Acid dissociation and solution equilibria of some pyridinecarboxylic acids
The acid–base equilibria of 2-hydroxynicotinic acid (hyna), 2-mercaptonicotinic acid (mena), 3-hydroxypicolinic acid (hypa) and pyridine-2,3-dicarboxylic acid (quinolinic acid, quina) have been studied spectrophotometrically in a water–ethanol medium containing 23.5 mol-% ethanol at 20 °C. The solution spectra of these compounds have been recorded at different pH values and the spectral changes are explained in terms of shifts in equilibria between the different species that exist in solution. Two equilibria have been established in solutions at pH values of approximately 3–10.5, based on the acid dissociation of the non-ionized form (LH2) of the reagents. The existence of these equilibria is confirmed by potentiometric pH titrations. The release of the proton from the cationic form (LH3+) of substituted nicotinic acids was verified by measurements of the spectra at pH 0.5–2. The pKa values corresponding to various acid-base equilibria have been determined and are discussed in relation to molecular structure. Tautomeric equilibria of the type: neutral species ⇌ zwitterionic species, in solutions of hyna and mena, have also been characterized using the ionization constants of the two acids.