The possibility of using thiocyanate to determine iron(II) and/or iron(III) in water-acetone mixture has been re-examined as part of a systematic and comparative study involving metallic complexes of pseudohalide ligands. Some parameters that affect the complete oxidation of the ferrous cations, their subsequent complexation and the system stability have been studied to optimize the experimental conditions. Our results show the viability and potentiality of this simply methodology as an alternative analytical procedure to determine iron cations with high sensitivity, precision and accuracy. Studies on the calibration, stability, precision, and effect of various different ions have been carried out by using absorbance values measured at 480 nm. The analytical curve for the total iron determination obeys Beer's law (r = 0.9993), showing a higher sensitivity (molar absorptivity of 2.10x10(4) L cm-1 mol-1) when compared with other traditional systems (ligands) or even with the "similar" azide ion [1.53x10(4) L cm-1 mol-1, for iron-III/azide complexes, in 70% (v/v) tetrahydrofuran/water, at 396 nm]. Under such optimized experimental conditions, it is possible to determine iron in the concentration range from 0.5 to 2 ppm (15-65% T for older equipments, quartz cells of 1.00 cm). Analytical applications have been tested for some different materials (iron ores), also including pharmaceutical products for anemia, and results were compared with atomic absorption determinations. Very good agreement was obtained with these two different techniques, showing the potential of the present experimental conditions for the total iron spectrophotometric determinations (errors < 5%). The possibility of iron speciation was made evident by using another specific and auxiliary method for iron(II) or (III).
Dissertação apresentada à Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto da USP, como parte das exigências para a obtenção do título de Mestre em Ciências, Área: Química RIBEIRÃO PRETO -SP Agradecimentos Aos colegas do laboratório: Adriano, Ana Paula, Cíntia, Cristina, Denise, Olímpia, Leandro, pelo companheirismo inigualável. Aos amigos e colegas que fiz durante a graduação, em especial à Vânia, Daniela e Cristiano. Ao apoio de todos os amigos do Departamento de Química, Roberta, Luciene, Luiza, dentre outros. Aos funcionários do Departamento de Química, e em especial, à Seção de Pós-Graduação, pela atenção dispensada. Aos docentes do Departamento de Química, pelos conhecimentos e experiências compartilhados. A todos meus familiares, pela paciência e carinho. Ao CNPq, pela bolsa fornecida. Ao Departamento de Física, pela disponibilidade do programa de interfaceamento computacional. A todos aqueles que, direta ou indiretamente, contribuíram para a realização deste trabalho. ÍNDICE página ÍNDICE DE FIGURAS i ÍNDICE DE TABELAS iv LISTA DE ALGUNS SÍMBOLOS E ABREVIATURAS v RESUMO vii ABSTRACT ix I -INTRODUÇÃO 01 I.1. CONSIDERAÇÕES GERAIS 01 I.2. PSEUDO-HALETOS E HALOGENÓIDES 11 I.2.1. Aspectos Gerais 11 I.2.2. Tiocianato, SCN 15 I.2.3. Tiocianogênio, (SCN) 2 16 I.2.4.O sistema Ferro/ Tiocianato 18 II -OBJETIVOS 20 III -PARTE EXPERIMENTAL 22
IntroductionAzide and thiocyanate complexes of several metallic cations have been systematically studied in our laboratories with a view to understanding their coordination chemistry (equilibria) [1][2][3] and developing their analytical applications [4][5][6][7][8][9][10][11][12]. There is great analogy between the azide and the thiocyanate ligands (pseudohalides), since they develop the same red color in acid solution containing iron(III). Some of our studies have been of a comparative character, showing that the azide system is much more stable than the thiocyanate one. As part of these continuous investigations, it is expected that chromium(III) will form complexes with the pseudohalide azide in the same way that other transition metals such as iron, copper, nickel, and cobalt.In the literature, studies carried out by Sherif and Orab [13,14] in aqueous medium using the visible region revealed the existence of a relationship between the several free ligand (azide) and chromium(III) concentrations with absorbance, producing colorings ranging from violet to green. Moreover, the solution was reported to develop blue tones under great ligand excess. In the work of Templeton & King [15], the effect of perchloric acid concentration on the formation of pentaaquoazidochromium(III) compounds was also reported. The main objective of the latter work was to determine the kinetics and equilibrium of this system. The authors reported maximum absorption at 270 nm, under highly acid concentrations, which was attributed to interactions between azidochromium(III) and/or hydrogen azidochromium(III) and the perchlorate ion. The Abstract: A sensitive and alternative method for the spectrophotometric determination of chromium(III) based on the formation of chromium(III)/azide complexes was established by investigating a new band in the ultraviolet region. The best experimental conditions for the analytical determination of this metallic ion were: ligand and perchloric acid analytical concentration = 493 and 12.0 mmol L -1 , respectively; aqueous medium; T = 25.0 ºC; contact time = 1 hour. The maximum molar absorptivity coefficient occurred at 287 nm (average 1.481 ± 0.008 × 10 4 L mol -1 cm -1 ), leading to the determination of metal ion concentrations one hundred times lower than the ones formerly determined in the visible region. The system obeys Beer's Law and is suitable for chromium determination in the 0.702-2.81 mg L -1 concentration range (15-65% T, 1.00 cm-width quartz cells).Analytical applications of the current method were tested with a nutritional supplement containing chromium. Results were compared with those obtained with atomic absorption spectrometry.
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