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

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

JOURNAL OF PHARMACEUTICAL AND BIOMEDICAL ANALYSIS

Elsevier

Año: 2009 vol. 49 p. 923 - 930

0731-7085

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 pK 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 pK a 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 pK by means of the Van?t Hoff approach.