Electrochemical determination of pKa of N-bases in ionic liquid media

Two electrochemical techniques have been used to measure the pK(a) of N-bases in several ionic liquids (ILs). The first method corresponds to a potentiometric titration of a strong acid with the N-base using a platinized Pt indicator electrode immersed in the IL solution and maintained under dihydrogen atmosphere via gas bubbling. The second approach involves performing cyclic voltammetry at a platinized Pt electrode in a solution containing both strong acid and the conjugate weak acid of the N-base. Values of pK(a) obtained by one or the other approach are in good agreement with each other. The experimental data clearly demonstrated that acid/base chemistry in ILs is similar to that observed in molecular nonaqueous solvents; i.e., the relative strengths of the bases were in the right order and spaced (ΔpK(a)). It was also observed that the strength of N-bases is highly dependent on the anion of the ionic liquid; this observation indicates that pH-dependent reactions could be controlled by the appropriate choice of anion for bulk ILs or as an added co-ion to bulk IL.