Enthalpies and constants of dissociation of several neutral and cationic acids in aqueous and methanol/water solutions at various temperatures

Evaluation of ionic equilibria in mixed-buffer isothermal titration calorimetry and continuously stirred tank reactors

Determination of an equilibrium pH value in complex aqueous solution and deconvolution of this equilibrium to evaluate phenomena related to mixing, dilution, or progress of reaction is increasingly important in areas ranging from water quality to pharmaceutical formulations and manufacturing. Linearization of pH problems by simple algebraic substitution enables equilibria within complex buffered aqueous solutions to be modeled as an eigenvalue problem. This formulation approach makes rigorous determination of equilibrium pH values and reactor dynamics more accessible than with previous calculation methods, even when activity coefficients and non-ideality are considered. This work demonstrates how such calculations can enable detailed modeling of enthalpic changes in an isothermal titration calorimeter. In support of this work, the acid dissociation constants for three furancarboxylic acids (2-furancarboxylic acid, FA; 5-formyl-2-furancarboxylic acid, FFA; and 2,5-furandicarboxylic acid, FDCA), two of them novel, were determined and compared with multi-wavelength ultraviolet-visible spectrophotometry. The thermodynamic pKa values were determined to be 3.1 for FA, 2.2 for FFA, and 2.1 and 3.4 for the first and second ionization steps of FDCA, respectively.

Determination of acidity constants for weak acids and bases by isothermal titration calorimetry

The advantage of isothermal titration calorimetry (ITC) to determine the acid dissociation constant (pK_a value) is the simultaneous determination of the binding constant and binding enthalpy, as well as being precise and easy to use. The pK_a can be calculated from the binding constant, and the temperature dependency of the pK_a can be calculated from the binding enthalpy. The use of ITC to study protonation reactions is less common compared to its more conventional use of studying macromolecules and ligands. Water will influence the equilibrium due to autoionization, meaning that both the conjugate base and acid will exist in the sample cell at the beginning of the experiment. These differences are accounted for by optimizing the theoretical model used to estimate the binding constant and binding enthalpy. Through simulations and experimental measurements, we show that ITC can be used to determine the pK_a for ibuprofen, ascorbic acid, 2-morpholin-4-ylethanesulfonic acid and paracetamol. The pK_a values were consistent with potentiometric or spectrophotometric determinations as well as literature values. Optimizing the theoretical model does not lead to an improved determination, so the "one set of sites" model is adequate for the determination of pK_a values.

Photoacidity as a tool to rationalize excited state intramolecular proton transfer reactivity in flavonols

Determination of acidic strengths at the electronic ground and excited states of flavonol derivatives.