Real Time Monitoring of Hydrogen Peroxide Consumption in an Oxidation Reaction in Molecular Solvent and Ionic Liquids by Hydrogen Peroxide Electrochemical Sensor

An efficient electrochemical protocol to monitor hydrogen peroxide consumption during metal-catalyzed oxidation by using screen-printed electrodes modified with Prussian blue is presented. In particular, cyclooctene oxidation to cyclooctene oxide, catalyzed by a vanadium(V)–salophen complex (H2salophen=N,N′-o-phenylenebis(salicylideneimine)), in molecular and ionic media was tested. Initially, a protocol for batch analysis was developed for a monophasic system in acetonitrile, and subsequently, an in situ protocol was developed for a biphasic system of 1-butyl-3-methylimidazolium hexafluorophosphate/phosphate buffer. Calibration curves were performed in amperometric mode by applying −50 mV versus an Ag pseudo-reference. The calibration curve of hydrogen peroxide showed a linear correlation from 1×10−6 up to 5×10−3 mol L−1 with satisfactory inter- and intra-electrode reproducibility (relative standard deviation (RSD) values of 5 and 13 %, respectively, for the monophasic system and 11 and 13 %, respectively, for the biphasic system). Kinetic studies to investigate the oxidation reaction for both the mono- and biphasic systems have been carried out in amperometric mode as well. Firstly, the decomposition of hydrogen peroxide was examined, which showed that, in 1-butyl-3-methylimidazolium hexafluorophosphate, it completely decomposed in 300 min, whereas in acetonitrile, in the same time frame, 20 % of the initial amount was still active. In the presence of 1 % of the catalyst the decomposition rate increased in both solvents. Finally, the complete oxidation of cyclooctene was followed and the effective conversion was determined. The developed protocols showed high reproducibility, with the advantage that the environmentally friendly biphasic system could also be recycled. The good analytical performance obtained, coupled with a short analysis time, the possibility of in-line automation and the use of ionic liquids instead of molecular solvents, made this system a very attractive choice for monitoring oxidative reactions.