Acidity and basicity of aqueous mixtures of a protic ionic liquid, ethylammonium nitrate

Ethylammonium nitrate (EAN) is composed of C(2)H(5)NH(3)(+) and NO(3)(-) ions, which behave as an acid and a base, respectively. The ionic liquid thus involves small amounts of C(2)H(5)NH(2) and HNO(3) molecules owing to proton transfer from C(2)H(5)NH(3)(+) to NO(3)(-). The equilibrium constant K(s) (= [C(2)H(5)NH(2)][HNO(3)]), which corresponds to the autoprotolysis constant of water, was obtained to be ca. 10(-10) mol(2) dm(-6) by potentiometry using an ion-selective field-effect transistor and hydrogen electrodes at 298 K. The value indicates that C(2)H(5)NH(2) and HNO(3) 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 C(2)H(5)NH(2), or the acid-dissociation constant of C(2)H(5)NH(3)(+), in aqueous solution. This implies that the reaction C(2)H(5)NH(3)(+) + H(2)O --> C(2)H(5)NH(2) + H(3)O(+) 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 H(3)O(+) is a stronger acid than HNO(3) in an EAN solution, unlike water.