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

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Acidity and Basicity of Aqueous Mixtures of a Protic Ionic Liquid, Ethylammonium Nitrate ANALYTICAL SCIENCES OCTOBER 2008, VOL. 24 1347 2008 © The Japan Society for Analytical Chemistry Notes 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 –5 –3 electrodes at 298 K. The value indicates that C2H5NH2 and HNO3 molecules of ca. 10 mol dm 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 Æ 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. (Received March 4, 2008; Accepted June 24, 2008; Published October 10, 2008) On the other hand, in an EAN–water mixture, the hydrogen- Introduction bonded network structure among ions may be broken, and other types of hydrogen-bonded structure among cation, anion and Ethylammonium nitrate (EAN), a liquid electrolyte at room water may be extensively formed. The physicochemical temperature (the melting point of 285 K), involves dissociable properties may thus strongly depend on the solvent composition, protons, and is a so-called “protic ionic liquid”. Protic ionic or the solvent structure in the mixture. liquids (PILs), potential proton conductors,1–3 can be used as Acidity (or basicity) is one of basic properties of a solvent, medium electrolytes for fuel cells4 and polymer membrane and we have to take the solvent acidity into account when an separators.5 The Grotthuss mechanism has been suggested to be acid and base reaction is involved. This applies to not only a model for proton-transfer dynamics in imidazolium-based conventional solvents but also PILs. However, the acid-base PILs, as well as in water.6 Also, PILs and their aqueous property of PILs has not yet been established. So far, the mixtures can be used as reaction media for biocatalysis,7 and are amounts of molecular species in a given PIL composed of A– + expected to be tunable solvent media for enzymatic activity of and BH has been estimated from DpKa, the difference acid proteins.8 Furthermore, a dissolution of nonpolar gases shows dissociation constants of their conjugate acid AH and base B 9 1,16,17 negative enthalpy and entropy, and hydrocarbons tend to obtained in water. However, it is evident that the DpKa favorably form aggregates in EAN.10 EAN thus seems to behave value gives only a qualitative measure of solvent acidity and like water. The properties and applications of EAN were basicity, because all species, particularly ions, are strongly recently reviewed in the literature.11 hydrated in water, but not in neat PIL. A direct determination of Ethylammonium nitrate is miscible with water to form solvent acid-base strength is thus essential to establish the mixtures at any composition, and both of the component ions, solution chemistry in PILs. In the present study, the equilibrium + – C2H5NH3 and NO3 , can favorably form hydrogen bonds with constant, KS (= [B][AH]), for a proton-transfer reaction water. Physicochemical properties, such as the molar volume,12 BH+ + A– = B + AH has been determined by potentiometry in heat capacity,12,13 and vapor pressure14 have so far been neat EAN and EAN–water mixtures at 298 K. examined over the whole range of the solvent composition. The observed variation profiles of the quantities show that the EAN– water mixture is almost an ideal solution, and is considered to Experimental be an ideal model system for studying a highly concentrated electrolyte solution, or a mixture of ionic and molecular Materials liquids.15 Indeed, the ethylammonium ion involves three –NH Ethylammonium nitrate (EAN) has been prepared from – protons capable of hydrogen-bonding, and the NO3 ion binds to aqueous ethylamine (C2H5NH2, 70%) and nitric acid (HNO3, the proton through the O atom. Ions in neat EAN are thus 69%) (both Kishida Chemical, Japan) by mixing equimolar linked together to form a hydrogen-bonded network structure. amounts of C2H5NH2 and HNO3 in an ice bath. The thus prepared solution (pH = 5 – 6) was dried in vacuo at room † To whom correspondence should be addressed. temperature to give a colorless or pale-yellow liquid, which was E-mail: [email protected] then treated with activated charcoal, and further dried over 1348 ANALYTICAL SCIENCES OCTOBER 2008, VOL. 24 molecular sieves 