Kanzaki Ryo, Uchida Kuniaki, Song Xuedan, Umebayashi Yasuhiro, Ishiguro Shin-ichi
Department of Chemistry, Faculty of Science, Kyushu University, Higashi, Fukuoka 812-8581, Japan.
Anal Sci. 2008 Oct;24(10):1347-9. doi: 10.2116/analsci.24.1347.
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.
硝酸乙铵（EAN）由C(2)H(5)NH(3)(+)和NO(3)(-)离子组成，它们分别表现为酸和碱。由于质子从C(2)H(5)NH(3)(+)转移到NO(3)(-)，因此该离子液体含有少量的C(2)H(5)NH(2)和HNO(3)分子。通过在298K下使用离子选择性场效应晶体管和氢电极的电位滴定法，得到平衡常数K(s)（= [C(2)H(5)NH(2)][HNO(3)]），其对应于水的自质子解离常数，约为10(-10) mol(2) dm(-6)。该值表明纯EAN中含有约10(-5) mol dm(-3)的C(2)H(5)NH(2)和HNO(3)分子。另一方面，在EAN-水混合物中，水分子表现为碱。在各种溶剂组成的EAN-水混合物中测定了表观pK(s)值。有趣的是，在EAN摩尔分数为0.05 - 0.9的范围内，混合物中的pK(s)值保持在10.5。该值接近水溶液中C(2)H(5)NH(2)的pK(a)，即C(2)H(5)NH(3)(+)的酸解离常数。这意味着反应C(2)H(5)NH(3)(+) + H(2)O --> C(2)H(5)NH(2) + H(3)O(+)在很宽的溶剂组成范围内决定了pK(s)。因此，纯EAN中的pK(s)值略小于混合物中的pK(s)值，这意味着在EAN溶液中，H(3)O(+)是比HNO(3)更强的酸，这与水不同。
