Analytical Sciences

Abstract − Analytical Sciences, 24(10), 1347 (2008).

Acidity and Basicity of Aqueous Mixtures of a Protic Ionic Liquid, Ethylammonium Nitrate
Ryo KANZAKI, Kuniaki UCHIDA, Xuedan SONG, Yasuhiro UMEBAYASHI, and Shin-ichi ISHIGURO
Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki, Higashi, Fukuoka 812-8581, Japan
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 Ks (= [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 pKs value was determined in EAN-water mixtures of various solvent compositions. Interestingly, the pKs 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 pKa 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 pKs over a wide range of solvent composition. The pKs 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.