Dissociation Constants of Some Amino Acid and Pyridinecarboxylic Acids in Ethanol–H2O Mixtures

Abstract

Abstract The dissociation constants of nicotinic, picolinic, benzoic, and o-, m-, and p-aminobenzoic acids and those of glycine, dl-alanine, β-alanine, aniline, and pyridine were determined potentiometrically and spectrophotometrically in 0, 10, 30, 50, and 72% (w/w) ethanol–water mixtures at 25 °C at various ionic strengths (0.01–0.5 mol dm−3). The pK’s of benzoic, o-, m-, and p-aminobenzoic acids increased with increase of EtOH contents while those of nicotinic acid, picolinic acid, glycine, dl-alanine, and β-alanine passed through minimum (upto 30% w/w EtOH contents). The pK’s of aniline and pyridine were almost the same as those found in literature. The variation in pK’s with the solvent composition is discussed in terms of the free energy of transfer from H2O to EtOH–H2O mixtures. The zwitterion to neutral molecule ratio of these acids is also discussed in terms of variation of composition of mixtures.