Electrooxidation of sulfite at carbon paste electrode modified with ionic liquids derivated of N-octyl-pyridinium hexafluorophosphate with different substituents in the cation

Abstract

Carbon paste electrodes modifed with a family of ionic liquids (as binders) derived from N-octyl-pyridinium hexafluorophosphate, (composite-ionic liquid electrodes (CILEs)) were studied toward the oxidation of sulfte as an inner-sphere probe reaction and compared to a conventional carbon paste electrode (CPE) in which the binder was mineral oil. The ionic liquids were modifed at para-position with substituents that are electron withdrawing: -CN (CILE/CN) and -CF3 (CILE/CF3) and substituents that are electron donating: -CH3 (CILE/CH3) and -OCH3 (CILE/OCH3), and compared with the ionic liquid without substituents (CILE/OPy). The results showed that CILEs are capable of catalyzing the sulfte oxidation, shifting the oxidation potential to more negative values compared to CPE. Also, they showed linear correlations between increasing sulfte concentration and increasing current density. The best system in terms of sensitivity was the electrode modifed with the 4-methyl-N-octylpyridiniumhexa?uorophosphate CILE/CH3 and, then, that modifed with the non-substituted ionic liquid CILE/OPy measured by amperometry. In terms of potential, the best systems are the CILEs modifed with ILs with inductive-substituents, -CH3 and -CF3 and OPy, indicating that the delocalization of the charge of the cation produced by the mesomeric-substituent (-OCH3, -CN) diminishes the electrocatalytic behavior of these binders for that oxidation. On the other hand, the analytical parameters of all the amperometric sensors studied here (all CILEs excepts CILE/OCH3) are good enough to be applied in food industry for samples without polyphenols at concentrations higher than ca. 1mM of sulfte. Their stability is very high (at least 100 cycles after obtaining a stable response without changing its current, exposed to air and humidity) and they can be used to remove sulfte from wastewaters. Finally, to our knowledge, there are not comparative studies about the e?ect of changing the substituent of the cation in electrocatalysis of modifed IL-electrodes.