Thermodynamics of proton ionization in aqueous solution :|bI. A calorimetric method for the determination of microconstants : pK, [Delta]H, and [Delta]S values for proton ionization from o-, m-, and p- aminobenzoic acids and their methyl esters at 25C : II. The extension of the entropy titration method to intermediate pK ranges (4


Calorimetric ΔH° values are given for stepwise proton dissociation from the protonated o-, m-, and p-aminobenzoic acids together with ΔH° values for proton ionization from the three corresponding methylaminobenzoates. The pK_1 values for the m- and p-aminobenzoic acids and pK values for the methylaminobenzoates were determined both by the entropy titration procedure and by the pH titration method. The pK_1 value for o-aminobenzoic acid was determined by entropy titration and the pK_2 values for o- and m-aminobenzoic acids were calculated from pH titration data. The ratios (K_Z) of the molar concentrations of zwitterion and neutral molecule for the three aminobenzoic acids were determined by both a pK method and a calorimetric method. An error analysis is made to provide insight into the relative merits of the two methods, and criteria are given which provide the best method for K_Z determination for any specific case. Microconstant, microenthalpy change, and microentropy change values are calculated for proton ionization from protonated m- and p-aminobenzoic acids from their respective K_Z and macrothermodynamic values. Previously, the calculation from calorimetric titration data of free energy, enthalpy, and entropy change values for protonation reactions in aqueous solution using stong acid or base solution as the titrant has been limited to substances whose pK values are either less than four or greater than ten. In this study, it is shown that the entropy titration method can be extended using weak acid or base solutions as titrants to cover the intermediate pK range ( 4>pK>10). To illustrate the applicability of this method using a weak acid titrant, pK, ΔH°, and ΔS° values for proton ionization from metanilic acid, pyridinium ion, imidazolium ion, protonated tris(hydroxymethyl)aminomethane and glycine have been determined from calorimetric titration data using a solution of acetic acid as titrant. Criteria are presented for titrant selection so the pK, ΔH°, and ΔS° values for any substance may be precisely and accurately determined by this method. A calorimetric study has been made of proton ionization from 71 protonated amines in aqueous solution at 25°. The resulting ΔH° values were combined with pK values to calculate ΔS° values. pK values were taken from the literature for 53 substances and new values were determined for 18 substances. A compilation of pK, ΔH° and ΔS° values from the present study and the literature is given for proton ionization from 171 amines. Hydrocarbon chain lengthening and branching effects on ΔH° and ΔS° values for proton ionization from primary and secondary aliphatic amines can be described by simple linear equations. Proton ionization from amines was not found to follow the linear relation between ΔG° and ΔS° predicted by the Bjerrum theory of electrostatics. The changes in ΔH° and ΔS° from the first to the second step of ionization for 37 diamines have been examined using the Bjerrum and Kirkwood-Westheimer theories of electrostatic interactions in aqueous solution. A deviation for the diamines, identical to that previously found for dicarboxylic acids, was found between the results derived from the experimental data and those predicted by the Kirkwood-Westhiemer theory. Possible reasons for this deviation are discussed.



College and Department

Chemistry and Biochemistry



Date Submitted


Document Type





Protons, Ionization



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