Acidity and Basicity of Aqueous Mixtures of a Protic Ionic Liquid, Ethylammonium Nitrate

Abstract

Ethylammonium nitrate (EAN) is composed of C₂H₅NH₃⁺ and NO₃^- ions, which behave as an acid and a base, respectively. The ionic liquid thus involves small amounts of C₂H₅NH₂ and HNO₃ molecules owing to proton transfer from C₂H₅NH₃⁺ to NO₃^-. The equilibrium constant K_s (= [C₂H₅NH₂][HNO₃]), which corresponds to the autoprotolysis constant of water, was obtained to be ca. 10^-10 mol² dm^-6 by potentiometry using an ion-selective field-effect transistor and hydrogen electrodes at 298 K. The value indicates that C₂H₅NH₂ and HNO₃ molecules of ca. 10^-5 mol dm^-3 are involved in neat EAN. On the other hand, in an EAN-water mixture, a water molecule behaves as a base. The apparent pK_s value was determined in EAN-water mixtures of various solvent compositions. Interestingly, the pK_s value is remained at 10.5 in mixtures over the range of an EAN mole fraction of 0.05 - 0.9. The value is close to the pK_a of C₂H₅NH₂, or the acid-dissociation constant of C₂H₅NH₃⁺, in aqueous solution. This implies that the reaction C₂H₅NH₃⁺ + H₂O → C₂H₅NH₂ + H₃O⁺ is responsible for the pK_s over a wide range of solvent composition. The pK_s value in neat EAN is thus slightly smaller than that in the mixtures, implying that H₃O⁺ is a stronger acid than HNO₃ in an EAN solution, unlike water.