Enthalpy and entropy of transfer of hydrogen ion from water to mixed solvents

Abstract

Attempts have been made to determine the enthalpy and entropy of transfer of H -t-ion from water to mixed solvents using the calorimetric data of earlier experiments. The results are in qualitative agreement with the facts that ~HO(H +) passes through an exothermic maximum and T~,S 0 passes through a minimum at about 20 to 30 wt% of organic solvent indicating the initial structure formation and the ultimate breakdown of the solvent structure with the addition of organic solvent.The importance and use of the free energies of transfer [~G~ or medium effects of the H + ion in different mixed or non-aqueous solvents are well-known. The medium effect of the H + ion is a quantitative measure of the differences in ion-solvent interactions which are the controlling factors in dilute solutions where ion-ion interactions are absent. Moreover, medium effects of H + ions in different solvents would measure the relative basicities of the solvents directly.Measurements of the medium effects of electroneutral combinations are relatively simple, but the thermodynamics provide no method to divide them into single-ion contributions. Though a number of theoretical and semi-empirical extrathermodynamic methods have been attempted to divide the free energies of transfer into single-ion contributions, all the methods have some inherent limitations and the results obtained are usually divergent [1][2][3][4][5].However, it has been widely admitted that the free energy of transfer of ions can be regarded as composed of a neutral part and an electrostatic part, i.e. ~GO(ion) = AGO(neut) + AGO(el)(ion)The values of the free energies of transfer of ions from one solvent to another are reliable in view of the compensation of the ~d-/0 and TZarS O terms, where the structural contributions of unknown magnitudes largely cancel out [5].However, the separation of ~d-/0 into single-ion contributions is difficult, as the enthalpies of solvation are mainly dependent on complex structural contributions arising from the making or breaking of bonds. Considerable disagreements (as high as 50 Kcal) in the single-ion enthalpy values in water have been reported [5]. Consistent results are not given by methods such as [6]

Solubilities of salts and kinetics of reactions involving inorganic complex ions in aqueous acetone mixtures. Derivation of transfer chemical potentials for ions in these aqueous mixtures at ambient pressure and 298.2 K

“…We will deal with points raised by Wellsz3 in a later paper which draws together estimates of transfer parameters for several binary aqueous systems. We do not accept the transfer parameters for ions in aqueous mixtures reported by Lahiri et al [24][25][26][27][28][29][30] It is claimed2'. 30 that the reported transfer parameters are not based on an extrathermodynamic assumption.…”