In situ electrochemical determination of free Cu(II) ions in biodiesel using screen-printed electrodes: Direct correlation with oxidation stability

Abstract

This work presents a simple and sensitive method to measure in situ free Cu(II) ions in biodiesel using organic-resistant screen-printed gold electrodes (SPGEs) and demonstrates the direct correlation of the content of Cu(II) ions with oxidation stability of biodiesel. Biodiesels contaminated with copper due to corrosion, spiking with standard solutions, and naturally occurring from the oil source, were analyzed after simple dilution in 90% (v/v) ethanol and 10% (v/v) water containing 0.1 mol L−1 HCl as electrolyte (final concentration). Square-wave anodic stripping voltammetry under hydroethanolic media enabled the determination of Cu(II) ions in biodiesel, while the total Cu content was determined by atomic absorption spectrometry. The Rancimat method measured the oxidation stability of biodiesels based on induction period (IP) values, and the results indicate a decrease in IP values due to the catalytic effect of copper in biodiesel, which present direct correlation with the free content of Cu(II) rather than the total copper content. Hence, we recommend the measurement of free metal ions in biodiesel to predict its lower oxidation stability using disposable SPGEs for in-loco analysis. This voltammetric sensor responds to Cu(II) ions in hydroethanolic media for 80 consecutive measurements before its replacement and it is considered highly-repetitive (RSD < 5% for n = 10) with a low detection limit (1.5 µg L−1 corresponding to 1.70 µg kg−1).