Determination of Na, K, Mg and Ca in biodiesel by LS F AAS and HR-CS F AAS: studies that supported the proposal of the ABNT NBR 15556 norm

Abstract

Recentemente, a Associação Brasileira de Normas Técnicas (ABNT) lançou uma série de normas, relacionadas com a determinação de Na, K, Mg e Ca em biodiesel, recomendando procedimentos similares àqueles das normas europeias e americanas, mas também introduzindo uma norma alternativa (ABNT NBR 15556), que possibilita a determinação dos 4 elementos usando a técnica de espectrometria de absorção atômica com chama (F AAS). Neste trabalho, apresentam-se os estudos de otimização e validação que dão suporte a essa norma, bem como a comparação do desempenho de um equipamento com fonte de linhas com outro de fonte contínua. Entre os fatores otimizados, os baixos valores de taxa de aspiração e o uso de uma solução de óleo mineral em xileno para o ajuste da viscosidade entre soluções das amostras e de calibração devem ser destacados. Testes de precisão (repetitividade e precisão intermediária) mostraram valores adequados, comprovando a robustez do método, e os limites de detecção estiveram confortavelmente abaixo daqueles propugnados pelas legislações brasileira e internacional. A exatidão foi confirmada por testes de recuperação e pela participação em exercícios interlaboratoriais de proficiência promovidos pela Agência Nacional de Petróleo, Gás Natural e Biocombustíveis (ANP). A determinação dos 4 elementos por F AAS traz evidentes vantagens econômicas ao produtor, uma vez que se trata de um equipamento mais barato na sua aquisição e manutenção.Recently, ABNT (Brazilian Association of Technical Standards) has presented a set of norms related to the determination of Na, K, Mg and Ca in biodiesel, recommending procedures similar to those of the European and American norms. However, it has also introduced an alternative norm (ABNT NBR 15556) wherein these 4 elements can be determined by flame atomic absorption spectrometry (F AAS). This work presents the studies that have supported this proposal and compares the performance of linear source equipment with a continuous source one in the application of the norm. Among the optimized factors, the low aspiration rate and the use of mineral oil solution in xylene to adjust the viscosity between samples and calibration solutions are highlighted. Precision tests (repeatability and intermediary precision) have shown adequate values, confirming the robustness of the method. Limits of detection were suitably below the legislation limits. The accuracy was confirmed by recovery tests as well as by the participation in interlaboratory proficiency tests promoted by the National Agency on Oil, Natural Gas and Biofuels (ANP, Brazil). The determination of these 4 elements by F AAS brings evident economic advantages to the producer, once F AAS is less expensive to purchase and maintenance.Keywords: biodiesel, ABNT NBR 15556 norm, metals, flame atomic absorption spectrometry Oliveira et al. 1401 Vol. 23, No. 7, 2012 Introduction Nowadays biodiesel represents a real alternative to diesel for internal combustion engines. The use of biodiesel has the potential to lower the environmental impact o...

Determination of Silicon in Vegetable Oil and Biodiesel by High-Resolution Continuum Source Flame Atomic Absorption Spectrometry Using Sample Dilution with Xylene

“…For external calibration, as occurs for other procedures in which sample pretreatment consists only of a dilution with an organic solvent, it is necessary to adjust the viscosity of the calibration solutions. On the basis of studies reported by Oliveira et al a 120 g L –1 mineral oil solution in xylene was chosen as the diluting medium, because this way the calibration solutions showed a final viscosity similar to those of the diluted samples. Another important point to be considered is the stability of the solutions in xylene.…”

“…Metal solutions in organic medium are unstable, and the analytical signal tends to decrease with time. Thus, because of this lack of stability, it is important to analyze the calibration and sample solutions immediately after their preparation, making the use of the whole automatic sampler carrousel difficult. , …”

“…Approximately 1.0 g of vegetable oil or biodiesel was accurately weighed in a 15 mL polypropylene flask and diluted to 10 mL with xylene. Calibration solutions were prepared from the intermediary (100.0 mg L –1 ) Si organometallic solution and adequately diluted with the stock solution of oil in xylene (120.0 g L –1 ) to match the viscosity of these solutions with those of the diluted samples …”

“…Also, a well-established cattle meat productive chain assures the availability of large amounts of animal fat, a raw material for biodiesel production as well . The diversity of feedstock compels the development of advanced techniques to ensure the quality of the resulting biodiesel fuel. − …”