Ethylammonium nitrate (EAN) is a protic room-temperature ionic liquid and classified into an amphoteric solvent. The equilibrium constant KS = [EtNH2][HNO3] has directly been obtained for the first time by using an ion-selective field effect transistor (ISFET) electrode. The obtained KS value is of 10−10.0 mol2 dm−6, indicating that the neutral EAN involves HNO3 and EtNH2 molecules as much as 1.0 × 10−5 mol dm−3, or the pH (= −log[HNO3]) is 5 at 298 K.

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