Sistema de análise em fluxo contínuo para determinação de acidez em amostras de vinagre e óleos vegetais

Abstract

This work describes the development of an analytical method to determine the acidity index in vinegars and vegetable oils using Flow Injection Analysis systems (FIA). As the sensor, a portable colorimeter was constructed using an arduino uno® board for sign readings, a light emitting diode (LED) as a radiation source, photodiode as detector and an acrylic flow cuvette (optical pass of 2 cm). In the determination of acetic acid in Vinegar, the effects of chemical and flow factors were performed using the two-level factorial design in a multivariate optimization of the system. The linear response for the analytical curve, obtained with aqueous HCl, was 3.55x10-4 g L-1 to 1.10 g L-1, and the analytical frequency was approximately 30 readings per hour. The detection and quantification limits were 9.28x10-5 and 3.55x10-4 g L-1, respectively. Precision was evaluated in terms of the relative standard deviation (n = 10) for HCl concentrations of 0.005 mol.L-1 and 0.01 mol.L-1 as 0.58 and 1.2% respectively. Accuracy was checked using comparison with conventional titration, and the results were consistent (paired t-test, p = 0.05). It was not possible to use the FIA system to determine the acidity of vegetable oils. The sample immiscibility in the carrier solution caused perturbations in the flow, formation of emulsions and non-repeatability of the signals, becoming impossible the application of this method. Instead, it was used the sodium dodecyl sulfate (SDS) surfactant in substitution of the organic solvent (ether) in the standard titration. The results were satisfactory with the advantage of the SDS to be biodegradable and less aggressive to the environment. Determinations of the acid value, in samples of vegetable oils from local market using SDS, showed the same results as the standard titration, being a plausible alternative for this application.