Tentative Determination of the Acidity Level in Room Temperature Ionic Liquids by Electrochemical Methods

In our attempt to evaluate the acidity levels reached by acidified ionic liquids (BMImBF4, BMImNTf2 and BMImOTf + HOTf or HNTf2), the uncertainty on the pKas of the indicators needed for the Hammett spectrophotometric procedure was pointed out. As consequence another method is proposed, based on the H+/H2 couple potential measurement. In this purpose, if dynamic methods failed mainly for lack of sufficient reversibility, potentiometry with a hydrogen electrode gave meaningful results. The R0(H+) Strehlow function, could be calculated, using the Fc+-Fc couple as reference assumed as solvent independent. The obtained results show that i) the acidities are much higher than those in water; ii) the acidities measured by the hydrogen electrode are higher than those measured by the Hammett method; iii) the sequence of acidities for solutions of similar content of added acid is still BF4- > NTf2- > OTf- as previously measured with the Hammett method.