Determinação de etanol e metanol em álcool combustível e gasolina empregando métodos eletroanalíticos

Abstract

In this present work two analytical procedures are presented. The first deals with the simultaneous determination of ethanol (EtOH) and methanol (MetOH) in fuel ethanol samples using Cyclic Voltammetry (CV) and the second, the determination of EtOH in fuel ethanol and gasohol using Batch Injection Analysis with Multiple Pulses Amperometric detection (BIA-MPA). In both procedures, gold is used as working electrode and NaOH solution (0.5 mol L-1) as supporting electrolyte. In this condition, EtOH is electrochemically oxidized at +0.19 V (without interference of MetOH) and at +1.20 V, both compounds (EtOH and MetOH) are simultaneously oxidized. Employing CV, the strategy is the EtOH determination at +0.19 V and both compounds (EtOH and MetOH) at +1.20 V. By the difference of signal detected in the two potentials regions and using a correction factor, the MetOH quantification was also possible. In this procedure, the linear response range for simultaneous determination of EtOH and MetOH was between 0.070 and 0.346 % (v/v) (R = 0.999) and 0.030 to 0.149 % (v/v) (R = 0.993), respectively. The limit of detection (LD) and quantification (LQ) was calculated, respectively, 0.028 and 0.093 % (v/v) for EtOH and 0.045 and 0.149 % (v/v) for MetOH. Using the BIA-MPA procedure, the EtOH determination without MetOH interference was possible through the application of the following potentials pulses as a function of time: +0.18 V / 100 ms: EtOH detection without MetOH interference; +0.70 V / 300 ms: AuO formation on the working electrode surface; -0.25 V / 300 ms: AuO reduction formed in the previous step (cleaning / activation of the working electrode).

In this condition, repeatability studies showed RSD value lower than 1.6 % (n = 10). The method presented linear response range for EtOH determination by BIA-MPA between 0.1 to 0.5 % (v/v) (R = 0.998). LD and LQ were calculated as 0.0011 and 0.0033 % (v/v), respectively. The analytical frequency of the system was 180 injections h-1. Both proposed methods have the following characteristics: low cost, simple sample pretreatment procedures (only dilution or extraction followed by dilution) and use of only one reagent in the analysis (0.5 mol L-1 NaOH). The BIA-MPA procedure has two additional advantages: high analytical frequency and possibility of performing on-site analysis (outside the laboratory).