3A. A small amount of excess ethylamine in EAN thus prepared was separately determined by potentiometry in aqueous solution. The water content in EAN was checked by a Karl Fisher test to be less than 80 ppm. The density is close to the reported value (1.212 g cm–3).18 Ethylamine was dried by distillation. Bis(trifluoromethanesulfonyl)amide (CF3SO2)2NH (Htfsa) crystals were kept in a glove box and used without further purification. Measurements –3 An Htfsa solution (10 mmol dm ) and a C2H5NH2 solution (ca. 200 mmol dm–3) were prepared in a glove box using EAN or an EAN–water mixture as a solvent. An acid Htfsa solution 3 (5 cm ) was titrated with a C2H5NH2 solution in a cell thermostated at 298 K. The cell used for emf measurements in neat EAN is represented as Ag, AgCl | 0.1 mol dm–3 NaCl (aq) || neat EAN || Fig. 1 Potentiometric titration curves in EAN–water mixtures of x = test solution (EAN) | Pt(H2), (1) 0.5 (f) and 0.9 (a), and those in neat EAN (S, A, F). The solid lines show curves calculated using E˚ and K . where either a Pt or an ion-selective field effect transistor S (IS-FET) electrode (Horiba) was used as a pH sensor, together with an Ag, AgCl electrode as a reference. A test solution was connected with a reference solution through a salt bridge. The HNO3, Htfsa is completely dissociated to yield HNO3 in an cell gave a satisfactorily stable potential, and showed a good EAN solution. It is thus expected that an acid-base reaction in Nernstian response in neat EAN. Potentiometric measurements an EAN solution of Htfsa occurs between HNO3 and C2H5NH2 + – in EAN–water mixtures were carried out using a conventional to give medium ions, C2H5NH3 and NO3 . glass electrode (ORION 81-02). Emfs obtained at each titration point are plotted against volume of the titrant added in Fig. 1. An emf jump is clearly seen at a given volume, where an excess HNO3 may be Results and Discussion neutralized by the titrant C2H5NH2. The emf may be given by E = E˚ + (RT/F)ln[HNO3], where E˚ denotes the standard Proton-transfer reaction in neat EAN potential of the cell. Indeed, the Gran’s plot gave a straight line Ethylammonium nitrate is an ionic liquid composed of in both acidic and basic regions, and a pKS value of 9.83 was + – C2H5NH3 and NO3 ions (the melting point of 285 K). Proton obtained, which is in satisfactory agreement with the value transfer among ions leads to the formation of small amount of (10.0) obtained using an IS-FET electrode. The pKS value of –5 –3 C2H5NH2 and HNO3 molecules according to 9.83 does mean that 1.2 ¥ 10 mol dm C2H5NH2 and HNO3 –3 + molecules are present, together with 11.3 mol dm C2H5NH3 + – – C2H5NH3 + NO3 æÆ C2H5NH2 + HNO3. (2) and NO3 ions, in neutral EAN. + – Here, the product, a(C2H5NH2)a(HNO3)/a(C2H5NH3 )a(NO3 ), Acid-base properties in EAN–water mixture gives an equilibrium constant, where a(i) denotes the activity of In an EAN–water mixture, water molecules, as well as + + – species i. If we take into account that [C2H5NH3 ] >> C2H5NH3 and NO3 , play an essential role in the acid-base – – [C2H5NH2] and [NO3 ] >> [HNO3] in neat EAN, the product property of the solvent. Since H2O is a stronger base than NO3 [C2H5NH2][HNO3] also gives a constant, KS, because the in water, proton-transfer may occur more favorably to H2O – + activities of ionic species are kept approximately constant, and rather than NO3 from C2H5NH3 in an EAN–water mixture. We the activity coefficients of C2H5NH2 and HNO3 may be kept suppose that a following equilibrium is established in a mixture, practically unchanged in a self-ionic medium of EAN. KS + + corresponds to the autoprotolysis constant of water KW C2H5NH3 + H2O = C2H5NH2 + H3O . (3) + – (= [H3O ][OH ]), or that of an amphoteric organic solvent, KS + – + (= [H2S ][S ]). Here, note that an amphoteric organic solvent, The equilibrium constant KS = [H3O ][C2H5NH2] is thus + – + – HS, yields the lyonium H2S and lyate S ions, whereas an obtained in an EAN–water mixture, where C2H5NH3 , NO3 and + – + amphoteric ionic liquid composed of AH and B yields their H2O behave as the solvent species, i.e., the amounts of H3O and conjugate base A and acid BH, according to a proton-transfer C2H5NH2 are much less than the solvent species.
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