Effect of temperature on the protonation constants of some aromatic, heterocyclic nitrogen bases and the anion of 8-hydroxyquinoline

Abstract

Concentration-based association constants for the heterocyclic nitrogen atoms of 4,4′-bipyridyl (4,4′-bipy), 8-hydroxyquinoline (oxine), 8-aminoquinoline (amquin), pyridine (py) and the 2-(aminomethyl)pyridinium cation (2-ampy) with hydrogen ion have been determined in aqueous solution at various temperatures between 278 K and at least 373 K from spectrophotometric data. All the reactions are isoelectric and the constants decrease with increasing temperature. In two cases (oxine and 2-ampy) non-linear plots of pK_a against T^–1 were obtained. The reaction of base with oxine to yield the oxinate anion is also isoelectric but a non-linear plot of pK_b against T^–1 was obtained. The published, high-temperature pK_a values and associated thermodynamic quantities for 1,10-phenanthroline, some of its derivatives and 2,2′-bipyridyl have been re-evaluated using more realistic experimental errors.