Determinação voltamétrica de zinco empregando eletrodos quimicamente modificados com bismuto

This dissertation presents the development of voltammetric methods to zinc determination in multivitamin commercial samples, talc, and art materials for painting (soft pastel) combining an alkaline extraction with 1.0 mol L-1 NaOH aqueous solution and bismuth modified electrodes. Two electrodes were used to zinc quantification in the samples, bismuth film electrode (BiFE) plated in situ onto glassy carbon and carbon paste electrode chemically modified with strongly acidic ion exchange resin Amberlite® IR 120 and bismuth nanostructures (EPCAmbBi). It was verified that the best concentration of Bi3+ for Bi film deposition onto glassy carbon was 4.0 μmol L-1 using an 0.1 mol L-1 acetate buffer aqueous solution (pH = 4.5) as supporting electrolyte. The best condition to formation of Bi nanostructures in the EPC modified with 10 % Amberlite® IR 120 was the use of 30 s to pre-concentration (open circuit) in 0.5 mmol L-1 Bi3+ aqueous solution (pH 5.5) prepared with supporting electrolyte solution. The obtained analytical curve for Zn2+ using BiFE presented linear range from 0.5 to 5.0 μmol L-1, the limit of detection (LD) was 41 nmol L-1. For EPCAmbBi only one linear range was observed for the analytical curve varying the Zn2+ concentration from 0.05 to 8.2 μmol L-1, LD obtained in this curve it was equal to 10 nmol L-1. The EPCAmbBi presented the most intense and sharp anodic stripping peaks for Zn2+ presenting, therefore, a better voltammetric profile, with sensitivity higher than obtained with the BiFE. Moreover, the EPCAmbBi presented a LD lower than that obtained with the BiFE. Alkaline extraction was an efficient sample pretreatment to extract Zn2+ from solid samples, besides that, this procedure was less susceptible to interferences from Cu2+, since it remains at extracting vessel as insoluble Cu(OH)2. The combination of alkaline extraction with the EPCAmbBi is a simple, fast, efficient and low cost for the zinc determination in pharmaceutical formulations and art materials for painting (soft pastel) samples, which can be employed as a low-cost alternative method to the atomic absorption spectroscopy.

Avaliação das condições de precipitação do azul da Prússia sobre pó de grafite visando o desenvolvimento de eletrodos de pasta de carbono para a quantificação de nitrito

An amperometric FIA method for nitrite quantification based on nitrite electroreduction and employing a carbon paste electrode (CPE) chemically modified with iron hexacyanoferrate (HCF) as an amperometric detector was developed. The influence of experimental conditions on the preparation of the electrode materials was evaluated and the materials obtained in each study were used for the development of modified electrodes. The electrochemical sensors were prepared by a fast, simple, and inexpensive procedure, and the long-term performance of the electrodes were quite satisfactory as the stability was maintained over one year. HCF was an effective redox mediator for nitrite electroreduction in acidic media, allowing nitrite detection at +0.2 V vs. Ag/AgClsat, which is a potential free of possible interfering species that are normally present in food and water samples. The electrochemical cell used in the FIA system was similar to a batch injection analysis cell, enabling recirculation of the carrier solution. This is an attractive feature because it allows the use of a high flow rate (6 mL min-1) leading to high sensitivity and analysis speed, while keeping reagent consumption low. The proposed method had a detection limit of 9 μmol L-1 and was successfully employed for nitrite quantification in spiked water and sausage samples. The obtained results were in good agreement with those provided by the spectrophotometric official method. At a 95 % confidence level it was not observed statistical differences neither in nitrite content nor in the precision provided by both methods. The experimental conditions for the synthesis of HCF were optimized and the best electrode material was prepared by mixing FeCl3, K4[Fe(CN)6] and carbon powder subjected to an acid and thermal treatment (400 ºC), followed by ultrasonic agitation at 4 °C. This material was used to construct an electrode with improved analytical performance to reduce nitrite, which presented greater stability compared to HCF film electrodeposited on the EPC, showing that the preparation procedure of the electrode material is an effective strategy for the development of HCF modified electrodes.

Projeto iterativo, simulação, análise e otimização de máquina a relutância variável monofásica

Variable reluctance motors have been increasingly used as an alternative for variable speed and high speed drives in many industrial applications, due to many advantages like the simplicity of construction, robustness, and low cost. The most common applications in recent years are related to aeronautics, electric and hybrid vehicles and wind power generation. This paper explores the theory, operation, design procedures and analysis of a variable reluctance machine. An iterative design methodology is introduced and used to design a 1.25 kW prototype. For the analysis of the machine two methods are used, an analytical method and the finite element simulation. The results obtained by both methods are compared. The results of finite element simulation are used to determine the inductance profiles and torque of the prototype. The magnetic saturation is examined visually and numerically in four critical points of the machine. The data collected in the simulation allow the verification of design and operating limits for the prototype. Moreover, the behavior of the output quantities is analyzed (inductance, torque and magnetic saturation) by variation of physical dimensions of the motor. Finally, a multiobjective optimization using Differential Evolution algorithms and Genetic Algorithms for switched reluctance machine design is proposed. The optimized variables are rotor and stator polar arcs, and the goals are to maximize the average torque, the average torque per copper losses and the average torque per core volume. Finally, the initial design and optimized design are compared.

Efeito da concentração de sólidos e de variáveis operacionais no desempenho de um hidrociclone concentrador

The hydrocycloning operation has a goal to separate solid-liquid suspensions and liquid-liquid emulsions through the centrifugal force action. Hydrocyclones are equipment with reduced size and used in both clarification and thickening. This device is used in many areas, like petrochemical and minerals process, and accumulate advantages like versatility and low cost of maintenance. However, the demand to improve the process and to reduce the costs has motivated several studies of equipment optimization. The filtering hydrocyclone is a non-conventional equipment developed at FEQUI/UFU with objective to improve the hydrocycloning separation efficiency. The purpose of this study is to evaluate the operating conditions of feed concentration and underflow diameter on the performance of a filtering geometry optimized to minimization of energy costs. The filtration effect was investigated through the comparison between the performance of the Optimized Filtering Hydrocyclone (HCOF) and the Optimized Concentrator Hydrocyclone (HCO). Because of the resemblance of hydrocyclones performance, the filtration did not represent significant effect on the performance of the HCOF. It was found that in this geometry the decrease of the variable underflow diameter was very favorable to thickening operation. The suspension concentration of quartzite at 1.0% of solids in volume was increased about 42 times when the 3 mm underflow diameter was used. The increase on the feed solid percentage was good for decreasing the energy spent, so that a minimum number of Euler of 730 was achieved at CVA = 10.0%v. However, a greater amount of solids in suspension leads to a lower efficiency of the equipment. Therefore, to minimize the underflow-to-throughput ratio and keep a high efficiency level, it is indicated to work with dilute suspension (CVA = 1.0%) and 3 mm underflow diameter (η = 67%). But if it is necessary to work with high feed concentration, the use of 5 mm underflow diameter provides a rise in the efficiency. The HCO hydrocyclone was compared to the traditional family of hydrocyclones Rietema and presented advantages like higher efficiency (34% higher in average) and lower energy costs (20% lower in average). Finally, the efficiency curves and project equation have been raised for the HCO hydrocyclone each with satisfactory adjust.