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

Carbon paste electrodes modified 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 sulfite 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 modified at Para-position with substituents that are electron withdrawing: -CN (CILE/CN) and -CF3 (CILE/CF3) and substituents that are electron donating: -CH3 (CILECH3) and -OCH3 (CILE/OCH3), and compared with the ionic liquid without substituents (CILE/OPy). The results showed that CILEs are capable of catalyzing the sulfite oxidation, shifting the oxidation potential to more negative values compared to CPE. Also, they showed linear correlations between increasing sulfite concentration and increasing current density. The best system in terms of sensitivity was the electrode modified with the 4-methyl-N-octylpyridiniumhexafluorophosphate CILE/CH3 and. then, that modified with the non-substituted ionic liquid CILE/OPy measured by amperometry. In terms of potential, the best systems are the CILEs modified 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 hinders 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 he applied in food industry for samples without polyphenols at concentrations higher than ca. 1 mM of sulfite. 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 sulfite from wastewaters. Finally, to our knowledge, there are not comparative studies about the effect of changing the substituent of the cation in electrocatalysis of modified IL-electrodes.