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

E. SHOGHI; C. RÁFOLS; E. BOSCH; L. ROMERO; M. RETA

Gdansk

Congreso; 19th International Symposium on Pharmaceutical & Biomedical Analysis; 2008

The acidic dissociation enthalpies and constants of anilinium, protonated tris(hydroxymethyl)aminomethane (HTris+), benzoic and acetic acids, have been determined at several temperatures in pure water and in methanol/water mixtures by potentiometry and by isothermal titration microcalorimetry (ITC). The pKa values determined by both techniques are in accordance when the dissociation process involves large amounts of heat. However, for the neutral acids the ITC technique gave slightly lower pKa values than those from potentiometry at the highest temperatures studied due to the small amounts of heat involved in the acidic dissociation. The dissociation enthalpies have been determined directly by calorimetry and the obtained values slightly decrease with the increase of temperature. Therefore, only a rough estimation of the dissociation enthalpies can be obtained from potentiometric pKa by means of the Van’t Hoff approach.