence of PAH´s, evaluate the impact of the formed interaction in molecular properties (e.g dipole moment, Raman scattering of the fundamental modes) and evaluate the reorientational kinects of these complexes. Universidade Federal da Bahia Dissertação