Caracterizações e aplicações analíticas de eletrodos compósitos modificados com Azul da Prússia e determinações simultâneas em sistemas de análise por injeção em batelada empregando somente um eletrodo de trabalho

Abstract

The present thesis demonstrates some potentialities of flow injection analysis (FIA) and batch injection analysis (BIA) coupled with amperometric detection to the development of routine methods applicable in industry (food, pharmaceutics and biofuels). In this way, a composite electrode modified with Prussian blue was used to determination of hydrogen peroxide in milk and antiseptic mouthwash. Moreover, a boron-doped diamond electrode was employed in simultaneous determinations of pharmaceutics presents in comercial tablets (paracetamol and caffeine; dipyrone and caffeine) and three biodiesel preservatives: tert-butylhydroquinone (TBHQ), butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT). The Prussian-blue bulk modified graphite-composite electrode was made from a fluid graphite-composite prepared in laboratory (graphite / epoxy adhesive / cyclohexanone / graphite modified with Prussian blue). The best results were obtained when the graphite modified with Prussian blue were added in the fluid composite in proportion of up to 30 % (m/m). This modified composite was inserted in distinct supporting materials: (1) micropipette tip (Ø = 1.6 mm, three-electrode-integrated sensor) and (2) polyamide tube (Ø = 7.2 mm). Respectively, these electrodes were used in FIA and BIA systems and hydrogen peroxide was selectively detected by amperometry. Employing both electrochemical systems and appropriate experimental conditions (supporting electrolyte: 0.1 mol L-1 KCl and 0.05 mol L-1 phosphate buffer solution (pH = 7.2); injected volume: 100 μL; flow rate (FIA): 16.7 μL s-1; dispensing rate (BIA): 100 μL s-1), recovery values close to 100 % were obtained for milk and antiseptic mouthwash samples. The FIA and BIA methods showed high analytical frequency (100 h-1 and 80 h-1) and low limits of detection (0,8 μmol L-1 and 0,19 μmol L-1). In another aim, the methods for simultaneous determinations (pharmaceutics and antioxidants) were based on batch injection analysis with multiple pulse amperometric detection (BIA-MPA). In the simultaneous determination of paracetamol and caffeine, each optimized potential pulse was applied in sequence on boron-doped diamond working electrode for a specific purpose: a) +1.20 V (40 ms): paracetamol oxidation and selective quantification; b) +1,55 V (40 ms): oxidation of both compounds. Caffeine can then be quantified by subtraction of the currents obtained from both potential pulses and using a correction factor. In the simultaneous determination of dipyrone and caffeine the same strategy was employed. However, the potential pulses adopted were +1.1 V and +1.55V. The experimental conditions optimized to the analysis of pharmaceutics were: 0.1 mol L-1 acetate buffer (supporting electrolyte), injected volume = 100 μL and dispensing rate = 156 μL s-1. In these conditions, BIA-MPA presented high-throughput analysis (160 injections per hour), elevated precision (RSDs < 2%, n = 10) and low limits of detection (paracetamol: 1.72 μmol L-1; caffeine: 0.84 μmol L-1; dipyrone: 0.72 μmol L-1). In the simultaneous determination of TBHQ, BHA and BHT by BIA-MPA, the following potential pulses were applied in the BDD working electrode: a) + 0.6 V (50 ms): TBHQ oxidation and selective quantification; b) + 0.8 V (50 ms): TBHQ and BHA oxidations without interference of BHT; c) + 1.3 V (50 ms): oxidation of TBHQ, BHA and BHT. Simple mathematic equations (and correction factors) were used to obtain the currents of each compound. In the simultaneous determination of these compounds in biodiesel, hydroethanolic (50 % ethanol, v/v) containing 0.1 mol L-1 KCl + HCl solution was used as supporting electrolyte. The optimized experimental conditions were: injected volume: 200 μL; dispensing rate: 256 μL s-1; stirring rate of BIA cell: 2200 rpm. In these conditions, the BIA-MPA exhibited the following results, respectively to TBHQ / BHA / BHT: limits of detection of 2.30 μmol L-1 / 0.18 μmol L-1 / 0.54 μmol L-1, RSD (n = 20) of 0.68 % / 0.99 % / 1.75 % and analytical frequency of 120 injections per hour. After the simultaneous determination of these antioxidants in soybean biodiesel we obtained recovery close to 100 % to TBHQ and BHA and close to 50 % to BHT.