Análise por injeção em batelada associada à detecção amperométrica: desenvolvimento de métodos rápidos para a determinação de antioxidantes em biodiesel

Abstract

This dissertation presents, for the first time, the application of Batch Injection Analysis (BIA) with amperometric detection for the determination the phenolic antioxidants butylated hydroxy-anisole (BHA) and tert-butylhydroquinone (TBHQ) in biodiesel samples. The developed methodologies have high potential to be applied in the field and also to be miniaturized. The biodiesel instability due to oxidation processes caused during storage, transport and use, makes necessary the addition of antioxidants in the biofuel. The content of antioxidants in biodiesel can be associated with its oxidative stability, which is typically measured by the Rancimat method and requires hours (minimum time of analysis of a sample within the specification is 6 hours). In the dissertation, first, it is presented the development of a method for the individual determination BHA and subsequently for the simultaneous determination of BHA and TBHQ. In the BIA system, the samples were injected directly onto the working electrode (boron-doped diamond or glassy-carbon) immersed in 50% (v/v) hydroethanolic solution with 0.1 mol L-1, using an electronic micropipette. Importantly, the only preparation step required for biodiesel analysis is a simple dilution of the sample in the electrolyte hydroethanolic. After optimization of the BIA system and the choice of the working potential (1.0 V), BIA with amperometric detection was applied for the determination of BHA in biodiesel using a boron-doped diamond electrode. Among the several advantages of the developed method for routine analysis of biodiesel, include: a low relative standard deviation between injections (0.29%, n=20), high analytical frequency (120 h−1), adequate recovery values (93 101%) for spiked samples, satisfactory accuracy (based on determinations by high-performance liquid-chromatography), and a low detection limit (50 nmol L-1). In order to perform the simultaneous determination of TBHQ and BHA, the multiple pulse amperometry (MPA) was chosen as detection technique. The technique allows the application of a sequence of potential pulses and continuous monitoring of the current in each pulse. We chose the potential of 0.6 V for the selective determination of TBHQ in the presence of BHA and, the potential of 1.0 V, at which occurs the electrochemical oxidation of BHA and TBHQ. The current contribution from the oxidation of TBHQ can be subtracted from the current response of BHA, and thus BHA could be quantified. The amperometric method for the simultaneous determination of BHA and TBHQ in the BIA system using a glassy-carbon electrode produced satisfactory results for its application in routine analysis and in field (on-site analysis), including: low relative standard deviation between injections (RSD <1%; n=10), fast (170 injections per hour), satisfactory accuracy (recoveries between 100 and 110%), and low detection limit (73 and 75 nmol L-1 for BHA and TBHQ, respectively).